
POWER-ASSISTED STEERING 397
24.2 PRINCIPLES OF POWER-ASSISTED
STEERING
Basic requirements
At the heart of a conventional PAS system is a hydraulic
servo mechanism, which is brought into operation
whenever the resistance to turning the steering wheel
exceeds a predetermined amount. It then furnishes
additional effort to assist the manual operation of the
steering and thus greatly reduces the steering effort that
would otherwise be imposed upon the driver.
An engine-driven pump circulates fluid around the
closed circuit of the servo mechanism and provides a
build-up of hydraulic pressure when needed. As the
steering wheel is turned, a hydrauli ...