Power Steering System Pump and Rotary Valve
Power Steering System
Key parts of the power steering system include the rack-and-pinion or recirculating-ball mechanism.
A rotary-vane pump (see diagram below) provides the hydraulic power for the steering. This pump is driven via a belt and pulley. It has a set of retractable vanes that turn inside an oval chamber.
As the vanes turn, they draw hydraulic fluid from the return line at low pressure and push it into the outlet at high pressure. The amount of flow from the pump often varies based on the engine’s speed. The pump is designed to send a regular flow when the engine is idling. But when engine is running at higher speeds, the pump moves much more fluid than necessary.
The Power Steering Pump
The pump also has a pressure-relief valve to ensure the pressure doesn’t get too high, particularly at high engine speeds when there’s so much more fluid being pumped.
A power steering system assists drivers when force is applied to the steering wheel (for example, when starting a turn). When the driver isn’t exerting any force (like when you’re driving in a straight line), the system doesn’t need to help at all. The part that senses force on the steering wheel is called the rotary valve.
The rotary valve contains a torsion bar, a thin rod of metal that twists when torque is applied to it. The top of the bar connects to the steering wheel and the bottom of the bar connects to the pinion or worm gear (this turns the wheels). So, the level of torque in the torsion bar is equal to the level of torque the driver is applying to turn the wheels. As the driver applies more torque to turn the wheels, the bar twists even more.
The input from the steering shaft forms the inner part of a spool-valve assembly. It also hooks up to the top of the torsion bar. At the bottom, it connects to the outer part of the spool valve. The torsion bar also turns the output of the steering gear, connecting to either the pinion gear or the worm gear.
As the bar twists, it spins the inside of the spool valve. Since the inner part of the spool valve is hooked up to the steering shaft (and therefore to the steering wheel), the rotation between the inner and outer parts of the spool valve is based on how much torque you apply to the steering wheel.
When the steering wheel is at rest and not turning at all, both hydraulic lines give equal pressure to the steering gear. Whenever the spool valve is adjusted one way or the other, ports open to provide high-pressure fluid to the correct line.
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