Machine thrusts with different counterbalance valve pilot options
An internally piloted counterbalance valve controls cylinder extension smoothly, but reduces thrust during the working portion of the cycle.
Figure 5-13 shows the maximum force from different types of counterbalance circuits while acting on a part with the cylinder stalled. System pressure of 2000 psi acting on the cylinder cap end produces 100,540 lb of thrust. (50.27 in.2 X 2,000 psi = 100,540 lb.) The 15,000-lb weight on the rod end increases the resulting downward force to 115,540 lb. The 716 psi acting against the 26.51-in.2 rod end area produces an upward acting force of 18,981 lb. (716 psi X 26.51 in.2 = 18,981 lb.) The net effective downward acting force is 96,559 lb. If the upward acting force could be reduced or eliminated, the cylinder could do more useful work.
The counterbalance valve more than cancels the weight of platen and tooling that gives an energy loss of approximately 16%. Approximately 7.5 tons of force from the rod end weight must be raised during every cycle but does not do any work as the cylinder extends.
Externally piloting the counterbalance valve, Figure 5-14, requires about twice as much pressure to extend the cylinder. However, upon reaching the work, the loss of backpressure on the cylinder increases the cylinder force and more than makes up for the loss.
The schematic shown in Figure 5-14 has the same downward force as Figure 5-13 -- a total of 115,540 lb. The difference is that there is no upward force in Figure 5-14. The resultant downward force of 115,540 lb is an increase of 16% over the circuit with an internally piloted counterbalance valve. This saves most of the energy expended to raise the load on the return cycle.
If at all possible, a counterbalance valve should be externally piloted. As explained previously, there are some instances where a cylinder might chatter as it extends if its circuit uses an externally piloted counterbalance valve. This chatter usually applies to circuits with high load-induced pressure or when the counterbalance valve is mounted at a distance from the cylinder port. The best practice is to mount the counterbalance valve directly on or very close to the cylinder port. Note that if a conductor between the cylinder port and valve breaks, the cylinder will free fall. That is why it is always good practice to use an external safety device to protect the operator and machine.
Figure 5-15 shows a schematic diagram of the best counterbalance circuit. This circuit has a counterbalance valve with internal and external pilot supply. As the cylinder extends, the lower-pressure internal pilot gives a smooth descent at reduced pump pressure. The end result is the same as the externally piloted valve of Figure 5-14. When the cylinder contacts the work, all upward force is eliminated, minimizing energy loss.
Hydraulic motor brake valve
Excessive backpressure can damage a fast-turning hydraulic motor during an emergency stop situation. An open-center valve will eliminate backpressure, but the motor will continue to turn until it coasts to a stop. For a fast, non-shock stop, use a special counterbalance valve, called a brake valve. Figures 5-16, 5-17, and 5-18 illustrate a hydraulic motor circuit that uses a brake valve.
The brake valve is an internally/externally piloted valve with different pilot areas. Some designs take one eighth of the pilot pressure at the external pilot port as that set at the internal pilot port. This means, for example, that setting the internal pilot at 900 psi requires only 113 psi at the external pilot to open the valve. The actuator could be a hydraulic motor or a fast moving horizontally mounted cylinder with an over running load. In either case, a brake valve eliminates damage from stopping the actuator abruptly.
With the hydraulic motor moving, Figure 5-17, external pilot supply opens the brake valve at low pressure. As long as pressure required to move the motor is greater than the external pilot pressure needed, there is little or no energy loss. A brake valve appears virtually nonexistent as the motor runs under load. If the hydraulic motor tries to run away, say on a loaded winch, a brake valve holds against the load until the motors down port sees at least 113 psi. The load will lower only as fast as fluid enters the motor down port. A brake valve counterbalances when necessary and allows almost free flow under load.
NOTICE: Using a counterbalance or brake valve in a hydraulic motor circuit will not keep the motor from creeping when stopped. No matter how leak-free the counterbalance valve is, the internal bypass in the motor will let it slowly turn. Use an external braking system to hold any overrunning load driven by a hydraulic motor.
When the directional valve shifts back to its center position, Figure 5-18, external pilot pressure drops and the brake valve begins to close. The hydraulic motor now acts like a pump, trying to force oil through the brake valve. As the brake valve starts closing, internal pilot pressure builds to 900 psi, forcing fluid through the brake valve at a 900-psi pressure drop. This 900-psi backpressure decelerates the actuator smoothly and rapidly. Setting the valve pilot pressure higher makes the stop faster and more abrupt. A lower pilot pressure setting makes the stopping time longer but smoother. In any case, stopping action is smoother and quicker than it would be without the brake valve.
The difference between this circuit and a setup using a cross-port relief valve is that setting the brake valve at a pressure lower than system pressure does not affect normal actuator operation. Also, it eliminates the danger of cavitating an externally drained hydraulic motor. Note how the path around the brake valve with a bypass check valve allows reverse free flow for opposite rotation. To stop the motor quickly in the opposite direction of rotation, install another brake valve in the opposite motor line.