How To Troubleshoot Hydraulic Cylinder Drift

作者：Brendan Casey

A popular misconception abouthydraulic cylinders is that if the piston seal is leaking, the cylinder willdrift down. Fact is, if the piston seal is completely removed from adouble-acting hydraulic cylinder, the cylinder is completely filled with oiland its ports are plugged, the cylinder will hold its load indefinitely –unless the rod-seal leaks.

What happens under theseconditions – due to the unequal volume either side of the piston, is fluid pressureequalizes and the cylinder becomes hydraulically locked. Once this occurs, theonly way the cylinder can move is if fluid escapes from the cylinder via therod seal or its ports.

If you grasp the theory at workhere, you’ll probably realize there are a couple of exceptions. The first is adouble-rod cylinder – where volume is equal on both sides of the piston. Andthe second is when a load is hanging on a double-acting cylinder. In thisarrangement, the volume of pressurized fluid on the rod side can be easilyaccommodated on the piston side.

This is not quite the end of thestory though, but it’s important to at least grasp this theory before we moveon. Notwithstanding the two exceptions mentioned above, if a double-actingcylinder’s service ports are blocked – by a closed-to-actuator or cylinderspool and the piston seal does bypass, pressure will eventually equalize onboth sides of the cylinder. As already stated, at this point a hydraulic lockis effected and no further creep can occur – unless fluid is allowed to escapefrom the cylinder or the cylinder circuit.

But because of the loss ineffective area – due to pressure now acting on the rod-side annulus area, thestatic pressure in the cylinder must increase to support the same load. Forexample, if the load-induced pressure on the piston side of the cylinder was2,000 PSI and zero on the rod side when the DCV closed, assuming no leakagepast the spool, the equalized pressure may be 3,000 PSI – depending on theratio of the areas.

But what if this circuit has aservice port relief valve set at 2,500 PSI? As pressure starts to equalizeacross the piston seal and the increasing static pressure on the piston side ofthe cylinder reaches the cracking pressure of the port relief, the cylinderWILL creep down.

So while the root cause of theproblem IS the leaking piston seal, the physics that applies is fundamentallydifferent to what many people believe. And if you understand the theory, youcan see how the humble pressure gauge can be extremely useful whentroubleshooting cylinder drift.

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How To Troubleshoot Hydraulic Cylinder Drift