Operation Principle of Counterbalance Valve
These are basically relief valves but used in a particular application to set up a back-pressure in a circuit. They are frequently employed to ‘counterbalance' a load as shown in the circuit in Figure 1. Here the valve creates a back-pressure to prevent the load running away when the cylinder is retracting. The usual pressure setting is 1.3 times the load-induced pressure.
Figure 1 Counterbalance valve. (a) Section. (b) Circuit.
A check valve is incorporated in the circuit in Figure 1 to allow free-flow in the reverse direction (i.e. to bypass the counterbalance valve when raising the load). Care must be taken if using a conventional relief valve for this application as at some stage of operation the tank port will be subject to maximum circuit pressure. This is not permissible with many relief valves. The counterbalance valve shown has an integral check valve. A separate drain connection from the spring chamber is unnecessary because the pressure section of the valve is inoperative when the T port is pressurized (flow is through the check valve). When it is counterbalancing the back pressure at T should be kept to a minimum.
A disadvantage of a counterbalance valve is that it reduces the available force. Consider the press circuit in part (a) of Figure 2 where the valve is used to counteract the weight of the press tools whilst they are closing. During'the forming operation, part of the possible pressing force will be lost in overcoming the back-pressure set up by the counterbalance valve.
Figure 2 Press circuit. (a) With counterbalance valve. (b) With over-center valve.
Figure 3 Over-center valve (pilot-operated counterbalance valve or brake valve).
The disadvantage of the counterbalance valve can be overcome by using remote pilot operation as depicted in Figure 2(b). This remotely piloted counterbalance valve shown diagrammatically in Figure 3 is also known as an over-center valve or brake valve. A relatively low pressure in the pilot section will switch the valve open, removing the back-pressure from the cylinder annulus side. When the piston tries to run away, pilot pressure is lost and the counterbalance section switches back into circuit. During the forming part of the press operation, the valve is piloted open removing the back pressure and all the pressure on the full bore side is then available for pressing.
The over-center valve is frequently used in motor circuits (hydrostatic transmissions) as a brake valve. In Figure 4, the circuit shows a simple winch driven by a hydraulic motor; the over-center valve will:
(a) Hold the load in the neutral position. .
(b) Prevent over-run during lowering.
(c) Gently brake the motor to a halt on switching from ‘lewering' to 'neutral'.
The ratio between the pilot pressure to the direct pressure necessary to open the valve is generally from 2:1 to 10:1 according to the application.
Figure 4 Over-center valve used in a winch circuit.
Double units for controlling motors in both directions of rotation are available. A particular variation incorporates a series of check valves and is known as a 'motion control and lock valve'. It has a port for the make-up oil input for closed-circuit transmissions and in the event of the motor stalling it functions as a cross-line relief valve.