CA1207214A - Fluid control valve - Google Patents

Abstract

Abstract of the Disclosure An improved heavy duty motor clutch drive system is disclosed which includes a motor adapted to be drivingly connected to apparatus to be driven by means of a clutch unit of the oil shear type which may be selectively actuated by application of fluid pressure.
Improved electrically actuated control valve means are provided for controlling the application of actuating fluid pressure to the clutch unit and includes associated control circuitry for operating the control valve so as to accelerate the driven apparatus in a predetermined manner. Additionally, in some applications a plurality of such drive systems may be utilized to drive a common apparatus in which case the clutch actuating controls may be interconnected so as to provide progressive predetermined delay actuation of the various clutch units.
Safety override means are also incorporated into the control circuitry which are designed to deactuate the clutch unit so as to disconnect the motor from the driven apparatus in response to abnormal operating conditions.

Description

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The present invention relates to a fluid control valve, and more particularly, to valves for use in drive systems which include fluid actuated oil shear type clutch units for operatively connecting a driving motor to apparatus to be driven thereby and associated control apparatus for selectively controlling the application of actuating fluid to the clutch unit.
This is a division of copending Canadian Patent Application Serial number 383,885, filed on August 14, 1981.
Jo In numerous applications it is highly desirable to be able to smoothly accelerate a driven apparatus from a sta-tic condition to full operating speed without subjecting -the apparatus to the often excessive stress and strain associated with an uneven or abrupt start. Counteracting this desire to achieve a smooth gradual startup of the driven apparatus is the desire to achieve full speed operation as rapidly as possible as well as the desire to avoid any excessive wear resulting from unnecessarily prolonged slippage of the clutch unit as it is being engaged.
Additionally, smooth startups may be~particularly desirable in apparatus having multiple spaced drives provided thereon such as for example overland conveyor systems utilized in various types of mining operations so as to avoid shifting or spillage of the load thereon as well as to avoid excessive tensioning of the conveyor belt and the potential backlash which may result. In such conveyor systems it is normally necessary to progressively start the various drives in succession with a slight delay between each successive start sb/i "I

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so as to enable the sections of conveyor belt between each of the drives to be properly tensioned. The application of driving forces by proper clutch actuation so as to achieve a soft start enables the conveyor belt to tension in a generally even manner whereby any backlash or rebounding thereof is minimized. In order to insure such proper tensioning of the conveyor system, it is important that the acceleration curves of each drive unit be consistent, predictable and very accurately controllable.
Additionally, because the clutch actuating controls must be located in close proximity to the drive units and hence the conveyor equipment, they are subjected to extremely adverse operating conditions due to the high level of contaminants in the surrounding atmosphere. This problem is particularly prevalent in systems used for transporting coal both at the mines and at coal fired power plants. As a result, problems have been encountered as a result of dirt contamination of clutch actuating fluid causing plugglng of control valves thereby resulting in loss of full operating control as well as increased maintenance cost and equipment downtime. Therefore, it is highly desirable to provide control valving which is substantially less senstive to such dirt contamination.
According to the present invention there is provided a fluid control valve which has a housing provided with an inlet opening and at least one outlet opening. A
valve member is rotatably and movably disposed within the housing, the valve member having a cam surface positioned in radial alignment with the inlet opening, the cam surface sb/~

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belng movable from a Eirst posi-~ion in which the cam surface is radially spaced from the opening to a second position in which the cam surface effectively restricts fluid flow through the inlet opening. Drive means is provided for driving the valve member between the first and second positions in pulsed steps, the pulsed step movement of the valve member being operative to dislodge contaminants from the inlet opening.
Additional advantages and features of the present invention will become apparent from the subsequent description and the appended claims taken in conjunction with the accompanying drawings.
Brief Description of the Drawings -Figure 1 is a perspective view of an overland conveyor system incorporating a plurality of motor clutch drive system;
Figure 2 is a schematic diagram of a clutch unit and associated control valve connected in circuit with pump means;
Figure 3 is a sectioned view of a control valve for use in the drive system shown in Figure 1 in accordance with the present invention, the section being taken along a radial plane extending parallel to the axis of rotation of the valve unit;
Figure 4 is a sectioned view of the valve assembly of Figure 3, the section being taken along line 4-4 thereof;
Figure 5 is an enlarged fragmentary section view of the cam member forming a part of the valve assembly of Figure 3;

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Figure 6 is a transverse section view of the valve assembly of Figure 3 showing the stop arrangements incorporated therein, the section being taken along line 5-5 thereof; and Figure 7 is a block diagram of the control circuit used to operate the stepping motor which operates to drive the control valve shown in Figure 3.
Description of the Preferred Embodiment Referring now to the drawings and in particular to Figure l, there is shown an overland conveyor system 10 which includes a rather long conveyor belt 12 movably supported on a plurality of roller means 14 and including a plurality of driving means 16 spaced along the length of the conveyor system. As shown, each of the drive means includes a motor means 18 such as for example an internal combustion engine or electric motor which is operatively connected to a clutch unit 20 which operates to selectively transmit the driving forces from the motor means 18 to the conveyor belt 12. Such conveyor systems may range in length from several hundred feet to several miles depending on the particular application.
Clutch units 20 are preferably of the oil shear type such as disclosed in U.S. Patent 3,696,898, issued October 10, 1972 and U.S. Patent 4,171,038, issued October 16, 1979 and assigned to the assignee of the present invention.
Such clutch units comprise a plurality of alternating discs and plates, the discs being secured for rotation with one of the input and output shafts of the clutch and the plates being secured for rotation with the other of said sb/"-input and output shafts. An actuating cylinder is normally provided to which pressurized fluid such as an oil for example is supplied which aerates to move the alteTna~ing discs and plates into mutual operative relationship whereby the driving force of the motor means may ye transmit*ed to the output shaft so as to driYe the apparatus connected thereto, the cons~ructian and operation of the clutch is described more fully in the above TefeTenced patent.
In order to cc~ntTol actuatic)n of c:lutch unit 20 and hence provide a smooth "soft start of the cunveyor system or other appaTatus being driven, pump means 22 aTe pTo~ided as shown diagram-matically in tune 2 which aTe operative to provide a supply of pressurized fluid lam a reservoir snot shown to the clutch unit 20 via cc)nduit 24 so as to enable actuatisn of the clutch unit 20.
In operation, the pump means may be continuously opeTated so as to lS provide lubrication and cooling fluid to the clutch with only a portion of the output being used for control puTposes, AlteTnatively, separate pump means may be provided for control puTposes~ In either event, the pump means is first started so as to create a source of pressurized fluid and a pressure relief control valve assembly 26 is provided being connected to conduit 24 by conduit 28 and which operates to allow fluid output from the pump means 22 to be directed thereth~ou~h back to a Teservoir when in a fully open position so as to substantially pTevent any actua~inQ pressure being applied to the clutch unit 20, As the valve assembly 26 closes, fluid flow there-thIDu~h will ye prDFressi~ely Testricted theTeby resultinQ in increasing actuatiD~ fluid pTessule being aPPlied to the clutch unit 20, As this fluid actuatin pressuTe is progressively increased, the plates and discs of the clutch unit will be Pro~TessiYely moved axially into toTque transmittina relationshiP with relative rotation there~etween being pro~Tessively decreased until suck time as the outPut shaft is being rotated at fLll operating speed.

~!L2~)7;2 Referring now to Figures 3 ~hTough 6, a pressure relief control valve assembly 26 in accordance with the present invention is illustrated and includes an elongated geneTally Tectangular shaped housing 30 having a central bore 32 extending longitudinally theTe-through which mcludes enlarged diameter poTtions 34 and 36 a opposite ends 38 and 40 theTeof respectively. An inlet opening 42 is provided in the uppel sidewall 44 which opens radially ~IwaTdly into bore 32 and which is adapted to have a fluid supply line connected theTeto such as cGnduit 28 shown in FiguTe 2. A paiT of Tadially extending outlet openings 44 and 46 are also pTovided on the lower sidewall 48 of housing 30 being substantially equally longitudinally spaced in opposite diTections from inlet opening 42. Outlet openings 44 and 46 each open into a manifolding cavity 50 folmed in the loweT
sidewall 48 of housing 30. A plate 52 is secuTed by suitable fasteners 53 to the lower sidewall 48 of housing 30 which in part defines and encloses manifolding cavity 50 and has a single outlet opening 54 pTovided therein which is adapted to have a fluid supply line connected th~Teto JOT TetuTning actuating fluid to the reservoil.
on elongated rotatable valve core membeT 56 is moYably positioned within lcngitudinally extending boTe 32 and includes an enlarged diameter flange portion 58 adjacent the outeT end 60 which engages a Tadially extending flange portion 62 interconnecting boTe 32 and enlarged diameteT portion 34 so as to axially position valve coTe x mber 56 with respect to housing 30. A Tetaining plate 64 is secured by suitable fasteners 65 to the outeT end 38 of hcusing 30 and has a poltion 66 extending into enlarged diameter poTti~n 34 of bore 32 and into engagement with the outeT surface 68 of flange 58 so as to securely restrain valve core member against axial movement.
An annular groove 70 is also provided on portion 66 within which suitable sealing means 72 such as an O-ring is positioned which 2C)7~
sealingly engages the sidewall of enlarged diameter poTtion 34 thereby preventing leakage of fluid theTefrGm. An opening 74 is also provided in housing 30 extending radially outwardly fTom the lower part of enlarged diameter portion 34 an opening into cavity 50 which operates S to al70w actuating fluid accumulating within enlarged diameter poTtion 34 Jo be Tetu med to the reservoiT.
Valve core member 56 also has a pair of axially spaced bearing journals 76 and 78 adjacent opposite ends theTeof which engage the sidewalls of the longitudinally extending bore 32 so as to Totatably sl~poTt valve core m~mbe~ therein. A cam lobe 80 is also pTovided on valve core membeT 56 being positioned approximately midway between journals 76 and 78 and in Tadial alignment with inlet opening 42, As best seen wqth reference to Figures 4 and 5~ cam lobe 80 has a notched portion 82 on the circumference thereof which is positionable in alignment with inlet opening 42 so as to allow substantiaLly unrestricted fiLl fluid flow therethrough when valve assen~:Ly 26 is in a fully open position. C2m lobe 80 also has a substantially uniform rise OT increasing radius circumf.erential surface portion 84 extending n a counterclockwise direction fTom notch 82 as shown in Figure S and through approximately 270 of Totaticn as measured from approximately the center line of notch 82 with the Temaining circumferential surface portion 86 thereof being of substantially constant radius. In.the enbodiment shown, the total cam rlse is approximately .09 of an inch although it should be noted what both the contour and total rise may be vaTied so as to provide any desiTed acceleration c w e. For example, should an extre'mely gradual initial star*up be desired, the cam love could be provided with a first more gentle wise poTtion followed by a steepeT rise portion which would allow the driYen equipment to be more rapidly accelerated after initial movement had begun, Other vaTiations suitable foT the particulaT application may easily be provided by selecting ~20~Z~4 an appropTiate cam lobe contour. The maximum Tadius circumferential surface portion 86 OT cam lobe 82 is such that when valve core member 56 is Totated so as Jo position the n~imum radius circumferential surface portion 86 in alignment with inlet cpening 42, fluid flow into valve assembly 26 will be substantially elinina~ed. Suitable sealing means 88 are also positioned within enlarged dianeteT portion 36 through which end poTtion 90 of valve core member 56 extends outwardly of housing 30. Also, in oTdeT to prevent accumulation of fluid between journal 78 and sealing neans 88, an opening 92 is provided extendlng radially outwardly fTon bore 32 and opening into nanifolding cavity 50 so as to enable fluid to be retu med to the reservoir.
As shown in Figure 3, valYe housing 30 is supportingly secured by means of a plurality o fastening means 93 to a coupling housing 94 which in tu m is supportingly secured to a control housing 96. A stepping motor 98 and associated control circuitry described in greater detail below aTe disposed within control housing 96 with stepping motor 98 having a dlive shaft 100 extending outwardly there prom through opening 102 provided in sidewall portion 104 and into coupling housing 94. End portion 90 of valve core member ~6 also projects into coupling housing 94 through an opening 106 provided in sidewall 108 thereof and is positioned in axial alignment with stepping motor dTi~e shaft 100. Suitable coupling means 110 aTe provided for dTiYingly connecting end portion 90 of valve coTe nember 56 to drive shaft 100 and includes a first portion 112 secured to end portion 9~ and a second potion 114 secured to drive shaft 100.
Coupling means 110 will preferably ye of the type which utilizes a suitably resilient Tubber or elastome~ic composition bushing 111 positioned between overlapping axially extending anm segments provided on first and second portions 112 and 114 Tespectively, Such couplings are readily commeTcially available and hence further description theTeof is believed unnecessary.
As best seen with Teference to Figures 3 and 6, irst poTtion 112 of coupling means 110 is provided with a Tadially autwa~dly extending arm 116 which is adapted to engage a stop member 118 projecting into the in~e~ior of coupling housing in substantially parallel spaced relationship to end poTtion 90 of valve coTe member 56 so as to limit the Totational movement of valve core member. In operation, arm 116 will be posi*ioned relative to cam lobe 80 so as to position notch 82 in alignment with inlet opening 42 when arm 116 engages stop member 118 in a counterclockwise direction of rotation as shown in Figule 6 and to position max~num radius circumfeTential suTface portion 86 in aligmnent with inlet opening 42 when aTm 116 engages stcp member 118 in the opposite direction of rotation. The provision o rubber or elastom~ric bushing 111 within coupling means 110 will opeTate to insulate stepping motor 98 from the shock encountered when arm 116 is driven against stop membeT 11~.
Stepping motor US will pTeferably haze a relatively large number of steps per revolution; on the ordeT of 200 and has a relatively low torque output sufficient to rotate valve coTe membeT
56 but yet small enough to avoid any damage Tesulting from Totation against stop men~eT, A control circuit 120, as illustrated in block diagram in Figure 7, is pT~vided for controlling the opeTation of stepping motor 9~ and hence pressure relief contTol valve assembly 26. Control CiTCUit 120 ~sludes both forwaTd and reverse drive means 122 end 124 connected to stepping motoT 98 which when actuated aTe operative to provide stepping pulses to drive stepping motor 98 in ei~hel clockwise (foTwaTdl or counterclockwise (~eveTsel directions. CiTcuit reset means 126 are provided which upon energization will opeTate to activate -~9-~L2C)7 reverse dTive means 124 so as to sequence stepping ~otoT 9B through a pull series of steps in a counterclockwise direction to insure valYe coTe member 56 is positioned in the fully open OT fUll bypass mode with the notch portion 82 of cam love 80 positioned in alignment s with inlet opening 42 on valve housing 30 so as to theIeby insuTe that no actuating fluid pressuTe is applied to the clutch unit 20.
When valve coTe member 56 is in this position, substantially the entiTe fluid output of pump meals 22 will be pumped from a reserYoir (not shcwn) through conduits 24 and 28 into valve inlet cpening 42 across the valve core member 56 on both sides of Gam lobe 80 th m ugh the two outlets 44 and 46 into manifolding cavity S0 and outlet opening 54 into a condlit which will Tet~n the fluid to ale reservoir.
Because notch portion 82 on cam lobe 80 of the valve core member 56 is positioned in alignment with inlet opening 42~ fluid flow therethrough will be substantially unrestricted and substantially no actuating pressure will be applied to the clutch unit. It should also be noted thaw because of the pTovision of two geneTally parallel fluid flow paths around cpposite axial sides of cam lobe 80 of valve core member 56 any axially diIected ~o~ces exerted on the valve core member 56 fTom the fluid flow which could result in binding thereof so as to pTevent or inhibit rotation of valve cove nember 56 are substantially balanced.
Once this reset operation has been completed and the location of valve core member ~6 nelative to the control circuit sequencing has teen ~eIified, low speed clock means 128 is actuated and cpe~ates to pIovidb a sîgnal to the forward dTive means 122 so as to thereby begin advancing the stepping motor 98 in a forward direction which in turn will operate to begin moving the valYe core member 56 out of the fully open position. During the initial mcvement of notch 82 on cam lobe 80 of valve cove member 56 out of alignment 72~4 with the inlet opening 42 the rate of increase of actuating fluud pressure applied to the clutch will be relatively high because relatively slight movement of the valve core member 56 will create a significant increase in the restriction of the inlet opem ng 42 theleby reducing the volume of fluid flowing through the pTessuTe relief control valve 26.
Low speed clock means 128 will continue to signal the foIwaTd drive means 122 to provide dTiving pulses Jo the stepping motor 98 thereby further progressively closing pressure relief control valve 26 and increasing the actuating pTessure applied to the clutch unit 20 until such time as the pressure sensing means 130 connected in conduit 24 signals pressure set means 132 that a predetermined actuating fluid pTessure has been applied to clutch unit 20, This pTedeteTmined pressure may be set at any desired level but pre:ferably will be at a plessuTe corresponding at least to a valve core member 56 position in which notch 82 provided on cam lobe 80 has been moved completely out of the area of the inlet opening 42. Thereafter, pTessure set means 132 will deactivate low speed clock n~ans 128 and activate a high speed clock means 134 which will continue to drive stepping rotor 98 in a folwaTd diTectian until the pressure relief control valve means 26 is moved into a fully closed position and full operating pressure is applied to the clutch unit 20. I~hen full actuating pressure is applied Jo clutch unit 20 the alternating discs and pla*es will both be rotating at substantially the same speed so as to transnat the full dliving powel of the motor means to the driven apparatus.
As shoMn, the output of the high speed clock means 134 is also connected to the reverse dTive means 124 via an inverter 136.
Invertel 136 is incorporated in the CiTCUitTy because both forward 3Q and reverse drive means 122 and 124 are responsive to only positive Z0729L~
gating signals Thus, when it is desired to open pressuTe relief contTol valve 26, high speed clock means 134 will be activated to provide a series of negative pulses which will be supplied to both the foTward and Teverse drive means 122 and 124. However9 because the polarity of the negative pulse will be changed to positive by inverted 136, only the reverse drive means 124 will be actuated so as to operate stepping motor 98 in a reverse or counterclockwise direction thereby causing the pressure relief control valve 26 to move sequentially fTom a fully closed position to~lard a fully open position resulting in increas mg fluid flow therethrough and a decrease in actuating fluid pressure applied to the clutch unit 20.
The contTol system of the present invention is also provided wnth safety override means opeTative to abort a startup of the driven appaTatus in the event of abnoTmal conditions such as may occur in the event the apparatus to be driven is jammed OT overloaded.
The safety oveTTide means includes a fiTst circuit moans adapted to abort a startup in the event there is no Totational movement of the clutch unit output shaft afteT stepping motor 98 has advanced a pTedetenn m ed number of steps This circuit includes speed sensor means 138 assocîated with the output shaft of the clutch unit 20 which senses rotational vement thereof in response to which a signal is sent to a countercircuit n~ans 140. Counter means 140 also receives the same pulses from the foTward drive 122 as are being transmitted to the stepping motor 98. In the event counter neans 140 has not received any signal fT~m the speed sensor means 138 after the foIwa~d ~IiYe 122 has advanced the stepping motor 98 through a pTedeteTmined number of steps, counter means 140 will actuate the reverse drive 124 so as to pulse stepping motor 98 back to the start position in which the pTessure relief control valve 26 is in a fully open position and substantially no actuating fluid pressure is being I_ -12-~z~z~
applied to clutch unit 20. If desired, counter means 140 may also operate to activate alarm means on a central remotely located annunciatoT panel so that the abnormality may be investigated and repairs effected.
The safety override moans also includes circuity operative to about a staTtup in the event the dTiven apparatus is not acceleTated to full operating speed within a pIcdeteTmined time period such as may occur it the apparatus is overloaded for example.
This circuitry includes full speed set means 142 which also Teceives a speed responsive signal from speed sensor means 138 which is compared to a predeteTmined full speed setting. If the signal fT~m speed sensor moans 138 fails to indicate that the apparatus has achieved full operating speed within a pTedeteTmined time period full speed set means 142 will actuate reverse drive means 124 so lS as to pulse stepping motor 98 in a counteTclockwise direction thereby n~ving the pressuTe Telief control valve 26 into a fully open position and relieving the actuating fluid pressuTe being applied to clutch unit 20 so as to prevent possible damage to the clutch OT otheT
associated dTive system components. The full speed set means 142 may also be tied into an alaTm system whereby the need for corrective action may be signaled.
In operation, the pTessure relief control valve assembly ~6 and the associated control d TcUit~y 120 will opeTate to enable clutch unit 20 to be actuated in such a manner as to provide a Telatively "soft" staTt yet still allow ~elati~ely rapid engagement of the clutch plates and discs so as to minimize slippage and hence wear and fIicticnal heating theTeof. this drive system of the preset Lnvention is thus particulaTly well suited foT a numbeT of applications in which very large loads aTe encountered dhring startup such as for example conveyor systems. Additionally, ~C~7 Z
the pTessure Telief control valve 26 of the present invention is particularly suited foT use in aTeas of high dust or dirt contaminations in which the pressure actuating fluid may become contaminated therewnth~
Because the pTesent pressure relief contTol valve utilizes a relatively large orifice plus the fact what it is designed to be driven in incremental discrete steps, it has been found Jo be substantially less susceptible to clogging or plugging as a lesult of highly contaminated fluid. FuTther, even when such clogging ox plugging does occur such a5 during prolonged periods of operation with the valve in the fully closed position, the initial stepping motion of the valve toward the open position has Tesulted in rapid clearing of the ccntaminant accumulation thus providing precise reliable contTol.
Also, both the pressure relief control valve and the associated control circuitry may be easily fabricated at relatively low cost and yet provide a highly reliable and durable clutch control system which may be easily utilized in both single and multiple drive systems. For example, in overland conveyor systems which may extend for several miles and which may have a number of belt drive means spaced along the length thereof, it is necessaTy to sequence the startup of the drives so as to assure proper belt tensioning.
This may be easily accomplished with the present control system by merely delaying the pulses to the respective stepping motors driving the pressuTe relief contTol valves of successîve clutches by any number of counts necessary to enable ploper belt tensioning.
Additionally, the safety override moans proviaes integral protection within the clutch control system which will abort staTtups and shut dnwn the system undeT a~n~mal conditions so as to thereby prevent excessive slipping ox the clutch unit which may result in premature failure thereof as well as other possible damage to the driven or 3Q driving equipment associated thelewith.

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The above described drive system including the fluid actuated clutch means and the control means is also described and is claimed in above-identified parent application serial number 383,885.
While it will be apparent that the preferred embodiment o the invention disclosed is well calculated to provide the advantages and features above stated, it will be appreciated that the invention is susceptible to modification, variation and change without departing from the proper scope or fair meaning of the subjoined claims.

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Claims (11)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A fluid control valve comprising:
a housing having an inlet opening and at least one outlet opening;
a valve member rotatably movably disposed within said housing, said valve member having a cam surface positioned in radial alignment with said inlet opening, said cam surface being movable from a first position in which said cam surface is radially spaced from said opening to a second position in which said cam surface effectively restricts fluid flow through said inlet opening; and drive means for driving said valve member between said first and second positions in pulsed steps, said pulsed step movement of said valve member being operative to dislodge contaminants from said inlet opening.

2. A fluid control valve as set forth in Claim 1 wherein said valve member includes a pair of axially spaced bearing journals provided thereon, said cam surface being positioned substantially centrally between said bearing journals.

3. A fluid control valve as set forth in Claim 2 wherein said valve housing has a pair of spaced fluid outlets, each of said outlet openings being positioned between one of said bearing journals and said cam surface whereby axially directed forces exerted on said valve member by fluid flow therethrough are substantially balanced.

4. A fluid control valve as set forth in Claim 1 wherein said cam surface includes first portion having a notch therein positionable in radial alignment with said inlet opening so as to allow substantially unrestricted fluid flow through said control valve.

5. A fluid control valve as set forth in Claim 4 wherein said cam surface includes a second portion having a substantially constant rate of rise.

6. A fluid control valve as set forth in Claim 1 wherein said housing has an elongated bore extending therethrough, said valve member comprises an elongated shaft rotatably positioned within said bore, said drive means being connected to one end of said shaft.

7. A pressure relief valve for controlling actuating fluid pressure applied to a clutch comprising:
a housing having an elongated generally cylindrically shaped valve chamber provided therein;
an inlet opening provided in said valve housing, said inlet opening defining an orifice opening into said valve chamber;
an outlet opening provided in said housing and opening into said valve chamber;
a valve member rotatably movably disposed within said valve chamber, said valve member having a cam surface positioned in radial alignment with said orifice and operative to control fluid flow through said orifice; and pulsating drive means for driving said valve member through a plurality of discrete pulsed steps between first and second positions, said pulsed movement of said valve member being operative to dislodge contaminants in said actuating fluid lodged within said orifice whereby the flow rate through said orifice may be accurately controlled by the relative position of said cam surface.

8. A pressure relief valve as set forth in Claim 7 wherein said pulsating drive means comprises a stepping motor and control means for selectively energizing said stepping motor.

9. A pressure relief valve as set forth in Claim 7 wherein said valve means includes stop means for limiting rotational movement of said valve member between open and closed positions.

10. A pressure relief valve as set forth in Claim 7 wherein said valve member includes a pair of axially spaced bearing journals provided thereon, said cam surface being positioned substantially centrally between said bearing journals.

11. A pressure relief valve as set forth in Claim 10 wherein said valve housing has a pair of spaced fluid outlets, each of said outlet openings being positioned between one of said bearing journals and said cam surface whereby axially directed forces exerted on said valve member by fluid flow therethrough are substantially balanced.