Abstract:
A valve for controlling flow of hydraulic fluid. The valve includes first and second relatively movable valve members (60 and 62). The valve members (60 and 62) include cooperating surfaces (112, 142, 144) which define first and second orifices for metering fluid from a pressurized fluid source (44) toward first and second locations (22 and 24) in a fluid utilization device (12). The valve members (60 and 62) have a displaced position in which the first orifice is increased in size and the second orifice is decreased in size. The increased size of the first orifice reduces a pressure drop across the first orifice. The decreased size of the second orifices increases a pressure drop across the second orifice. The valve members (60 and 62) include means (114 and 146) for increasing a pressure on a downstream side of the second orifice to reduce the pressure drop for controlling valve noise.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a valve for controlling flow of hydraulic fluid. The valve is used to control the flow of hydraulic fluid to a power steering motor. 
     A known power steering control valve for controlling flow of hydraulic fluid to a power steering motor includes a hollow valve sleeve and a valve core. The valve core is positioned within the valve sleeve. The valve core and the valve sleeve are relatively rotatable. Each of the valve core and valve sleeve has a plurality of lands and grooves. 
     When the valve core and valve sleeve are in a neutral position fluid is communicated to opposite chambers of the power steering motor at equal pressures. When the valve core and valve sleeve are relatively rotated from the neutral position, fluid flow is variably restricted. Restriction of the fluid flow causes pressurized fluid to be delivered to one of two chambers of a hydraulic power assist motor to cause motor actuation. 
     The restriction is provided by lands on the valve core and valve sleeve which define flow orifices. Variation of the flow orifices, and the amount of restriction, is provided by end surface segments of the lands which are formed such that varied amounts of relative rotation between the valve core and valve sleeve cause the end surface segments to be positioned at varying distances apart. Due to a high volume of hydraulic fluid flow and pressure differential changes as the hydraulic fluid flows through the flow orifices, noise may be created in the control valve. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a valve for controlling flow of hydraulic fluid. The valve includes first and second relatively movable valve members. The valve members include cooperating surfaces for defining first and second orifice means for metering fluid from a pressurized fluid source toward first and second locations in a fluid utilization device, respectively. The valve members have a displaced position in which the first orifice means is increased in size and the second orifice means is decreased in size. The increased size of the first orifice means reduces the magnitude of a pressure differential across the first orifice means. The decreased size of the second orifice means increases the magnitude of a pressure differential across the second orifice means. The valve members include means for increasing a pressure on a downstream side of the second orifice means to reduce the magnitude of the pressure differential across the second orifice means for controlling valve noise. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other features of the present invention will become apparent to one skilled in the art to which the present invention relates upon consideration of the following description of the invention with reference to the accompanying drawings, wherein: 
     FIG. 1 is a longitudinal cross-sectional view of a power steering gear with a valve which embodies the present invention; 
     FIG. 2 is a partial sectional view of the valve taken along line 2--2 of FIG. 1 and includes schematic hydraulic connection representations; 
     FIG. 3 is a view similar to FIG. 2, with valve members relatively rotated; 
     FIG. 4 is an enlargement of a portion of the valve shown in FIG. 3; 
     FIG. 5 is an enlargement of another portion of the valve shown in FIG. 3; 
     FIG. 6 is a view similar to FIG. 2, showing a second embodiment of the invention; and 
     FIG. 7 is a view similar to FIG. 3, showing a third embodiment of the invention, with valve members relatively rotated. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     The fluid control valve 10 (FIG. 1) of the present invention may be used to control fluid flow associated with mechanisms of a variety of constructions and uses. Preferably, the control valve 10 is utilized in a power steering gear 12 for turning dirigible wheels (not shown) of a vehicle (not shown) to effect steering of the vehicle. The preferred power steering gear 12 is a TAS Integral Power Steering Gear manufactured and marketed by TRW Inc., Commercial Steering Division of Lafayette, Ind., and identified as TAS40, TAS55 or TAS65. 
     The power steering gear 12 includes a housing 14 having an inner cylindrical surface 16 defining a chamber 18. A piston 20 divides the chamber 18 into opposite chamber portions 22 and 24 located at opposite ends of the piston 20. An O-ring 26 carried in a groove 27 in the piston 20 provides a fluid seal between the chamber portions 22 and 24. 
     A series of rack teeth 28 are formed on the periphery of the piston 20. The rack teeth 28 mesh with teeth 32 formed on a sector gear 34. The sector gear 34 is fixed on an output shaft 38 which extends outwardly from the steering gear 12 through an opening (not shown) in the housing 14. The output shaft 38 is typically connected to a Pitman arm (not shown) which in turn is connected to a steering linkage (not shown) of the vehicle. Thus, as the piston 20 moves in the chamber 18, the output shaft 38 is rotated to operate the steering linkage as is understood by those skilled in the art. 
     The housing 14 includes a fluid inlet port 40 and a fluid return port 42. The inlet port 40 and return port 42 are adapted to be connected in fluid communication with hydraulic circuitry (schematically illustrated) including a power steering pump 44 and a power steering pump fluid reservoir 46. The control valve 10 can direct pressurized fluid from the inlet port 40 to one or the other of the chamber portions 22 and 24. Fluid from the other of the chamber portions 22 and 24 is simultaneously directed by the control valve 10 to the return port 42 which is connected with the reservoir 46. 
     The control valve 10 is actuated by a rotatable shaft 48. The shaft 48 is supported for rotation relative to the housing 14 via bearing member 50. An outer end portion 52 of the shaft 48 is splined for receiving a portion of a shaft 54 thereon. The shaft 54 is connected with a steering wheel (not shown) which is manually turned by the operator of the vehicle to effect steering of the vehicle. 
     The control valve 10 includes a valve sleeve 60 and a valve core 62. The valve core 62 is integrally formed with the shaft 48. The valve core 62 is located within the valve sleeve 60 and is coaxial with the valve sleeve 60. The valve core 62 is rotatable relative to the valve sleeve 60 about a common axis 64 (FIG. 2) of the valve core 62 and valve sleeve 60. 
     The valve sleeve 60 (FIG. 1) is supported for rotation by bearings 66 and 68 located between the valve sleeve 60 and the housing 14. The bearing 66 is located between an annular projecting portion 70 of the valve sleeve 60 and a radial wall 72 of the housing 14. The bearing 66 is a ball bearing. Also, a seal ring 74 is located between the outer surface of the valve sleeve 60 and the housing 14. 
     The bearing 68 is a thrust bearing and is located between a radial surface 76 of the annular projecting portion 70 of the valve sleeve 60 and a retaining nut 78. The nut 78 is threaded into the housing 14 and holds the control valve 10 in position in the housing 14. A seal ring 80 is located between the nut 78 and an outer surface of the valve sleeve 60. Another seal 82 is disposed in a groove in the housing 14. 
     The valve sleeve 60 is integrally formed with a follow-up member 86 which has a screw thread portion 88 formed in its outer periphery. The valve sleeve 60 and the follow-up member 86 form an integral one-piece unit 90. A plurality of balls 92 are located in the screw thread portion 88. The balls 92 are also located in an internally threaded portion 94 formed in a bore 96 of the piston 20. Axial movement of the piston 20 corresponds to rotation of the follow-up member 86, as is known. 
     A torsion bar 98 is connected between the input shaft 48 and the follow-up member 86 by pins 100 and 102, respectively. During a steering maneuver, the torsion bar 98 can transfer rotational force from the input shaft 48 to the follow-up member 86. However, if the follow-up member 86 is resistant to rotation in unison with the input shaft 48, the torsion bar 98 twists and the valve core 62 is rotated relative to the valve sleeve 60, away from the neutral position. When the valve core 62 is rotated relative to the valve sleeve 60, the piston 20 moves due to application of pressurized fluid to one of the chamber portions 22, 24. When the steering maneuver is terminated, the one-piece unit 90, and thus the valve sleeve 60, will rotate relative to the valve core 62 and return the valve core 62 and the valve sleeve 60 to the neutral position. 
     The valve sleeve 60 (FIG. 2) has three pairs of diametrically opposed lands, 106, 108, and 110, respectively, which have end surfaces 112, 114, and 116, respectively. The valve sleeve 60 has two radially directed passages 118 extending from its outer circumference to its inner circumference. The passages 118 are diametrically opposed about the valve sleeve 60 and extend through respective end surfaces 112 of lands 106. The passages 118 communicate with an annulus 120 (FIG. 1) in the housing 14. The annulus 120, in turn, is connected with the inlet port 40, and is thus subjected to the fluid pressure from the power steering pump 44. 
     The valve sleeve 60 (FIG. 2) has on its inner surface two axially extending grooves 122. The two grooves 122 are diametrically opposed about the inner circumference of the valve sleeve 60. Each groove 122 is located between a respective land 106 and a respective land 110. Each of the grooves 122 communicates with a respective passage 124. The passages 124 are spaced 180° apart about the valve sleeve 60. The passages 124 (FIG. 1 shows only one passage 124, in phantom) communicate with the chamber portion 22. 
     The valve sleeve 60 (FIG. 2) has two axially extending grooves 126 on the inner surface thereof. The grooves 126 are diametrically opposed about the inner circumference of the valve sleeve 60. Each groove 126 is located between a respective land 106 and a respective land 108. Each of the grooves 126 communicates with a respective radially extending passage 128. The passages 128 are spaced 180° apart about the valve sleeve 60. 
     The passages 128 (FIG. 1 shows only one passage 128, in phantom) communicate with an annulus 130 in the housing 14. The annulus 130 communicates with a suitable housing passage 132 which, in turn, communicates with the chamber portion 24. The valve sleeve 60 (FIG. 2) also has two axially extending grooves 134 on the inner face thereof. The grooves 134 are diametrically opposed. Each groove 134 is located between a respective land 108 and a respective land 110. 
     The valve core 62 has three pairs of diametrically opposed lands 136, 138 and 140, respectively, which have end surfaces 142, 144 and 146 respectively. The valve core 62 has two axially extending grooves 148 (FIG. 2) in its outer circumference. The grooves 148 are diametrically opposed about the outer circumference of the valve core 62 and communicate with the passages 118 in the valve sleeve 60. Each groove 148 is located between a respective lands 136 and 138. The extent of the grooves 148 around the outer circumference of the valve core 62 is such that each of the grooves 148 communicates equally with respective grooves 122 and 126 when the valve core 62 is in a centered or neutral position relative to the valve sleeve 60 (as shown in FIG. 2). 
     The valve core 62 has two axially extending grooves 150 in its outer circumference. The grooves 150 are diametrically opposed about the outer circumference of the valve core 62. Each groove 150 is located clockwise (as shown in FIG. 2) about the valve core 62 from an adjacent groove 148 and between a respective land 138 and a respective land 140. The extent of the grooves 150 around the outer circumference of the valve core 62 is such that each of the grooves 150 communicates with respective grooves 126 and 134 when the valve core 62 is in the neutral position relative to the valve sleeve 60. 
     The valve core 62 has two axially extending grooves 152 in its outer circumference. The grooves 152 are diametrically opposed about the outer circumference of the valve core 62. Each groove 152 is located counterclockwise (as shown in FIG. 2) about the valve core 62 from an adjacent groove 148 and between a respective land 136 and a respective land 140. The extent of the grooves 152 around the outer circumference of the valve core 62 is such that each of the grooves 152 communicates with respective grooves 122 and 134 when the valve core 62 is in the neutral position relative to the valve sleeve 60. 
     The valve core 62 has two radially extending internal passages 154. Each passage 154 extends through a respective land 140 and communicates with a respective groove 134 on the valve sleeve 60. The passages 154 communicates with an internal passage 156 of the valve core 62. The internal passage 156 of the valve core 62 also communicates with a plurality (four) of radially extending passages 158 (FIG. 1, only two shown) which extend through the valve core 62. The radially extending passages 158 communicate with an annulus 160 in the housing 14. The annulus 160, in turn, communicates with the return port 42 in the housing 14. 
     Each land corner on the valve sleeve 60 (FIG. 2) and the adjacent land corner on the valve core 62 define a variable flow orifice for fluid flow between respective adjacent grooves in the valve sleeve 60 and the valve core 62. Relative rotation of the valve sleeve 60 and the valve core 62 varies the size of each flow orifice. The lands 136 and 138 each have a chamfered corner 164 (FIG. 4) adjacent to an associated groove 148. Each corner 164 has two facets 166 and 168. Each facet 166 is located adjacent to a respective groove 148 and each facet 168 is located intermediate a respective facet 166 and an outer circumference diameter of the valve core 62. 
     In a preferred embodiment, the outer diameter of the valve core 62 is 1.1241 inch and the radial depth of the modification from the outer diameter to each facet 166 surface adjacent to the respective groove 148 is 0.0106 inch. Each two associated facets 166 and 168 are inclined relative to each other at an angle A. In the preferred embodiment, the angle A is 16°. 
     The lands 140 (FIG. 2) each have two chamfered corners 170 (FIG. 5). Each chamfered corner 170 has a facet 172 and a tapering curved segment 174. Each facet 172 is located adjacent to a respective groove, either 150 or 152, and each segment 174 is located intermediate a respective facet 172 and an outer circumference diameter of the valve core 62. Each segment 174 has a radius of curvature which changes as the segment 174 extends from the facet 172 toward the outer diameter. In a preferred embodiment, the depth of the modification from the outer diameter to each facet 172 adjacent the respective groove, either 150 or 152, is 0.106 inch. 
     In operation, the amount of fluid flow from the grooves 148 (FIG. 2) to either the grooves 122 or 126 is dependent upon the proximity of either the lands 136 or 138 to the respective land 106, due to relative rotation of the valve sleeve 60 and the valve core 62. Further, the amount of fluid flow from either the grooves 122 or 126 toward the grooves 134 is dependent upon the proximity of the lands 136, 138 and 140 to the lands 108 and 110, respectively, due to relative rotation of the valve sleeve 60 to the valve core 62. 
     When the valve sleeve 60 and valve core are in the neutral position (FIG. 2), each land 136 is radially centered relative to a respective groove 122, each land 138 is radially centered relative to a respective groove 126, and each land 140 is radially centered relative to a respective groove 134. Fluid from the pump 44 flows into the grooves 148. The fluid flows substantially equally into the grooves 122 and 126. The fluid flows into the respective grooves 152 and 150 and then into the grooves 134. The pressure of the fluid drops (changes) as it flows through each flow orifice. Thus, a pressure differential occurs across each flow orifice. 
     The fluid is circulated from the pump 44 through the control valve 10 without being directed toward the steering gear 12. Thus, in the neutral position, the pressures in the chambers 22 and 24 are substantially equal. Therefore, the piston 20 is not moved. 
     Upon rotation of the steering wheel in a first direction (FIG. 3), the valve sleeve 60 and the valve sleeve 62 are relatively rotated in a first direction away from the neutral position. The valve core 62 is rotated counterclockwise (as shown in FIG. 3) relative to the valve sleeve 60. The flow orifices between the grooves 148 and the grooves 122 are increased in size and the flow orifices between the grooves 148 and the grooves 126 are decreased in size. 
     The pressure drop (differential) across the flow orifices between the grooves 148 and the grooves 122 is decreased and the pressure drop (differential) across the flow orifices between the grooves 148 and the grooves 126 is increased. The flow orifices between the grooves 148 and 126 for flow fluid 175 (FIG. 4) from the grooves 148 to the grooves 126 are convergent. The flow orifices between the grooves 122 (FIG. 3) and the grooves 152 are decreased in size, to substantially zero, and the flow orifices between the grooves 126 and the grooves 150 are increased in size. 
     The flow orifices between the grooves 150 and the grooves 134 are decreased in size. The pressure drop (differential) across the flow orifices between the grooves 150 and the grooves 134 is increased. The flow orifices between the grooves 150 and the grooves 134 for fluid 176 (FIG. 5) from the grooves 150 to the grooves 134 are convergent. Thus, pressurized fluid is directed toward the chamber 22 (FIG. 3) and the fluid is exhausted from the chamber 24. However, an increased pressure drop across the flow orifices between the grooves 148 and 126 can produce fluid flow noise. 
     The problem of fluid flow noise is alleviated by the simultaneous decrease in size of the flow orifices between the grooves 150 and the grooves 134 which creates a back pressure in the fluid in the grooves 150 and grooves 126. The back pressures mitigates the pressure drop across the flow orifices between the grooves 148 and grooves 126. Specifically, a chamber region defined by the grooves 148 and 122 has fluid with a relatively high pressure. A chamber region defined by the grooves 126 and 150 has fluid with a relatively intermediate pressure. A chamber region defined by the grooves 134 and 152 has fluid with a relatively low pressure. 
     The facets 166 and 168 (FIG. 4) provide two different rates of change of the size of the orifices between the grooves 148 and the grooves 126 per unit of relative rotation between the valve sleeve 60 and the valve core 62. The rate of change of the orifice size per unit of relative rotation is greater when the facets 166 are radially aligned with respective edges on the lands 106 than when the facets 168 are radially aligned with the respective edges on the lands 106. 
     The facets 172 (FIG. 5) provide a first rate of change of the size of the orifices between the grooves 150 and grooves 134 per unit of relative rotation between the valve sleeve 60 and the valve core 62 when aligned with respective edges on the lands 108. Similarly, the tapering curved segments 174 provides a diminishing rate of change of the size of the orifices between the grooves 150 and the grooves 136. Thus, as the valve sleeve 60 (FIG. 3) and the valve core 62 are relatively rotated from the neutral position, the size of the orifices between the grooves 150 and the groove 136 is first decreased and then retained at a constant. This provides the desired back pressure in the grooves 126 and 150 and yet does not prevent sufficient fluid flow during large angle relative rotation between the valve sleeve 60 and the valve core 62. 
     Upon relative rotation of the valve sleeve 60 and the valve core 62 in a second direction from the neutral position of FIG. 2, fluid from the pump 44 is directed to the chamber 24 and fluid is exhausted from the chamber 22. The flow orifices are changed in size obversely to that which occurred during relative rotation of the valve sleeve 60 and the valve core 62 in the first direction. Thus, valve noise is also suppressed. 
     In another embodiment of the invention (FIG. 6, wherein structure similar to that of the first embodiment is identified by the same reference numeral) the valve core 62 has two bores 178 and two bores 180. Each bore 178 extends from a respective groove 150 to the passage 156. Each bore 180 extends from a respective groove 152 to the passage 156. 
     The bores 178 are sized to permit the desired back pressure to be created in the grooves 126 and 150 to suppress flow noise at the orifices between the grooves 148 and the grooves 126 during counterclockwise rotation of the valve core 62 relative to the valve sleeve 60 from the neutral position. Yet, the bores 178 permit a minimum fluid flow from the groove 150 to the passage 156 for all relative rotational positions of the valve sleeve 60 and the valve core 62, even large angle relative rotation. The bores 180 function similarly for back pressure creation in the grooves 122 and 152 during clockwise rotation of the valve core 62 relative to the valve sleeve 60 from the neutral position. 
     In a third embodiment of the invention (FIG. 7 wherein structure similar to that of the first embodiment is identified by the same reference numerals), the valve sleeve 60 has four pairs of diametrically opposed lands, 186, 188, 190, and 192, respectively. The valve sleeve 60 has two radially directed passages 118 extending from its outer circumference to its inner circumference. The passages 118 are diametrically opposed about the valve lands 186 and extend through a respective land 186. 
     The valve sleeve 60 has on its inner surface two axially extending grooves 202, which are diametrically opposed about the inner circumference of the valve sleeve 60. Each groove 202 is located between a respective land 186 and a respective land 190. The valve sleeve 60 has on its inner surface two axially extending grooves 204, which are diametrically opposed about the inner circumference of the valve sleeve 60. Each groove 204 is located between a respective land 186 and a respective land 188. 
     The valve sleeve 60 has on its inner surface two axially extending grooves 206, which are diametrically opposed about the inner circumference of the valve sleeve 60. Each groove 206 is located between a respective land 190 and a respective land 192. Each of the grooves 206 communicates with a respective passage 124. 
     The valve sleeve 60 has on its inner surface two axially extending grooves 208, which are diametrically opposed about the inner circumference of the valve sleeve 60. Each groove 208 is located between a respective land 188 and a respective land 192. Each of the grooves 208 communicates with a respective radially extending passage 128. 
     The valve core 62 has four pairs of diametrically located lands 210, 212, 214 and 216, respectively. The valve core 62 has two axially extending grooves 226, which are diametrically opposed about the outer circumference of the valve core 62 and communicate with the passages 118 in the valve sleeve 60. Each groove 226 is located between respective land 210 and a respective land 212. The extent of the grooves 226 around the outer circumference of the valve core 62 is such that each of the grooves 226 communicates equally with respective grooves 202 and 204 when the valve core 62 is in a centered or neutral position relative to the valve sleeve 60 (as shown in FIG. 2). 
     The valve core 62 has two axially extending grooves 228, which are diametrically opposed about the outer circumference of the valve core 62. Each groove 228 is located clockwise (as shown in FIG. 4) about the valve core 62 from an adjacent groove 226 and between a respective land 212 and a respective land 216. The valve core 62 has two axially extending grooves 230 in its circumference. The grooves 230 are diametrically opposed. Each groove 230 is located counterclockwise (as shown in FIG. 4) about the valve core 62 from an adjacent groove 226 and between a respective land 210 and a respective land 214. 
     The valve core 62 has two axially extending grooves 232, which are diametrically opposed about the outer circumference of the valve core 62. The extent of the grooves 232 around the outer circumference of the valve core 62 is such that each of the grooves 232 communicates with respective grooves 206 and 208 when the valve core 62 is in the neutral position relative to the valve sleeve 60. The valve core 62 has two radially extending internal passages 154. Each passage 154 communicates with a respective groove 232 on the valve core 62. 
     The lands 210 and 212 each have a chamfered corner adjacent to an associated groove 226. The lands 214 each have a chamfered corner adjacent to an associated groove 230. The lands 216 each have a chamfered corner adjacent to an associated groove 228. 
     In operation, the amount of fluid flow from the grooves 226 toward either the grooves 206 or 208 is dependent upon the amount of relative rotation of the valve sleeve 60 and the valve core 62. Further, the amount of fluid flow from either the grooves 206 or 208 toward the grooves 232 is dependent upon relative rotation of the valve sleeve 60 to the valve core 62. 
     When the valve sleeve 60 and valve core 62 are in the neutral position fluid from the pump 44 flows into the grooves 226. The fluid flows substantially equally into the grooves 202 and 204 and then into grooves 230 and 228, respectively. The fluid flows into the respective grooves 206 and 208 and then into the grooves 232. The pressure of the fluid drops (changes) as it flows through each flow orifice. Thus, a pressure differential occurs across each flow orifice. 
     The fluid is circulated from the pump 44 through the control valve 10 without being directed toward the steering gear 12. Thus, in the neutral position, the pressures in the chambers 22 and 24 are substantially equal. Therefore, the piston 20 is not moved. 
     Upon rotation of the steering wheel in a first direction, the valve sleeve 60 and the valve sleeve 62 are relatively rotated in a first direction away from the neutral position. The valve core 62 is rotated counterclockwise (as shown in FIG. 4) relative to the valve sleeve 60. The flow orifices between the grooves 226 and the grooves 202 are increased in size and the flow orifices between the grooves 226 and the grooves 204 are decreased in size. The pressure drop (differential) across the flow orifices between the grooves 226 and the grooves 202 is decreased and the pressure drop (differential) across the flow orifices between the grooves 226 and the grooves 204 is increased. 
     The flow orifices between the grooves 202 and the grooves 230 are decreased in size, slightly. The flow orifices between the grooves 204 and the grooves 228 are increased in size. The flow orifices between the grooves 230 and the grooves 206 are increased in size. The flow orifices between the grooves 206 and the grooves 232 are essentially blocked. The flow orifices between the grooves 228 and the grooves 208 are decreased in size. The flow orifices between the grooves 208 and 232 are increased in size. Thus, pressurized fluid is directed toward the chamber 22 and the fluid is exhausted from the chamber 24. However, an increased pressure drop across the flow orifices between the grooves 226 and 204 can produce fluid flow noise. 
     The problem of fluid flow noise is alleviated by the simultaneous decrease in size of the flow orifices between the grooves 228 and the grooves 208 which creates a back pressure in the fluid in the grooves 228 and grooves 204. The back pressures mitigates the pressure drop across the flow orifices between the grooves 226 and grooves 204. Specifically, a chamber region defined by the grooves 226, 202, 230 and 206 has fluid which is relatively high pressure. A chamber region defined by the grooves 204 and 228 has fluid which is relatively intermediate in pressure. A chamber region defined by the grooves 208 and 232 has fluid which is relatively low in pressure. 
     Upon relative rotation of the valve sleeve 60 and the valve core 62 in a second direction from the neutral position, fluid from the pump 44 is directed to the chamber 24 and fluid is exhausted from the chamber 22. The flow orifices are changed in size obversely to that which occurred during relative rotation of the valve sleeve 60 and the valve core 62 in the first direction. Thus, valve noise is also suppressed. 
     From the above description of the invention, those skilled in the art will perceive improvements, changes and modifications. Such improvements, changes and modifications within the skill of the art are intended to be covered by the appended claims.