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Hydraulic power steering (HPS) is a hydraulic system for reducing the steering effort on vehicles by using hydraulic pressure to assist in turning the wheels. It is intended to provide for easier driving direction control of the car while preserving "feedback", stability and unambiguity of the trajectory specified.
The steering booster is arranged so that should the booster fail, the steering will continue to work (although the wheel will feel heavier).
Construction
The steering booster consists of the following basic elements:
- Steer torque detector
- Controlled pressure distributor case
- Hydraulic booster pump
- Tank with a working liquid
- Connection hoses
The working liquid, also called "hydraulic fluid" or "oil", is the medium by which pressure is transmitted. Common working liquids are based on mineral oil.
The steering booster works as follows:
The working liquid travels from a tank to the pump inlet opening due to gravity. The liquid travels under pressure from the pump to the distributive gear. The distributive gear has an elastic element, a torsion bar or a spring, which causes the cross-section area of bypass holes (drain ports) to vary in proportion to the effort applied to the steering wheel.
In a neutral position section of the bores passing a liquid in the right and left part of the hydrocylinder is equal to section of bores issued a liquid from them in a tank. The pressures in the right and the left parts of the hydrocylinder are equal too.
When the wheel is turned, because of forces of friction and other forces there is an effort deforming an elastic element, changing the section of bores of the distributive gear and, thereby, pressure in the right and left parts of the hydrocylinder. For pressure restriction in the pump there is a restrictive valve, which is adjusted on different cars in a range from 7 to 13 MPa.
Operation
To avoid dangerous situations under abnormal conditions where the car's steering system suddenly stops working, it is necessary to check the amount of oil in the HPS tank periodically. At an appreciable drop of its level which is not explained by temperature, angle of rotation of wheels, car inclination, etc., it is necessary to check leakproofness of the hydraulic circuit: hoses, places of their inputs etc. To increase the lifetime of the HPS elements and system as a whole, it is recommended to replace the power steering oil completely once every 1-2 years. Manuals of the majority of cars prohibit keeping the wheels in an extreme position for more than 5 seconds as it can lead to the oil heating up to its boiling point, and to the breakdown of the HPS[citation needed].
See also
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