Steering Gear: Description and Operation

GEARBOX ASSEMBLY
1. Power Cylinder





The gearbox is integrated with a built-in control valve and power cylinder. The rack shaft is used as a power cylinder piston and a rotary control valve is located in such a manner as to enclose the pinion shaft.
The control valve and power cylinder are connected to each other by two pipes through which hydraulic oil flows.

2. Control Valve





The control valve consists of a rotor (which rotates together with the steering shaft), a pinion (which is connected to the rotor and torsion bar), and a sleeve (which rotates together with the pinion). Oil grooves C and D are located in the rotor and sleeve to form oil flow passages V, through V4.
The pinion and rotor are meshed with adequate clearance. They utilize a fail-safe design.
- Operating principle
When the torsion bar twists in relation to the steering force, a relative rotational displacement occurs between the rotor and sleeve. This displacement changes the cross-sectional area of oil passages V1, V2, V3 and V4, which in turn switches oil passages and controls oil pressure.





- When no steering force is applied:
The rotor and sleeve are held at the neutral position. Oil passages V1, V2 and V3, which are formed by valve grooves C and D are open equally. Under this condition, oil delivered from the oil pump returns to the oil reservoir so that neither oil pressure builds up nor does the power cylinder activate.





- When steering force is applied:
When the steering wheel is turned to the right, for example, oil passages V, and V3 open while oil passages V2 and V4 nearly close. At this point, oil under pressure in chamber A increases in response to the throttle position of oil passages V2 and V4 so that the rack piston moves to the right. Oil in chamber B, on the other hand, is discharged through oil passage V3, returning to the oil reservoir.
- Fail-safe function
If oil pressure fails to build up due to a broken oil pump drive belt, torque is transmitted from the valve rotor to the pinion by way of the fail-safe function.