Power Steering System With Vehicle Speed-Sensitive Flow

My invention is an improvement in power steering mechanisms of the kind shown in U.S. Pat. No. 4,063,490 although the invention is capable of being used in other kinds of power steering gears for automotive vehicles. The steering gear structure shown in U.S. Pat. No. 4,063,490 comprises a rack-and-pinion steering gear mechanism in which the gear rack is connected operatively to a steering gear linkage and to the piston of a fluid motor. The rack engages drivably a drive pinion that is connected to a driver operated steering shaft through a torsion rod. A rotary valve mechanism comprising a valve sleeve and an inner valve member controls distribution of pressure from a power steering pump to the fluid motor portions of the rack-and-pinion steering gear mechanism.

The inner valve member of the rotary valve mechanism is connected to the steering shaft and the valve sleeve with which the inner valve member cooperates is connected to the pinion. Angular deflection of the torsion rod results in relative displacement of the inner valve member with respect to the valve sleeve thereby establishing pressure distribution to the fluid motor.

A power steering pump of the kind adapted for use with the present invention is shown in U.S. Pat. No. 4,199,304. Both this patent and U.S. Pat. No. 4,063,490 are assigned to the assignee of this invention.

The pump of U.S. Pat. No. 4,199,304 contains a flow control valve that establishes a constant flow delivery to the power steering gear regardless of the vehicle speeds. The rotor of the pump is connected to the crankshaft of the engine through a belt drive, but the varying engine speed during vehicle operation will not cause a corresponding change in the rate of delivery of pressurized fluid to the steering gear because of the operation of the flow control valve.

In the present invention I have made provision for reducing the flow delivery to the steering gear by introducing a solenoid operated bypass valve in cooperation with a constant flow valve so that a constant part of the pressurized fluid delivered by the pump to the steering gear is subtracted thereby causing an increased road feel and reduced power consumption at high road speeds. The solenoid bypass valve may be triggered by a speed sensitive switch which causes the valve to close at zero speed and during operation of the vehicle at speeds less than a predetermined value but which opens the valve at higher speeds.

FIG. 1 is a chart that shows the relationship of pressure to torque for various effective values of flow of pressurized fluid to the steering gear.

FIG. 2 is a schematic representation of a valve system for use in a pump and steering gear circuit.

FIG. 3 is a cross-sectional view of a steering gear mechanism of the rack-and-pinion type to which the improvements of my invention may be applied.

In FIG. 3 reference character 20 identifies an end housing at one end of a power cylinder sleeve. Located at the other end of the cylinder sleeve is a remote end housing, not shown, with a central opening within which one end of the cylinder sleeve is received. Valve housing 22 is secured to the housing 20. It contains a rotary valve structure.

Pressure distribution to the right turn and left turn passages 58 and 60 is effected by the rotary valve assembly. The valve housing 22 is formed with an internal cylindrical valve cavity 64. A rotary valve sleeve 66 positioned within cavity 64 is provided with radial pressure supply ports 68 and pressure distribution ports 70 and 72 which communicate respectively with left turn pressure passage 60 and right turn pressure passage 58. The pressures in passage 60 and in port 70 are isolated, one with respect to the other, by ring seals 74 and 76 which surround the cylindrical valve element 66 corresponding to ring seals 78 and 80 surrounding valve element 66 thereby isolating the pressure in port 72 and passage 58. Seals 76 and 78 are also effective to isolate pressure in pressure supply ports 68, the latter communicating with fluid pressure inlet conduit 82 through port 84 in housing 22. A fluid fitting 86 provides a fluid tight connection between the conduit 82 and port 84.

Valve element 66 has a cylindrical internal cavity within which is positioned inner rotary valve member 88.

Valve member 88 is formed with an internal cavity 90 through which a torsion rod 92 extends. The right-hand end of the valve element 88 defines a steering shaft 94, shown in FIG. 3, which is splined or otherwise connected to a driver operated steering shaft and journalled within bearing opening 96 in housing 22 by means of a steering shaft bearing 98. Fluid seal 100 surrounds steering shaft 92 and is held in place by a snap ring as indicated.

The right-hand end of the torsion rod 92 is pinned securely to the right-hand end of steering shaft 94. The left-hand end of the torsion rod 92 is pinned at 114 or otherwise securely connected to a drive pinion 102 journalled within housing 20. The pinion 102 is formed with end bearing portion 104 and an intermediate bearing sleeve 106. Bearing portion 104 is journalled within bearing opening 108 in the housing 20 and bearing portion 106 is journalled within bearing opening 109 at the opposite end of the housing 20.

Pinion 102 forms a part of an assembly that includes sleeve 106 which is positioned pinion seal 112. The end of sleeve 106, in the embodiment of FIG. 3, is secured by pin 114 to the pinion 102. The end of the sleeve 106 engages thrust reaction ring 116. Located on either side of the reaction ring 116 are radial needle bearings 118 and 120 which engage, respectively, thrust washers 122 and 124. Washer 122 is fixed in abutting relationship with respect to the end 126 of the housing 20. A Belleville thrust washer 128 is located between thrust washer 124 and an adjacent shoulder 130 formed in the interior valve opening of the housing 22. The preload on the Belleville washer 128 is sufficient to maintain the pinion 102 in fixed axial disposition with respect to the housing 20. Pinion 102 engages rack teeth 132 formed on the rack 50.

The pinion 102 is formed with helical teeth. When torque is applied to the pinion 102, the resulting axial thrust in either one direction or the other is absorbed by the thrust bearing assembly shown at 116, 118 and 120. The preload on Belleville washer spring 128 is sufficient to prevent axial displacement of the pinion 102 relative to the housing 20 for any design torque to which the pinion 102 may be subjected. The teeth 132 of the rack 50 are held in preloaded, zero-backlash engagement by a preload plunger 134 slidably situated in cylindrical opening 136 in housing 20. Preload force is applied to the plunger 134 by compression spring 138 which is seated on closure 140 threadably received within the end of the opening 136.

The solenoid windings are schematically shown in FIG. 2 at 206. When the valve 200 is opened, a constant flow control valve 208 is supplied with fluid through branch passage 210. That branch passage comprises a flow restricting orifice 212 and the pressure differential across the orifice is applied to opposite sides of a valve element 214. The valve element normally is urged in a left-hand direction by valve spring 216.

The outlet flow Q.sub.3 established by the constant flow valve is delivered to the low pressure side of the pump through branch passage 218.

It is thus seen that at high speeds a constant Q.sub.3 flow is subtracted from the flow Q.sub.1 delivered from the power steering pump to the steering gear shown in FIG. 3. This causes constant flow Q.sub.2 and a change in the characteristic curve of pressure versus torque as seen in FIG. 1. When the flow is relatively high, the characteristic curve is as indicated in the left-hand part of FIG. 1 and at lower flows the charactertistic curve is shifted in a right-hand direction. Thus the torque applied to the steering shaft for any given pressure is greater at low flow than it is at high flow. This results in an increased road feel and a reduction in the power assist during straight ahead high speed operation and in a decrease in road feel and an increase in power assist during steering maneuvers at low speeds and during parking maneuvers.