Hydraulic System

Hydraulic System

Hydraulically operated power assisted systems work on either a constant pressure or constant flow layout. The former incorporates a hydraulic accumulator to store the pressure. The latter uses flow of fluid around the system continuously until assistance is required.

The Components.

The essential components required to operate a constant flow system are shown in Fig. 27.64. The system incorporates a pump, control valve and ram cylinder in addition to the normal steering components.

Pump.

An eccentric rotor pump driven by a vee belt from the engine crankshaft is normally used. The pump is located either on the front of the engine or at the front of the crankshaft. A fluid reservoir is attached to the pump and generally stores a low-viscosity mineral oil similar to the type used in an automatic transmission. A pressure relief valve limits the maximum

pressure of 7 MN/m2, and once this pressure is attained the oil passes from the pump outlet back to the reservoir.

Control Valve or Reaction Valve.

In the layout shown in Fig. 27.64, the valve is located between the two halves of the drag link. It can also be positioned together with the ram cylinder in the steering box. A spool type valve is attached to one half of the drag link and is held centrally in the valve body by two reaction springs. Flexible pipes are used to connect the valve to the ram cylinder, pump and reservoir.

Ram Cylinder.

A double acting ram piston is connected to the steering arm to provide appropriate force in either direction to assist the drive. One side of the cylinder is vented and the other side is pressurized for the force to act in either direction. Assistance given by the ram depends on the fluid pressure available to it by the control valve.

Fig. 27.65. Control valve principle.

The pressure surface in most power steering systems is located in the steering gear, called integral type. Also in some power steering systems it is located in an exterior power cylinder connected between the steering linkages and vehicle frame, called link type. This is similar to a power rack and pinion arrangement.

Operation.

Operation of the system depends on the driver's input torque applied to the steering wheel. Under low torque conditions, represented in Fig. 27.65, no assistance is required. Application of low torque on the wheel is insufficient to overcome the tension of the reaction springs in the control valve, so that fluid flows back to the reservoir. In this phase of operation, the control valve remains in the neutral position and hence offers no resistance to the oil flow to or from the ram cylinder.

The torque applied by the driver is controlled by the strength of the reaction springs. When this torque reaches a predetermined value, the greater force acting in the drag link operates the valve. The valve connects one side of the ram to the pump and the other side vents to the reservoir. Interruption of the fluid circuit in this manner allows the pump pressure to build up quickly. Consequently, the thrust on the ram also builds up until the movement of the ram overcomes the resistance of the road wheel. At this stage the force in the drag link reduces, so that the valve returns to the neutral position and the system pressure drops to the original value. Thus in this sequence of operation, steering wheel torque opens the valve and the ram exerts a proportional thrust to close the valve.

Turning the steering wheel in the opposite direction also produces a similar action, but in this case the movement of the control valve directs fluid to the other side of the ram piston. There are different designs of reaction valve and spring. One from incorporates a small torsion

Fig. 27.66. Operation of a power steering control valve.

bar to transmit the drive between the inner column and the gear in the box. When the torque exceeds a given amount the bar twists to move a valve, which activates the ram.

27.8.3.