WO2005049229A2

WO2005049229A2 - Apparatuses and methods for screening a product

Info

Publication number: WO2005049229A2
Application number: PCT/IB2004/003781
Authority: WO
Inventors: Giorgio Brighenti
Original assignee: Cuccolini S.R.L.
Priority date: 2003-11-21
Filing date: 2004-11-19
Publication date: 2005-06-02
Also published as: EP1697064A2; WO2005049229A3

Abstract

A screening apparatus comprises screen means (17) for screening a product and weight detecting means (24) operationally associated with said screen means (17); a screening apparatus comprises sieve means (103; 103') and further sieve means (103a; 103'') between which there is interposed barrier means (107; 107') arranged in such a way as to prevent a product screened by said sieve means (103; 103') from reaching said further sieve means (103a; 103''), an outlet (106; 106') of said sieve means (103; 103') through which a fraction of said product retained by said sieve means (103; 103') can be evacuated and an inlet (105a; 105'') terminating in said further sieve means (103a; 103''), between said outlet (106; 106') and said inlet (105; 105') connecting means (116; 119) being interposed; a method comprises pouring a product to be screened onto sieve means (103; 103') and furthermore pouring said product onto further sieve means (103b; 103''), said pouring comprising completely occupying said sieve means (103; 103') and said further pouring comprising partially occupying said further sieve means (103b; 103'').

Description

Apparatuses and methods for screening a product

The present invention relates to apparatuses and methods for screening a product.

Screening units are known comprising a first vibrating screen provided with a casing, suitable for receiving a product to be screened, that is delimited at the bottom by a first sieve consisting for example of a net made of metal wire or of plastic fibres and having meshes of preset dimensions.

The first sieve is provided with a motor-powered vibrator that vibrates the casing to facilitate the passage through the first sieve of a fraction of the product to be screened having smaller dimensions than those of the meshes.

The casing is supported on a base by means of a plurality of springs that enable the casing to oscillate in relation to the base.

The first screen is furthermore provided with a discharge opening arranged to evacuate from the casing a fraction of material that, having dimensions that are greater than the dimensions of the meshes, is unable to pass through the first sieve.

Furthermore, below the casing a gathering collector is provided that receives the fraction of product that has passed through the first sieve and directs it to a supply opening of a second vibrating screen that is structurally similar to the first vibrating screen with which the screening unit is provided.

The second screen is provided with a further sieve made with a further net having meshes that are larger than or the same as the size of the meshes of the first sieve.

The second screen is arranged downstream of the gathering collector of the first screen, in such a way that the fraction of material that has passed through the first screen is made to interact with the second screen.

The second screen therefore acts as a control device of correct operation of the first screen.

If the first screen performs the envisaged separating function effectively, the fraction of material coming from it crosses the further sieve without any particle of material being substantially retained by the latter.

On the other hand, if the net that constitutes the first sieve has breakages and/or lacerations and it is not therefore able to retain particles of material having a greater dimension than that of the meshes, part of the fraction of material coming from the first screen is retained by the further sieve.

In this case, an operator assigned to monitoring the screening unit, noticing the presence of particles of material retained by the further sieve, is alerted to the presence of the breakages in the first sieve and may repair and/or replace the net that makes it up.

In order to enable the operator to easily check for the presence of coarse particles of material on the further sieve, the second screen may be free of a closing lid.

The screening units disclosed above are usually used in productive systems of many different types, to screen materials that have been produced by apparatuses, or which have to supply apparatuses installed inside the system.

Such screening units are for example used in systems for the manufacture of ceramic products.

In such systems in fact the ceramic material that has been wet milled by appropriate mills, for example ball mills, must be screened before being sent to the atomiser.

A drawback of prior-art screening units consists of the fact that they require the constant attendance of an operator entrusted with monitoring for the presence of material on the further sieve.

This is particularly disadvantageous if the screening units are part of systems that operate continuously.

In this case, workshifts have to be organised that ensure the constant presence of an operator entrusted with the supervision of the screening units.

As a result, prior-art screening units have high running costs.

A drawback of the screening unit disclosed above consists of their poor efficiency that arises from the limited quantity of material that can be screened by the first vibrating screens over a given period of time.

As a result, in the event of a noticeable increase in the productivity of the apparatuses that the screening unit disclosed above have to supply, or by which the screening unit have to be supplied, it is necessary to considerably increase the number of the screening unit. This involves major redesigning of the systems in which the screening units are installed inasmuch as the zones intended to receive the screening units have to be decisively enlarged.

Owing to this, the general layout of the above-mentioned systems may have to be significantly revised.

In order to solve the above problem, screening units have been devised that are provided with a plurality of superimposed casings in such a way as to increase the screening surface without proportionally increasing the area occupied by the screens.

Each of the above casings comprises a sieve, at the bottom of which a baffle is arranged for collecting the fraction of material that has passed through the sieve to prevent this fraction from passing into a further casing located further downstream.

Each casing is therefore provided with its own supply conduit and with its own discharge conduit.

The supply conduits are supplied by a distributor of the material to be screened whereas the discharge conduits deposit the material through the respective sieves inside the same gathering collector.

Each casing is furthermore provided with a discharge opening through which the material is expelled that, having larger dimensions than the dimensions of the meshes of the sieve, is unable to pass through the sieve.

A drawback of such screening units consists of the fact that, although they are equipped with a screening surface having a larger extent than that of the first vibrating screens of the screening units disclosed previously, they do not enable satisfying productivity to be obtained, if they are used to screen fluid materials comprising a suspended solid, in particular fluid materials having high density and/or viscosity and from which a coarse part has to be separated that constitutes a relevant fraction of the above-mentioned solid part.

In the case of separation of coarse particles from a fluid, it is in fact desirable for the discharge from the sieve of the particles having larger dimensions that those of the meshes of the sieve to occur when the particles have been drained in such a way that the liquid part is prevalently separated therefrom.

For this purpose, the sieves of the screening units disclosed above have to be supplied with a head of liquid that affects only a central zone of the sieves in such a way that an annular ring remains defined in the sieves at which the coarse particles separate from the liquid and are free of dripping liquid. In other words, the screening units disclosed above, although comprising a plurality of superimposed screening surfaces, do not enable great productivity to be achieved inasmuch as the screening surfaces are provided with an active extension zone significantly less than the theoretical one. An object of the present invention is to improve prior-art screening apparatuses.

A further object is to obtain screening apparatuses that do not require the constant supervision of an operator entrusted with monitoring correct operation of the screening apparatuses. A further object of the invention is to obtain screening apparatuses that have reduced dimensions such as to contain its overall dimensions and to optimise exploitation of the spaces in which such apparatuses have to be installed.

In a first aspect of the invention, a screening apparatus is provided comprising screen means and support means arranged to support said screen means, characterised in that it further comprises weight detecting means interposed between said screen means and said support means .

Owing to this aspect of the invention, the weight detecting means enables checking of weight increases of the screen means, these weight increases being indicating of the granulometry of the material with which these screen means is supplied. In particular, if the screen means is located downstream of further screen means and is supplied with a fraction of material that has passed through the further screen means, a weight increase of the screen means is indicating of ineffective screening action by the further screen means, due, for example, to laceration of a screening net with which it is equipped. The weight detecting means may generate a signal indicating a screening apparatus operating fault.

Furthermore, the screen means may perform the sieving action that the further screen means is unable to perform, due to the above- mentioned laceration of the screening net, until the screening net is repaired or replaced.

The apparatus according to the invention does not therefore require the constant supervision of an operator entrusted with monitoring its correct operation. In a second aspect of the invention, a screening apparatus is provided, comprising screen means, evacuation means arranged to evacuate a fraction of a product retained by said screen means and detecting means associated with said evacuation means and arranged to detect a leak of said fraction from said screen means through said evacuation means.

Owing to this aspect of the invention, it is possible to detect the presence of a material retained by screen means and exiting it through the evacuation means.

If the screen means is located downstream of further screen means, as disclosed above, and is supplied with a fraction of material that has passed through further screen means, the exit of material from the evacuation means is indicative of an ineffective screening action by the further screen means.

This enables monitoring means of correct operation of a screening apparatus to be provided, which monitoring means is constructionally rather simple.

In a third aspect of the invention, an apparatus is provided comprising monitoring means suitable for monitoring screening surface means provided with openings, characterised in that said monitoring means comprises conduit means provided with orifice means having dimensions substantially the same as, and not less than, the dimensions of said openings.

Owing to this aspect of the invention, a screening apparatus can be provided that has limited overall dimensions inasmuch as it does not require a monitoring screen arranged downstream of the screen means.

The monitoring means according to the invention furthermore enables the apparatus to operate with great reliability.

It is furthermore possible to obtain a screening apparatus that does not require the constant attendance of operators to monitor correct operation of the apparatus. In this way, the need to provide personnel to supervise the apparatus is reduced or eliminated, thereby significantly reducing running costs.

In a fourth aspect of the invention, a method is provided for monitoring a screening apparatus, comprising picking up a product screened by said apparatus, advancing said product through a monitoring zone, characterised in that it further comprises detecting the pressure inside said monitoring zone, a variation of said pressure beyond a preset limit value indicating a breakage of screening surface means of said apparatus.

Owing to this aspect of the invention it is possible to obtain a screening apparatus that enables faults to be detected, if any in a very reliable manner, thus eliminating the possibility of malfunctions due to distraction and/or human errors.

Owing to these aspects of the invention disclosed above, it is possible to obtain a monitoring screen device associable with screens installed in an already existing productive system.

In particular, a single monitoring screen device can be supplied with a material originating from a plurality of screens, by means of suitable channels.

In a fifth aspect of the invention, a screening apparatus is provided, comprising sieve means and further sieve means between which there is interposed barrier means arranged in such a way as to prevent a product screened by said sieve means from reaching said further sieve means, an outlet of said sieve means through which a fraction of said product can be evacuated that is retained by said sieve means and an inlet terminating in said further sieve means, characterised in that connecting means is provided interposed between said outlet and said inlet.

In a sixth aspect of the invention, a method is provided comprising pouring a product to be screened onto sieve means and furthermore pouring said product onto further sieve means, characterised in that said pouring comprises completely inundating said sieve means and said further pouring comprises partially inundating said further sieve means.

Owing to these aspects of the invention, it is possible to obtain an apparatus equipped with high productivity and limited surface area.

In the apparatus according to the invention it is possible to superimpose a plurality of screening elements, in such a way as to cope with increased productivity of machines that supply the apparatus itself, or which have to be supplied by the latter.

Furthermore, the screening element, or the screening elements, located further upstream can be supplied with a considerable quantity of material to be screened that, in the case of a fluid material, may give rise to a head of significant height that affects the entire extent of the screening elements. On the other hand, the head of fluid material that is inserted in the screening element located further, downstream affects only a central portion of the sieve associated with the screening element so that an annular crown is defined therein on which a coarser fraction of the material to be screened can lose the liquid part dripping therefrom before being deposited in a gathering collector.

This enables the efficiency of the screening elements to be maximised that are located further upstream without affecting the efficacy of the discharge of the coarser material that constitutes the residue that accumulates on the sieve of the screening element located further downstream.

It should be noted that this is allowed by the fact that all the coarser material, retained by the different screening elements, is poured into a gathering collector only from discharge means associated with the screening element located further downstream, whereas further discharge means associated with screening elements located further upstream pour the coarser material, together with part of the material still to be screened, inside supply means of a screening element located downstream of them.

In a seventh aspect of the invention, a screening apparatus is provided, comprising screening net means, deflecting means arranged to guide a product to be screened along a path preset on said screening net means and support means supporting said deflecting means, said screening net means being interposed between said deflecting means and said support means.

Owing to this aspect of the invention, it is possible to obtain a screening apparatus in which the path followed by the particles of material to be screened on the screening net means has a considerable extent.

In the case of screening of a solid suspended product this enables the coarser fraction of this product to be separated almost completely from the liquid fraction before being evacuated from the screening net means, with subsequent improvement of apparatus performance.

In one embodiment, the deflecting means is shaped in such a way as to extend at least partially along a portion of the support means.

In this way the deflecting means, in addition to the task of directing the product to be screened, also performs the function of locking the screening net means, thereby prolonging the operating life of the latter.

The invention may be better understood and implemented with reference to the enclosed drawings that illustrate some embodiments by way of non-limitative example, in which:

Figure 1 is a schematic section, taken along a vertical plane, of a screening apparatus comprising three screening elements;

Figure 2 is a section like the one in Figure 1, showing a screening apparatus comprising two screening elements;

Figure 3 is a plan view of the apparatus in Figure 2;

Figure 4 is a side view of the apparatus in Figure 2;

Figure 5 is a schematic plan view of the apparatus in Figure 4;

Figure 6 is a schematic plan view of another embodiment of the apparatus in Figure 4;

Figure 7 is a side view of a screening apparatus;

Figure 8 is a plan view of the screening apparatus in Figure 7;

Figure 9 is a schematic side view of weight detecting means of the apparatus in Figure 7;

Figure 10 is an enlarged and partially sectioned detail of the weight detecting means in Figure 9;

Figure 11 is a view like the one in Figure 9, showing another embodiment of the weight detecting means;

Figure 12 is an enlarged and fragmentary detail of a screening apparatus comprising the weight detecting means in Figure 11;

Figure 13 is a view like the one in Figure 9, showing a further embodiment of the weight detecting means;

Figure 14 is a side view of a screening apparatus comprising the weight detecting means in Figure 13;

Figure 15 is a side view of a screening apparatus comprising a yet further embodiment of the weight detecting means;

Figure 16 is a fragmentary side view of a screening apparatus made according to one version;

Figure 17 is a side view of screen means of the screening apparatus in Figure 16;

Figure 18 is a plan view of the screen means in Figure 17;

Figure 19 is an enlarged and partially sectioned detail of Figure

17;

Figure 20 is a fragmentary side view of another embodiment of the screen means; Figure 21 is a fragmentary side view of a further embodiment of the screen means;

Figure 22 is an enlarged and fragmentary detail of Figure 21;

Figure 23 is a fragmentary side view of a yet further embodiment of the screen means;

Figure 24 is a fragmentary plan view of the screen means in Figure

23;

Figure 25 is an enlarged and fragmentary detail of the apparatus in Figure 17;

Figure 26 is a fragmentary side view of the detail in Figure 25.

Figure 27 is a fragmentary side view of a yet further embodiment of the screen means;

Figure 28 is an enlarged and fragmentary detail in Figure 27;

Figure 29 is a fragmentary side view of a yet further embodiment of the screen means;

Figure 30 is a fragmentary side view of screen means made according to a yet further embodiment;

Figure 31 is a fragmentary side view of a further embodiment of the screen means;

Figure 32 is an enlarged and fragmentary detail of Figure 31;

Figure 33 is a partially sectioned side view of an apparatus for screening a product made according to a further version;

Figure 34 is a side view of a screening apparatus made according to another version;

Figure 35 is a schematic view of monitoring means of the screening apparatus in Figure 34;

Figure 36 is a side view of a screening apparatus made according to a further version;

Figure 37 is a schematic view of monitoring means of the screening apparatus in Figure 36.

Figure 1 shows a screening apparatus 100 comprising a first casing

102 inside which a first screening element is positioned 103 consisting of a first net 104 having meshes of a preset size.

The first casing 102 furthermore comprises a first supply opening

105 by means of which the material to be screened is conveyed to the first screening element 103 and a first discharge conduit 106 arranged to evacuate from the first casing 102 the coarser material on the first net 104, i.e. the material that, having larger dimensions than those of the meshes of the first net 104 has been unable to pass through the first screening element 103.

The first casing 102 furthermore comprises a first baffle 107 that is impermeable to the material to be screened and arranged downstream of the first screening element 103 to collect the finer material that, having smaller dimensions than the dimensions of the meshes of the first net 104, has passed through the first screening element 103.

The first baffle 107 directs the finer material to a first outlet

109.

The screening apparatus 100 furthermore comprises a second casing

102a substantially analogous to the first casing 102 e comprising a second screening element 103a equipped with a second net 104a, a second supply opening 105a and a second discharge conduit 106a.

The second casing 102a is equipped with a second baffle 107a that collects the finer material that has passed through the second net

104a to direct it to a second outlet 109a.

The screening apparatus 100 is furthermore provided with a third casing 102b comprising a third screening element 103b equipped with a third net 104b, a third supply opening 105b and a third discharge conduit 106b.

The third casing 102b is provided with a lower portion 110 shaped as an upturned conical trunk that directs the finer material that has passed through the third net 104b to a third outlet 109b.

The first casing 102, the second casing 102a and the third casing

102b have a substantially circular section.

The first net 104, the second net 104a and the third net 104b have meshes with substantially the same dimensions.

The first casing 102, the second casing 102a and the third casing

102b are arranged so as to be superimposed, so as to minimise the surface occupied by the screening apparatus 100.

The apparatus furthermore comprises an eccentric-mass vibrator 111 driven by an electric motor 113.

The lower portion 110 of third casing 102b, which in addition to the third screening element 103b also supports the first casing

102 and the second casing 102a, is connected to a base 114 of the screening apparatus 100 by means of springs 115 that enable the first casing 2, the second casing 102a and the third casing 102b to oscillate in relation to the base 114. The first baffle 107 prevents the finer material that has passed through the first net 104 from falling inside the second casing

102a located downstream of the first casing 102.

Similarly, the second baffle 107a prevents the finer material that has passed through the second net 104a from falling inside the third casing 102b located downstream of the second casing 102a.

The first discharge conduit 106 is connected to the second supply opening 105a by means of a collector 116, whereas the second discharge conduit 106a is connected to the third supply opening

105b by means of a further collector 117.

During operation, a material to be screened is introduced inside the first casing 102 through the first supply opening 105.

Part of the material is screened by the first screening element

103 in such a way that a finer fraction of the material passes through the first net 104, whereas a coarser fraction of the material is retained by the first net 104.

The finer fraction retained by the first baffle 107, is conveyed to the first outlet 109.

A further part of the material to be screened is transferred, together with the coarser fraction retained by the first net 104, inside the second casing 2a by means of the collector 116.

In the second casing 102a a further finer fraction passes through the second net 104a, whereas a further coarser fraction, retained by the second net 104a, is transferred together with the coarser fraction and a yet further part of the material to be screened inside the third casing 102b, by the further collector 117.

The further finer fraction retained by the second baffle 107a is conveyed to the second outlet 109a.

The third screening element 103b separates a yet further finer fraction that is gathered inside the lower portion 110 of the third casing 102b and is expelled through the third outlet 109b.

The first outlet 109, the second outlet 109a and the third outlet

109b terminate in the same gathering container.

The third discharge conduit 106b expels from the screening apparatus 100 the coarser material comprising the coarser fraction separated by the first screening element 103, the further coarser fraction separated by the second screening element 103a and the yet further coarser fraction separated by the third screening element 103b. If the material to be screened comprises a fluid suspension of solid particles, it is possible to supply the first casing 102 and the second casing 102a in such a way as to obtain in each of them a head of fluid suspension of a desired height, this head being sufficiently high to enable the entire surface of the first net 104 and of the second net 104a to be wet by the fluid suspension. The fluid suspension can be introduced inside the screening apparatus 100 in a controlled manner such that the head of fluid suspension inside the third casing 102b is such as to affect only a central zone 118 of the third net 104b, in such a way that in the latter an annular crown 120 is defined that is not affected by the passage of the fluid suspension.

In particular, the collector 116 and the further collector 117 may be provided with a vertically movable shutter that enables regulation of the quantity of material to be screened, in particular fluid suspension, that can pass from the first casing 102 to the second casing 102a and from the second casing 102a to the third casing 102b, respectively.

The annular crown 120 in this way defines a gathering zone of the particles of solid material having dimensions greater than those of the meshes of the third net 104b.

In this gathering zone the particles of solid material can thus become completely separated from the fluid inside which they were immersed.

This enables the discharge of the solid particles by the third screening element 103b to be made simpler than is the case with particles of solid material still mixed with the fluid. The productivity of the screening apparatus 100 is considerably increased because it is not necessary to limit the extent of the head inside all the casings that make it up, but only inside the casing located further downstream, at which the entire coarser fraction of the material to be screened is expelled from the screening apparatus 100.

Furthermore, providing a head of liquid that completely wets the sieves housed in the casings located further upstream enables screening speed to be increased thanks to a greater quantity of material that, with its own weight, presses down on the sieves. As Figures 2 to 4 show, the screening apparatus 100 may comprise two superimposed screening elements rather than three, in particular an upper casing 102' and a lower casing 102''.

The upper casing 102' and the lower casing 102'' are structurally similar to the first casing 102 and to the third casing 102b described in Figure 1.

In Figures 2 and 4 the details of the upper casing 102' corresponding to those of the first casing 102 are indicated by the same numeric reference with which they were indicated in

Figure 1, said numeric reference being followed by a single apex.

Similarly, the details of the lower casing 102'' corresponding to those of the third casing 102b are indicated by the same numeric reference with which they were indicated in Figure 1, said numeric reference being followed by a double apex.

The screening apparatus 100 shown in Figures 2 and 4 furthermore comprises a yet further collector 119 that connects a supply opening 105'' arranged to supply the lower casing 102'' with the material to be screened with a discharge conduit 106' arranged to evacuate the coarser fraction of the material to be screened that has been retained in the upper casing 102'.

Similarly to what has been disclosed in Figure 1, the yet further collector 119 may be provided with a vertically movable shutter that enables the quantity of fluid suspension to be adjusted that can pass from the upper casing 102' to the lower casing 102''.

Alternatively, the screening apparatus may comprise a number of casings greater than three, depending on the productivity that is desired.

Figures 4 and 5 show a screening apparatus 100 made according to one version and provided with a upper casing 102' and with a lower casing 102'' each one of which comprising a support structure 74 provided with a circular portion 75 and with radial portions 76 on which an upper net 104' and a lower net 104'' rest respectively.

Inside the upper casing 102' and the lower casing 102'' flow deflectors are provided 73, each one of which is provided with walls 79 having a preset height.

Alternatively, a single flow deflector can be provided associated with the upper net 104', or with the lower net 104''.

Each flow deflector 73 is arranged above the upper net 104', or above the lower net 104'', respectively so that the walls 79 protrude from the upper net 104', or from the lower net 104'', by an amount that is such that during operation the material to be screened is forced to follow a path - shown by arrows F5 in Figure 5 - that is longer than the path that the material to be screened would follow if the flow deflector 73 were absent.

In this way, the solid particles of material remain on the upper net 104', on the lower net 104'', for a prolonged period in such a way that the fraction of liquid material is separated completely from the particles before the expulsion of the coarser fraction. Each flow deflector 73 comprises a further radial portion 78 and a further circular portion 77 that extends for an angle that is less than a round angle, in such a way as to define a passage gap 80. The further circular portion 77 is shaped in such a way as to couple in a joined manner with the circular portion 77, located immediately below the net. Similarly, the further radial portion 78 is shaped in such a way as to couple in a joined manner with one of the radial portions 76.

The apparatus furthermore comprises locking means, for example bolts, arranged to tighten the upper net 104', or the lower net 104'', between the flow deflector 73 and the support structure 74. Owing to this configuration, in the zones near the walls 79, the net is well tightened between the support structure 74 and the flow deflector 73.

In this way, the tendency of the upper net 104 ' and of the lower net 104'' to detach themselves from the walls 79 through the effect of the weight of the material to be screened is counteracted.

Figure 6 shows one embodiment of the flow deflector 73 comprising a yet further radial portion 82 that interconnects the further circular portion 77 with a yet further circular portion 83. In this way, in the upper casing 102' and in the lower casing 102'' a path is defined for the material to be screened having a greater extent than the path that the material has to travel in the apparatus embodiment in Figures 4 and 5.

In the yet further circular portion 83 a further passage gap 84 is obtained that enables the material to be screened, initially poured to the inside the yet further circular portion 83, to flow through a channel 85 identified between the yet further circular portion 83 and the further circular portion 77. Subsequently, after passing through the passage gap 80, the material to be screened flows into a further channel 86 defined between the further circular portion 77 and the upper casing 102', or the lower casing 102''.

As a result, yet more time will be available for achieving a separation of the liquid fraction from the solid fraction before evacuation of the coarser material from the apparatus 1.

Figures 7 and 8 show a screening unit 1 comprising a screening apparatus 100 of the type shown with reference to Figures 2 to 6 and a monitoring device 58, associated with the screening apparatus 100, that enables correct operation of the screening apparatus 100 to be monitored.

In Figures 7 and 8 the details of the screening apparatus 100 are indicated by the same numerical references with which they were indicated in Figures 2 to 6.

Alternatively, the screening unit 1 may comprise a screening apparatus of any type, in particular a usual screen provided with a sole net associated with a sole casing.

The monitoring device 58 comprises a screen 17 provided with a casing 18 inside which a further net is obtained.

The further net has meshes having dimensions the same as, or greater than the dimensions of the meshes of the upper net 104' and of the lower net 104''.

The casing 18 is equipped with a further supply opening 19 through which the fraction of material that has passed through the upper net 104' or the lower net 104'' and has been conveyed to a gathering conduit 9 is introduced inside the screen 17.

As the further net is provided with meshes of the same size as, or greater than the size of the meshes of the upper net 104' and of the lower net 104'', the screen 17 acts as a monitoring device of correct operation of the screening apparatus 100.

In fact, if the coarser fraction of material is retained by the upper net 104' and by the lower net 104'', the material that interacts with the further net pass through the meshes of the latter without being retained.

On the other hand, if the screening apparatus 100 does not perform its sieving action correctly, in particular due to lacerations that affect the upper net 104' and/or the lower net 104'', part of the material coming from the gathering conduit 9 is retained by the further net.

The casing 18 is furthermore provided with an evacuation conduit

20 through which a finer fraction of material that has passed through the upper net 104', the lower net 104'' and the further net is expelled from the screen 17.

The casing 18 is furthermore provided with a further discharge opening 57 through which a coarser fraction of material is expelled that has been intercepted by the further net in the event of breakage of the upper net 104' and/or of the lower net 104''.

The further discharge opening 57 thus enables the further net to be suitably cleaned.

During normal operation of the screening unit 1, i.e. when the upper net 104' and the lower net 104'' are whole, the further discharge opening 57 is shut by suitable closing means, for example a plug, that can be removed from the latter to enable evacuation of the coarser material retained by the further net if breakages of the upper net 104' and/or of the lower net 104'' have occurred.

Similarly to what has been disclosed with reference to the screening apparatus 100, the casing 18 is vibrated by further motor-powered vibrator 22.

The casing 18 is associated with a support ring 23 by means of further springs 21.

Between the support ring 23 and a frame 15 of the screening unit 1 weight detecting means 24 is interposed that, once it has been suitably calibrated, detects an increase in the weight of the screen 17 due to certain quantity of material retained by the further net with which it is provided.

As a result, this weight increase occurs if the screening apparatus 100 is unable to retain a coarser fraction of the material.

In this way, incorrect operation of the screening apparatus 100 is indicated by an increase in the weight of the screen 17 and it is therefore not necessary to provide for the attendance of an operator entrusted with checking that no coarser fraction of material is retained and expelled from the screen 17, as occurs in prior-art screening apparatuses. The weight detecting means 24 may be associated with signalling means, that are not shown, that produces a signal indicating incorrect operation of the screening apparatus 100.

As Figures 8 to 10 show, the weight detecting means 24 comprises three hydraulic cylinders 25 arranged at equidistant angles from one another.

Each hydraulic cylinder 25 is connected to the frame 15 ed e internally provided with a piston from which a stem 26 leads away that is fixed to a first end 29 of a rod 27.

The rod 27 is slidable inside a tubular element 28 that acts as a guide, the tubular element 28 being fixed to the frame 15 by means of screws 30.

Inside the tubular element 28 there is provided a pair of bushes

31, interposed between the tubular element 28 and the rod 27, which facilitate sliding of the rod 27 in relation to the tubular element 28.

At a further end 32, opposite the first end 29, the rod 27 is connected to the support ring 23 by a further screw 33.

With the further end 32 a cover 34 is associated that partially covers the rod 27 and the tubular element 28.

With the cover 34 a bellows sheath 35 is associated extending between the cover 34 and the frame 15.

The cover 34 and the sheath 35 prevent polluting particles from infiltrating between the rod 27 and the tubular element 28, thus preventing sliding of the rod 27 in relation to the tubular element 28.

Each hydraulic cylinder 25 is provided with an opening 36 connected to a conduit 56 by means of a further conduit 37.

The conduit 56, the hydraulic cylinders 25 and the respective further conduits 37 define a hydraulic circuit 38 inside which a fluid is contained.

The hydraulic circuit 38 is delimited by a first stopcock 39 and by a second stopcock 40 arranged at opposite ends of the conduit

56.

The hydraulic circuit 38 furthermore comprises a gauge 41 that enables the pressure of the fluid inside the hydraulic circuit 38 to be checked and a control unit 42 provided with a pressure switch arranged to detect whether the pressure is above a preset threshold value. During an initial calibration phase, a certain quantity of fluid is introduced inside the hydraulic circuit 38, in such a way that the pressure of the fluid is such as to bear the weight of the screen 17.

Once an equilibrium configuration has been reached between the pressure of the fluid and the weight of the screen 17 the first stopcock 39 and the second stopcock 40 are shut.

In this way it is not necessary to provide a constantly operating pump that keeps the fluid at a desired pressure.

The threshold value of the pressure switch is set in relation to the size of the value of the pressure inside the hydraulic circuit

38 reached at the end of calibration operations.

During operation, in the event of breakage of the upper net 104' and/or of the lower net 104'' a certain fraction of coarser material is deposited on the further net.

The weight of the screen 17 thus increases, thus causing a corresponding increase of pressure inside the hydraulic circuit

38.

When pressure inside the hydraulic circuit 38 exceeds the threshold value at which the pressure switch is calibrated, the control unit 42 produces an alarm signal.

Figures 11 and 12 show one embodiment of the monitoring device 58 in which the support ring 23 is arranged at a lower vertical height than the frame 15 and is appended to the latter by means of the hydraulic cylinders 25.

In the configuration disclosed above, each hydraulic cylinder 25 has its own further opening 43 connected to the conduit by means of a respective further conduit 37.

Advantageously the hydraulic cylinders 25 may be dual-effect hydraulic cylinders - in which the opening 36 communicates with a first chamber whereas the further opening 43 communicates with a second chamber - that are nevertheless used as single-effect hydraulic cylinders, i.e. only one chamber of them is filled with the fluid.

In particular, in the embodiment of the screening unit 1 in

Figures 9 and 10 only the first chamber contains the fluid.

On the other hand, in the embodiment of the screening unit 1 in

Figures 11 and 12 only the second chamber contains the fluid. In the embodiment of the screening unit 1 in Figures 11 and 12 an increase in the weight of the screen 17, caused by the presence of a coarser fraction of material retained by the further net causes a leak of the stem 26 from the hydraulic cylinder 25.

In this case, the possible presence of polluting particles in contact with the movable parts of the hydraulic cylinder 25 does not prevent sliding of the stem inasmuch as a portion of it that has been previously received inside the cylinder and is therefore clean interacts with a part of the cylinder.

Furthermore, the structure of the weight detecting means 24 is simplified inasmuch as it is not necessary to provide guide devices such as the rod 27 and the tubular element 28.

Figures 13 and 14 show a further embodiment of the screening unit

1 in which the weight detecting means 24 comprises load cells 44, for example of the electric resistance variation type, interposed between the frame 15 and the support ring 23.

The load cells 44 are arranged in such a way as to be at equidistant angles from one another, in other words, they are positioned on the support ring 23 at angular intervals of 120°.

Alternatively, the load cells 44 can be positioned in such a way as not to be equidistant from one another, in other words, the angular spaces between the adjacent load cells may be different from one another.

During operation, the load cells 44 detect an increase in the weight of the screen 17 and communicate this weight increase to the control unit 42.

The control unit 42 in turn generates an alarm signal.

Figure 15 shows a yet further embodiment of the screening unit 1, in which the weight detecting means 24 comprises mechanical scale means 45.

The mechanical scale means 45 comprises a lever 46, shaped like a plate hinged, at an intermediate portion 47 thereof, on the frame

15.

The lever 46 furthermore comprises an end portion 49 hinged on the support ring 23 and an extended portion 48 on which a counterweight 50 is slidable, as indicated by the arrow F.

The lever 46 is furthermore provided with a further end portion

51, opposite the end portion 49, shaped in such a way as to interact in operation with a microswitch 52 connected to a control unit 42.

Alternatively, instead of the microswitch 52, there can be provided a photocell or another detecting device, such as a proximity sensor.

During operation, in an initial calibrating phase, the screen 17, on the further net of which no coarser fraction of material is substantially present, is positioned in an equilibrium configuration by acting on the counterweight 50.

In the equilibrium configuration, the screen 17 is supported at a set zone 54 of the support ring 23 by the lever 46, and at further zones 55 of the support ring by abutting elements 53 on which the support ring 23 is supportingly received.

During operation, an increase in the total weight of the screen

17, caused by the presence of a coarse fraction of material retained by the further net, causes the lever 46 to rotate, as indicated by the arrow R, and consequent actuation of the microswitch 52.

The microswitch 52 transmits information to the control unit 42 on the basis of which the latter generates an alarm signal.

Figures 16 to 19 show a screening unit lb comprising a screening apparatus of the disclosed type referring to Figures 2 to 6 with which a monitoring device 58 is associated equipped with a screen

17 provided with a casing 18 inside which a further net 189 is obtained (Figure 29) .

Alternatively, the screening unit lb may comprise a screening apparatus of any type, in particular a usual screen provided with a sole net associated with a sole casing.

The further net 189 has meshes having dimensions the same as, or greater than the dimensions of the meshes of the upper net 104' and of the lower net 104''.

The casing 18 is associated with a support plate 123 by means of further springs 21.

Between the support plate 123 and the frame 15 weight detecting means 24 is interposed that detects an increase in the weight of the screen 17 due to a certain quantity of material retained by the further net 189 with which it is provided.

Such a weight increase may occur if the screening apparatus 100 is unable to retain a coarser fraction of the material. The weight detecting means 24 may be associated with signalling means that is not shown that produces a signal indicating incorrect operation of the screening apparatus.

The support plate 123 is rotatably connected to the frame 15 by means of hinge means 130 that enable the screen 17 to rotate around a hinge means 130 axis in relation to the frame 15.

In Figure 19, the weight detecting means 24 comprises a further hydraulic cylinder 125, provided with a base body 137 fixed to the support plate 123 inside which a chamber 143 is obtained that receives a fluid, for example oil. The base body 137 receives within itself a piston 138 from which a stem 126 leads away that is fixed to a first end 129 of a rod 127.

Inside the base body 137 a bush 131 is provided interposed between the base body 137 and the stem 126, which facilitates sliding of the stem 126. A seal ring 128, interposed between the stem 126 and the base body 137 prevents oil leaks from the chamber 143. To the first end 129 a cover 134 is connected with which a sheath 135 is associated that partially covers the stem 126 and prevents polluting particles from entering the further hydraulic cylinder

125.

The rod 127, at its own second end 132, opposite the first end

129, is provided with protruding elements 133 arranged at right angles to it and perpendicularly to a plane A, shown in Figure 18, passing through the centre line of the screen 17.

The protruding elements 133 engages in slot means 139 obtained in the frame 15, and extending in a direction B parallel to plane A and substantially perpendicular to a longitudinal axis C of the further hydraulic cylinder 125. Owing to this configuration, following rotation of the support plate 123 around the hinge means

130, the protruding elements 133 can slide inside the slot means 139 in direction B thus enabling the further hydraulic cylinder 125 to be placed in retracted configuration or in extended configuration.

The further hydraulic cylinder 125 is provided with an opening 136 associated with a hydraulic circuit that delivers the oil into the chamber 143. The base body 137 comprises, at one of its upper ends 140, a bleed valve 141, for example a micrometric valve, that is used during calibration operations of the further hydraulic cylinder 125 to expel any air bubbles that may be present inside the chamber 143 without however expelling the oil, such air bubbles being capable of adversely affecting correct operation of the further hydraulic cylinder 125.

As the bleed valve 141 is located at a greater height than the opening 136, the operation of expulsion of the air bubble is rapid and effective as the air bubbles tend to be arranged above the oil contained inside the chamber 143.

In a first installation phase of the screening unit lb, as the further hydraulic cylinder 125 does not contain oil, an upper part of the piston 138 is in contact with a bottom wall of the chamber

143.

Subsequently, as the oil is progressively delivered inside the further hydraulic cylinder 125 there is a corresponding rotation of the screen 17 around the hinge means 130.

This rotation is permitted by sliding of the protruding elements

133 along the slot means 139.

At the end of delivery of the oil, the further hydraulic cylinder

125 is arranged in such a way that its longitudinal axis C is directed substantially vertically.

The hydraulic circuit comprises a gauge that enables checking of the pressure of the fluid inside the hydraulic circuit and a control unit provided with a pressure switch arranged to detect if this pressure is greater than a threshold value.

During an initial calibration phase, a certain quantity of oil is delivered inside the hydraulic circuit, such that the pressure of the oil is able to bear the weight of the screen 17.

Once an equilibrium configuration has been achieved between the pressure of the oil and the weight of the screen 17, the hydraulic circuit is shut, for example by means of a first stopcock located downstream of the further hydraulic cylinder 125 and a second stopcock arranged upstream of the further hydraulic cylinder 125.

In this way, it is not necessary to provide a continuously operating pump that keeps the oil at a desired pressure.

The threshold value of the pressure switch is set according to the pressure value inside the hydraulic circuit at the end of calibration operations.

During operation, in the case of breakage of the upper net 104' and/or of the lower net 104'' a certain fraction of coarser material is deposited on the further net 189. The weight of the screen 17 therefore increases, causing a corresponding pressure increase inside the hydraulic circuit.

When the pressure inside the hydraulic circuit exceeds the threshold value at which the pressure switch is calibrated, the control unit produces an alarm signal.

Figures 21 and 22 show another embodiment of the monitoring device

58, which is provided with weight detecting means 24 comprising yielding support means cooperating with shift detecting means.

The yielding support means comprises a pneumatic cylinder 142 equipped with a further chamber 145, fixed to the support plate

123 and arranged to receive inside itself a compressible fluid, for example air.

Inside the further chamber 145 a piston element 180 is slidable that is connected to the frame 15 by means of a further rod 146, provided with further protruding elements 147 engaging in further slot means 148.

The pneumatic cylinder 142 is structurally similar to the further hydraulic cylinder 125.

The support plate 123 is hinged on the frame 15 in a similar manner to what has been disclosed with reference to the embodiment of the monitoring device 58 specified in Figures 16 to 19.

The shift detecting means comprises sensor means 150, for example switches, or photocells, or proximity sensors, or a linear encoder that detect a significant position of an actuating element 149 integral with the support plate 123 and positioned near the further chamber 145.

The actuating element 149 takes on a preset position when no particle is retained inside the screen 17, namely in conditions of correct operation of the screening apparatus 100.

In the event of breakage of the upper net 104' and/or of the lower net 104'' of the screening apparatus 100, particles of dimensions that are greater than the dimensions of the meshes of the upper net 104' and of the lower net 104'' are retained by the further net 189 of the screen 17.

The progressive accumulation of such particles causes a weight increase that weighs down on the support plate 123, thereby compressing the air contained inside the pneumatic cylinder 142 and causing a progressive approach of the actuating element 149 to the sensor means 150. In this configuration, the actuating element 149 is in a further position, in which it faces the sensor means 150.

At this point, the sensor means 150, by detecting the presence of the actuating element 149, sends information to a control unit.

As the pneumatic cylinder 142 uses a compressible fluid rather than oil the monitoring device 58 has the advantage of not requiring a circuit for the operating fluid, thus enabling overall dimensions to be reduced.

In one embodiment that is not shown, the sensor means 150 may comprise a switch that is actuatable by the actuating element 149.

In a further embodiment that is not shown a plurality of pneumatic cylinders rather than just one sole pneumatic cylinder may be provided.

Figures 23 to 26 show a further embodiment of the monitoring device 58, wherein the yielding support means comprises a spring

151, for example of the helical type.

The spring 151 comprises a first end portion 152, which is integral with the support plate 123, and a second end portion 153, opposite the first end portion 152, with which a yet further rod

154 is connected. The yet further rod 154 is connected below to the frame 15, by means of yet further protruding elements 155 that engage in yet further slot means 156, in a manner that is completely similar to what has been disclosed with reference to

Figures 16 to 19.

Near the first end portion 152, buffer means 158 is provided that is fixed on a face 160 of the support plate 123 facing the second end portion 153 and made of a deformable material, for example rubber.

In one zone of the frame 15 facing the buffer means 158 stop elements 159 are fixed that prevent the buffer means 158 and therefore the first end portion 152 of the spring 151 from advancing beyond a preset lower height.

In this way the stroke of the spring 151 is limited, thus preventing the latter being damaged during operation because of excessive weight acting on the support plate 123.

The shift detecting means comprises further sensor means 162 connected to the frame 15 and arranged in such a way as to interact, during use, with a further actuating element 161 fixed to the face 160 of the support plate 123, near the first end portion 152.

The further sensor means 162 may comprise, for example, switches, or photocells, or proximity sensors, or a linear encoder that detect a significant position of the further actuating element

161, and therefore of the support plate 123. The further sensor means 162, operates in a similar manner to what has been disclosed previously with reference to the sensor means 150.

Figures 27 and 28 show a yet further embodiment of the monitoring device 58, which is provided with yielding support means and with shift detecting means and in which the screen 17 is directly supported on the frame 15 by means of the further springs 21.

This embodiment of the monitoring device 58 differs from the previous embodiments disclosed, with reference to Figures 16 to 19 and 21 to 26 inasmuch as the support plate 123 and the hinge means

130 that connects the latter to the frame 15 are not provided.

The shift detecting means comprises yet further sensor means 163 provided in one zone of the frame 15 and arranged in such a way as to interact with a yet further actuating element 164 that is connected to a bottom 165 of the screen 17.

The yet further actuating element 164 can be fixed in relation to the bottom 165 or be adjustable to approach or move away from the bottom 165 in an adjusting direction D.

Similarly, also the position of the yet further sensor means 163 in relation to the frame 15 can be varied according to the amount of the shift to which the screen 17 can be subjected in the event of a weight increase due to the accumulation of particles on the further net 189 with which it is provided.

As an alternative to the possibility of adjusting the position of the yet further actuating element 164 and/or of the yet further sensor means 163 in the adjusting direction D, the stroke of the screen 17 can be varied by using springs having appropriate stiffness values.

In this way, by appropriately choosing the stiffness value of the springs and/or by varying the reciprocal position of the yet further sensor means 163 and of the yet further actuating element

164, it is possible to adjust the degree of sensitivity and also the speed with which the weight detecting means 24 detect a variation in the total weight of the screen 17. Further buffer means 166 is furthermore provided shaped in such a way as to interact during use with further stop elements 167, in such a way as to limit the stroke of the further springs 21 during compression, to prevent damage to the weight detecting means 24. This embodiment of the monitoring device 58 has the advantage of being cheaper than the previous embodiments. It is in fact not necessary to provide further yielding elements - for example the pneumatic cylinder 142, or the spring 151 - in addition to the further springs 21 and the support plate 123 is not provided connected to the frame 15 by means of the hinge means 130. Figure 20 shows a yet further embodiment of the monitoring device 58 in which the screen 17 is arranged on the support plate 123 by means of the further springs 21 and in which the weight detecting means 24 comprises a load cell 168, for example of the type with variation of electrical resistance, interposed between the frame 15 and the support plate 123.

During operation, the load cell 168 detects an increase in the weight of the screen 17 and communicates this weight increase to a control unit that is in turn able to generate an alarm signal. Figure 29 shows a yet further embodiment of the monitoring device 58 in which the screen 17 is directly supported on the frame 15 by means of the further springs 21.

On a portion of the casing 18 a discharge gap 169 is obtained that is arranged at a vertical height E that is greater than further vertical height F at which the further net 189 of the screen 17 is arranged.

The discharge gap 169 enables a fraction of coarser material to be evacuated that is retained by the screen 17.

With the discharge gap 169 there is associated a conveying conduit 170 suitable for conveying the fraction of coarser material to a gathering container 171, arranged at a vertical height below the casing 18.

Inside the gathering container 171 a level indicator 172 is provided comprising an elongated body 173 provided at one of its ends 174 with a float element 176. The elongated body 173 is rotatable around a hinge 175 fixed to the gathering container 171.- The elongated body 173 is provided at a further end opposite the end 174, with a contact portion 177 suitable for interacting with a sensor 190, for example a microswitch, or a photocell, or a proximity sensor, or a linear encoder.

Alternatively, float-type level indicators can be provided according to constructional solutions that are different from those disclosed in detail.

Also alternatively, the level indicator may be of the capacitive type, such an indicator detecting an increase in level by measurements of conductivity of the product received inside the gathering container.

Alternatively, the level indicator may be of the vibration type.

Such an indicator is provided with a vibrating element, the oscillations of which are dampened by the presence of a product received inside the gathering container.

Also alternatively, the level indicator may be of the ultrasound type.

During operation, the fraction of coarser material exiting the screen 17 through the discharge gap 169 is conveyed by the conduit conveyor 170 to the gathering container 171. An increase in the level of the product contained in the gathering container 171 causes a shift upwards of the float element 176. In this way, the elongated body 173 is rotated around the hinge 175, thereby causing the contact portion 177 to interact with the sensor 190 connected to a control unit.

In one version of the monitoring device 58 disclosed above, the gathering container 171 is not fixed to the frame 15 but can be supported by a weight sensor operationally connected to the control unit, which detects material contained inside the gathering container 171.

In another version of the monitoring device 58, the gathering container 171 is not fixed to the frame 15 but can be supported by a yielding element, for example a spring, associated with a sensor, for example a proximity sensor, or a microswitch.

The material received inside the gathering container 171 causes compression of the yielding element in such a way that the sensor, detecting the variation in the position of the gathering container

171, can send information to the control unit.

Figure 30 shows a yet further embodiment of the monitoring device

58 in which the screen 17 is provided with a further conveyor conduit 178 and with a discharge gap 169, arranged similarly to what has been disclosed for the embodiment in Figure 29.

Inside the further conveying conduit 178 an intercepting element

179 is arranged comprising a lamina body 181 having an extent such as to substantially occupy the entire transverse section of the further conveying conduit 178.

The lamina body 181 is able to rotate around a hinge 183 fixed to a wall of the further conveying conduit 178.

A counterweight 182 is furthermore provided that is fixed to the lamina body 181, which counterweight 182 maintains the lamina body

181 in a rest position X indicated by a continuous line drawing in

Figure 30.

In the rest position X the lamina body 181 identifies a substantially horizontal position.

As an alternative to the counterweight 182, the lamina body 181 can be maintained in the rest position X by a spring.

During operation, in the event of damage to the upper net 104 ' and/or to the lower net 104'' of the screening apparatus 100, a fraction of coarser material accumulates on the further net 189.

Tale fraction of coarser material passes through the discharge gap

169, runs through the further conveying conduit 178 and interacts with the lamina body 181.

As a result, the lamina body 181 is rotated in the direction indicated by the arrow R, passing from the rest position X to an operating position Y shown in Figure 30.

In the operating position Y, the lamina body 181 interacts with a sensor 191, that may comprise a microswitch, or a proximity sensor, or a photocell.

The sensor 191 sends information to a control unit that in turn produces an alarm signal.

Figures 31 and 32 show a yet further embodiment of the monitoring device 58, in which the screen 17 is provided with a discharge gap

169 with which a yet further conveying conduit 184 is connected that extends along a substantially vertical axis 185.

Inside the yet further conveying conduit 184 a wheel 186 is arranged from which blades 194 leave radially, rotatable around a rotation axis 192 arranged perpendicularly to the axis 185.

A fraction of coarser material expelled from the screen 17 through the discharge gap 169 runs along the yet further conveying conduit 184 until it interacts with the blades 194 of the wheel 186 that is thus rotated.

The wheel drives 186, by means of a drive element 187, for example a belt, a detecting wheel 188, or a phonic wheel.

With the detecting wheel 188 a sensor element 193 is associated that may for example be a photocell, which sends information to a control unit that in turn produces an alarm signal.

Alternatively, the detecting wheel 188 may comprise a wheel provided with cam elemnts that during rotation drive microswitches operationally connected to the control unit.

Figure 33 shows a screening unit Id comprising a screening apparatus 100a provided with a net 310. At a height near the net

310 a discharge opening 311 is provided that conveys outside the coarser material retained by the net 310.

The screening unit Id comprises a monitoring device 58 provided with a monitoring screen 302 supplied, by means of a conduit 305, with a screened material that has passed through the net 310.

The monitoring screen 302 is provided with a further net 320 having meshes the dimensions of which are the same as or larger than the meshes of the net 310.

The monitoring screen 302 is shaped in such a way that the flow of screened material coming from the screening apparatus 100a can in conditions of correct operation of the screening apparatus 100a be completely evacuated without creating accumulations or clogging on the further net 320. In other words, the screening surface of the further net 320 is sufficiently extended to receive without accumulations or clogging all the screened material thereupon that is directed from the conduit 305.

With the further net 320 a further discharge opening 321 is associated that is arranged above the further net 320 at a set distance from the latter. In this way, the particles of material that have dimensions larger than the dimensions of the meshes of the further net 320 are retained inside the monitoring screen 302 until they reach the height of the further discharge opening 321 .

A level indicator device 303 is fitted on an adjustable support

304 fixed to a base 322 of the monitoring screen 302. The adjustable support 304 enables the height position of the level indicator 303 device to be adjusted in relation to the surface of the further net 320. In particular, the level indicator 303 device is positioned in such a way that a sensitive element with which it is provided is arranged at a distance from the further net 320 that is less than the preset distance that separates the further discharge opening 321 from the further net 320.

The level indicator 303 device enables an alarm signal to be sent if the material that accumulates above the further net 320 has reached a preset height. In particular, this height is comprised between the height of the further net 320 and the height of the further discharge opening 321.

If the screening apparatus 100a operates correctly, the entire flow of material screened by the screening apparatus 100a passes through the monitoring screen 302.

If damage to the net 310 occurs, the conveyed material that reaches the monitoring screen 302 through the conduit 305 has coarse particles that are retained by the further net 320. These particles accumulate on the further net 320 and, before reaching the height of the further discharge opening 321, interact with the level indicator device 303, which indicates that the net 310 has been damaged.

In one embodiment that is not shown, the monitoring screen is delimited on the side by a casing provided with openings intended to enable the discharge of a fraction of coarser material retained by the further net.

In this case, the fraction of coarser material accumulates inside the casing and interacts with the level indicator device.

In the event of poor operation of the screening apparatus 100a, an operator who has been alerted by signalling means actuated by the level indicator device, manually empties the monitoring screen, overturning at least partially the casing, and furthermore repairs the net of the screening apparatus.

In a further embodiment that is not shown, the further discharge opening may be provided immediately upstream of the further net

320.

In this case, during normal operation of the screening unit Id, i.e. when the net 310 is whole, the further discharge opening is closed by suitable closing means, for example a plug, that can be removed from the latter to evacuate the fraction of coarser material retained by the further net, if breakages of the net 310 have occurred. Figures 34 to 37 show a screening unit la comprising a screening apparatus 100 of the type shown in Figures 2 to 6 and a monitoring device 58, associated with the screening apparatus 100 that enables correct operation of the screening apparatus 100 to be monitored.

In Figures 34 to 37, the details of the screening apparatus 100 are indicated by the same numeric references with which they were indicated in Figures 2 to 6.

Alternatively, the screening unit la may comprise a screening apparatus of any type, in particular an usual screen provided with a sole net associated with a sole casing.

In particular, the monitoring device 58 enables monitoring of the presence if any, in the product part that has passed through the upper net 104' and/or the lower net 104'', of particles having dimensions greater than the dimensions of the meshes of the upper net 104' and of the lower net 104'', i.e. due to undesired laceration of the upper net 104' and/or of the lower net 104''.

The monitoring device 58 comprises conduit means 70 through which part of the material is passed that has passed through the upper net 104' and/or the lower net 104'' and which is conveyed by a first conduit 59 and a second conduit 60 to a gathering collector

61. This part of the material is picked up by the collector 61 by means of a drain conduit 62.

This part may comprise just one portion of the entire quantity of material that has passed through the upper net 104' and/or the lower net 104 ' ' .

Alternatively, this part of material comprises a first part taken directly from the first conduit 59 and a second part taken directly from the second conduit 60.

The conduit means 70 comprises a pipe 68 that is provided at the bottom with a nozzle 66 comprising a calibrated orifice 67, the dimensions of which are the same as, or greater than the dimensions of the meshes of the upper net 104' and of the lower net 104' ' .

As Figures 34 and 35 show, the monitoring device 58 furthermore comprises a pump 63, for example of the pneumatic type, interposed between the drain conduit 62 and the pipe 68. In particular, the pump 63 is provided with a suction opening connected to the drain conduit 62 and with a delivery opening connected to the pipe 68. Downstream of the pump 63 a pressure measurer, for example a gauge

64 is provided that is associated with a control unit 65 provided with a pressure switch.

During operation, part of the material that has passed through the upper net 104' and/or the lower net 104'' is aspirated by the pump

63 and is then made to flow to the nozzle 66.

In the case of correct operation of the screening apparatus 100, all the part of the material drained from the gathering collector

61 that is conveyed in the pipe 68 from the pump 63 is able to pass through the calibrated orifice 67, inasmuch as it does not comprise particles having dimensions greater than the dimensions of the meshes of the upper net 104' and of the lower net 104'', and therefore of the dimensions of the calibrated orifice 67.

All the part of drained material, after passing through the monitoring device 58, is conveyed by a pipe 69, to a collecting station where it is added to the finer material conveyed by the gathering collector 61.

On the other hand, if a screening apparatus 100 fault arises due to breakages in the upper net 104' and/or the lower net 104'', particles having greater dimensions than the dimensions of the meshes of the upper net 104' and of the lower net 104'' are retained inside the nozzle 66 causing a blockage of the calibrated orifice 67.

In such circumstances, the gauge 64 indicates a pressure increase inside the pipe 68 and sends a message to the control unit 65 to generate an alarm signal and to stop operation of the screening apparatus 100 so as to enable the repair and/or replacement of the damaged components.

As Figures 36 and 37 show, the nozzle 66 is positioned at a top end of the pipe 68 in such a way as to be directly connected to the drain conduit 62. In this case the nozzle 66 is positioned upstream of the pump 63. Between the nozzle 66 and the pump 63 the gauge 64 is arranged, which is associated with the control unit

65.

In the case of correct operation of the screening apparatus 100, a part of the material drained from the gathering collector 61 is aspirated by pump 63 and is passed through the calibrated orifice

67 of the nozzle 66. Subsequently, all the part of drained material is conveyed by the pipe 69 to a collecting station so as to be added to the fine material conveyed by the gathering collector 61.

On the other hand, if a screening apparatus 100 fault arises due to breakages in the upper net 104' and/or the lower net 104'', particles having greater dimensions than the dimensions of the calibrated orifice 67 - present in the part of material- they cause a blockage of the latter. In such circumstances, inside the pipe 68 a pressure fall occurs that is detected and indicated by the gauge 64. The latter sends a message to the control unit 65 to generate an alarm signal and to stop operation of the screening apparatus 100 so as to enable the repair and/or replacement of the damaged components.

In the embodiments of screening unit la disclosed above, an operator, alerted by the control unit 65, must check in which of the nets there is a laceration before proceeding to repair or replacement .

Alternatively, a monitoring device 58 may be provided that is associated with the upper casing 102' (i.e. with the upper net

104'), and a further monitoring device associated with the lower casing 102'' (i.e. with the lower net 104'').

In this the control unit 65 will inform the operator directly of the net affected by the laceration.

The monitoring device 58 disclosed above can also be fitted to existing screens.

Claims

1. Screening apparatus, comprising screen means (17) for screening a product, characterised in that it further comprises weight detecting means (24) operationally associated with said screen means (17).

2. Apparatus according to claim 1, and furthermore comprising frame means (15), said weight detecting means (24) being interposed between said screen means (17) and said frame means (15) .

3. Screening apparatus according to claim 1, or 2, wherein said weight detecting means (24) comprises hydraulic cylinder means (25; 125) .

4. Apparatus according to claim 3, wherein said hydraulic cylinder means (25; 125) is supplied with a fluid contained in hydraulic circuit means (38) .

5. Apparatus according to claim 4, wherein said hydraulic cylinder means (25; 125) supports said screen means (17), in such a way that a weight increase of said screen means (17) is matched by an increase in the pressure of said fluid means .

6. Apparatus according to claim 4, or 5, wherein said hydraulic circuit means (38) comprises pressure switch means.

7. Apparatus according to any one of claims 4 to 6, wherein said hydraulic circuit means comprises gauge means (41) .

8. Apparatus according to any one of claims 3 to 7 as claim 3 is appended to claim 2, wherein, during use part of said frame means (15) with which said hydraulic cylinder means (25) is associated is arranged below a portion (23) of said screen means (17) with which said hydraulic cylinder means (25) is associated.

9. Apparatus according to any one of claims 3 to 7 as claim 3 is appended to claim 2, wherein, during use part of said frame means (15) with which said hydraulic cylinder means (25) is associated, is arranged above a portion (23) of said screen means (17) with which said hydraulic cylinder means (25) is associated in such a way that said screen means (17) is hung to said frame means (15) by means of said hydraulic cylinder means (25) .

10. Apparatus according to any one of claims 3 to 9, wherein said hydraulic cylinder means comprises three hydraulic cylinders (25) positioned in such a way as to be angularly equidistant from one another.

11. Apparatus according to any one of claims 3 to 9, wherein said hydraulic cylinder means comprises a single hydraulic cylinder (125) .

12. Apparatus according to claim 11, wherein said screen means (17) comprises a portion hinged on framework means (15) and a further portion that interacts with said hydraulic cylinder (125) .

13. Apparatus according to any one of claims 10 to 12, wherein said three hydraulic cylinders (25) and said single hydraulic cylinder (125) comprises chamber means (43) provided with bleed valve means (41) suitable for enabling the expulsion of air from said chamber means (43) .

14. Apparatus according to claim 1, or 2, wherein said weight detecting means (24) comprises load cell means (44; 168).

15. Apparatus according to claim 14, wherein said load cell means (44; 168) is of the electric resistance variation type.

16. Apparatus according to claim 14, or 15, wherein, said load cell means comprises three load cells (44) positioned in such a way as to be angularly equidistant from one another.

17. Apparatus according to claim 14, or 15, wherein said load cell means comprises a single load cell (168) .

18. Apparatus according to claim 17, wherein said screen means (17) comprises a portion hinged on framework means (15) and a further portion that interacts with said load cell (168) .

19. Apparatus according to claim 1, or 2, wherein said weight detecting means (24) comprises yielding support means (142; 21; 151) arranged to yieldingly support said screen means (17) and sensor means (150; 163; 162) arranged to detect a shift of said screen means (17) .

20. Apparatus according to claim 19, wherein said screen means (17) comprises a portion hinged on framework means (15) and a further portion that interacts with said yielding support means (142; 151) .

21. Apparatus according to claim 19, or 20, wherein said sensor means comprises photocell means.

22. Apparatus according to claim 19, or 20, wherein said sensor means comprises switch means .

23. Apparatus according to claim 19, or 20, wherein said sensor means comprises proximity sensor means.

24. Apparatus according to claim 19, or 20, wherein said sensor means comprises encoder means.

25. Apparatus according to any one of claims 19 to 24, wherein said yielding support means (21) is fixed to a casing (18) of said screen means (17) inside which screening means (189) is housed.

26. Apparatus according to any one of claims 19 to 24, wherein said yielding support means (142; 151) is fixed to support means (123) that supports, by shock absorbing means (21) , a casing (18) of said screen means (17) within which there is lodged screening means (189) .

27. Apparatus according to any one of claims 19 to 26, and furthermore comprising actuating means (149; 164; 161) suitable for cooperating with said sensor means (150; 163; 162) .

28. Apparatus according to claim 27 as appended to claim 25, wherein said actuating means (164) is associated with said casing (18) .

29. Apparatus according to claim 27 as appended to claim 26, wherein said actuating means (149; 161) is associated with said support means (23) .

30. Apparatus according to any one of claims 19 to 29, wherein said yielding support means comprises spring means (21; 151).

31. Apparatus according to claim 30, wherein said spring means comprises a spring (21; 151) .

32. Apparatus according to any one of claims 19 to 29, wherein said yielding support means comprises pneumatic cylinder means (142) .

33. Apparatus according to claim 32, wherein said pneumatic cylinder means comprises a pneumatic cylinder (142) .

34. Apparatus according to claim 1, wherein said weight detecting means (24) comprises mechanical scale means (45).

35. Apparatus according to claim 34, wherein said mechanical scale means (45) comprises lever means (46) hinged at one of their intermediate zones (47) to frame means (15).

36. Apparatus according to claim 35, wherein said lever means (46) comprises an end zone (49) on which is hinged a portion (54) of said screen means (17), said screen means (17) comprising at least a further portion (55) restingly received on said support means (15) .

37. Apparatus according to claim 36, wherein said lever means (46) comprises a further end zone (51) , opposite said end zone (49), arranged to interact with sensor means (52).

38. Apparatus according to claim 37, wherein said sensor means comprises a microswitch (52) .

39. Apparatus according to claim 37, wherein said sensor means comprises a photocell.

40. Apparatus according to claim 37, wherein said sensor means comprises a proximity sensor.

41. Apparatus according to any one of claims 35 to 40, wherein said lever means (46) comprises an elongated element (48) on which a counterweight (50) is slidable.

42. Apparatus according to any preceding claim, and furthermore comprising further screen means (100) arranged upstream of said screen means (17) .

43. Apparatus according to claim 42, wherein said further screen means (100) comprises transfer means arranged to supply said screen means (17) with a material screened by said further screen means (100) .

44. Apparatus according to claim 43, wherein said transfer means comprises pipe means terminating inside a supply opening (19) of said screen means (17) .

45. Apparatus according to any preceding claim, wherein said screen means (17) comprises vibrating screen means provided with screening net means (189) .

46. Apparatus according to any one of claims 42 to 44, or according to claim 45 as appended to any one of claims 42 to 44, wherein said further screen means (100) comprises further vibrating screen means provided with further screening net means (104' , 104' ') .

47. Apparatus according to claim 46 as appended to claim 45, wherein said screening net means (189) comprises meshes having dimensions greater than or the same as, dimensions of further meshes of said further screening net means (104', 104' ') .

48. Screening apparatus, comprising screen means (17), evacuation means (169; 170; 178; 184) arranged to evacuate a fraction of a product retained by said screen means (17) and detecting means (172; 179; 186) associated with said evacuation means (169) and arranged to detect a leak of said fraction from said screen means (17) through said evacuation means (169) .

49. Apparatus according to claim 48, and furthermore comprising containing means (171) arranged downstream of said evacuation means (169; 170) and suitable for receiving said fraction.

50. Apparatus according to claim 49, wherein said detecting means comprises level indicating means (172) arranged to detect an increase in the level of said fraction inside said containing means (171) .

51. Apparatus according to claim 50, wherein said level indicating means (172) comprises a body (173) at an end of which a float element (176) is fixed arranged to interact with said fraction.

52. Apparatus according to claim 51, wherein said body (173) comprises a further end (174) arranged to interact with a sensor element (190) .

53. Apparatus according to claim 49, wherein said detecting means comprises weight detecting means arranged to detect a weight increase of said containing means (171).

54. Apparatus according to claim 53, wherein said weight detecting means comprises yielding support means arranged to yieldingly support said containing means (171) and sensor means arranged to detect a shift of said containing means (171) .

55. Apparatus according to claim 54, wherein said yielding support means comprises pneumatic cylinder means.

56. Apparatus according to claim 54, wherein said yielding support means comprises spring means.

57. Apparatus according to claim 53, wherein said weight detecting means comprises hydraulic cylinder means.

58. Apparatus according to claim 53, wherein said weight detecting means comprises load cell means.

59. Apparatus according to claim 48, wherein said detecting means comprises intercepting means (179; 186) received inside conduit means (178; 184) arranged downstream of said evacuation means (169) .

60. Apparatus according to claim 59, wherein said intercepting means (179) comprises shutter means (181) arranged to intercept said fraction and movable between a rest position (X) , wherein it substantially occupies a transverse section of said conduit means (178), and an operating position (Y) in which it interacts with sensor means (191) .

61. Apparatus according to claim 59, wherein said intercepting means comprises blade means (194) extending from wheel means (186) .

62. Apparatus according to claim 61, wherein said wheel means (186) is rotationally supported on an internal portion of said conduit means (184) .

63. Apparatus according to claim 61, or 62, wherein said wheel means (186) is connected by drive means (187) to detecting wheel means (188) .

64. Apparatus according to claim 63, wherein said detecting wheel means (188) comprises a phonic wheel.

65. Apparatus according to claim 63, wherein said detecting wheel (188) is provided with protruding portions arranged to interact with further sensor means (193).

66. Apparatus according to any one of claims 48 to 65, and furthermore comprising further screen means (100) arranged upstream of said screen means (17).

67. Apparatus according to claim 66, wherein said further screen means (100) comprises transfer means arranged to supply said screen means (17) with a material screened by said further screen means (100) .

68. Apparatus according to any one of claims 48 to 67, wherein said screen means (17) comprises vibrating screen means provided with screening net means (189) .

69. Apparatus according to claim 66, or 67, or 68 as appended to claim 66 or 67, wherein said further screen means (100) comprises further vibrating screen means provided with further screening net means (104', 104'').

70. Apparatus according to claim 69 as appended to claim 68, wherein said screening net means (189) comprises meshes having dimensions greater than or the same as, dimensions of further meshes of said further screening net means (104', 104 ' ' ) .

71. Apparatus, comprising monitoring means (58) suitable for monitoring screening surface means (104', 104'') provided with openings, characterised in that said monitoring means (58) comprises conduit means (68) provided with orifice means (67) having dimensions substantially the same as, and not less than, the dimensions of said openings.

72. Apparatus according to claim 71, wherein said conduit means (68) is arranged downstream of said screening surface means (104', 104'') in such a way as to interact with particles of said product that have passed through said screening surface means (104', 104' ') .

73. Apparatus according to claim 72, wherein said conduit means (68) is supplied with only a part of the entire quantity of said product that has passed through said screening surface means (104', 104 ' ' ) .

74. Apparatus according to any one of claims 71 to 73, wherein said monitoring means (58) comprises pressure-measuring means (64) arranged to detect the pressure inside said conduit means (68) .

75. Apparatus according to claim 74, wherein said pressure measuring means comprises a gauge (64).

76. Apparatus according to claim 74, or 75, wherein said monitoring means (58) furthermore comprises a control unit (65) operationally connected to said pressure measuring means (64) .

77. Apparatus according to claim 76, wherein said control unit (65) comprises a pressure switch.

78. Apparatus according to any one of claims 71 to 77, and furthermore comprising pump means (63) arranged to direct said particles to said conduit means (68) .

79. Apparatus according to claim 78, wherein said pump means (63) is connected to said conduit means (68) in such a way as to be arranged upstream of said orifice means (67) .

80. Apparatus according to claim 79 as claim 78 is appended to any one of claims 74 to 77, wherein said pressure measuring means (64) detects a pressure value greater than a limit value, if said orifice means (67) is obstructed by a particle of said product having dimensions greater than the dimensions of said orifice means (67) .

81. Apparatus according to claim 78, wherein said pump means (63) is connected to said conduit means (68) in such a way as to be arranged downstream of said orifice means (67).

82. Apparatus according to claim 81, as claim 78 is appended to any one of claims 74 to 77, wherein said pressure measuring means (64) detects a pressure value below a further limit value if said orifice means (67) is obstructed by a particle having dimensions greater than the dimensions of said orifice means (67) .

83. Apparatus according to any one of claims 71 to 82, and furthermore comprising motor-powered vibrator means (111) arranged to vibrate said screening surface means (104', 104' ') .

84. Method for detecting a screening apparatus (100), comprising picking up a product screened by said apparatus (100) , advancing said product through a monitoring zone (58), characterised in that it further comprises detecting the pressure inside said monitoring zone (58), a variation of said pressure beyond a preset limit value indicating a breakage of screening surface means (104', 104'') of said apparatus (100) .

85. Method according to claim 84, wherein said picking up comprises picking up only a portion of said product.

86. Method according to claim 84, or 85, wherein said advancing comprises pumping said product.

87. Method according to claim 84, or 85, wherein said advancing comprises aspirating said product.

88. Method according to any one of claims 84 to 87, wherein said advancing comprises directing said product to orifice means (67).

89. Screening apparatus, comprising sieve means (103; 103') and further sieve means (103a; 103'') between which there is interposed barrier means (107; 107') arranged in such a way as to prevent a product screened by said sieve means (103; 103') from reaching said further sieve means (103a; 103''), an outlet (106; 106') of said sieve means (103; 103') through which a fraction of said product retained by said sieve means (103; 103') can be evacuated and an inlet (105a; 105'') terminating on said further sieve means (103a; 103' characterised in that connecting means is provided (116; 119) interposed between said outlet (106; 106') and said inlet (105; 105') .

90. Apparatus according to claim 89, wherein said connecting means (116; 119) comprises adjusting means arranged to monitor the quantity of said product that can be transferred from said sieve means (103; 103') to said further sieve means (103a; 103'').

91. Apparatus according to claim 89, or 90, wherein said sieve means (103; 103') is arranged upstream of said further sieve means (103a; 103' ' ) .

92. Apparatus according to any one of claims 89 to 91, wherein said sieve means (103; 103') is superimposed to said further sieve means (103a; 103'').

93. Apparatus according to any one of claims 89 to 92, wherein said sieve means (103; 103') comprises filtering net means (104; 104') having meshes with dimensions substantially the same as the dimensions of the meshes of further filtering net means (104a; 104'') of said further sieve means (103a; 103' ') .

94. Apparatus according to any one of claims 89 to 93, wherein said sieve means (103; 103') and said further sieve means (103a; 103'') have a substantially circular section.

95. Apparatus according to any one of claims 89 to 94, and furthermore comprising yet further sieve means (103b) arranged downstream of said further sieve means (103a) .

96. Apparatus according to claim 95, and furthermore comprising yet further connecting means (117) interposed between a further outlet (106a) of said further sieve means (103a) through which a further fraction of said product retained by said further sieve means (103a) can be evacuated and a further inlet (105b) terminating on said yet further sieve means (103b) .

97. Apparatus according to claim 95, or 96, and furthermore comprising further barrier means (107a) interposed between said further sieve means (103a) and said yet further sieve means (103b) in such a way as to prevent a product screened by said further sieve means (103a) from reaching a said yet further sieve means (103b) .

98. Apparatus according to any one of claims 89 to 97, and furthermore comprising evacuation means (109; 109') arranged to evacuate from said sieve means (103; 103') part of said material that has passed through said sieve means (103; 103') .

99. Apparatus according to any one of claims 89 to 98, and furthermore comprising further evacuation means (109a; 109'') arranged to evacuate from said further sieve means (103a; 103'') a further part of said material that has passed through said further sieve means (103a; 103'').

100. Apparatus according to any one of claims 95 to 97, or according to claim 98, or 99, as appended to any one of claims from 95 to 97, and furthermore comprising yet further evacuation means (109b) arranged to evacuate from said yet further sieve means (103b) a yet further part of said material that has passed through said yet further sieve means (103b) .

101. Apparatus according to claim 100 as appended to claims 98 and 99, wherein said evacuation means (109; 109'), said further evacuation means (109a; 109'') and said yet further evacuation means (109b) terminate inside a same gathering collector.

102. Apparatus according to claim 93, or according to any one of claims 94 to 101 as appended to claim 93, and comprising deflecting means (73) arranged to guide a product to be screened along a path preset on said filtering net means (104; 104') and/or on said further filtering net means (104a; 104'') and support means (74) supporting said deflecting means (73), said filtering net means (104; 104') and/or said further filtering net means (104a; 104'') being interposed between said deflecting means (73) and said support means (74).

103. Apparatus according to claim 102, and furthermore comprising locking means arranged to tighten said filtering net means (104; 104') and/or said further filtering net means (104a; 104'') between said deflecting means (73) and said support means (74) .

104. Apparatus according to claim 102, or 103, wherein said deflecting means (73) is shaped in such a way as to extend along at least a portion (75, 76) of said support means (74) .

105. Apparatus according to claim 104, wherein said portion (75, 76) occupies a zone inside said filtering net means (104; 104') and/or of said further filtering net means (104a; 104' ') .

106. Apparatus according to any one of claims 102 to 105, wherein said path extends from a central zone of said filtering net means (104; 104') and/or of said further filtering net means (104a; 104'') to a peripheral zone of said filtering net means (104; 104') and/or of said further filtering net means (104a; 104'') at which there is provided evacuation means of a coarser fraction of said product that was retained by said filtering net means (104; 104') and/or by said further filtering net means (104a; 104'').

107. Apparatus according to any one of claims 102 to 106, wherein said path has a coil shape.

108. Apparatus according to any one of claims 102 to 107, wherein said path has a labyrinth shape.

109. Apparatus according to any one of claims 102 to 108, wherein said deflecting means (73) comprises wall means (79) protruding from said filtering net means (104; 104') and/or from said further filtering net means (104a; 104'').

110. Method, comprising pouring a product to be screened onto sieve means (103; 103') and furthermore pouring said product onto further sieve means (103b; 103''), characterised in that said pouring comprises completely occupying said sieve means (103; 103') and said further pouring comprises partially occupying said further sieve means (103b; 103'').

111. Method according to claim 110, wherein said partially occupying comprises depositing said product on a central portion (118) of said further sieve means (103b; 103'').

112. Method according to claim 111, wherein said depositing comprises identifying in said further sieve means (103b; 103'') an annular portion (120) arranged to receive a fraction of said product retained by said further sieve means (103b; 103'').

113. Method according to claim 112, and furthermore comprising evacuating said fraction from said annular portion (120) .

114. Method according to any one of claims 110 to 113, wherein said further pouring comprises transferring part of said product from said sieve means (103; 103') to said further sieve means (103b; 103'').

115. Method according to claim 114, wherein said transferring comprises picking up said part from an outlet (106; 106') of said sieve means (103; 103') by connecting means (116, 119) and directing said part to an inlet (105a; 105'') of said further sieve means (103b; 103'').

116. Method according to claim 114, or 115, and furthermore comprising shifting shutter means associated with said connecting means (116; 119) to monitor said transferring.

117. Method according to any one of claims 110 to 116, and furthermore comprising preventing a further fraction of said product that has passed through said sieve means (103; 103') from reaching said further sieve means (103b; 103'').

118. Method according to claim 117, wherein said preventing comprises interposing barrier means (107; 107') between said sieve means (103; 103') and said further sieve means (103b; 103' ' ) .

119. Method according to claim 117, or 118, wherein said preventing furthermore comprises picking up said further fraction from said sieve means (103; 103').

120. Screening apparatus, comprising screening net means (104'; 104''), deflecting means (73) arranged to guide a product to be screened along a path preset on said screening net means (104'; 104'') and support means (74) supporting said deflecting means (73), said screening net means (104'; 104'') being interposed between said deflecting means (73) and said support means (74).

121. Apparatus according to claim 120, and furthermore comprising locking means arranged to tighten said screening net means (104'; 104'') between said deflecting means (73) and said support means (74).

122. Apparatus according to claim 120, or 121, wherein said deflecting means (73) is shaped in such a way as to extend along at least a portion (75, 76) of said support means (74) .

123. Apparatus according to claim 122, wherein said portion (75, 76) occupies a zone inside said screening net means (104'; 104 ' ' ) .

124. Apparatus according to any one of claims 120 to 123, wherein said path extends from a central zone of said screening net means (104'; 104'') to a peripheral zone of said screening net means (104'; 104'') at which evacuation means of a coarser fraction of said product is provided that has been retained by said screening net means (104'; 104'').

125. Apparatus according to any one of claims 120 to 124, wherein said path has the shape of a coil.

126. Apparatus according to any one of claims 120 to 125, wherein said path has a labyrinth shape.

127. Apparatus according to any one of claims 120 to 126, wherein said deflecting means (73) comprises wall means (79) protruding from said screening net means (104'; 104'').

128. Apparatus according to any one of claims 1 to 47 and/or according to any one of claims 48 to 70 and/or according to any one of claims 71 to 83 and/or according to any one of claims 89 to 109 and/or according to any one of claims 120 to 127.

129. Method according to any one of claims 84 a 88 and/or according to any one of claims 110 to 119.