POWER STEERING
Power Steering helps drivers steer vehicles by augmenting steering effort of the steering wheel. Most power steering systems work by using a hydraulic system to steer the vehicle's wheels. It does all this by adding controlled energy to the steering mechanisms steering rack or steering box, so the driver needs to provide only modest effort, regardless of the road conditions. Power steering especially helps when a vehicle is stopped or moving very slowly.
Power steering systems for cars augment steering effort via an actuator which is a hydraulic cylinder, which is part of a servo system. These systems have a direct mechanical connection between the steering wheel and the linkage that steers the wheels. This means that power steering system failure still permits the vehicle to be steered using great amounts of manual effort.
Other power steering systems have no direct mechanical connection to the steering linkage as they require power for example large trucks and abnormal construction vehicles. Systems of this kind, with no mechanical connection, are sometimes called "drive by wire". In this context, "wire" refers to electrical cables that carry power and data to servos that are used to move in one direction or the other.
In other power steering systems, electric motors provide the assistance instead of hydraulic systems. As with hydraulic types, power to the actuator motor is controlled by the rest of the power steering system.
The hydraulic pressure for a steering system typically comes from a rotary vane pump, the power steering pump driven by the vehicle's engine. A double-acting hydraulic cylinder applies a force to the steering gear, which pushes the wheels to the left or the right, in turn steering the wheels of the vehicle.
One design for measuring the torque applied to the steering wheel has a torque sensor or a torsion bar at the lower end of the steering column. As the steering wheel rotates, so does the steering column, as well as the upper end of the torsion bar. Since the torsion bar is relatively thin and flexible, and the bottom end usually resists being rotated, the bar will twist by an amount proportional to the applied torque. The difference in position between the opposite ends of the torsion bar controls a valve. The valve allows fluid to flow to the cylinder which provides steering assistance and the greater the "twist" of the torsion bar, the greater the force.
Since the hydraulic pumps are positive-displacement type, the flow rate they deliver is directly proportional to the speed of the engine. A pressure relief valve prevents a dangerous build-up of pressure when the hydraulic cylinder's piston reaches the end of its stroke. Pressure build up would be undesirable if the pump produced more pressure at high speeds of engine rotation to low speeds so a restricting orifice and flow-control valve direct some of the pump's output back to the hydraulic reservoir at high engine speeds.
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