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How Power Steering Works

A Training Manual Provided by Wingate Motors


Power steering really means power assisted steering. Every time you turn the steering wheel, most of the hard work is done for you automatically by a sophisticated hydraulic mechanism incorporating a pump, drive belts, bearings, valves, hoses and seals.

The hydraulic power for the steering is provided by a pump that's driven by the car's engine via a belt and pulley. The pump pulls hydraulic fluid from the return line at low pressure and force it into the outlet at high pressure. The amount of flow provided by the pump depends on the car's engine speed. The pump must be designed to provide adequate flow when the engine is idling. As a result, the pump moves much more fluid than necessary when the engine is running at faster speeds. The pump contains a pressure-relief valve to make sure that the pressure does not get too high, especially at high engine speeds when so much fluid is being pumped.

The power-steering system should assist the driver only when he is exerting force on the steering wheel (such as when starting a turn). When the driver is not exerting force (such as when driving in a straight line), the system shouldn't provide any assist. The device that senses the force on the steering wheel is the rotary valve.


The key to the rotary valve is a torsion bar, a thin rod that twists when torque is applied to it. The top of the bar is connected to the steering wheel, and the bottom of the bar is connected to the pinion or worm gear (which turns the wheels), so the amount of torque in the torsion bar is equal to the amount of torque the driver is using to turn the wheels. The more torque the driver uses to turn the wheels, the more the bar twists.

The input from the steering shaft forms the inner part of a spool-valve assembly. It also connects to the top end of the torsion bar. The bottom of the torsion bar 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 depending on which type of steering the car has. As the bar twists, it rotates the inside of the spool valve relative to the outside. Since the inner part of the spool valve is also connected to the steering shaft (and therefore to the steering wheel), the amount of rotation between the inner and outer parts of the spool valve depends on how much torque the driver applies to the steering wheel.

When the steering wheel is not being turned, both hydraulic lines provide the same amount of pressure to the steering gear. But if the spool valve is turned one way or the other, ports open up to provide high-pressure fluid to the appropriate line. The resulting hydraulic pressure differential assists the driver in turning the wheels into the desired direction.

Maintenance


As with any other part of your car, things can go wrong if the power steering system is not properly maintained. This seven-point quick check procedure will pick up many problems before they interfere with power steering performance:

  1. Check the fluid level. Ensure that the outer surface of the reservoir is clean before opening it to avoid contaminating the fluid. If the level is low, add fluid, if very low, check for leaks.
  2. Check the hoses and fittings for leaks, cracks, or abnormally soft spots. Replace where necessary.
  3. Check the metal fittings for looseness or damaged threads. Replace where necessary.
  4. Check the pump V-belt for improper tension, cracks, oiliness, peeling, glazing or separation. Adjust or replace the belt if necessary.
  5. Check the pump for leaking seals. Replace them if necessary.
  6. Check the power cylinder for leaking seals. Replace if necessary.
  7. Check the front tyres for evidence of misalignment or under inflation. Correct if necessary.
  8. Fit a Magnefine filter for cleaner oil.

Spotting Worn Hoses

To check for exterior hose damage, look for the following:

  1. Brittleness or hardness, usually resulting from age or close contact with a hot exhaust manifold.
  2. Softness caused by internal leaks, often at or near the metal end fittings.
  3. Abrasion resulting from the hose rubbing against a metal surface.
  4. Wet spots stemming from pin-hole leaks, or from loose or improperly fitted couplings.
  5. A hose can deteriorate from the inside also as a result of age, heat, and vibration. This can cause contamination and pump damage. For complete safety, power steering hoses should be replaced every 30,000km.