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 V₁ through V₄ .

The pinion and rotor are meshed with adequate clearance. They utilize a fail-safe design.

Fig 1: Identifying Control Valve

• 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.

  Fig 2: Operating Principle (When No Steering Force Is Applied)

  
• When steering force is applied:

  When the steering wheel is turned to the right, for example, oil passages V₁ 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.

  Fig 3: Operating Principle (When Steering Force Is Applied)

  
• 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.