The fourth and last normally closed pressure control valve found in hydraulic circuits is the counterbalance valve. Cylinders with external forces — such as weight from a platen, machine members, or tooling — acting against them will overrun when cycled if oil flowing out of them is not restricted. A meter-out flow control circuit is one way to control overrunning loads but it has one main drawback. A flow control’s speed is fixed except for manual adjustment or when using an infinitely variable proportional type. Because flow is fixed, the actuator will continue at the same speed – even when working flow to it increases or decreases. Thus, control is minimal and there could be high energy waste. (Figure 13-8 shows a meter-out flow control circuit for running away loads.)
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A counterbalance valve keeps an actuator from running away regardless of flow changes because it responds to pressure signals, not flow. A counterbalance valve is almost the same as a sequence valve except it normally does not have an external drain connection. The cutaways and symbols in Figure 14-6 depict the physical makeup of three different counterbalance valves and how they are represented on a schematic drawing.
The two cutaways and symbols on the left are spool designs with internal and external pilots. The valve on the right is a poppet design that is both Internally and externally piloted. Each valve type has advantages in different circuit arrangements that will be discussed later. A counterbalance valve usually has a bypass check valve for reverse flow because its most common use is in controlling actuators with running away or overrunning loads.
An internal pilot-operated counterbalance valve shifts to allow excess fluid to flow to the outlet when pressure at the inlet increases to the pressure set by the pressure adjustment. Pressure at the inlet never drops below set pressure when there is flow at the outlet. Flow from the inlet to the outlet is just enough so that backpressure on the actuator never drops below set pressure. This means the actuator moves only as fast as it is supplied and stops when Inlet flow ceases.
Pressure adjustment on the Internal-piloted counterbalance valve is usually made by first screwing the pressure adjustment all the way in. To assure that the valve is capable of high enough pressure, start the pump and raise the load a small amount. Then center the directional valve — which connects the cylinder rod-end port to tank — to see if it holds. If the load holds, next raise the load in increments — checking for load stop every few inches. With the load suspended, start reducing set pressure on the counterbalance valve slowly until the load creeps forward. When the load starts drifting down slowly, increase pressure until movement stops, then turn the pressure adjustment another quarter to half turn higher. This method of adjusting usually wastes less energy while it always stops and holds the load.
The main disadvantage of an internal pilot-operated counterbalance valve is that backpressure is constant and it holds back even when the actuator needs maximum force. Another disadvantage is that to maintain optimum performance, an Internal-piloted counterbalance valve must be readjusted every time the load changes. The valve’s main advantage is that it produces smooth cylinder action while advancing to the work.
An external pilot-operated counterbalance valve shifts to allow excess fluid flow to the outlet when pressure at the opposite cylinder port reaches the pressure set by the pressure adjustment. Pressure at the inlet never drops below load-induced pressure plus pressure set on the pressure adjustment when there is flow at the outlet. Flow from inlet to outlet is just enough that the actuator moves only as fast as it is supplied and stops when flow to the actuator ceases.
Pressure adjustment on the external pilot-operated counterbalance valve can be made on a test stand by setting the pressure adjustment at 100 to 200 psi. If pressure must be set on the machine, set the pressure adjustment higher than 200 psi and lift the load a small distance to make sure it stops and holds. If it holds, continue to raise the load high enough to have some time for the next step. Now, power the load down and observe pump pressure. Pump pressure while lowering the load should not exceed 200 psi. Continue this action until pump pressure is between 100 and 200 psi while the load is lowering. This method of adjusting usually wastes less energy while always stopping and holding the load.
The main disadvantage to an external pilot-operated counterbalance valve is that it may cause lunging or even stop cylinder action while advancing to the work. The main advantage is that backpressure is only present when the actuator is advancing to the work. At work contact, pressure at the actuator inlet increases and forces the counterbalance valve wide open, thus eliminating all backpressure. Another advantage is that an external pilot-operated counterbalance valve does not need to be readjusted when the load changes.
Internal and external pilot-operated counterbalance valves shift when pressure at the internal pilot area reaches the pressure set on the pressure adjustment and allows excess flow to go to the outlet. Pressure at the Inlet never drops below set pressure when there is flow at the outlet. Flow from the inlet to the outlet is just enough that backpressure on the actuator never drops below set pressure. This means the actuator moves only as fast as it is supplied and stops when Inlet flow ceases.
Pressure adjustment on an internal and external pilot-operated counterbalance valve is usually made by first screwing the pressure adjustment all the way in. To assure that the valve is capable of high enough pressure, start the pump and raise the load a small amount. Then center the directional valve that has the cylinder rod-end port connected to tank — to see if it holds. If the load holds, then raise the load in increments — checking for load stop every few inches. With the load suspended, start reducing set pressure slowly until the load creeps forward. When the load starts drifting down slowly, increase pressure until movement stops, then turn the pressure adjustment another quarter to half turn higher. This method of adjusting usually wastes less energy while always stopping and holding the load.
An internal and external pilot-operated counterbalance valve lowers loads smoothly and opens fully when pressure at the actuator inlet increases upon contact with the work. The valve does need to be readjusted when loads change, but this is a small price to pay for good control.
Figure 14-7 depicts a vertically oriented cylinder with rod facing down and a load trying to extend it. To keep the cylinder from running away, the counterbalance valve must resist the load-induced pressure from the weight. The load-induced pressure can be calculated and the counterbalance valve could be preset at 100 to 150 psi higher on a test stand, but pressure adjustment is usually done at the machine (as mentioned earlier).
Notice that the directional control valve has ports A and B connected to tank in the center condition. There is no chance of extra pressure buildup in the pilot line while the circuit is at rest. If ports A or B were blocked, pressure could build and pilot the counterbalance valve open, allowing the cylinder to drift.
Energizing solenoid A1 sends pump flow to the cylinder cap end. As pressure builds there, pressure also increases in the rod end. When pressure at the cylinder rod end reaches 100 to 150 psi above the load-induced pressure, the cylinder starts to extend as fast as the pump fills the cap end. When flow increases, cylinder speed increases and when flow decreases, cylinder speed decreases.
As stated in the counterbalance valve explanation, backpressure at the cylinder rod end is present during the entire extend stroke. As a result, at work contact cylinder force is reduced by counterbalance pressure times the cylinder’s rod-end area. The total weight of the platen and tooling on a press plus the amount of added pressure at the counterbalance valve cannot be used to do work. Energy is expended to raise the weight but it is not recouped during the work cycle. Energizing solenoid B1 sends fluid around the counterbalance valve through the bypass check valve and on to the cylinder rod end to retract it.
The circuit in Figure 14-8 shows the same cylinder with an external pilot-operated counterbalance valve. An externally piloted valve can be set at approximately 100 to 200 psi regardless of load-induced pressure in the cylinder. This is especially convenient in applications where loads constantly change. It is also the best use of energy because the counterbalance valve opens fully when the cylinder meets resistance so the weight is able to do some work. Because backpressure on the cylinder rod end is zero, more force is available.
Energizing solenoid A1 sends fluid to the cylinder’s cap end to start it extending. As pressure builds in the cylinder cap end, it pressurizes the external pilot and opens the counterbalance valve The valve only opens enough to let fluid out when the cap end is at pilot pressure. If pilot pressure is set too low, the counterbalance valve may quickly open too far — allowing the cylinder to run away and pilot pressure to drop. At this point, the counterbalance valve shuts abruptly and the cylinder stops. Almost immediately, pressure again builds at the cylinder cap end, the counterbalance valve reopens, and the same scenario repeats until the cylinder meets resistance. A meter-in flow control in the external pilot line can help, but is very difficult to set. Energizing solenoid B1 sends fluid around the counterbalance valve through the bypass check valve and on to the cylinder rod end to retract it.
The internal and external pilot-operated counterbalance valve in Figure 14-9 incorporates the best features of both valves. The internal pilot provides a smooth advance stroke at low force, while the external pilot opens the valve fully to eliminate backpressure from the cylinder rod end when it contacts the workpiece. (Like the internally piloted valve. this version must be reset at each load change to maintain its efficiency and keep energy losses low.)
The symbols in these example circuits show a direct-acting pressure control valve. Several suppliers offer a pilot-operated version that is more stable and has less pressure differential between cracking and full flow operation.
The circuits shown here work equally well with hydraulic motors, except that a counterbalance valve will not stop and hold a running away load on a motor without creep. All hydraulic motors have internal leakage that increases as the motor wears. The counterbalance valve may not have any bypass but fluid will slip by the motor parts no matter what its design.
There are no counterbalance valves for air circuits. Air circuits depend on meter-out flow controls to keep an actuator from running away. Usually an air circuit uses a 2-position valve that keeps pressure on the retract side at rest so it stays in place at end of stroke. When a load must be stopped in mid-stroke, a 3-position valve with cylinder ports blocked in center is the common method of trying to do this. There also is available a pilot-operated check valve for air service that gives some control for stopping and holding a pneumatic cylinder in mid-stroke.