A popular misconception involving hydraulic cylinders is that if the piston seal is leaking, the cylinder will drift. While a leaking piston seal can be the root cause of cylinder drift, the physics involved are often misunderstood.

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

In this condition, due to 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.

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Figure 1. Double-rod cylinder - oil volume is equal on both sides of the piston

Exceptions to the Rule
There are two exceptions to this theory. The first is a double-rod cylinder (Figure 1) where volume is equal on both sides of the piston.

The second exception involves a load hanging on a double-acting cylinder (Figure 2). In this arrangement, the volume of pressurized fluid on the rod side can easily be accommodated on the piston side. But 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 that arrests further drift.

This is not the end of the cycle, but it's important to at least grasp this theory before continuing.

Notwithstanding these two exceptions, if a double-acting cylinder's service ports are blocked by a closed-to-actuator spool (Figure 3), and the piston seal does bypass, pressure will eventually equalize on both sides of the cylinder. At this point, a hydraulic lock is effected and no further drift can occur, unless fluid is allowed to escape from the cylinder or cylinder circuit.

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Figure 2. Load hanging on double-acting cylinder

Loss of Effective Area
Because of the loss in effective area due to pressure now acting on the rod-side annulus area, the static pressure in the cylinder must increase to support the same load. Remember, force developed by a cylinder is a product of pressure and area.

For example, if the load-induced pressure on the piston side of the cylinder was 2,000 PSI and zero on the rod side when the directional control valve closed, assuming no leakage past the spool, the equalized pressure may be 3,000 PSI depending on the ratio of the piston and annulus areas.

Now consider what can happen if this circuit has a service port relief valve (Figure 4) set at 2,500 PSI. As pressure equalizes across the piston seal and the increasing static pressure on the piston side of the cylinder reaches the cracking pressure of the port relief, however the cylinder will still not retract.

A similar situation can occur in circuits with a load control (counterbalance) valve installed. In this circuit, shown in Figure 5, the directional control valve has a float center spool (service ports A and B open to tank).

As previously stated, if the piston seal leaks, unequal volumes of oil on the rod and piston sides of the cylinder indicates hydraulic lock will prevent any noticeable drift. But once again, due to the loss of effective area as a result of the same pressure now acting on the piston and rod-side annulus areas, the static pressure in the cylinder must increase to support the same load.

The magnitude of this pressure increase depends on the ratio of the cylinder's piston and annulus areas. If the increase in static pressure exceeds the set maximum load of the counterbalance valve, the valve will open allowing oil from the piston side of the cylinder to flow to the tank and the cylinder to retract.

Diagnosing Cylinder Drift
Therefore, while the root cause of the problem in both examples is the leaking piston seal, the physics is fundamentally different from the general belief. And if the theory 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.