system to aid the steering of an automobile by use of a hydraulic device (driven from the engine) that amplifies the turning moment, or torque, applied to the steering wheel by the driver. To reduce the torque required from the driver as cars became heavier and tires softer, gears were introduced between the steering wheel shaft and the linkage that turns the wheels. The gears multiplied the torque supplied by the driver to a much greater torque on the shaft that drives the front wheels to right or left. A disadvantage arose, however, in the higher steering ratio required; i.e., amount of turn of the steering wheel needed.

To make steering easy for the driver without using high steering ratios, power steering devices were introduced in the early 1930s. Most modern power-steering systems consist of hydraulic boosts applied to either the steering linkage or the steering gear. Rotation of the steering wheel activates a valve that directs oil, pressurized by a pump driven by the engine, to act on a piston. The hydraulic boost acts only while the steering wheel is moving.