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Helpful Technical Tips From the President of Applied Motion Technologies


TECHNICAL TIPS FOR JANUARY 2009

Cylinder failure & burnt oil – —

We hear about this problem several times each year. The complaint usually centers on a premature failure of a cylinder. Mechanics replacing the cylinder observe oil coming out of the cylinder that is black in color. The oil has a ‘burnt’ odor.

This cylinder failure can occur over time because of cavitation during cylinder retraction and/or while the cylinder is decelerating to a stop in the retracting direction. There are a number of scenarios that could cause a vacuum to develop within the cylinder’s head end. Without elaboration, sources of cavitation include rapid deceleration with meter-in flow controls or bore side port reliefs without make up check valves on the cylinder’s rod side (head end).

Cylinder rod seals are designed to keep hydraulic fluid inside the cylinder but may not work very well when the cylinder’s head end develops a deep vacuum. Then, the rod seal can give way and let air enter the cylinder to replace the vacuum. During the next cylinder retracting cycle when the cylinder head end pressure is high, combustion can occur within the cylinder. This is sometimes called Dieseling. The components for combustion are all there – fuel (oil based hydraulic fluid), air and compression (pressure) sufficient for combustion. Over time a substantial portion of the hydraulic fluid is reduced to carbon – “char”. The black color confirms this. This is not explosive – There is not enough air.

The problem can be aggravated when the cylinder’s displaced rod side volume is less than the pipe volume between the cylinder’s rod side port and the directional control valve’s working port.

The hydraulic circuit should be reviewed with the possibility of modification so that vacuum cavitation (Dieseling) does not occur. This is usually requires the installation of an anti-cavitation check valve between the reservoir and the cylinder’s head end (rod end) port.