What is in this article?:
- BOOK 2, CHAPTER 5: Counterbalance Valve Circuits
- Externally piloted counterbalance valve
- Machine thrusts with different counterbalance valve pilot options
Externally piloted counterbalance valve
Figure 5-7 shows an externally piloted counterbalance valve circuit. (This is the same cylinder arrangement shown previously.) Notice the counterbalance setting of 150 psi with this circuit. Because this is an externally piloted counterbalance, it operates at a much lower setting. If the load changes, the counterbalance setting does not change. Load-induced pressure does not affect set pressure. Theoretically, set pressure could be 1 psi and the load would not move. The cylinder would only extend when the counterbalance valve’s external pilot port sensed 1 psi at the cylinder cap end.
On most externally piloted counterbalance valves, the minimum pressure setting is 100 to 200 psi. This keeps the counterbalance valve from hunting. Hunting starts when the valve sees enough pressure to open, but then opens too wide. The cylinder runs away when the valve opens too much and pilot pressure drops. When pilot pressure drops, the counterbalance valve closes and the cylinder stops. After the cylinder stops, pilot pressure builds again. The process repeats and continues to the end of stroke. The higher the load-induced pressure, the greater the hunting problem. (On some systems it is possible to add an orifice in the pilot line to slow the pilot supply response and reduce hunting. This orifice fix is difficult to get right and may cause other circuit problems.)
Energizing the directional valve to extend the cylinder in Figure 5-8 sends pilot pressure to the counterbalance valve from the cap end cylinder line. Once pressure in the cap end cylinder line reaches 150 psi, the counterbalance opens and the cylinder extends. As long as there is enough pilot pressure to keep the counterbalance open, the cylinder moves forward. Increasing, decreasing, or stopping pump flow causes the cylinder to respond accordingly, but never to run away.
When the cylinder meets the load, pressure in the pilot port of the counterbalance continues to increase. When pilot pressure goes above the counterbalance setting, the valve opens fully and drops all backpressure on the cylinder rod end. With no backpressure on the rod end, the weight energy generates extra downward force. The externally piloted system saves energy by eliminating all backpressure when performing work. Figure 5-9 shows the flow paths after the directional control valve shifts to retract the cylinder.
Internally/externally piloted counterbalance valve
Some manufacturers make counterbalance valves with internal and external pilots. These internally/externally piloted valves provide the best of both systems. They use the internal pilot to lower the load smoothly and the external pilot to drop all backpressure when performing work, thus avoiding loss of down force. In addition, internally/externally piloted counterbalance valves don’t hunt.
Figure 5-10 shows a schematic drawing with an internally/externally piloted counterbalance valve. In the at rest condition, the external pilot drains to tank through the directional valve. The internal pilot has static pressure from the load-induced pressure on the rod end area. Setting the counterbalance pressure approximately 25% higher than static pressure (1.25 X 566 = 707 psi) means that when pressure at the cylinder cap end rises to approximately 75 psi, the cylinder starts to stroke.
When the directional valve shifts, Figure 5-11, the cylinder begins to extend. Internal pilot pressure opens the counterbalance valve enough for the cylinder to move. Pilot pressure built by the pump pushing against the cylinder keeps the counterbalance valve open. The cylinder continues to extend smoothly at a controlled rate. If flow to the cylinder cap end changes or even stops, cylinder speed responds accordingly. When the cylinder meets resistance, the external pilot takes over, Figure 5-12, and opens the counterbalance fully at approximately 250 psi. With the counterbalance valve open to tank, backpressure against the cylinder rod end drops, allowing full thrust.
To see how cylinder thrust changes with different counterbalance valve pilot options, look at Figures 5-13, 5-14, and 5-15. These circuits show each type of counterbalance piloting system in the working condition -- with pressures, forces, and effective force listed.