w some bypass, so a cylinder with an outside force working against it slowly moves out of position when stopped. Installing pilot-operated check valves in the cylinder lines and connecting the directional valve’s A and B ports to tank in center position assures that the cylinder will stay where it stops (unless the piston seals leak).
The circuit in Figure 10-5 shows a pilot-operated check valve holding a load on the rod end of a vertically mounted cylinder. Pilot-operated check valves can hold potential runaway loads in place without creep, but this circuit usually has problems on the extend stroke. This is because a pilot-operated check valve opens the rod end of the cylinder to tank, letting it run away. When the cylinder moves faster than the pump can fill it, pressure in the cap end and pilot pressure to the pilot-operated check valve’s pilot port drops and the valve closes quickly. This can generate high-pressure spikes that may cause pipe and part damage. Almost immediately, pressure to the pilot-operated check valve’s pilot port builds again and the runaway/stop scenario repeats until the cylinder meets resistance or something fails. The best valve to control runaway loads is the counterbalance valve explained in Chapter 14.
Figure 10-6 illustrates another problem with using a pilot-operated check valve to hold back a runaway load: a pilot-operated check valve may not open when signaled to let a cylinder with an oversize rod and heavy load extend. When the directional valve shifts to extend the cylinder, load-induced pressure can hold the pilot-operated check valve poppet closed. It may take 300 to 400 psi to force the poppet open, even with its 3:1 or 4:1 area difference. Pressure builds at the pilot port, but at the same time it increases in the cylinder cap end. With a 2:1 rod-differential cylinder, it can add 600 to 800 psi to the load-induced pressure. The additional downward force causes pilot pressure to increase, which causes more downward force, which causes more pilot pressure -- until the circuit reaches maximum pressure. At that point, the relief valve bypasses or the pump compensator kicks in to stop flow. The cylinder simply cannot start to extend . . . and even if it could, the action would be erratic, as in Figure 10-5.