Steering: Description and Operation
STEERING SYSTEM DESCRIPTIONPower Steering System:
The steering system incorporates a compression joint in the lower shaft and is designed to collapse on impact. The misalignment of the upper steering column with the steering box and the inclusion of two universal joints, is also designed to prevent the column moving towards the driver under frontal impact.
The steering box is located behind the first chassis cross member and is connected to the road wheel swivel housing by a drag link and track rod. An hydraulic damper absorbs shocks in the steering, caused by road wheel deflections when operating on rough terrain.
POWER STEERING SYSTEM
The power steering system comprises an hydraulic pump which is belt driven from the engine and supplied with fluid from a reservoir that also acts as a cooler.
The steering box houses a self neutralizing rotary valve which is part of the worm/valve assembly and an hydraulic piston/rack to assist the mechanical operation. The rotary valve which is operated by movement of the steering wheel, directs fluid pressure to the appropriate side of the hydraulic piston/rack to provide assistance.
Rotary Valve Operation
The rotary valve assembly comprises a worm (1), valve sleeve (2), input shaft (4) and torsion bar (5).
The valve sleeve is retained inside the worm by a trim screw (3), and incorporates valve ports in its inner bore. The input shaft is attached to the steering wheel via a steering shaft and steering column and incorporates valve ports in its outer diameter to align with those in the sleeve. The torsion bar which is secured to the worm and input shaft with pins (6) at each end holds the valve ports in neutral alignment when there is no demand for assistance.
No Demand For Assistance (Valve At Neutral)
Rotory Valve At Neutral:
When there is no demand for assistance, the torsion bar holds the input shaft and sleeve valve ports in neutral relationship to one another, allowing equal pump pressure -A- to both sides of the piston/rack (9). Any excess fluid flow from the pump returns to the reservoir via -B-.
Demand For Assistance (Valve Misaligned)
Rotary Valve Misaligned:
When the steering wheel and input shaft is turned, steering resistance transmitted to the worm causes the torsion bar to be twisted and the valve ports to be misaligned for a right or left turn. The misalignment of the valve ports directs all fluid pressure -A- to one side of the piston only and allows displaced fluid -B- on the other side.
When demanding maximum assistance, any excessive fluid output from the pump due to high pump speed, will circulate through the regulator valve located in the pump unit, causing the temperature of the fluid and the pump to rise rapidly.
CAUTION: To avoid excessive fluid temperatures which could damage the oil seals, the steering must not be held on full lock for more than 30 seconds in one minute.
Only when the steering wheel, and hence the demand for assistance is released, will the torsion bar return the valve to neutral, allowing the fluid to circulate through the reservoir where it is cooled.
In the unlikely event of mechanical failure of the torsion bar, a coarse splined connection (7) between the input shaft and worm, ensures steering control is maintained sufficient to allow the vehicle to be recovered.
PUMP AND REGULATOR VALVE OPERATION
The pump which is belt driven from the engine is an eccentric roller type and also houses the pressure regulator and flow control valve. The pressure is controlled by a spring loaded ball valve (3) which is housed inside the flow control valve piston (4).
No Demand For Assistance, High Flow Through Box - Low Pressure
Control Valve - Closed:
1. Reservoir
2. Pump
3. Pressure control valve and spring
4. Flow control valve and spring
5. Press fit plug (ball bearing)
6. Restrictor
With no demand for assistance the rotary valve in the steering box acts as a pressure relief valve, allowing fluid -A- to flow freely through the steering box and back to the reservoir and pump inlet -B-.
NOTE: The ball plug (5) is pressed into the valve 4 during manufacture and determines the opening pressure of pressure relief valve (3).
No Flow Through Box - High Pressure
Control Valve - Open:
When the steering is turned, the rotary valve effectively stops all fluid flow through the steering box, thus causing an increase in pressure -A-. This increase in pressure is felt in the flow control valve spring chamber where, at a pre-determined pressure the relief valve (3) will open and allow the pressure to escape. The fall in pressure in the flow control spring chamber, allows the flow control valve to move to the right, which in turn allows pump output -A- to escape directly into the pump inlet -B-.
Assistance Demanded
As soon as the steering wheel is released after making a turn, the system reverts to the free flow condition and the road wheels are returned to the straight ahead position by the mechanical steering geometry.
In the event of any hydraulic failure steering control, though heavy, will be maintained through the mechanical components in the steering box.