There is a misconception about hydraulic cylinders that if the piston seal leaks, the cylinder will drift. Although piston seal leakage can be the cause of cylinder drift, the specific physics are usually misunderstood.

In fact, if the piston seal is completely removed from a double-acting cylinder and the cylinder is filled with oil while the ports are plugged, the cylinder will hold its load indefinitely unless the rod-seal leaks.

Under such condition, because of the unequal volume on either side of the piston, fluid pressure equalizes and the cylinder becomes hydraulically locked. Once this occurs, the cylinder can move only if fluid escapes from the cylinder via the rod seal or its ports.

However, there are two exceptions to this theory. The first is a double-rod cylinder where volume is equal on both sides of the seal gasket. The second involves a load hanging on a double-acting cylinder. In this exception, the volume of pressurized fluid on the hood side can easily be accommodated on the piston side. However, as the cylinder drifts, a vacuum will develop on the piston side due to unequal volumes, and depending on the weight of the load, this vacuum may eventually result in equilibrium which arrests further drift.

Notwithstanding these two exceptions, if a double-acting cylinder’s service ports are blocked by a closed-to-actuator spool, and the piston hydraulic seal does bypass, pressure will eventually equalize on both sides of the cylinder. At this point, a hydraulic lock is affected and no further drift can occur, unless fluid is allowed to escape from the cylinder or cylinder circuit.

Therefore, while the root cause of the problem in both examples is the leaking piston rubber seals, the physics is fundamentally different from the general belief. And if it is understood, a pressure gauge can be a useful tool for establishing the cause of cylinder drift. In either of these examples, if the cylinder is drifting but there is no equalization of pressure across the piston seal, the directional control valve or load control valve is the source of the problem.