The counterbalance valve, also called a holding valve, is used to prevent a weight from falling uncontrollably. As an example, consider a press with a two-ton platen (Fig. 3.27). When the directional control is shifted, the platen will fall unless there is a means for creating an opposing hydraulic force. A counterbalance valve accomplishes this task.
Suppose the cylinder in Fig. 3.27 has a six-inch bore and 2:1 area ratio. What pressure setting should be used for the counterbalance valve? The area of the rod end is 14.1 in2. Pressure to hold a two-ton platen is Generally, the valve is set about 50 psi higher than the pressure required to hold the load. A setting of 340 psi is selected.
What pressure must be developed to extend the cylinder? The hydraulic force on the rod end is
Any pressure above 28 psi causes the platen to move downward. Note that the cylinder friction force was not considered in this problem.
Often, it is desirable to use the weight of the platen to help build the total pressing force. In this case, a counterbalance valve with remote pilot line connection is used (Fig. 3.28). If pressure at the cap end, and thus the pilot line, drops below the set point, the counterbalance valve partly closes to slow the platen.
There is an additional reason for using a counterbalance valve other than to prevent uncontrolled motion of the actuator. When the platen is moving down too fast, the pump cannot keep the cap end filled with fluid, and a negative pressure can develop. It is possible to suck the fluid out of the pump fast enough to cause void spaces in the pump. The condition when the pump is not completely filled with fluid is called cavitation. One of the problems caused by cavitation is a breakdown of the lubrication film between moving parts. A higher pressure setting on the counterbalance valve is needed to achieve the desired counterbalance and limit actuator speed.