GB2171332A - Grinding machine - Google Patents

Abstract

The present invention provides a grinding machine of the type comprising a pair of heavy rollers which define a nip through which the material, such as powdered clay, to be ground is passed. One of the rollers is movable towards and away from the other roller in order to avoid damage to the machine in the event of an uncrushable foreign body passing into the roller nip. The control of the moving away of the roller is by means of a snap action torque limiting mechanism comprising a pair of links between which the torque limiting device is connected. The invention also provides a means for setting the gap between the rollers by driving the rollers together and then moving them back from the zero position. <IMAGE>

Description

SPECIFICATION Grinding machine This invention relates to a roller grinding machine of the type comprising a pair of grinding rollers of substantial mass and which are arranged with their axes parallel and horizontal. The rollers have grinding surfaces of a hard metallic material, such as the nickle iron alloy known as Ni-hard. The roller surfaces are ground to a high surface finish. Typically, the rollers are of the order of 1,000 to 1,200 mm dia.

and may be set to grind material to a fineness of the order of 0.5 mm. The rollers are however movable towards and away from each other so as to define a grinding gap to suit the material to be ground.

In the normal arrangement, one of the rollers is mounted in fixed bearing, whilst the other roller is mounted in bearings which are capable of movement towards and away from the other roller in order to provide said adjustability, but more particularly to enable the roller carried by the movable bearings to move sharply away from the other roller in the event of an overload condition arising.

Grinding machines of the type to which the present invention relates are capable of grinding materials which are pasty or fluent in nature such as pasty clay, sometimes referred to as plastic clay, which contains small stones and fossils which require to be ground to pu Iverised form to make the plastic clay in a desirable condition for manufacturing for example bricks and pipes. The grinding machine can also be used for pulverising dry granular material such as coke and stone. The material is ground by introducing same into the top side of the nip formed by the two rollers and by driving at least one of the rollers, and sometimes both rollers at the same or differential speeds, so that the material passes through the grinding nip where the material experiences a high load compression and, in the case of differential rotation of the rollers, shearing forces.The crushing forces involved may give rise to loading as much as 30 tons on each bearing supporting the ends ofthe shaft of the movable roller.

An overload condition exists when a foreign and uncrushable body enters the nip between the rollers and is of a size greater than the nip dimension. Such a foreign body may comprise a piece of metal, or wood or rubber. When such a condition exists, rather than risk the danger of damage being done to the machine, the machine is designed so that in such conditions the movable roller will move away from the fixed roller to a sufficient extent, against a pre-determined resistance, to allow the foreign body to pass between the rollers.

The conventional method of providing resistance to overload condition comprises a breaker plate or brust plate or disc on which a plunger or rod acts in normal operation of the machine, but which is punctured by the plunger or rod in the event of overloading arising. As the disc or plate is punctured, so the resistance to movement of the movable roller away from the fixed roller is lost, the movable roller snaps away from the fixed roller and the foreign body can pass between the rollers as a result of the moving away of the movable roller from the fixed roller.

The overload pressure may be adjusted by using different brust discs or plates.

One problem of using brust discs or plates in that when an overload condition exists, and the brust disc or plate is punctured, not only is there the expense of providing a new brust disc, but the replacement procedure is also time consuming and tedious, which represents a loss in productivityfrom the machine. Thus, in one arrangement wherein the movable roller is carried by self-aligning bearings in bearing clocks, the bearing blocks have attached thereto rods the ends of which bear on the burst discs. The bearing blocks are slidable horizontally in slide guides, and when an overload condition exists, one or both bearing blocks are displaced away from the fixed roller, puncturing one or both discs.It should be mentioned incidentally, that in an overload condition it is usual for only one of the discs to puncture, which results in the movable roller in fact slewing relative to the fixed roller. To remove the punctured brust disc, a sleeve surrounding the puncture rod has to be turned so as to retract the disc from the end of the puncturing rod and to enable it to be removed, then a new disc has to be inserted. Subsequently, a power device such as a fluid pressure operated cylinder is coupled to the machine and a force is applied to the new brust disc which in turn is applied to the puncturing rod, and the whole assembly including the bearing block is forced back into the operating position.The sleeve finally is turned so as to be screwed towards the fixed roller until a setting stop connected to the sleeve abuts a fixed portion of the machine, which re-sets the bearing block position to what it was before the overload condition followed by bursting of the burst disc took place.

It can be seen therefore that the replacement of the burst disc is time consuming and tedious.

In another arrangement, the movable roller is carried by a pivot assembly in that the bearing blocks are supported on parallel plates of the assembly, the plates being pivotable about an axis which is parallel to the axis of the movable roller.

The plates are rigidly connected and are coupled at a point centrally of the movable roller to operate a punch rod which in turn bears upon the puncture disc so that when there is an overload condition, the pivot assembly will rock about the axis of pivoting, and force the punch rod through the punch disc.

A screw mechanism coupled to the pivot assembly can be operated to move the pivot assembly for the purposes of retracting the punch rod to enable removal of the punctured burst disc for the replacement of same. The arrangement using a pivot assembly has the advantage that the movable roller cannot slew relative to the fixed roller, but nevertheless the process of replacing the punctured burst disc is time consuming and represents down time of the machine.

The present invention relates to the machines as described, but in one aspect seeks to provide a means whereby the utilisation of a burst disc is rendered inessential, and in accordance with this aspect of the invention, a grinding machine of the type as described comprises an overload mechanism in the form of two relatively rotatable members between which is a resettable torque limiting device, the overload mechanism being coupled to the movable roller so that upon an overload condition arising, the said members are loaded to vary the angle there-between, and the torque limiting device yields resulting in sharp relative angular movement between the members enabling sharp movement of the movable roller away from the fixed roller.

The said members are preferably relatively rotatable about a common axis and define a toggle type mechanism, said torque limiting device preferably comprising plates with rolling elements between same and spring loading keeping the rolling elements in recesses until a pre-determined torque corresponding to the overloading condition is applied between the plates in which condition the balls no longer prevent relative rotation of the plates.

The torque limiting device is preferably such that the said relatively rotatable members have two distinct relative positions, the first being the normal operation position, and the second being the overload position to which the members snap in the event of overloading condition of the machine arising.

Preferably there is also means for returning the members to the normal operating position after the foreign body causing an overload condition has passed between the rollers, and such means may typically be a fluid pressure operated cylinder which acts on the said members in the vicinity of the common axis.

The machine may be arranged so that when an overload condition does arise this is detected and the operation of the machine is automatically controlled in accordance therewith. Thus, as the material being ground after it passes between the rollers drops onto a conveyor belt, the detection of an overload condition may result in the conveyor belt direction being reversed, for a pre-determined time, in order that the foreign body will be moved on the conveyor belt in a direction opposite to the normal conveying direction of properly ground material so thattheforeign body can be dumped into a collecting bin or other suitable location for disposal.

Secondly, the supply of material to be ground to the machine may be temporarily halted, and after the pre-set time, the means for returning the movable roller to the operating position may be operated, and the supply of material to the machine and driving of the conveyor belt in the normal direction, resumed.

The invention can be applied either two a machine in which the movable roller bearing blocks slide horizontally, or to the pivot assembly type machine, and in the latter case, the overload mechanism may comprise a first link pivotally connected between the pivot assembly and common axis between the links, and a second link pivotally connected between said common axis and a fixed or normally fixed portion of the machine. The torque limiting device would comprise a coupling assembly mounted on the said common axis, and the torque limiting device is preferably of a type so that or is mounted so that the limiting torque can be adjusted to accommodate different overloadings.In the case of the pivot assembly machine, only a single overload assembly would be required, but in the case ofthe machine wherein the bearing blocks slide horizontally, an overload device for each bearing may be necessary.

In the case of the horizontally sliding bearing blocks, again a toggle type mechanism would be used coupled between a movable yoke and a fixed or normally fixed part of the machine, with a device comprising pivotally inter-connected links with the torque limiting device on the common axis between the links, the links being respectively pivotally connected to the said movable yoke and fixed or normallyfixed partofthe machine.

Machines of the type to which the invention relates experience high loading as has been indicated above, and also can experience substantial vibration insofar as the rollers may typically rotate at a speed in the range 100-350 revs per minute. The machines work at fine clearances between the rollers, and taking into account the machine's requirement and operating conditions, any play in pivot points or shaft bending can considerably affect the machine's performance. In an advantageous feature of the invention however, it may be desirable to provide a means applying loading across the toggle type overload mechanism in order to take up clearance in the pivot joints of the toggle mechanism. Such a loading device may comprise a strong spring or a fluid pressure operated cylinder.

According to another aspect of the ivnention, there is provided a means for setting the gringing gap between the rollers. This gap must be set accurately and existing machines do not have a means for achieveing this. Typically, in existing machines the gap may be set simply using a feeler gauge or manually adjustable viewer gauges.

The present invention in this aspect provides a power system which provides for the accurate setting of the gap between the rollers, and in this aspect of the invention the gap is set by driving the rollers together through a transmission means, and then powering the rollers apart and monitoring the spacing of the rollers by means of a control system involving a visual display. The said system involving the visual display may comprise control panels having appropriate controls by which the gap between the rollers can be set as required. In one example, the spacing between the rollers is monitored by means of a proximity switch sensing the angular rotation of a disc coupled to the drive which drives the rollers together and apart, and the proximity switch may provide an indication, for example in millimeters, of the roller spacing.

The advantage of driving the rollers together in the first instance for the setting of the gap enables the control system to be set to zero when the condition of zero gap has been established, and then the operator can set on the control panel the size of gap required. The drive mechanism will automatically stop when the rollers have been moved apart by the amount equal to the pre-selected gap size. In another arrangement, the operator pre-sets the gap by a decade switch. An auto button is then pressed and the rollers close and datum automaticaly (contact) before opening to the pre-set gap.

The changing of the gap size to any particular value therefore is a simple matter, and the gap size can be set to any degree of fineness much more effectively than performing the operation by hand using a feeler gauge.

Additionally, by powering the rollers together to a zero position, much of the slack in the transmission mechanism, and the roller bearings can be absorbed so that the machine will run accurately when the rollers are operated.

In a particularly suitable arrangement, the first and second aspects of the invention are combined in that the drive and transmission are connected to one end of the toggle arrangement serving as the overload device, in particular that end of the device which is normally fixed to a fixed or normally fixed machine part. The transmission would normally be fixed, and would be operated only when it is required to set the gap between the rollers. When that gap has been set, the machine operates as described above in that if an overload condition takes place, the toggle overload device will operate enabling the foreign matter to pass between the rollers, followed by the automatic return of the rollers to the operating position.The transmission for setting the gap between the rollers is operated only when it is required to change the operating gap or to adjust the rollers to compensate for surface wear thereof.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanyoing diagrammatic drawings, wherein: Figure 1 is a diagrammatic side view of the machine according to the first embodiment of the invention; Figure 1A is an enlarged view of a detail shown in Figure 1; Figure 2 is a diagrammatic side elevation of a machine according to a second embodiment of the invention; and Figure 3 is a plan view of part of the machine shown in Figure 2.

Referring to the drawings, the elements of a machine according to the invention are illustrated diagrammatically in side elevation, and comprise a fixed grinding roller 10 and a second grinding roller 12 which is mounted for pivotal movement on a lever arm frame 14, the axis of pivoting of the frame being indicated by numeral 16. The machine has a fixed frame 18 supporting the lever arm 14 and the roller 10, and the rollers 10 and 12 are rotatable about parallel axes 20 and 22, which axes are also horizontal in use. The roller nip 24 defines the vicinity of the grinding operation, and the material to be ground is to be introduced into a hopper (not shown) so the material enters the nip 24 as indicated by the arrow 26. The ground material issues from the nip as indicated by arrow 28 and may be collected on a horizontally moving conveyor belt (not shown).

The roller 12 is mounted on lever arm 14so asto be capable of moving away from the roller 10 as indicated by arrow 30 in the event of an overload condition as described herein, occuring.

The embodiment of the invention ofthe machine illustrated comprises an overload toggle device in the form of two links 32 and 34 respectively pivotally connected about axis 36, the links 32 and 34 being coupled by mens or a torque limiting coupling 38.

Figure 1Ashows the toggle overload device diagrammatically, but to a larger scale, and the normal operating position of the overload device is shown in Figure 1A in full lines and it will be seen that the links 32 and 34 form a shallow V. When an overload condition occurs, the resistance of the torque limiter 38 is overcome, and the links 32 and 34 snap to the dotted line position in Figure 1A which the links 32 and 34 form a much sharper V. When this overload occurs, the frame 14 can swing increasing the gap at the nip 24, and allowing the foreign body which causes the overload condition to fall between the rollers 10 and 12.The link 32, see Figure 1A, is connected at its free end by a pivot 40 to a frame 42 fast with the lever frame 14, whilst the free end of the link 34 is connected via a pivot 44 to an adjustment block 46 the function of which will be explained hereinafter. For the present, it can be assumed that the adjustment block 46 is fast with the machine frame 18.

A re-set air cylinder 48 is mounted on the lever arm 14 and when operated acts via a reaction block 50 connected to the piston of the device 48 on the pivot 36 in order to return the toggle to the full line position in Figure 1A, which automatically returns the roller 12 to the normal and correct working position establishing the correct nip gap between the rollers, and the design of the torque limiting device 38 is such that when the links 32 and 34 are restored to the normal operating position, the torque limiting resistance is restored, and therefore the machine is quickly and efficienctly re-set.

In order to take up any clearance or slack in the pivots 36, 40 and 44, there may if desired be a loading means between the lever frame 14 and the machine frame 18 to urge same together. This is desirable, because the nip clearance in region 24 must be maintained accurately, and as this clearance can be as small as 0.5 mm, it is very important that there should be as little play between the various machine components as possible as any such play could lead to a large percentage error at the roller nip.

The block 46 is as shown in Figure 1 connected to a motor 52 and gearbox 54, the output of which comprises a screw shaft on which the block 46 is screwed, so that when the motor is driven the screw shaft is driven by the gearbox 54, and the block 46 is moved axially of the shaft either to rock the lever frame 14to reduce the nip gap 24 orto increase same. The motor 52, gearbox and screw shaft together with appropriate control means comprise an effective device for setting the gap 24 in accordance with the material to be ground, and the method of setting the gap is as follows.The motor 52 is driven so asto rock the frame 14to bring roller 12 into face-to-face contact with roller 10, i.e. zero gap, and then the motor is reversed and the amount of reversal is monitored by means of a proximity switch device which is in fact a counting device. This count is translated into a gap dimension in millimeters or fraction of a millimeter, and to this end the proximity device is callibrated to a fine resolution.

For example a disc on the screw shaft may be provided with a plurality of closely spaced holes which are sensed as they pass the sensing device to measure the degree of rotation of the disc. The amount of that rotation can be translated into a gap size at the nip 24. The control system may be provided with a means whereby an operator can set manually on a digital display the gap size required, and as the roller 12 is moved away from the roller 10 after being zeroed in the contact position, it counts increments corresponding to increments of movement of the roller 12 away from roller 10 until the size of the gap corresponds to that set originally in said digital display, when movement of the roller 12 automatically stops.When the gap has been set in this way, the roller 12 will automatically be returned to the correct position after an overload condition displacement of the roller 12 as described herein.

The arrangement of Figure 2 is a linear version of the pivot arrangement shown in Figure 1, and therefore similar reference numerals to those already used in relation to Figure 1 are used in Figure 2 to facilitate an understanding of the Figure 2 embodiment. The roller 12 is shown, and one bearing block 13 at the shown end of the roller is shown. It should be mentioned that there are two bearing blocks, and the control arrangement illustrated in Figure 2 will be duplicated at the other side of the roller. The bearing block 13 bears upon a thrust shaft 15, which in turn is connected to a slidableframe oryoke 14. The link32 is pivotally connected at axis 40 to frame 14, whilst link 34 is pivotally connected at axis 44 to the screw block 46.

The motor 52 and transmission 54 are carried on a machine frame 18, and frame 14 is slidable relative to frame 18 as indicated by arrow 19.

In normal operation, the frame 14 is in effect rigid with frame 18, but when overloading occurs, the links 32 and 34 snap to the dotted line position as shown in Figure 2 by pivoting relative about the axis 36, the torque resistance of the torque limiting device 38 being overcome, and the block 13 can move to the left in Figure 2, allowing the foreign body causing the overload to pass between the rollers 10 and 12. in order to return thejacknifed toggle linkage 32 and 34 to the full line position, the air cylinder 48 carried by frame 14 is provided, and by extending the cylinder, the block 50 on the end of the piston engages the toggle linkage in the region of the axis 36 and returns same to the full line position, whereby the machine is automatically and correctly re-set.Adjustment of the gap and setting of the gap are achieved in the manner described in relation to Figure 1, using the motor 52 for displacing the screw blocks 46.

Support rollers 56 serve to permit the frame 14to slide freely relative to frame 18 in overload condition, whilst horizontal rollers 58, rotatable about vertical axes which may be eccentric to the peripheries of rollers 58, serve to act between the frame 14 and frame 18, to prevent the roller 12 from slewing as it moves away from roller 10 in an overload condition. The rollers 58 serve to keep the axis 22 of the roller 12 parallel to that of roller 10 as the roller 12 moves away from roller 10 in the overload condition.

Finally, the frame 14 may be provided with a large rubber bumper 60 for damping the movement of the links 32 and 34 as they snap into the overload condition. A similar and suitably located bumper may be provided in the Figure 1 embodiment.

It can be seen that an effective overload control device has been provided, which does not require the use of burst plates or pads, and re-setting of the machine can take place automatically and immediately after the overload condition has passed.

Furthermore, by the second aspect of the invention, the setting and adjusting of the gap between the rollers can be performed in a simple and effective manner.

Claims (23)

1. A grinding machine of the type as described comprising an overload mechanism in the form of two relatively rotatable members between which is a resettable torque limiting device, the overload mechanism being coupled to the movable roller so that upon an overload condition arising, the said members are loaded to vary the angle therebetween, and the torque limiting device yields resulting in sharp relative angular movement between the members enabling sharp movement of the movable roller away from the fixed roller.

2. A machine according to Claim 1, wherein said torque limiting device is such that the said relatively rotatable members have two distinct relative positions, the first being the normal operation position, and the second being the overload position to which the members snap in the event of overloading condition of the machine arising.

3. A machine according to claim 2, including means for returning the members to the normal operating position after the foreign body causing an overload condition has passed between the rollers, and such means may typically be a fluid pressure operated cylinder which acts on the said members.

4. A machine according to Claim 2 or 3, wherein said members are relatively rotatable about a common axis and define a toggle type mechanism.

5. A machine according to claim 4, wherein the machine is a pivot assembly type machine, and the overload mechanism comprises a first member pivotally connected between the pivot assembly and the common axis between the members, and a second member pivotally connected between said common axis and a fixed or normally fixed portion ofthe machine.

6. A machine according to claim 4, wherein the machine is of the type wherein the movable roller slides horizontally in bearing blocks, and the toggle type mechanism is coupled between a movable yoke and a fixed or normally fixed part of the machine, the torque limiting device on the common axis between the members, the members being respectivelypivo- tally connected to the said movable yoke and fixed or normally fixed part of the machine.

7. A machine according to claim 4,5 or 6, wherein the torque limiting device comprises a coupling assembly mounted on the said common axis, and of a type so that or is mounted so that the limiting torque can be adjusted to accommodate different overloadings.

8. A machine according to claim 7, wherein said torque limiting device comprises plates with rolling elements between same and spring loading keeping the rolling elements in recesses until a predetermined torque corresponding to the overloading condition is applied betwen the plates in which condition the balls no longer prevent relative rotation of the plates,

9. A machine according to any preceding claim, wherein the machine is arranged so that when an overload condition does arise this is detected and the operation of the machine is automatically controlled in accordance therewith.

10. A machine according to claim 9, including a conveyor belt onto which the material drops after it passes between the rollers, and the detection of an overload condition results in the conveyor belt direction being reversed, for a pre-determined time, in order that the foreign body will be moved on the conveyor belt in a direction opposite to the normal conveying direction of properly ground material so that the foreign body can be dumped into a collecting bin or other suitable location for disposal.

11. A machine according to claim 10, when taken with claim 9 and 3, wherein the machine is set so that the supply of material to be ground to the machine is temporarily halted, and after the predetermined time, means for returning the movable roller to the operating position may be operated, and the supply of material to the machine and driving of the conveyor belt in the normal direction, resumed.

12. A machine according to any preceeding claim including means for applying loading across the overload mechanism in order to take up clearance in the joints in the mechanism.

13. A machine according to any preceding claim including means for driving the rollers together through a transmission means, and then powering the rollers apart and monitoring the spacing of the rollers by means of a control system involving an indication.

14. A machine according to claim 13, wherein the said system involving the indication comprises control panels having appropriate controls by which the gap between the rollers can be set as required.

15. A machine according to claim 13 and 14, wherein there is a proximity switch for monitoring the spacing between the rollers by sensing the angular rotation of a drive coupled to the drive which drives the rollers together and apart so as to be rotated thereby.

16. A machine according to claim 13 when taken with claim 12, wherein said means for driving comprises also said means for applying loading and comprises an electric motor and gearbox connected to a screw driving a block to which one of the said members is pivoted.

17. A machine according to any of claims 13 to 16, wherein the machine enables the operator to set the size of gap required so that the means for driving will automatically stop when the rollers have been moved apart by the amount equal to the pre-selected gap size.

18. A machine according to any of claims 13 to 17 including control means for automatically returning the rollers of the rollers due to an overload condition.

19. A grinding machine of the type described including means for driving the rollers together through a transmission means and then powering the rollers apart and monitoring the spacing of the rollers by means of a control system involving an indication.

20. A machine according to claim 19, wherein the said system involving the indication comprises control panels having appropriate controls by which the gap between the rollers can be set as required.

21. A machine according to claim 19 or 20, wherein there is a proximity switch for monitoring the spacing between the rollers by sensing the angle of rotation of a disc coupled to the drive which drives the rollers together and apart so as to be rotated thereby.

22. A machine according to any of claims 19 to 21, wherein the machine enables the operator to set the size of the gap required so that the means for driving will automatically stop when the rollers have been moved apart by the amount equal to the pre-selected gap size.

23. A machine according ot any of claims 19 to 22, including control means for automatically returning the rollers to the pre-set gap size after moving apart of the rollers due to an overload condition.