There are a couple of key components in a steering system which provides assistance in addition to the rack-and-pinion. The hydraulic power for the steering is provided by a rotary-vane pump. This pump is driven by the car's engine by a belt and pulley system. It contains several vanes that spin inside a chamber.
The turning vanes pull hydraulic fluid through the line at a low pressure and then push it out the outlet at a much higher pressure. As the engine turns faster so does the pump, increasing the amount of fluid as necessary to provide assistance to the steering of the vehicle for the driver. The pump contains a relief valve to make sure that the pressure does not get too high even when the engine is running at high speeds.
Another type is the rotary valve pump. This contains a series of rods that twist as the steering wheel is turned, providing assistance to the driver. The steering wheel inside the car turns the steering rod under the bonnet which in turn applies torque to the system attached to the rack and pinion, turning the wheels of the car.
When the steering wheel is not being turned, the hydraulic pump or rotary valve pump provide the same amount of pressure to the steering system. As soon as the steering wheel is turned, the ports inside the pump open and provide the necessary high pressure to the lines, assisting the driver with the steering.
What is the difference between hydraulic power steering and electric power steering?
The biggest difference between hydraulic and electric steering is the motor that is used to supply the assistance. In a hydraulic system, the additional assistance is provided by the spinning of vanes inside a casting to create additional high pressure fluid so it can be pushed through the steering system. In an electric system, an electric motor is used to provide the pressure necessary for the steering system.
Electric steering is thought to be better than hydraulic for a few reasons. First, it seems to be more responsive than hydraulic at different speeds providing more assistance as needed. Hydraulic steering needs a short period of time to adjust during quick movements and turns where as electric uses sensors to decide when more or less power is necessary.
Electric steering appears to be less prone to problems and flaws. Hydraulic steering is an older technology than electric. Although it has improved greatly since the first units were introduced, the use of electric technology has given steering a new dimension. Because it uses sensors to provide power rather than relying on the opening and closing of ports and valves, it appears to be faster at responding.
However, like all new technology electric steering is more costly to repair than hydraulic and also provides more of a stress on the battery and electrical components of the vehicle. Hydraulic steering uses the engine itself for power, requiring no input from the battery whatsoever.