The piston in the power cylinder is positioned on the rack, and the rack moves due to fluid pressurized by the vane pump acting on the piston in either direction. Fluid pressure leakage is prevented by a seal ring on the piston. Also, there is an oil seal on both sides of the cylinder to prevent external leakage of the fluid. The control valve shaft is connected to the steering wheel. When the steering wheel is in the neutral (straight-ahead) position, the control valve is also in the neutral position, so the fluid from the vane pump does not act on either chamber but flows back to the reservoir tank. However, when the steering wheel is turned in either direction, the control valve changes the passage so the fluid flows into one of the chambers. The fluid in the opposite chamber is forced out and flows back to the reservoir tank by way of the control valve. Currently, there are three different types of control valves which perform this changeover action of the passage; spool valves, rotary valves and flapper valves. All types have a torsion bar between the control valve shaft and pinion, and the control valve functions in accordance with the amount of twist applied to the torsion bar.
2. Type of control valve
A control valve is located in the gear housing. The gear housing houses a rackand- pinion type power steering mechanism or a recirculating- bail type power steering mechanism. The control valve is one of three types: a rotary valve type, a spool valve type, or a flapper valve type. Currently, rotary valve types are used in many models.
Here, the rotary valve type is explained. The control valve in the gear housing determines to which chamber the fluid from the vane pump goes. The control valve shaft (to which steering wheel torque is applied) and the pinion gear are connected by means of a torsion bar. The rotary valve and pinion gear are secured by a pin and rotate integrally. If no vane pump pressure is applied, the torsion bar is fully twisted and the control valve shaft and pinion gear make contact at the stopper so the control valve shaft torque is applied directly to the pinion gear.
A restriction in the hydraulic circuit is formed by rotary movement of the control valve shaft in relation to the rotary valve. When the steering wheel is turned to the right, pressure is restricted at orifices X and Y. When it is turned to the left, a restriction is formed at X’ and Y’. When the steering wheel is turned, the control valve shaft rotates, turning the pinion gear via the torsion bar. In contrast to the pinion gear, as the torsion bar twists in proportion to road surface force at this time, the control valve shaft rotates only to the extent of the amount of twist, and moves to the right or left in relation to the rotary valve. Thus orifices X and Y (or X’ and Y’) are formed and a difference in hydraulic pressure between the right and left cylinder chambers is created. In this manner, rotation of the control valve shaft directly performs changeover of the passages and regulates the fluid pressure. The fluid from the vane pump enters from the outer circumference of the rotary valve, and the fluid returning to the reservoir tank passes between the torsion bar and the control valve shaft.
(1) Neutral position
As the control valve shaft does not revolve, it is in a neutral position in relation to the rotary valve. Fluid supplied by the pump returns to the reservoir tank through port “D” and chamber “D”. The right and left chambers of the cylinder are slightly pressurized but as there is no pressure difference between the two, no power steering assist occurs.
(2) Turning right
When the vehicle makes a right turn, the torsion bar is twisted and the control valve shaft revolves to the right accordingly. Fluid from the pump is constricted by orifices X and Y of the control edge in order to stop flow to ports “C” and “D”. As a result, fluid flows from port “B” to sleeve “B” and then to the right cylinder chamber, causing the rack to move to the left and resulting in power steering assist. At the same time, the fluid in the left cylinder chamber flows back to the reservoir tank via sleeve “C” -> port “C” -> port “D” -> chamber “D”.
(3) Turning left
In the same manner as for a right turn, when the vehicle makes a left turn, the torsion bar is twisted and the control shaft rotates to the left accordingly. The fluid sent from the pump is constricted by orifices X’ and Y’ of the control edge in order to stop flow to ports “B” and “D”. As a result, fluid flows from port “C” to sleeve “C” and then to the left cylinder chamber, causing the rack to move to the right and resulting in power steering assist. At the same time, the fluid in the right cylinder chamber flows back to the reservoir tank via sleeve “B” -> port “B” -> port “D” -> chamber “D”.