In automobiles, power steering (also known as power-assisted steering (PAS) or steering assist system) helps drivers steer by augmenting the steering effort of the steering wheel. Hydraulic or electric actuators add controlled energy to the steering mechanism, so the driver must provide only modest effort regardless of conditions. Power steering helps considerably when a vehicle is stopped or moving slowly.
Also, power steering provides feedback of forces acting on the front wheels to give an ongoing sense of how the spins interact with the road; this is typically called “rοad feel.” Representative power steering systems for cars augment steering effort via an actuator, 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 (to augment effort) still permits the vehicle to be conducted using manual effort alone.
Other power steering systems (such as those in the most oversized off-road construction vehicles) have no direct mechanical connection to the steering linkage; they require electrical power. Methods of this kind, with no mechanical connection, are sometimes called “drive by wire” or “steer by wire” by analogy with aviation’s “fly-by-wire.” In this context, “wire” refers to electrical cables that carry power and data, not thin-wire-rope mechanical control cables. In other power steering systems, electric motors assist instead of hydraulic systems.
As with hydraulic types, power to the actuator (motor, in this case) is controlled by the rest of the power-steering system. Some construction vehicles have a two-part frame with a rugged hinge in the middle; this hinge allows the front and rear axles to become non-parallel to steer the car. Negative hydraulic cylinders move the halves of the frame relative to each other to run.