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Power steering is a system to make steering the car easier. This is acheived by using an external power source to assist in turning the wheels. Most power steering systems work by using a hydraulic system. The hydraulic pressure is usually provided by a gerotor or rotary vane pump driven by the vehicles engine. A double-acting hydraulic cylinder applies a force to the steering gear, which in turn applies a torque, or twist, to the steering axis of the roadwheels. The flow to the cylinder is controlled by valves operated by the steering wheel; the more torque the driver applies to the steering wheel and the shaft it is attached to, the more fluid the valves allow through to the cylinder, and so the more force is applied to steer the wheels in the appropriate direction.
Power Steering Pumps are positive displacement pumps. The flow rate they deliver is directly proportional to the speed of the engine. At high engine speeds steering would naturally operate faster than at low engine speeds if left unchecked. Because of this, a restricting orifice and flow control valve are used to direct some of the pumps output back to the hydraulic reservoir when the engine is at high speed. A pressure relief valve is also used to prevent a potentially dangerous build-up of pressure when the hydraulic cylinders piston reaches the end of the cylinder.
Some modern implementations also include an electronic pressure relief valve which can reduce the hydraulic pressure in the power steering lines as the vehicles speed increases (this is known as variable assist power steering).