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Steering unit designs / options
The type of steering unit of greatest interest to Off-Road Vehicles is the Open Center / Non Load Reaction
It is the simplest and most economical design, has all the features we typically need, and uses a fixed displacement pump.
Non-Load Reaction
A non-load reaction steering unit blocks the cylinder ports in neutral, holding the axle position whenever the operator releases the steering wheel.
Load Reaction
A load reaction steering unit couples the cylinder ports internally (in the neutral position) with the meter gear set. Axle forces are then allowed to return the steering wheel to its approximate original position. Comparable to automobile steering, gradually releasing the wheel mid turn will allow the steering wheel to spin back as the vehicle straightens. The cylinder system used with load reaction units must have equal oil volume displaced in both directions. The cylinders should be a parallel pair (as shown) or one double rod end unit. Do not use with a single unequal area cylinder system
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Open Center / Non Load Sensing P - Pump. The high pressure input from the pump T - Tank. The low pressure return line to the reservoir (tank) L - Left. The high pressure output line to the left steering cylinder port i.e. when you turn the steering wheel to the left (counter-clockwise) this port carries fluid under pressure to the steering cylinder(s) R - Right. The high pressure output line to the right steering cylinder port i.e. when you turn the steering wheel to the right (clockwise) this port carries fluid under pressure to the steering cylinder(s) Note also the splined shaft on the left that is the operator input (the steering column connects here) |
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Closed Center / Load Sensing The closed center design is not suitable for our use, cannot be used with automotive power steering pumps, and is used on large construction equipment. In this system, a pump can rest when the oil is not required to operate a function. This means that the steering unit valve is closed in the center, stopping the flow of the oil from the pump.
In neutral position pump and tank are disconnected Most suitable on large construction equipment |
Load Sensing
Let me get this out of the way right now. Load sensing has nothing to do with the wheels returning to center, or road feel through the steered wheels back to the operator. Many many people think that it does, but I'm afraid they are all wrong.
Load sensing is a type of hydraulic circuit that is again, of little to no use or interest to us or our needs, but rather is for agricultural / construction / mining equipment and the like that use many hydraulic functions (blades, lifters, implements, etc.)
The following description of load sensing circuits is taken from Eaton. More information for those interested is available in the various pdf catalogues and manuals available at the end of this part of the article , in the "resources" section.
Load Sensing Circuits
Char-Lynn® load sensing power steering uses conventional or load sensing power supplies to achieve load sensing steering. The use of a load sensing steering unit and a priority valve in a normal power steering circuit offers the following advantages:
Char-Lynn load sensing steering control units and priority valves can be used with open center, closed center or load sensing systems. Use in an open center system with a fixed displacement pump or a closed center system with a pressure compensated pump, offers many of the features of a load sensing system. Excess flow is available for auxiliary circuits.
Listed below are the components of a typical load sensing control circuit and a brief application description.
Load Reaction
THIS is the hydraulic circuit feature that is of interest to us. Load Reaction is the infamous "return to center" feature. Note however, that this feature only allows external forces on the wheels to cause a reaction in the steering wheel. That is, it is only permitting the transfer of force, not actually creating it. This means, how well the steered wheels will return to center after a turn will still very much be a factor of steering geometry / alignment - most notable caster. Without sufficient castor or proper geometry, even with a Load Reaction steering unit, "return to center" and road feel can still be very poor. On the other hand, with good geometry and alignment, and a Load Reaction steering unit, road feel and return to center can be excellent.
Note: Some manufacturers refer to these functions as "reversing" and "non-reversing."
Steering Unit Integral Valves
If you recall how the steering unit is in fact a combination of valve(s) and metering unit, it will not be a surprise that there are often (depending on manufacturer) a number of "optional" valving functions available on steering units. Most are not really required by us, except for one very important one - the Manual Steering Check Valve. Most, if not every steering unit I have ever read about that we might possibly use includes one of these. It is a small valve built in to the steering unit that allows the steering unit to act as a small hand operated pump (by turning the steering wheel), providing limited manual steering, should the engine, pump or a belt quit. In my experience, the result is that the steering operates almost identically to normal automotive Saginaw power steering with no power. It is extremely difficult, if not impossible, to manually turn 38" tires at low pressure in a spooled and slugged front axle while sitting still with the engine off. However, with, say, a black YJ belonging to a good friend towing you along at a reasonable clip, it's actually quite steerable. I know this, because the first day I was testing my hydro steering I fried an alternator and a coil!
Again, the experts at Eaton can tell you more about valving options on steering units (and again, there's plenty more info available in the pdf files at the end):
Integral valves are available for the Char-Lynn® steering control unit. Included are: Inlet Relief Valve, Cylinder Port Shock Valves, LS-Relief Valve, and Anti-Cavitation Valves for cylinder ports. In addition, a Manual Steering Check Valve for limited manual steering is included. The integral valves eliminate the need for a separate valve block, and provides versatility to meet any steering circuit standard.
Valve Description:
**Steering units with displacements larger than 185 cm3/r [11.3 in3/r] may require a separate power source for limited operation.
Summary
A hydraulic steering unit consists of a directional control valve and metering section. The valve directs the pressurized oil supplied to and from the cylinder and the metering section. The metering section “meters” out the pressurized oil to the steering cylinder.
Open Center - pump and reservoir are connected when the steering unit is in the neutral position (not being steered). Requires fixed displacement pump.
Open Center, Power Beyond (5-line)- The steering unit has an auxiliary fifth port as a Power Beyond feature to supply fluid to other functions downstream of the steering unit. The Hydraguide automatically takes priority flow for steering, with the remainder available for auxiliary functions. When not steering, all flow is available to auxiliary functions. Not really required for us, but useable.
Closed Center - Closed center systems utilize a variable displacement pump providing variable flow to the steering circuit. All ports of the steering unit are blocked when the vehicle is not being steered. Of no use to us.
Manual Emergency Steering—A ball check valve allows manual steering in emergencies when pump flow is interrupted.
Non-load reactive (non-reversing)— The non-load reactive steering unit keeps the steered wheels in the steered position when the operator releases the steering wheel. The cylinder ports are blocked in the neutral valve position. The operator must steer the wheels back to the straight ahead position.
Load reactive (reversing)—The load reactive steering unit allows the steered wheels to return to the straight ahead position after the operator releases the steering wheel. This happens only if the steering geometry exerts a centering force on the steering cylinder. The cylinder ports are interconnected with the metering section so that the steering wheel follows the wheels back to center position. Reversing steering units should only be used in systems where the opposing cylinder chambers are of equal volume. DO NOT USE with a single unequal area/volume cylinder.
Steering Unit specifications
Even once you have decided on a "type" of steering unit for your application (most likely an open canter, load reactive, non-load sensing unit) there are usually several options available in that type, depending on the specifications. There are a huge number of technical specification that describe a hydraulic steering unit, which is why they often have very specific 30 digit part numbers, from maximum system pressure, to flow requirement, to maximum permissible temperature differentials, to required ISO filtration levels and required input torque. It can get quite complicated, which is why ultimately it can be to your advantage to consult / source your components from a knowledgeable dealer like Performance Off-Road Systems. However, the important specifications for a steering unit are:
How each of these effects your steering performance can only really be fully understood in the context of all the other components of the system, like the pump and cylinder(s). This we will cover in the section on "System Design". However, a basic understanding of what the specifications are can be useful.
Flow - this will normally be listed in 2 parts - maximum continuous rating, and recommended. The maximum continuous rating doesn't really concern us, since even the smallest lawn tractor mini steering unit is rated at 8 gpm, and recall most Saginaw steering pumps will be delivering 2-3 gpm and the highest performance pumps only 3-5 gpm. This would only become a concern if you were running some sort of industrial or agricultural pump capable of many gpm. Of interest to us is the recommended flow. This specifies the required flow the pump used must be able to generate to operate the steering unit FOR A GIVEN STEERING INPUT SPEED. In other words, a certain flow from the pump is required so that the steering wheel can be turned at a certain speed without loss of power assist. Normally, the industry standard steering input speed is two turns of the steering wheel per second. That means, if you see a steering unit rated at 3.43 gpm recommended, that steering unit requires a pump deliver at least 3.43 gpm so that you can turn the steering wheel at a rate of 2 turns per second (incidentally, this is a pretty darn fast rate - try it some time and see. It would be most unusual if you could rotate your steering wheel at a rate exceeding 2 full turns per second. 1 to 1.5 turns per second is a more normal input rate). In this example, if you were only able to supply this steering unit with 2 gpm, you would either have to reduce the rate at which you turn the steering wheel to something less than 2 turns per second (yes - in the "design" section we will cover how to calculate allowable steering speeds for given flow delivery), or, if you do turn the wheel at more than 2 turns per second while delivering less than 3.43 gpm you would experience a loss of power assist.
Note also that how fast you want to be able to turn the steering wheel will depend a lot on how many turns lock to lock your steering system has (which is governed by the next steering unit spec - it's displacement). For example, if you use a low displacement steering unit so that your steering has 8 turns lock to lock, you would want to be able to turn the wheel faster (to navigate a fast twisty section, for example) than if you used a high displacement steering unit so that you only has 2 turns lock to lock.
Displacement - This spec is the volume of fluid that is metered to the cylinder per revolution of the steering unit. It is normally specified in cubic inches per revolution (cu.in./rev). It is important because it, along with the dimensions of the cylinder (volume) will determine at what rate the cylinder extends and retracts; which translates into how many turns of the steering wheel it takes to go from lock to lock. Again, we will discuss the exact calculations in the design section. For know - if steering unit displacement is to small (in relation to the cylinder specs), the steering will seem slow and unresponsive. If it is too great, the steering will be twitchy and hard to control.
Pressure - This spec will be listed as a maximum operating pressure. It must be greater than the maximum pressure the pump is capable of, or component damage may result. Values in the 1800-2500 psi range are common. When compared to the common 12-1500 psi of automotive power steering pumps, again, there shouldn't be a problem for us. However, some steering units are rated at
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