If there is one component on an excavator that mechanics love to over-maintain, it's the swing bearing (the massive slew ring that connects the house to the undercarriage). Operators are trained to pump grease into the swing bearing zerk every morning, assuming that if a little grease prevents wear, a lot of grease must be even better. This logic is destroying swing bearings.
The swing bearing has a channel between the internal gear teeth and the bearing balls, separated by thin, factory-installed rubber or plastic labyrinth seals. If a mechanic puts a pneumatic grease gun on the zerk and pumps it twenty times, the grease doesn't just fill the gaps. It builds immense hydraulic pressure. That pressure will physically blow the internal seals outward, or blow them inward into the gear case.
If the outer seal blows, water and silica (sand) will wash into the bearing when it rains, acting as a grinding paste that destroys the hardening on the bearing races. If the inner seal blows, the heavy moly grease migrates into the swing gearbox. Because the swing gearbox only holds a few liters of light gear oil, the influx of thick grease dilutes the oil, causes the gearbox to overheat, and eventually blows the gearbox seals out. The manual for a 20-ton excavator usually specifies only three to five pumps of a manual grease gun per fitting. You should pump slowly until you see clean grease just begin to weep out of the lower seal, and then immediately stop. Over-greasing is not a sign of diligent maintenance; it is a guaranteed way to cause a catastrophic five-figure failure.
