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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.
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