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Hydraulic Power Assist

Hydraulic Power Assist. Definition. Hydraulic power assist means that a hydraulic system is incorporated with mechanical steering. Full Time Part Time Power Steering. Part Time The force of the center springs of the valve gives the driver the “feel” of the road at the steering wheel.

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Hydraulic Power Assist

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  1. Hydraulic Power Assist

  2. Definition • Hydraulic power assist means that a hydraulic system is incorporated with mechanical steering

  3. Full Time Part Time Power Steering • Part Time • The force of the center springs of the valve gives the driver the “feel” of the road at the steering wheel. • Full Time • The valve is installed without centering springs. Any movement of the steering wheel results in hydraulic boost being applied. (Vickers, 1967)

  4. Hydro-mechanical Power Steering (HMPS) • Hydro-mechanical power steering was the first type to be used on agricultural tractors in the early 1950’s. • Suitable for small to medium tractors where power steering can be an option to manual steering. • Once the size of the tractor becomes too big for manual steering, hydro-mechanical steering is usually not cost effective. (Wittren, 1975)

  5. Hydro-mechanical Power Steering • There are many different types of hydro-mechanical power steering, but they all can be grouped into four basic groups. • Steering linkage mounted integral valve and actuator • Steering wheel mounted control valve, linkage mounted actuator • Separate control valve and actuator mounted in best position • Integral valve and actuator mounted at steering wheel

  6. HMPS Type 1 The integral valve and actuator coupled to steering linkage. • Easiest to adapt to an existing mechanical steering layout. • Only two hoses are needed. (Wittren, 1975)

  7. HMPS Type 2 Steering column mounted control valve with separate, remote actuator coupled to a linkage member. • Creates a highly congested area with all of the hoses. • Four Hoses are needed: supply, return and a pair to the actuator. • Creates noise, heat and vibration (Wittren, 1975)

  8. HMPS Type 3 Control Valve and actuator separately mounted in the steering linkage. • Keeps the area around the steering wheel from becoming too congested. • Four hoses are required. (Wittren, 1975)

  9. HMPS Type 4 The control valve and the actuator are mounted on the steering column, the actuator drives the pitman arm by rack and pinion or by crank arm means. • Most sophisticated design • Requires little or no steering linkage modification. • Requires larger space envelope than other types. (Wittren, 1975)

  10. Integral Linkage Power System-hydraulic assist • Pitman arm operates steering gear through drag link (B) • Power cylinder thrust at steering arm (C) • Boosters actuate left wheel steering arm, right wheel steered by cross steering arm (D) • Only lines to booster are pressure and tank • Frame absorbs shock instead of steering gear, easy to service (Vickers, 1967)

  11. Remote Linkage System • Steering valve is remote mounted, not with cylinder • This linkage system allows for mechanical steering (Vickers, 1967)

  12. Combined Integral Remote System • Two cylinders • One cylinder has integral linkage • Second cylinder is operated by the same valve • The steering valve has an extra set of ports for the connection (Vickers, 1967)

  13. Remote Dual System • Two cylinders operated by single valve • Valve connects pitman arm and left cylinder • Common on rear wheel steer (Vickers, 1967)

  14. Power Steering Circuits • General Circuit • Integral Steering Unit Circuit • Remote Linkage System Circuit (Vickers, 1967)

  15. General Circuit Components • Manual Steering Gear • Power Steering Pump • Power Cylinder • Valves • Relief • Flow Control • Steering • Filters • Oil • Air Breather • Oil Reservoir • Hydraulic Lines (Vickers, 1967)

  16. General Circuit • Manual Steering Gear • Transmits motion of the steering wheel to the turning of the wheel. • Could be eliminated, but there are two reasons for not doing so. • Hydraulic system failure. • The public is not ready for a 1 to 1 ratio steering system. (Vickers, 1967)

  17. General Circuit • Power Steering Pump • Usually a vane-type pump or similar. • Driven by the engine. • Power Cylinder • Double-acting differential cylinder. • Steering response to left and right turns is slightly different. Hardly noticeable. (Vickers, 1967)

  18. General Circuit • Valves • Relief Valve • Required to protect the pump. • Flow Control Valve • Helps maintain a constant flow. • Variations in engine speed would affect pump flow without flow control valve. • Steering Valve • A four way valve that functions as a positioning servo valve. • Most are open-center. (Vickers, 1967)

  19. General Circuit • Filters • Oil Filter • Preferably installed in the return line. • A 10-micron or smaller filter is recommended. • Air Breather Filter • The “breather” or vent in the reservoir. • A 3-micron filter is recommended. (Vickers, 1967)

  20. General Circuit • Oil Reservoir • Must be large enough to hold more than all of the oil for the system. • Should be capable of dissipating heat in oil. • Hydraulic Lines • Flexible hoses due to the steering components movements. (Vickers, 1967)

  21. Special Power Steering Circuits • Integral Steering Unit Circuit • A simple circuit with the valve and cylinder mounted together. • Remote Linkage System Circuit • The valve and cylinder are mounted separately. (Vickers, 1967)

  22. Integral Steering Unit • S20 Steering Unit • Consists of a power cylinder connected to a steering valve. • Two external and internal ports • The inlet port is connected to the pressure line. • The outlet port is the tank return. • The upper internal port connects between the coaxial tubes of the cylinder to the cylinder rod end. • The lower internal port connects to the head end of the cylinder. (Vickers, 1967)

  23. Integral Steering Unit • View A • The valve is in neutral position. The spool is centered, and the oil from the pump is directed back to the tank • View B • The valve is in retract position. The spool is pushed to the left and the oil is directed to the rod end of the cylinder, thus moving the steering unit to the left. • View C • The valve is in the extend position. The spool is moved to the right and the oil is directed to the head end of the cylinder. The steering unit is moved to the right. (Vickers, 1967)

  24. Integral Steering Unit • Check Valve • Helps to avoid hydrostatic lock and allow for manual steering • Relief Valve • Optional. • Can be incorporated if the flow control and relief valve is not used. • Ball Stud mounting • The control valve ball stud can be mounted in any four positions relative to the port connections. (Vickers, 1967)

  25. Remote Linkage System • Auxiliary Side Ports (Vickers, 1967)

  26. Remote Linkage System • Pitman Arm Stops • Helps to protect against overheating. • Adjusted so that the Pitman arm stops just before the wheels must stop. • Should be used with any system that has a separate steering valve. (Vickers, 1967)

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