A hydraulic arm holds a load mid-stroke. It slowly sinks. You pull the cylinder, replace the seals, and it still drifts. The seals weren’t the problem. The counterbalance valve was never adjusted.
Why a vertical cylinder sinks
On a vertical cylinder, the load wants to fall. The cap-end hose is supposed to hold pressure, but the directional valve leaks internally. Oil seeps from the high side to the low side, and the load creeps down. New cylinder seals don’t fix the valve’s internal leak.
A counterbalance valve mounts directly on the cap end and blocks the oil until head-end pressure opens it. That’s what holds the load.
Setting the valve
Set the cracking pressure to 1.3 to 1.5 times the pressure the full load generates. Too low and the load free-falls. Too high and the head end has to build huge pressure to open the valve, making the action jerky.
Turn it down until the load drifts, then tighten until it stops. Add a small turn. Too tight and the cylinder hunts. Too loose and it drops. This is a field adjustment, not a catalog setting.
Counterbalance vs hydraulic lock
A hydraulic lock (pilot-operated check) is shutoff. It seals hard, but when it opens the load drops freely and can run away. A counterbalance is a throttling device: it always holds back pressure, so the load moves smoothly.
Use counterbalance on vertical loads where gravity accelerates the mass. Use a hydraulic lock on horizontal cylinders that just need to stay put.
Keep the plumbing short
Long hoses between the valve and the cylinder compress and expand, giving the load a few millimeters of drop before the oil reaches the valve. Mount the counterbalance directly on the cylinder port. No long hoses, no extra drop.
