Mechanical and Power Steering
Turning Circle
When the front wheels turn they are not following the same path. Each wheel must follow a different circle with the inside wheel following a smaller turning radius. The steering link mechanism turns the inner wheel more than the outer wheel. There are 2 main Steering gear types: rack-and-pinion and re-circulating ball.
Manual Rack-and-pinion Steering
Rack-and-pinion gearing converts the circular motion of the Steering wheel into linear motion. Either end of the rack is connected to a front wheel by a steering arm and tie-rod linkage. As the rack moves side to side, the front wheels turn left and right via the steering arms connected to the wheel spindles. The Rack and Pinion also provides a gear reduction to make turning easier. Most cars take three or four complete turns of the steering wheel to make the wheels move from full left to full right lock.
Power Rack-and-pinion Steering
a power-steering rack system has an hydraulic cylinder attached to the rack. Hydraulic steering fluid can be pumped in or out of two fluid ports on either side of the piston. Supplying hydraulic fluid to one side under pressure forces the piston to move in the opposite direction, which in turn moves the rack thus providing power assisted steering.
Hydraulic Steering Pump
A rotary-vane pump contains a set of retractable vanes that spin inside an oval chamber. The pump receives its power from the engine using a belt and pulley system.
The vanes spin, pulling or sucking hydraulic fluid from the return line under low pressure and pressurise it into the outlet providing the turning force.
As the pump is connected to the engine, its flow depends on the engine speed. Since it needs to have sufficient power or flow when the engine is idling it can move vastly more fluid than needed when the engine is revving at speed.
To compensate, the pump has a pressure-relief valve to limit the pressure at high engine speeds when excessive fluid is being pumped.
Rotary Steering Valve
A torsion bar in a rotary valve senses when torque is applied and interprets the driver's steering movements of the steering wheel to control the hydraulic power needed to assist in turning the vehicle. When driving in a straight line or when no steering force is needed the system shouldn't provide any turning force
When the steering wheel is not being turned, both hydraulic lines are in equilibrium and provide the same amount of pressure to the steering gears. As the valve senses movement and direction; the appropriate ports open up to provide high-pressure hydraulic fluid to the correct side of the piston.
Drive-Wire or Drive-By-Wire Power Steering
Rather than have the power-steering pump run constantly and wasting fuel, it's possible to use an electronically interpreted system. Turning sensors on the steering column could operate small servo-motors and provide direct power turning thus removing the usual mechanical linkages.
Recirculating-ball Steering
The Re-circulating ball steering system is mostly used on larger vehicles and contains a worm gear filled with ball bearings that move or circulate through the gear as it turns.