Last updated on October 14th, 2024 at 08:12 am
For many years, mobile equipment like tractors and loaders uses unique hydraulic steering systems (also called, hydraulic power steering) to steer the machines. This unique system uses steering control unit (SCU) which is rotated by steering wheel and steering shaft.
This hydraulic steering system has no direct mechanical connection compared to the mechanical linkages found in automotive power steering, as hydraulic hoses make it easy to move oil from the pump to the steering control unit and then to the steering cylinder, even if the cylinder is far away from the SCU. This flexibility is key in making the system work smoothly across different setups.
Most of us use hydraulic steering systems, but to be honest a lot of techs and engineers treat them like tools. They focus on just using them and skip over how these amazing systems actually work. (P.S., I also used to be like this, joke on me). If you’ve ever wondered what’s really happening when you turn the wheel or wanted to get beyond the basic of schematics, you’re in the right place.
Knowing how the internal parts of the steering system work is really important for engineers and technicians. It’s not just about the basics as steering cylinders often become a major source of contamination, and let’s face it, orbitals (or orbitrols) can sometimes be low quality. So, when we disassemble steering unit for service, it’s good to understand what’s going on inside so we don’t feel clueless.
The Steering Control Unit (SCU) has a few other names, like:
- Hand metering unit (HMU)
- Hydrostatic steering
- Steering hand-pump
- Steering control valve
Hydraulic Steering System Layout in Tractor (Source: Ognibene)
I’m really excited to walk you through hydraulic steering because, honestly, even while putting these posts together, I learned a lot myself! I hope you gain some insights too. And hey, if you know anything beyond what I’ve covered, feel free to share—after all, as someone once said, ‘To keep knowledge alive, you must share it!’
We’ll start with the basics and gradually move into more advanced topics. I’ll cover the different components of SCU, how they’re designed, how they operate, the various types of Steering Control Units (SCUs), and some cool features you can add. We’ll also tackle troubleshooting and finish up with electro-hydraulic steering systems.
There’s a lot to cover, so to keep things interesting and not to make you sleepy, I’ll split it into 3-4 posts. This first one will focus on the components and design. Even if you think you know this stuff already, it’s always good to brush up on your knowledge.
So, let’s get started!
Steering Unit Component
The SCU is responsible for completing two tasks related to steering a tractor. The SCU must control the direction of oil flow (right steer and left steer) and the quantity of oil flow (dictates how fast the wheels are turned).
Primary components of SCU includes:
- Gerotor assembly
- Drive shaft
- Control sleeve
- Centering pin
- Control spool
The internal components of an SCU are rotated by means of a Steering wheel and steering shaft.
- The steering wheel’s drive shaft is splined to the control spool.
- The control spool is coupled to the control sleeve through a centering pin and centering springs.
- The centering pin fits loosely inside the control spool and has a tight fit inside the control sleeve (because of pin diameter), letting the sleeve follow the spool when the wheel is turned.
- When the steering wheel stops, the centering springs bring the control spool/sleeve assembly back to the center position.
- As the control spool/sleeve assembly turns, the centering pin makes the drive shaft rotate.
- This drive shaft is connected to the internal gerotor, which spins within the gerotor assembly.
- The SCU housing, control spool, and control sleeve together function as a rotary directional control valve (DCV).
The stationary parts in the SCU include the gerotor’s ring gear, the housing, the spacer/wear plate, and the end plate/cover plate.
Disassembled View of Steering Control Unit
Steering Unit Design
Alright!, So you know what are the Internal Components of SCU and how they work with respect to each other. But these components are basic and standard ones, and to make SCU work efficiently and more importantly protect, we must design it in a way that it serves the desire purpose. Let’s checkout how we can design with the help of schematic. Here I have added standard schematic of steering unit, with respect to standard configured SCU I have presented variety of internal steering valve options one by one which have their unique use. To make it more easier to identify, I have added Green Colour Text next to the valve option.
- Inlet Check Valve
- Inlet Relief Valve
- Check Valve for Emergency Steering
- Cylinder Port Relief Valve
- Anti Cavitation Check Valve
- Load Sensing Relief Valve
The last steering valve option – load sensing relief valve’s schematic is different from standard configuration. we will discuss working of load sense steering control unit in detail, but as we are discussing all valve options of steering unit, I have included ‘relief valve for Load Sense System’ here as well, so that all valve options are covered in a single place. and once again all six valve options including ‘relief valve for load sense system’ are optional and can be added to steering unit based on application.
Steering Unit Standard Configuration
Inlet Check Valve
Steering Unit with Inlet Check Valve
Inlet Relief Valve
Steering Unit with Inlet Relief Valve
Manual Check Valve
Steering Unit with Manual Check Valve
Port Relief Valve
Steering Unit with Port Relief Valve
Anti Cavitation CV
Steering Unit with Anti Cavitation Check Valve
Inlet Check Valve:
When external forces act on the steering cylinder, they can sometimes cause the steering pressure to rise higher than the main system pressure. This increase could force the steering oil to flow backward through the SCU, leading to what’s known as steering kickback—where the steering wheel moves abruptly in the opposite direction. To prevent this, the inlet check valve is used. It stops the oil from flowing back, ensuring the steering wheel remains stable and preventing any sudden, unwanted movements.
Inlet Relief Valve:
Most hydraulic steering systems include a dedicated relief valve specifically for the steering circuit, often located within the SCU. The pressure setting for the steering relief valve is usually set lower than the main system pressure. If the hydraulic pump is dedicated solely to steering, the main system relief valve may not be needed, as the inlet relief valve would be sufficient. However, in cases where a fixed-displacement pump’s flow is split by a proportional flow divider valve, having the main system relief valve becomes necessary to manage and protect the overall hydraulic system properly.
Manual Check Valve:
In a fully hydraulic steering system, the SCU has a feature called emergency or manual steering mode that helps you steer if the engine stalls or the hydraulic pump stops working. The manual check valve is located at the SCU’s inlet and connects the inlet port to the tank return port.
If the pump isn’t supplying oil, this valve allows the SCU’s gerotor to pull oil directly from the reservoir. This ensures the steering system still has oil for emergency use. As the gerotor spins, it pulls oil through the valve, then directs it through the control assembly and into the steering cylinder, allowing you to steer manually even when the engine or pump fails.
Port Relief Valve:
Whenever a directional control valve (DCV) is in the neutral position, the part of the circuit after the DCV has no protection against forces caused by external loads. To safeguard the steering cylinders and hoses, SCUs can include cylinder port relief valves. These valves prevent pressure overloads that could damage the components by providing relief if external forces push the pressure beyond safe limits.
Anti-Cavitation Valve:
Also called a make-up valve, this valve can be installed inside an SCU to stop cylinder cavitation. It works by pulling oil from the reservoir whenever the cylinders move faster than the hydraulic pump can supply. This ensures that there’s always enough oil, preventing damage and maintaining smooth steering operation.
Load Sensing Relief Valve:
If the steering pressure goes beyond a certain limit, the load-sensing relief valve kicks in and redirects the excess pressure back to the reservoir. In some systems, this valve is built right into the steering priority valve. By doing this, it ensures that the steering pressure stays within safe limits, helping to maintain smooth and safe operation of the steering system.
I will cover more in depth about Load Sensing Steering System in next post. But if you’re eager to learn more right away, I’ve attached a video that gives a great overview of the Load Sense Relief Valve built into the Priority Valve. I hope you find it helpful!
Load Sensing Relief Valve
Steering Unit with Load Sense Relief Valve
Alright, that wraps up this post! We’ve covered the components and design of the SCU.
Now, what I recommend is to check out the SCU manufacturer page which covers what are the latest technologies in the market.
Like Danfoss Steering, Ognibene Steering and M+S Steering.
Why I would recommend is, becuase this little thing make up your mind to learn more about steering or for highly professional it make them proud to say, “I know that” or “I’ve worked on that.”
In the next post, things are going to get more interesting as we are going to look into how the steering unit operates its internal parts when you turn the wheel. We’ll also explore various types of steering control units, including open center, closed center, and load sensing systems. I hope you’ve picked up some new insights or at least refreshed your knowledge a bit. If you enjoyed this post, I’d really appreciate it if you could subscribe. I’m 100% planning to dive deep into hydraulics and break it down into simple terms for you!
