What is in this article?:
- BOOK 2, CHAPTER 5: Counterbalance Valve Circuits
- Externally piloted counterbalance valve
- Machine thrusts with different counterbalance valve pilot options
Some actuators with running-away (or overrunning) loads will let the load free fall when the directional valve that controls the actuator shifts to lower the load. Cylinders with large platens and tooling or hydraulic motors on winch drives are two examples of such actuators. When the directional valve shifts, an overrunning load forces the actuator to move faster than pump flow can fill it. Oil at high velocity leaves one end while the opposite side starves for fluid. A vacuum void forms in the inlet side of the actuator that must be filled before applying force. Any running-away or overrunning load needs some method to retard its movement.
A meter-out flow control is one way to control a running-away load at a constant speed. Unless pump flow never changes, setting flow precisely on this type control is critical. Setting the flow control for minimum pump flow will waste energy when pump flow is high. Setting the flow control for maximum pump flow lets the cylinder run ahead when pump flow is low. Incorporating a pressure control valve called a counterbalance is a better way to control running-away loads. A counterbalance keeps an actuator from running away even with variable flow rates.
Figure 5-1 shows the symbol for an internally piloted counterbalance valve. Use an internally piloted counterbalance to hold a load back when the actuator does not need full power at the end of stroke. This type of counterbalance valve retards flow continuously, so it resists flow even after work contact stops the actuator. Note that it is necessary to adjust an internally piloted counterbalance valve every time the load changes. The following circuits show these characteristics and how to design around them.
Figure 5-2 shows the symbol for an externally piloted counterbalance valve. This valve’s pilot supply is from a source other than the controlled load. An externally piloted counterbalance does not waste energy at the end of stroke and does not require adjustment for changing loads. However, an externally piloted counterbalance valve does waste a little more energy when moving the load to the work.
Figure 5-3 shows the symbol for an internally/externally piloted counterbalance valve. This valve has the best of both systems. As the load extends, internal pilot supply gives smooth control with little energy loss. After work contact, as system pressure builds, the external pilot fully opens the counterbalance to relieve all backpressure in the cylinder.
Counterbalance valves are manufactured in both spool and poppet designs. Spool designs leak at a rate of 3 to 5 in.3/min, thus allowing actuator creep. Poppet designs leak at only 3 to 5 drops/min, minimizing cylinder creep. (Because hydraulic motors bypass internally, a counterbalance only works with a moving load. The designer should apply a braking method to hold a hydraulic motor at rest.)
Internally piloted counterbalance valve
Figure 5-4 pictures a circuit with a running-away load. This circuit demonstrates the operation of an internally piloted counterbalance valve. The cylinder in Figure 5-4 has a static pressure of 566 psi in the rod end due to the 15,000-lb load on the 26.51- in.2 area. (15,000 / 26.51 = 566 psi). An open-center directional valve unloads the pump and keeps backpressure off the counterbalance valve outlet and pilot port. The cylinder holds in any position if the counterbalance valve is set correctly and does not leak. Set the counterbalance approximately 100 to 150 psi higher than the load-induced pressure.
Normal procedure for setting a counterbalance valve is to turn the adjusting screw to its highest pressure before raising the cylinder. After starting the pump, energize the directional valve and carefully raise the load a short distance. With the load suspended, deenergize the directional valve. A working counterbalance will hold the load suspended and gauge PG3 will show the load-induced pressure. Now start lowering the counterbalance pressure setting slowly. When the cylinder begins to creep downward, increase the pressure until creeping stops. Then continue turning the adjusting control in the same direction another to turn. After setting the counterbalance this way, power the cylinder down and notice the pressure reading on gauge PG3. Pressure should be approximately 700 to 750 psi. Any time the cylinder loading changes, repeat the above process. Resetting the counterbalance valve keeps the cylinder from running away and reduces energy loss with a lighter load.
When the directional valve shifts to extend the cylinder in Figure 5-5, oil from the pump flows into the cap end of the cylinder and pressure starts to build. When pressure in the cap end of the cylinder reaches about 75 psi, the cylinder should start to stroke. (This is because it builds an extra 140 psi in the rod end, adding to the load’s 566 psi.) At this point the cylinder starts to extend and continues to move as long as the pump supplies oil at 75 psi or higher to the cylinder cap end. If pump flow changes, cylinder speed changes also.
While the cylinder is retracting, Figure 5-6, pump flow bypasses the counterbalance valve through the integral bypass check. The counterbalance valve offsets the potential energy of the weight on the rod end of a cylinder. The 15,000-lb force in this figure cannot do useful work when using an internally piloted counterbalance valve. (See Figures 5-13 through 5-15 for energy loss and heat generation for different types of counterbalance circuits.)