HydraForce Insider Blog

While there are many uses for L.S. priority valves, typically they are used to provide priority flow and/or pressure to certain components or functions depending on need. One of the most common uses is to apply them with L.S. steering orbitals.
The two most common types of L.S. steering orbitals (Static and Dynamic) are shown below. We use a static L.S. priority valve (ECxx-42) with a static steering orbital and a dynamic L.S. priority valve (ECxx-43) with a dynamic steering orbital. Notice the direction of the sense flow.

Static Steering Unit

In a static steering system, the sense flow goes from the steering orbital to the EC valve. The faster you turn the steering wheel, the more flow comes out the work ports.

There are several variable orifices in the steering orbital, which open and close proportionally based on how fast you spin the wheel. In neutral, the sense pressure vents to tank through the steering unit.

Dynamic Steering Unit

In a dynamic steering system, the sense flow goes from the EC valve to the steering orbital and back to tank through the variable bleed orifice in the steering orbital.

Turning the steering wheel opens the work ports and closes the bleed-off orifice, thus building pressure in the sense line pushing on the EC spool. This directs more oil to the orbital the faster you spin the wheel.

Most Steering Circuits Utilizing Orbitals Use a Dynamic Setup

Since the steering unit is really just a rotary style variable orifice, using a pre-compensator in conjunction with it makes the steering function compensated. With a given steering RPM, the flow will remain constant regardless of varying load pressure.

The boost orifice needs to be located in the shown position to prevent slow movement of the EC spool when the RV opens, thus preventing a pressure spike.

The objective is for the steering to work perfectly with as little pressure drop as possible, however sometimes we need to fine tune the responsiveness or the maximun steering flow by tweaking a few things.

Ways to Fine-Tune the Circuit to Optimize Steering Performance

The pressure differential between the EC valve and the L.S. port of the steering unit is called the margin pressure. This pressure differential controls the responsiveness and the maximum steering flow. This is determined by a combination of the bias spring of the EC and boost orifice.

The dynamic L.S. priority valves have two orifices in the spool, one for damping (PP), and the other for feeding (DS) oil to the orbital. The pilot flow to the L.S. port of the steering unit is determined by the spring value and the feed orifice. The spring value is typically either 80 psi or 100 psi, while the orifice is typically between 0.020" and 0.031" Dia., depending on which size EC valve you are using.

You can create any margin pressure you want by simply increasing or decreasing the size of the boost orifice. The smaller the orifice, the higher the margin pressure.

Pressure drop data from a steering unit catalog will help determine what is required to achieve the flow you need. However, the responsiveness or optimum feel is typically determined by an expert operator. By increasing or decreasing the boost orifice, you will be able to fine-tune the responsiveness or feel of the steering wheel.

An increase or decrease of as little as 0.002" Dia. can make a huge difference in the steering feel. Margin pressure that is too low will result in steering that is slow and sluggish. Margin pressure too high will result in jerky steering.

Proportional Steering and On/Off Lift Circuit

It is common to use proportional valves instead of steering orbitals to steer many types of equipment. This circuit simply gives priority to steering while allowing the excess flow to lift and lower a cylinder. The EC valve not only gives priority to steering, it also compensates the proportional valve so the same current value will achieve the same flow regardless of load.

Priority Valve Working with Manual Valve

It is becoming more common to use an EC valve mounted to the inlet of a manual valve to give priority to certain functions while also limiting the maximum flow and pressure to those functions.

Typical Priority-on-Demand
Flow Control Circuit

Most uses of the L.S. Priority Valves are for flow control. By sensing downstream of a fixed or variable orifice, you can compensate the circuit. The needle valve shown above could be any one of a number of components such as a ball valve, proportional valve, on/off valve or a simple orifice.

Pressure Control Valve

Many people don't realize you can use an
ECxx-42 or ECxx-43 to create a pressure control. Think of this as a pressure reducing valve with an excess flow port. The valve will modulate to maintain the spring value in the priority (CF) leg regardless of flow and/or the downstream pressure and flow in the bypass leg. Higher spring pressures can be achieved by using an ECxx-43 and boosting the sense line with an orifice or relief valve.