Low Level Leakage Experienced With The New Caterpillar Yellow Rod Seals Caterpillar

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Problem:

Several reports of low level leakage, weepage, or wetness around the new Caterpillar yellow rod seals have been received. This leads to rebuilding the hydraulic cylinders prematurely.

Solution:

This leakage is associated with rod motion, and will not be seen on a rebuild stand without stroking the cylinder. The cause of weepage is the high contaminant exclusion efficiency of the wiper seal. As the rod extends from the cylinder, a very thin film of oil adheres to the rod due to surface tension and fluid shear effects. The thickness of the oil film is a function of the rod velocity, oil viscosity, system pressure, and the lip contact pressure of the u-cup seal.

The new Cat wiper seal was designed to exclude all dirt and other external contaminants. As the rod retracts, the wiper seal scrapes off all the contaminants along with most of the thin oil film on the rod. This oily dirt accumulates around the wiper seal face, and in some cases, in rings near the rod eye. A thin film only 0.00001 inches thick is enough to appear as weepage if scraped off the rod during retraction.

Older style wiper seals allow the oil film back into the cylinder, along with some dirt and other contaminants. Dirt, water, or other external contaminants is detrimental to the long term life of the seal package and other components in the hydraulic system. This type of leakage is largely incidental, and as long as the hydraulic system is not being contaminated (monitor through regular SOS), does not necessarily mean the cylinder should be immediately resealed.

Oil dripping from the cylinder indicates other seal failure mechanisms have occurred, and resealing is probably needed. Several seal modifications to eliminate weepage and oil rings without compromising dirt exclusion efficiency are being investigated, and will be announced at a later date, after validation is completed.

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