Motion Control Valves
Motion control valves, also referred to as load holding valves, are used to control the motion of a load in the following ways:
· Prevent a load from dropping in case of hose or tube failure.
· Prevent a load from drifting caused by directional control valve spool leakage.
· Provide smooth, modulated motion when the load is in a lowering or run-away mode.
· Provide smooth, modulated motion when the directional control valve is suddenly closed.
There are two basic types of motion control valves:
· Pilot-operated, or pilot-to-open check valves will satisfy the first two of the above requirements.
· Counterbalance valves will satisfy all four of the above requirements.
A counterbalance valve provides several functions:
· Free flow in one direction.
· Leak-free load holding.
· Protection against hydraulic line failure.
· Protection against pressure shocks caused by external forces or overrunning loads
· Cavitation-free motion control to match speed to pump flow when a load could
cause loss of control of an actuator (cylinder or motor).
· Smooth, modulated motion control when the directional valve is suddenly closed.
Counterbalance valves will positively hold a pressurized load and will control the motion of the load based on application of a pressure signal to the pilot port. Counterbalance valves are available as individual cartridges or standard cartridge-in-body (CIB) packages.
A typical circuit application for a counterbalance valve contains a pump, directional control valve, and an actuator. Without a counterbalance valve the load will drift down due to spool leakage if the directional control valve is centered with the load raised. Additionally there is no protection against the load dropping in the event of hydraulic line failure.
Adding a counterbalance valve controls motion and provides protection against hose or tube failure. In this circuit, moving the directional control valve to the left causes the cylinder to extend, raising the load with free flow going through the check valve portion of the counterbalance valve. When the directional control valve is centered, the counterbalance valve will prevent leakage and lock the load in position. Moving the directional control valve to the right sends flow/pressure to the rod end of the cylinder. This pressure also acts to pilot open the counterbalance valve and allows the load to be lowered.
Should the load cause the cylinder to run away from the pump, pilot pressure to the counterbalance valve will decrease and the counterbalance valve will modulate to match the cylinder speed to the pump flow.
The pressure required to pilot open the counterbalance valve can be calculated as follows:
P = (Ps • Ab) - W (load retracts cylinder)
(Ab • R) + Ar
P = (Ps • Ar) - W (load extends cylinder)
(Ar • R) + Ab
W = Load
Ps = Counterbalance valve relief setting; see below for more information
Ab = Cylinder bore area
Ar = Cylinder rod area
R = Counterbalance valve pilot ratio; see below for more information
Note that these equations are idealized and do not consider any backpressure in the circuit, which is additive to the pressure required to pilot open the check valve.
Some additional guidelines for counterbalance valve applications:
- Specify the counterbalance valve relief setting high enough to stop any motion (flow) at the maximum expected actuator pressure. Generally it is recommended to use a setting of 1.3 multiplied by the maximum load pressure.
- Use low pilot ratios (3:1 and 4.5:1) for applications where loads may vary widely. Low pilot ratios require higher pilot pressure and are less efficient but provide stable, precise control for varying loads.
- Use high pilot ratios (8:1 and 10:1) for applications where loads are relatively constant. High pilot ratio valves require lower pilot pressure, have faster response, and are more efficient, but lack stability and precision in response to varying loads.
- Do not oversize counterbalance valves. There is no pressure drop operating limit for counterbalance valves and in fact some pressure drop is required to maintain valve operation.
- Locate counterbalance valves at or near the actuator to provide maximum load holding protection in the event of hydraulic line failure.
- Do not use counterbalance valves with closed-center directional control valves. Pressure trapped between the directional control valve and the actuator can pilot the counterbalance valve open and result in undesired load motion.
- Do not use counterbalance valves with tandem-center directional control valves.