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
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.
For part time power steering, or power boost, the steering valve is only actuated if the force required to steer the vehicle is greater than the force of the valve’s centering springs. Let’s look at two different steering situations. First, lets say we are driving at highway speed and we go through a gentle banked curve. The force required to steer the vehicle will be less than the force of the centering springs, so power boost will not be activated. Now let’s say we are making a sharp turn at low speed or parking. The force required for such a maneuver is greater than the force of the centering springs, so the springs become compressed. This moves the steering valve spool and power boost is activated.
The steering valve of a full-time power steering system has no centering springs. Instead, the valve is attached to the wheel such that hydraulic boost is always active.
(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.
In the 1950’s tractors starting becoming too large to steer with man power alone, so hydraulic assist or hydro-mechanical power steering was added to make steering effort easier. It is best suited for small and medium tractors that only require part time power steering. As tractors become larger and need full time power steering, it is more economical to use hydrostatic steering. Generally the hydraulic components are less costly that complicated mechanical gears that would be necessary for a large machine. Hydrostatic steering is also more space effcient than hydraulic assisted steering since the mechanical linkages can be eliminated by hoses and large mechanical parts can be replaced by smaller hydraulic compnents.
(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
We can separate our power steering systems into four basic layouts. The first layout has an integral valve and actuator mounted directly to the pitman arm. In the second layout, the actuator may be coupled to the steering linkage with the valve mounted at the 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.
Our fist system layout makes use of an integral valve and actuator coupled directly to the pitman arm and connected to the steering wheel mechanically. This system type is the easiest to use when converting an existing mechanical steering linkage to hydraulic power assist. It is very simple in that it only requires a supply and return line. Unfortunately, it may not be very aesthetic and hoses can be easily exposed to the surrounding moving parts.
(Wittren, 1975)

7. HMPS Type 2 Creates a highly congested area with all of the hoses.
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
The next type of steering mounts the valve directly at the steering wheel and the actuator at the steering linkage. While it is convienent to have the valve at the wheel and the actuator at the linkage, it requires four hoses all near the steering wheel. This can create a space problem, especially when there are several other control elements at the steering wheel. Of course, power steering systems create noise, heat, and vibration which can interfere with driver comfort, so it is favorable to mount the components farther from the driver’s station.
(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.
It is possible that the valve and the actuator be mounted separately from each other, but both within the linkage. While it still requires several hoses, it is possible to get the hoses and other components away from the potentially crowded around the steering wheel. It is possible to keep the valve, actuator, and pump close together so that the hose length can be kept relatively short.
(Wittren, 1975)

9. HMPS Type 4 Most sophisticated design
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.
The fourth type of system uses and integral valve and actuator mounted at the steering wheel. The actuator operates a rack and pinion or crank arm which drives the pitman arm. Using this type of design makes converting mechanical steering to hydraulic assisted steering fairly easy. This design replaces the entire steering gear on a manual system. It may require some modifications to create enough space for the hydraulic assembly an hoses. This can be a high priced investment, so the financial benefits of replacing the mechanical gear must be considered carefully.
This design works well on small tractors and passenger vehicles, but it is not suitable for large tractors, especially articulated tractors.
(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
Here we can see a system like type 1, an integral valve and actuator as part of the steering linkage. In this case, when the wheel is turned, the pitman arm is moved activating the steering valve through the drag link. The actuator moves the steering arm for the left wheel and the right wheel is steered with cross steering arm. You can see from the picture why this design good for adding hydraulic boost to a mechanical system as the integral steering unit can be fit into the linkage where it is convenient.
(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
Here we have a combination of types 1 and 3 so that two cylinders can be used. We have integral cylinder and valve on one side of the vehicle. The valve is remote linked to the other cylinder. When the vehicle is steered the valve is used to control both the cylinders.
(Vickers, 1967)

13. Remote Dual System Two cylinders operated by single valve
Valve connects pitman arm and left cylinder
Common on rear wheel steer
The remote dual system is like system 3 with two cylinders. The valve and cylinders are not integral, but are mounted in the linkage with the valve between the left steering arm and the pitman arm. This a common system lay out for rear wheel steered vehicles.
(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.
The manual steering gear transmits motion of the steering wheel to the Pitman Arm and provides some reduction of steering wheel movement.
First, in the event of a hydraulic system failure, the steering gear ratio reduces the heavy manual steering effort. Second, automotive vehicle drivers just aren’t ready for a 1 to 1 ratio steering system. One automobile manufacture, in fact, was criticized for a low-ratio power steering system until the public became used to it. There is initially a tendency to over-steer with a low ratio gear.
(Vickers, 1967)

17. General Circuit Power Steering Pump Power Cylinder
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.
A power steering pump is usually a vane-type pump or some similar construction of positive displacement. It is driven by the vehicle engine, usually through a pulley and V-belt or other type indirect coupling.
The power cylinder is double-acting. It is a differential cylinder, so that steering response may be slightly different in left or right turns with a controlled flow rate. The differential is slight, however, and unnoticeable to most people. When two cylinders are used, oil is pumped simultaneously to the rod end of one and the head end of the other, so the differential is cancelled.
(Vickers, 1967)

18. General Circuit Valves Relief Valve Flow Control Valve Steering 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.
A relief valve is required in the pressure line to protect the pump from overloads.
The flow control valve maintains a constant rate of flow to operate the power cylinders. Without this valve, variations in engine speed would affect the sensitivity of the steering unit by causing variations in pump flow. For safety, it’s best to have the unit respond with exactly the same sensitivity at all speeds.
The steering valve is a four way valve that functions as a positioning servo valve. It must direct fluid to either end of the power cylinder. Most steering valves are the open-center type. When the valve is in neutral, oil from the pump is recirculated freely through the valve back to the reservoir.
(Vickers, 1967)

19. General Circuit Filters Oil Filter Air Breather 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.
The filter is preferably installed in the return line and should have a bypass valve to prevent blocking flow if the element is clogged. A 10-micron or smaller filter is recommended in power steering systems to prevent damage to the pump and steering valve from metal particles and dirt.
Power steering systems operate with vented reservoirs. The “breather” or vent in the reservoir should be equipped with a 3-micron filter element.
(Vickers, 1967)

20. General Circuit Oil Reservoir Hydraulic Lines
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.
The reservoir must hold all the oil required by system during operation, plus a sufficient level to avoid a vortex at the suction line. It should be capable of dissipating heat generated in the steering system.
Working hydraulic lines for the most part are flexible hoses, since the steering components move during operation. Long lines may be partly flexible hoses and partly tubing where flexibility isn’t required.
(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.
Integral steering unit circuit
The hydraulic circuit for an integral unit system can be as simple as shown in the pictorial diagram to the upper right. Controlled flow originates in the pump and is routed through the pressure line to the steering valve. The valve directs the flow to the power cylinder and returns it through the return line to the tank.
Remote Linkage System
In a remote installation, the valve and cylinder are mounted separately. Each is equipped with an end cap threaded to accommodate a mounting ball stud. The caps also contain two ports apiece to make the hydraulic connections between the valve and cylinder. Otherwise, the cylinder and valve are the same construction as in the integral steering unit
(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.
In the figure to the right, there is cut-away view of a typical S20 series steering valve. It consists of a power cylinder bolted to a steering valve. The rod end (anchor) ball stud mounts the unit to the vehicle frame. The head end ball stud is connected to the steering linkage. The center or control ball stud is connected to the Pitman arm drag link to actuate the steering valve. A single centering spring is mounted between the ball stud and valve, and is flanked by centering washers. This arrangement gives the driver road feel in both directions.
(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.
Oil flow through the steering unit is shown in the figure to the right.
View A is the neutral flow condition. There is no relative movement between the spool and valve body; in other words, the spool is centered. Oil from the pump is directed back out to the tank.
View B shows the control ball stud actuated to retract the cylinder. The spool, as we see it, has been pushed to the left. Oil from the pump is directed to the rod end of the cylinder. Since the rod is anchored, pressure pushes against the rod packing to move the entire steering unit to the left. At the same time, oil pushed out the head end is returned to the tank. Flow continues as shown until the control ball stud stops. The valve body then immediately catches up with the spool and the flow condition again is as in View A.
When the ball stud moves to the right (View C), flow reverses. Pump delivery is routed to the head end of the cylinder and oil pushed out of the rod end is returned to the tank. Pressure in the cylinder pushes on the head and moves the steering unit to the right to follow the control ball stud.
(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.
The small check ball in the valve body is normally held seated by pressure at the valve inlet (pressure port). If there is a hydraulic failure or a loss of power, pressure drops and lets this valve unseat. Then, oil can circulate freely between the head and rod ends of the cylinder. This avoids a hydrostatic lock and permits manual steering. The control ball stud then simply moves the anchored rod. The steering unit thus acts as a drag link to steer the vehicle.
An optional relief valve (upper figure) can be incorporated in the steering valve if the flow control and relief valve is not used. This is actually a compound relief valve. It doubles as a check valve in case of power loss.
As shown in the bottom figure, the control ball valve ball stud can be mounted in any of four positions relative to the port connections in the valve.
(Vickers, 1967)

25. Remote Linkage System Auxiliary Side Ports (Vickers, 1967)
When two cylinders are operated from the same valve, a special body is available which incorporates auxiliary ports on the side. The figure on top shows the hydraulic connections when a separate cylinder is used together with an integral steering unit….that is, a combination integral-remote circuit. Connections to the remote circuit are shown from the valve’s side ports.
In the figure to the right, you see a dual remote circuit. This circuit can use the special side-ported body, but as shown the second remote cylinder can be “teed” into the lines to the opposite cylinder. Notice that the port connection to the cylinders are opposite, so that one will retract as the other extends.
(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.
Pitman arm stops are used in many power steering systems to protect against overheating when the steering wheel is “hard over”.
Without Pitman stops, the steering valve continues to supply some boost when the wheels are turned as far as possible and the driver is still pulling the steering wheel. The only place the oil can go is over the relief valve which will generate excessive heat. The Pitman arm stops are adjusted to stop the arm just before the wheel stops, thus giving the steering valve room to center.
Pitman arm stops can and should be incorporated with any system that has a separate steering valve. They cannot be incorporated in valve-on-gear or integral power gear systems.
(Vickers, 1967)