Here's a guide to selecting a hydraulic steering system for your rig and a few key points to consider, a couple things to keep in mind is this article will be written with the intention of using only full hydraulic systems, I will not be addressing hydraulic assist, nor will I address the uses of hydraulic systems used on the street. To start I will discuss the main types of steering options and the benefits and any issues with them
Single Ended Ram
This is the simplest and most cost effective system, a Single Ended Ram is a basic Ram that simply expands and retracts, most setups mount one end of the Ram to a fixed point on the Axle and the other end on the Tie-Rod. The Tie-Rod ties both Steering Knuckles together, run off the factory pump and through an Orbital valve this system works well and can be pieced together for a fairly reasonable cost.
The downside to this setup is the Ram does not use the same amount of fluid to extend and retract the Ram, this is because the shaft of the Ram displaces the volume in the Cylinder when retracting, this will net a couple different issues; First off, it will require more turns of your steering wheel to extend the Ram than it will to retract it, because you are losing surface area you are also losing hydraulic force meaning it will be easier to turn the wheel in the direction the Ram extends. From real world experience if the system is properly sized and the pump is up to the task the system will work fairly well.
Dual Single Ended Rams
This system uses 2 Single Ended Rams working together with a Tie-Rod tieing both Knuckles together, the benefits of this system is the 2 Single Ended Rams will balance each other out, there will not be any of the displacement issues mentioned with the Single Ram setup, the down side to this setup is very complex, doubling the Rams also doubles the number of connections, hoses and moving parts vs a single setup. Another common issue is plumbing and sizing the system properly, 2 individual Rams will require a lot of fluid volume to steer reasonably quick, if the Rams are too large this problem gets worse exponentially, also with all the extra parts this kit can be expensive.
Double Ended Ram
A Double Ended Ram is a Single Cylinder with a shaft out each end of the Ram, the Cylinder body is mounted solid to the Axle and each Tie-Rod is linked to each Steering Knuckle. This system does not have any of the displacement issues found with the Single Ram setup and is significantly simpler than the Dual and Single Ram setup, essentially there are only 4 pivot points in the setup, and no Tie-Rod is required as the Ram holds both Knuckles together. This is a very strong and simple setup, the downside is sometimes the large Ram body can be difficult to mount, also because these are not typically mass produced for other industries like Single Ended Rams they can be higher in cost.
Orbital Steering Valves
Orbital Valves are fairly simple, basically all they do is meter the amount of fluid that flows to your Steering Cylinder as you turn the steering wheel, they are available in many different sizes often measured in Cubic centimeters (CC) or milliliters (ML), the larger the displacement the Orbital is the more volume per turn will be sent to your Cylinder, essentially increasing the speed of your steering, BUT also increasing demand on your pump, this is why its critical to properly size everything in your steering system.
Another very important thing to mention about Orbital Valves is there are 2 versions of return ports. The First is referred to as an Open Center; how this version works is when the steering wheel stops turning, the fluid is returned back to the reservoir, this is what is required when running a setup used off road. The second version; is commonly found in heavy equipment like tractors or forklifts and is referred to as a Closed Center, when the steering stops turning on a Closed Center Orbital the pressure supply to the Orbital is blocked off and is not allowed to return to the reservoir. If used in an automotive setup this will deadhead your pump bringing it up to relieve pressure, this will cause excessive heat and damage or ruin your pump very quickly. If you plan on sourcing used hydraulic parts from heavy equipment, be sure to know exactly what you are putting on before hooking it up. The few dollars you save by running used parts can add up quickly, if they cause damage to other parts of your system not to mention the headache that comes with it.