GB2308986A - Fluid filter assembly - Google Patents

Abstract

A full-flow fluid filter assembly 10 (Figure 1) suitable for an internal combustion engine has a large-pore-size screen 14 and a small-pore-size screen 17 in tandem and a by-pass valve 30 to conduct fluid past the small-pore-size screen when the latter cannot transmit fluid at a desired rate and a pressure difference develops across it sufficient to open the valve against bias spring 32. Tendency for the by-pass valve to open temporarily when the fluid is subjected to a pressure pulse, for example, on engine start up whilst the screen begins to conduct fluid, is countered by valve damping means 40 in the form of a valve member 35, which includes a dished support portion 41 and a control portion 45 as a membrane overlying the dish and, fixed downstream thereof, non-filtering, restricted aperture means 50 which encloses a volume 55 of fluid. The energy of a pressure pulse displacing the control portion is dissipated by viscous drag of the liquid expelled from the enclosure before the control portion can abut the support portion and act thereon to open the valve. The damping means therefore responds to rapid creation of pressure difference to damp valve opening response by delaying the onset of such opening until the energy therefor is dissipated but does not interfere with normal by-pass functioning in response to a steady pressure difference.

Description

Fluld Filter Assemblv This invention relates to fluid filter assemblies and particularly to such assemblies of the full-flow types in which all of the fluid in a circuit desirably passes through porous filtration means at which solid contaminants are trapped; and in particular relates to by-pass valve arrangements for preserving a flow path in conditions where such porous filtration means presents an impediment to fluid delivery.

For convenience in this specification the term screen is used to refer to any porous element of such porous filtration means upon which solid contaminants are trapped and irrespective of its actual structure.

Such full-flow fluid filter assemblies are known particularly for internal combustion engines of vehicles and the like in which the fluid comprises a fixed quantity of lubricating oil circulated around the engine and through the filter assembly by a pump driven by the engine, that is, pumping is commenced upon engine start-up and ceases when the engine is stopped. Such filter assemblies contain one or more screens to trap solid contaminants that could cause wear of surfaces within the engine, such contaminants being in the form of metal debris resulting from manufacture or wear, or particulate combustion products.

Debris tends to exist as particles in excess of 20corm across whereas combustion contaminants tend to exist as particles considerably smaller. Efficacy of a filtering screen for a single pass of fluid is, of course, related to the pore size of the screen, but in practice the pore size for the filter screen is considered as a compromise between one having a small pore size that traps small particles but becomes sufficiently occluded to impede flow in a short interval and a large pore size that remains occluded for longer but passes small particles.

Such compromise has, in respect of internal combustion engines, traditionally favoured a single filter screen of a paper-like material having a pore size that can be expected to remain unoccluded by contaminants for at least the interval between lubricating oil changes specified for the engine and a porous surface area that does not impede the flow of the fluid (lubricating oil) therethrough at the supply and delivery rates required in normal running of the engine, and notwithstanding temperature-dependant viscosity changes in the fluid.

However, in any design it is essential that the delivery of fluid from the filter assembly shall not be reduced below a safe level by inability of the fluid to pass through a filter screen and to this end a pressure-operated by-pass valve is usually incorporated to cause the fluid to by-pass the filter screen if significant pressure develops across it due to a permanent or temporary inability of the fluid to pass through the screen at the rate it is supplied to the assembly.Furthermore, such screen material may be physically damaged by excessive pressure difference across it and such by-pass valve also serves to regulate the pressure difference to which such screen is exposed to below a level at which physical damage could occur, absent provision of a costincreasing structural support for the screen, and thereby is instrumental in providing the ability to use such fragile screen materials. Thus it is considered desirable for such filter assembly construction for the by-pass valve to respond not only to an excessive pressure difference across the filter screen but also to respond rapidly to mitigate pressure damage to the screen.

It has been recognised that such a by-pass valve opens not only when a filter screen becomes occluded by contaminants after prolonged use, but also as a result of a pressure pulse in the fluid, for example, when the engine is started up.

Prior such engine start, a filter assembly filled with fluid has all fluid at the same pressure, that is, there is no pressure difference across the filter screen. When the engine is running and fluid flows through the assembly by way of the screen there is a significant, but small, pressure difference across the screen between fluid inlet and outlet caused by its resistance to flow and dependent upon the viscosity of the fluid.However, when the engine is first started, a sudden increase in pressure difference is created across the screen, due to the sudden increase in inlet pressure, which decays as flow is established through the filter screen and outlet pressure rises also to the normal operating pressure difference, that is, the filter screen and the by-pass valve experience a pressure pulse and therefore, for a short period of probably of less than one second, the by-pass valve may open briefly and a small quantity of unfiltered fluid be delivered by the filter assembly to the engine. Furthermore it is exacerbated by high viscosity of cold lubricating oil at initial start up of any running period.

However, it will be appreciated that the most damaging contaminants comprise particles of debris resulting from machining of, or new components in, an engine, which debris is circulated when the engine is started. Thus at the time when filtration is essential it may be by-passed by such pressure pulse opening the by-pass valve.

In conventional full-flow fluid filter assembly designs this has been recognised and this specific risk dealt with by providing special filter assemblies for use in newly machined and/or assembled engines, which engines can be guaranteed to undergo a filter change after a closely monitored interval that is short compared with normal service intervals. A supplementary filter screen is disposed in line with the by-pass valve to trap any debris by-passing the main filter screen. However, as such supplementary filter screen compromises the by-pass function by introducing a risk of reduced outflow to the engine should it become occluded, it can only safely be used under well monitored conditions.

Co-pending United Kingdom application No. 9523936.4 describes a full-flow fluid filter assembly which is suitable for prolonged use in internal combustion engines and employs a tandem arrangement of a large-pore-size screen and a small-pore-size screen through which the fluid is directed to flow in turn, together with means responsive to occlusion of the small-pore-size screen to cause substantially all of the fluid to pass only through the large-pore-size screen, such means typically being one-way valve means which is normally biased closed but opened against the bias by a predetermined level of pressure difference across the small-pore-size screen.

In the present context and for the purposes of the rest of this specification "pore size" refers to the filtration capability in respect of particulate material. "Large" for present purpose means a pore size in the range 50 to lOOgm typically 70 m. "Small" means a pore size in the range 10 to 40 m typically less than 30m.

However, these values may vary substantially according to the particular end use/application.

Debris found in a newly machined or assembled engine tends to exist in sizes in excess of 20 m.

The aim of such a tandem screen filter is to be used for longer intervals than is normally accepted for conventional single screen filters and to this end the screens are preferably formed from a more robust wire mesh material and, for ultimate longevity, possibly equal to the life of the engine, may be used in combination with a centrifugal separator.

The structure and operation of the tandem screen filter is described in greater detail hereinafter, but briefly, when the engine is started and lubricating oil supplied to the filter assembly large contaminant particles are trapped by the largepore-size screen whereas small particles are trapped by the smallpore-size screen. The pore size of the latter is chosen relatively small such that those debris particles of a size which could damage a new engine are trapped very soon after the engine is operated and thereafter leaving relatively clean oil to circulate, notwithstanding that the pore size means that the screen may become occluded by debris during its period of use.At such time the by-pass valve becomes open for prolonged intervals, or completely, during normal flow conditions, the large-pore-size screen providing filtering of larger contaminant particles with little prospect of becoming occluded. However, at such time the oil is also relatively free of damaging particles of debris and to a large extent, further contamination comprises very small particles of combustion products which do not pose any instant threat of damage to the engine. Therefore as mentioned above, it is convenient to operate such a tandem screen filter in combination with a centrifugal separator, which is efficient at removing small particles over a period of time, and whereby the large-pore-size screen filter serves to trap any unexpectedly large debris.

However, as also indicated above, before the small-pore-size screen has trapped, and become occluded by, the most harmful debris the situation exists on engine start-up that a pressure pulse in the oil supply can temporarily operate the by-pass valve and permit debris of a particle size between the screen pore sizes to by-pass effective filtering.

It will be appreciated that such a full-flow fluid filter assembly may be employed with other than an internal combustion engine and/or fluid other than lubricating oil, and preserving such generality, it is an object of the present invention to provide a tandem screen full-flow fluid filter assembly which mitigates such effect of fluid pressure pulse in fluid delivered by the assembly.

According to the present invention a full-flow fluid filter assembly includes a large-pore-size screen and a small-pore-size screen which in use are disposed in tandem within the fluid flow, by-pass valve means responsive to a predetermined pressure difference across the small-pore-size screen to open to permit the fluid to by-pass the small-pore-size screen, and by-pass valve damping means operable to limit the opening response rate the bypass valve means.

The damping may be related to the rate of change of pressure difference, responding to a rapid change in pressure difference to significantly limit the opening rate response whereas a slow change in pressure difference encounters less, if any, limitation.

The by-pass valve damping means may be operable in response to a rapid change in pressure difference across the by-pass valve means from substantially zero pressure difference to limit the valve opening response rate by delaying the opening of the by-pass valve means to fluid passage for an interval corresponding to the duration of a pressure pulse extending from said creation of pressure difference across the by-pass valve means to the decay of said pressure difference due to flow of fluid through an unoccluded small-pore-size screen.

The energy of said pressure pulse may be dissipated during the delay, to prevent the valve means opening thereafter, by frictional impedance, such as viscous drag between the fluid and restricted aperture means.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which Figure 1 is a sectional elevation through a first form of tandem screen full-flow fluid filter assembly, in accordance with the invention, having by-pass valve means and valve damping means in the form of a seated valve member with relatively displaceable portions, and restricted aperture means fixed downstream of the valve member, Figure 2 is a sectional elevation through a second form of tandem screen full-flow fluid filter assembly in accordance with the present invention having a different form of by-pass valve means including a valve member which is displaceable a finite distance between closed and opened positions, and including valve damping means, Figure 3 is a sectional elevation through part of a third form of filter assembly similar to Figure 1 illustrating the by-pass valve means and an alternative form of valve damping means having a moveable restricted aperture means downstream of the valve member, Figure 4 is a sectional elevation of part of a fourth form of filter assembly similar to Figure 1 but illustrating the by-pass valve means and a further alternative form of valve damping means having restricted aperture means fixed upstream of the valve member, and Figure 5 is a sectional elevation of a fifth form of filter assembly in accordance with the present invention, the by-pass valve means comprising a single rigid valve member unseated by minimal displacement, and including damping means in the form of restricted aperture means coupled to the valve member for movement therewith.

Referring to Figure 1, a first form of filter assembly 10 suitable for an internal combustion engine of a small vehicle, such as a car, comprises a generally cylindrical container 11 enclosing, in a direction extending radially inwardly from its side wall 12, an annular inlet chamber 13, an annular large-pore-size screen 14 of pleated metal mesh (the pore size being of the order of 70 m) supported on perforated metal tube 15, an annular intermediate chamber 16, an annular small-pore-size screen 17 of pleated metal mesh (the pore size being of the order of 3Om) supported on perforated metal tube 18, and a tubular outlet chamber 19 centred on the longitudinal axis 20 of the container 11.The outlet chamber 19 is in connection with an axial outlet duct 21 in one end wall 22 of the housing, which includes a screw thread arrangement 23 by which the housing is secured to an internal combustion engine, and is surrounded radially outwardly by one or more inlet ducts 24 in the end wall which communicate with inlet chamber 13.

Adjacent the other end wall 25 of the container the intermediate chamber 13 extends about the end of the small-pore-size screen 17 and outlet chamber 19, being separated therefrom by a by-pass valve means 30. This valve means comprises a tubular insert 31 having an inlet passage 32 surrounded by an annular valve seat 33 and contains a bias spring 34 and a valve member 35 which has a seating portion 36 biased by the spring into abutment with the seat 33 to close inlet passage 32 from the outlet chamber 19.The end 37 of the tubular insert remote from the valve member is apertured such that when a predetermined valve-opening level of pressure difference exists across the valve member, that is, between the intermediate and outlet chambers, the fluid exerts a force on the valve member sufficient to overcome the spring bias and displace it, so that fluid from the intermediate chamber can enter the outlet chamber with little resistance to flow and bypassing the small-pore-size screen 17.

In use of the filter assembly within an internal combustion engine, wherein the fluid is a fixed and recirculated quantity of lubricating oil supplied to inlet ducts 24 and delivered at outlet duct 21, the by-pass valve means 30 opens when fluid delivery rate exceeds the transmission rate of the small-pore-size screen 17.

This will occur if the screen becomes occluded by debris, whereupon, if the supply rate remains substantially constant, the by-pass valve will tend to remain open for a prolonged period.

Such a continuous operating condition may be sensed in some way and the assembly replaced or the screens cleaned, or it may be tolerated, particularly when the assembly used in conjunction with a centrifugal separator (not shown) of very small particles, on the basis that by the time the small-pore-size screen has become occluded most harmful contaminants in the form of debris have been removed from the recirculated fluid and the large-pore-size screen can trap any subsequently occurring large debris particles.

In addition to opening due to a continuous pressure difference across an occluded small-pore-size filter screen, such by-pass valve means in general tends to open temporarily at engine start up when, as discussed hereinbefore, a pressure pulse in the fluid occurs as supplied fluid meets with resistance to instantaneous transmission even through an unoccluded small-pore-size screen caused by static fluid in the outlet chamber 19. The effect of such pulse is exacerbated by high fluid viscosity when the engine, and the fluid (lubricating oil) thereof, is cold.

The very first time that such engine is started, there may be no fluid in the filter assembly and thus virtually no resistance to flow that creates such pressure pulse, so that such pressure pulse effect only occurs if the filter assembly is primed with fluid or when the engine is started for a second or subsequent time. Such tendency towards temporary valve opening therefore occurs from at least the second time that a newly machined engine is started, and notwithstanding the presence of the large-pore-size screen harmful debris of intermediate size, that is between 30 m and vOpm, in the fluid may in such brief periods by-pass the small-pore-size screen altogether and be returned to the engine.

To prevent such temporary by-passing of the small-pore-size screen, in accordance with the present invention the filter assembly 10 includes valve damping means indicated generally at 40 and operable to limit the by-pass valve opening response rate.

The damping means 40 provides damping related to the rate of change of pressure difference across the valve means, responding to a rapid increase and decay representative of a pressure pulse but not slow increases indicative of the small-pore size screen becoming occluded or of minor variation in normal supply pressure during operation.

In order to respond specifically to such a pressure pulse as occurs at engine start up, and prior to which there is zero pressure difference across the closed by-pass valve means, the valve damping means limits the opening response rate by delaying opening of the valve means for an interval corresponding to the duration of a pressure pulse extending from creation of a pressure difference across the valve means to the decay of said pressure difference to a lower level, less than the aforementioned valveopening level, due to flow of fluid through said small-pore-size screen in unoccluded state.

It will be appreciated that the effective duration of such pressure pulse will vary with the degree of screen occlusion, being shorter for a new screen and extending as occlusion progresses, and also with the viscosity of the fluid, which is greater for a cold engine.

Thus in a practical implementation, and with the primary intent of avoiding by-passing of a substantially unoccluded (new) screen with fluid (oil) of maximum viscosity the delay may be tailored to such conditions.

In order to effect such delay in opening of the valve means the damping means 40 responds to the instantaneous pressure difference across the valve means but furthermore acts to prevent the force exerted by the pressure pulse from simply opening the valve means after the delay, by comprising dissipation means operable to dissipate energy from the pressure pulse other than in opening the valve means, namely as friction in the form of viscous drag between the fluid in the assembly and restricted aperture means.

The damping means 40, that is, pressure pulse energy dissipation means, comprises the valve means 30, in particular the structure of the valve member 35, and restricted aperture means indicated at 50 and described hereinafter.

The valve member 35 comprises a rigid support portion 41 overlying the inlet passage 32 and surrounding valve seat 33. The periphery 42 of the support portion overlies the valve seat 33 and centrally thereof it has a dished part 43, centrally apertured at 44. The support portion supports on the peripheral part 42 a flexible resilient membrane, or washer, 45 which overlies the dished part 43 and provides at said periphery 42 the resilient seating portion 36 of the valve member for sealing co-operation with the valve seat 33.The membrane 45 provides a control portion of the valve member and, when there is zero pressure difference across the valve member, the membrane is spaced from the apertured centre of the dished part, assuming a rest position with respect thereto that defines between them a damping reservoir 46 which fluid can be admitted to, and ejected from, by way of aperture 44. In response to a pressure difference across the valve member less than the valve-opening level the membrane is deformed and displaced into the dish to assume, when in conforming abutment with the support portion, an actuated position at which the damping reservoir is collapsed and fluid ejected. With the control portion in said actuated position the valve member behaves as a rigid member to further pressure increases.

The restricted aperture means 50 is disposed downstream of the valve means in outlet chamber 19 and comprises an apertured body 51 fixed with respect to the valve means in the form of a dome of mesh 52 with its concave surface 53 directed towards the valve means to receive fluid therefrom. The mesh dome 52 defines with the valve means an enclosed volume of fluid 55 downstream of the valve member which includes the damping reservoir 46 whereby displacement of the control portion of the valve member between its rest and actuated positions effects a reduction in the enclosed volume by the volume of the damping reservoir and displacement of the fluid through the mesh 52.

The mesh has a pore size greater than the large-pore-size screen 14 both to avoid acting as a filter screen and to give a specific flow rate greater than both screens, and provides an array of apertures of which the total aperture area is sufficient to permit fluid flow at a constant rate without significant pressure drop across the mesh, at least at lower viscosity levels associated with fluid that is engine oil and normal running temperature.

Preferably, the large-pore-size screen has a pore size no greater than 70m and the mesh has a pore size in the range 70-200 m, conveniently lOO m. Such a pore size represents a compromise between the mesh having a pore size large enough to cause little resistance to constant fluid flow and not act in a filtering capacity downstream of screen 14, whilst being able to provide resistance to rapid passage of liquid therethrough in conditions pertaining with the above described pressure pulse.

Thus, when in normal operation, a pressure difference exist across the small-pore-size screen and the valve means the fluid exerts a force on the membrane 45 which is displaced into abutment with the support portion, so that the valve member acts as a rigid body seated in a first position. A rise in the pressure difference, possibly due to the small-pore-size screen becoming occluded, may reach a level at which it causes the whole valve member to be displaced and become unseated.

However, when there is no fluid flow from a stationary engine the zero pressure difference across the valve member in its first, valve closed, position permits the membrane to be spaced from the central part of the dished part and define damping chamber 46 as an extension to enclosure 55 filled with the fluid. Upon engine start up, the instantaneous creation of the pressure difference begins to displace the membrane and collapse the damping chamber, but the membrane cannot bear on the support portion (and cause the valve member to become unseated) until a volume of fluid corresponding to the initial damping reservoir volume is displaced through the mesh.

The mesh represents an impedance to the passage of fluid which is dependant on the specific flow rate and area of the mesh and on the pressure difference and viscosity of the fluid. Just as the small-pore-size screen cannot conduct the fluid instantly the mesh likewise tends to delay the onset of through-flow but to a smaller extent given the large pore size. The initially maximum pressure difference across it caused by the rising edge of the pressure pulse serves to establish through-flow quickly, but as flow is established the pressure difference begins to decay as the small pore-size screen begins to deliver fluid to the outlet chamber.

Therefore the short-lived pressure pulse is found to have decayed before a surprisingly small volume of fluid is displaced from the enclosed volume by way of the mesh, permitting the valve member to define a corresponding small volume of damping reservoir.

However, because the impedance of the viscous drag between the displaced fluid and mesh dissipates the energy that the pressure pulse puts into the valve member, the displacement not only delays the onset of valve member unseating but serves to dissipate energy from the pressure pulse necessary to perform such unseating and the valve member remains seated. Clearly a pulse of longer duration could result in the valve means opening so in overall terms, the damping means 40 thus operates to damp the opening response rate of the by-pass valve means.

Clearly the form taken by the restricted aperture means and dimensions of such restricted apertures (pores) are open to choice based upon accommodating the above-mentioned factors governing through- flow, viscous drag and magnitude and duration of pressure pulse. The relationships are complex and difficult to relate mathematically, but it has been found straightforward to derive parameters empirically, such as the above described mesh pore size for a filter assembly having conventional dimensions for a vehicle engine. The restricted aperture means may take other forms in terms of shape and perforation structure, as a mesh, perforated sheet or body of porous material.

In addition to dimensional variations, the configuration of valve means contributing to the damping means may vary from that described.

The two-part valve member structure of assembly 10 may alternatively be implemented with a control portion that is rigid like the support part, with the damping reservoir and displaceability defined by flexible means, such as bellows, joining the two.

Alternatively the valve member may comprise a unitary structure.

Referring to Figure 2, which shows a second embodiment of tandem screen filter assembly 60 generally similar to the arrangement 10, and for which corresponding reference numbers are employed, it differs in respect of valve means 30' (described hereafter) and the inclusion of a barrier 61 extending axially and radially between the large-pore-size screen 14 and small-pore-size screen 17 to define a first intermediate chamber 16', at the outflow of the large-pore-size screen and a second intermediate chamber 16'2 surrounding, and providing the inflow to, the small-pore-size screen 17.The radially extending part of the barrier 61 includes by-pass valve means 30' comprising a bias spring 32' and a valve member 33' in the form of an open ended plug which extends axially from the first intermediate chamber, through the second intermediate chamber and into the tubular outlet chamber 19. The side wall 63 of the plug is apertured at 64 such that fluid entering the plug from first intermediate chamber l6' flows either into the second intermediate chamber 16'2 with the valve means biased closed (as shown) or into the outlet chamber 19 with the plug displaced axially through a predetermined distance, that is, the valve open.

The second intermediate chamber 16 t2 is separated from the outlet chamber 19 by a radially extending partition 66 defining an annular valve seat surrounding outer wall portion 67 of the valve member plug. The aperture is 64 spaced from the base of the plug such that when there is zero pressure difference across the valve means is disposed, as shown, in a first position, the aperture 64 is displaced axially by said predetermined distance from the partition 66 by length 67'.

When a pressure difference is developed across the valve member, the member is displaced axially against the spring bias as a function of the level of pressure difference until, when it reaches said predetermined valve-opening level, the aperture 64 passes the partition 66 and the fluid enters outlet chamber 19, by-passing screen 17 rather than being supplied to second intermediate chamber 16'2.

Damping means, indicated generally at 68, is defined also in part of valve member 33' and by restricted aperture means 50 fixed downstream thereof to define an enclosed volume 55'.

The part of the valve member plug defined by the wall portion length 67', which comprises the distance of aperture 64 from the partition 66 when there is zero pressure difference across the valve means, thus represents a control portion of the valve member. The control portion, along with the rest of the valve member, is displaceable in response to creation of a pressure difference less than the valve-opening level, and the volume of the valve member plug so displaced displaces a corresponding volume of fluid from enclosure 55' by way of restricted aperture means 50.

The operation of the damping means 68 is thus analogous damping means 40 described above with the control portion being required to undertake an impeded displacement before the valve means can open to redirect fluid and said impedance being related to the rate at which a volume of fluid can be displaced through restricted aperture means in accordance with prevailing pressure difference across it.

Referring to Figure 3, this shows in sectional elevation a portion of a third embodiment of tandem screen filter assembly 70 similar to Figure 1 except insofar as the damping means comprises a different form of impedance means, comprising restricted aperture means 72 in the form of an apertured body 73, possibly of mesh as described above, which is connected rigidly to the control portion 75 (membrane or otherwise) of the valve member 74 for displacement therewith. Operation is similar except that instead of displacement of the control portion collapsing a damping reservoir and displacing a volume of fluid through a fixed porous body, the porous body is moved directly with the control portion and through the fluid.

It will also be appreciated that the impedance means, insofar as it comprises a restricted aperture means in the fluid, may be disposed upstream of the valve means, as illustrated at 50' in a fourth embodiment 80 in Figure 4 which is otherwise similar to Figure 1.

It will also be appreciated that the energy dissipating, frictional, impedance to displacement of a control portion of the valve member prior to it unseating reduces the possibility of contaminants of a size to be trapped by the small-pore-size screen from by-passing it when it is substantially unoccluded. However, as such contaminants have been subjected to at least coarse filtering by the large-pore-size screen it may be permissible, if the anticipated pressure pulse duration is particularly short, to have valve means which does begin to open without delay but against an initially stronger resistance that is due to the damping means.

Referring to Figure 5, this shows a sectional elevation through valve means of a fifth embodiment of tandem screen filter assembly 90 similar to assembly 10 of Figure 1 but in which valve means 91 comprises a rigid valve member 92 which is biased against an annular seat 93 by bias spring 94. Downstream of the valve means is fixed restricted aperture means 95 defining enclosure 96. In response to a pressure pulse the valve member tends to displace but meets impedance not only from the bias spring but also from the fluid in enclosure 96 against too-rapid displacement therefrom, effectively restricting the flow through the valve means when it is initially opened rapidly.

Damping means which functions in accordance with the viscosity of the fluid, and which fluid viscosity varies as a function of its temperature, may be made to have an opening response rate that is temperature dependant as well, either to counter the effects of viscosity change or to give different operating characteristics at different temperatures.

It will be appreciated that the dissipation of pressure pulse energy may be effected by frictional means other than viscous drag involving the fluid passing through the filter assembly. Such frictional means could employ viscous drag effected with a closed volume of fluid, not necessarily the same as the fluid being filtered, or mechanical friction causes by contact between the control portion of the valve member (or an attachment thereto) and the assembly structure as it is displaced, or by mechanical work required to effect deformation of such control member in displacing part of it.

Clearly the damping means may operate independently of the fluid viscosity and/or temperature, driving the valve means between closed and open states in dependence upon pressure difference and rate of change thereof alone.

The damping means may instead of the passive forms described above, comprise transducer measurement of pressure or pressure difference at different parts of the assembly and active control of the valve means directly therefrom in respect of opening, commencement of opening and/or rate of opening in accordance with the above outlined criteria to avoid the temporary by-passing of the small-pore-size screen.

In some circumstances delaying opening of the valve means may be effected without dissipation of the pressure pulse energy, that is, by omitting the restricted aperture means or corresponding mechanical friction means so that the by-pass valve means retains a tendency to delayed, brief opening, relying upon the fact that flow is already established through the small-pore-size screen to minimise any such by-pass flow through the valve means.

It will also be appreciated that the tandem screen filter assembly may be configured with the screens disposed such that the largepore-size screen is downstream of the small-pore-size screen and valve means, but only if the damping means does not include restricted aperture means in a form that could become occluded by any contaminants that do pass through the valve means during its operation.

As mentioned above, the filter assembly is not confined to use with a vehicle internal combustion engine in which the fluid is lubricating oil which exhibits a change in viscosity throughout the range of working temperatures of such engine. It may be employed with varying degrees of utility on any fluid system in which stagnant fluid is subjected to pressure pulses and irrespective of the viscosity behaviour of the fluid.

Claims (21)

1. A full-flow fluid filter assembly including a large-pore-size screen and a small-pore-size screen, which in use are disposed in tandem within the fluid flow, by-pass valve means responsive to a predetermined valve opening pressure difference across the small-pore-size screen to open to permit the fluid to by-pass the small-pore-size screen, and by-pass valve damping means operable to limit the opening response rate of the by-pass by-pass valve means.

2. An assembly as claimed in claim 1 in which the valve damping means is operable to provide damping related to the rate of change in pressure difference across the by-pass valve means.

3. An assembly as claimed in claim 2 in which the valve damping means is operable in response to a change in pressure difference across closed by-pass by-pass valve means from substantially zero pressure difference.

4. An assembly as claimed in claim 3 in which the valve damping means is operable to delay opening of by-pass the by-pass valve means for an interval corresponding to the duration of a pressure pulse extending from creation of pressure difference across the by-pass valve means to the decay of said pressure difference across the by-pass valve means due to flow of fluid through an unoccluded small-pore-size screen.

5. An assembly as claimed in claim 4 in which the valve damping means is responsive to the instantaneous pressure difference across the by-pass valve means.

6. An assembly as claimed in claim 5 in which the valve damping means includes dissipation means operable to dissipate energy from the pressure pulse other than in opening said by-pass valve means.

7. An assembly as claimed in claim 6 in which the valve damping means in arranged to dissipate said pressure pulse energy by means of friction.

8. An assembly as claimed in claim 7 in which the valve damping means is arranged to dissipate said pressure pulse energy by means of viscous drag between fluid in the assembly and restricted aperture means.

9. An assembly as claimed in claim 7 or claim 8 in which the by pass valve means comprises a valve member arranged, with zero pressure difference across it, to adopt a first position in which the by-pass valve means is closed to fluid by-passing the small-pore-size screen and, in response to said predetermined valve-opening level of pressure difference across the valve member, to be displaced to open the by-pass valve means to fluid by-passing the small-pore-size screen, and the damping means comprises a valve member configuration including a control portion of the valve member displaceable in response to creation of a pressure difference, less than said valve-opening level, across the by-pass valve means as a said pressure pulse and impedance means to frictionally impede said displacement of the control portion and dissipate the pressure pulse energy.

10. An assembly as claimed in claim 9 in which the by-pass valve means comprises a valve seat and a valve member having a seating portion adopted to abut the seat to close the valve to fluid flow and be unseated therefrom to open the valve to fluid flow in response to said predetermined valve-opening level of pressure difference across the valve member, and in which said control portion of the valve member is arranged to adopt a rest position relative to the seating portion with the valve member in said first position and be displaced to an actuated position relative to the seating portion in response to said creation of a pressure difference across the valve member less than said valve-opening level.

11. An assembly as claimed in claim 10 in which the valve seat comprises an annular surround of a fluid inlet passage and the valve member comprises a substantially rigid support portion overlying said inlet passage and valve seat and said control portion secured to the support portion and movable relative thereto in a direction to and from the fluid inlet aperture, both the control portion and support portion being displaceable with respect to said seat in response to said predetermined valve-opening level of fluid pressure difference across the valve member and said control portion being displaceable with respect to said support portion in response to a pressure difference across the valve member less than said valve opening level.

12. An assembly as claimed in claim 11 in which a central part of the support portion in line with said fluid inlet passage is dished and supports a control portion comprising a flexible resilient membrane overlying the dished part thereof and spaced therefrom when subject to said zero pressure difference to define a damping reservoir therebetween, said membrane being arranged in response to a pressure difference less than said valve- opening level thereacross, to be deformed and displaced into the dish and thereby displace from the reservoir a corresponding volume of fluid.

13. An assembly as claimed in claim 12 in which said membrane is arranged to overlie the periphery of the dished part and define with the support portion thereat the seating portion of the valve member.

14. An assembly as claimed in any one of claims 9 to 13 when dependant upon claim 8 in which the small-pore-size screen and by-pass valve means are disposed downstream of the large pore-size screen and in which the impedance means comprises restricted aperture means disposed in the flow path of fluid through the by-pass valve means and fixed with respect thereto to define with said control portion of the valve member an enclosed fluid filled volume whereby displacement of said control portion changes the enclosed volume and causes displacement of fluid through the restricted aperture means.

15. An assembly as claimed in claim 14 in which the restricted aperture means is disposed downstream from the valve member.

16. An assembly as claimed in any one of claims 10 to 15 in which the restricted aperture means comprises an apertured body having an array of through-apertures of which the total aperture area is sufficient to permit fluid flow therethrough and a constant rate corresponding to a constant delivery rate of the assembly without significant pressure drop across the body and the individual aperture size is not less than the pore size of the large-pore-size screen.

17. An assembly as claimed in claim 16 in which the apertured body comprises a dome disposed downstream of the by-pass valve means with its concave surface thereof directed towards the by-pass valve means to receive fluid therefrom.

18. An assembly as claimed in claim 16 or claim 17 in which the apertured body comprises a mesh.

19. An assembly as claimed in any one of claims 16 to 18 in which the large-pore-size screen has a pore size no greater than 70 m and the apertured body has a pore size in a range 70 200corm.

20. An assembly as claimed in claim 19 in which the apertured body has a pore size of lOO m.

21. A full-flow fluid filter assembly substantially as herein described with reference to, and as shown in, any one of the accompanying drawings.