If there is one failure on an excavator that will absolutely ruin a mechanic's week, it's a leaking upper swing bearing center joint. Every excavator has a giant cast-iron doughnut sitting between the upper house (where the operator sits) and the lower frame (the tracks). Inside this doughnut are a series of rotating seals. These seals keep two completely separate worlds apart: the high-pressure hydraulic oil up top, and the heavy gear oil that lubricates the swing gear and the travel gearboxes down below.
When those rotating seals fail-usually due to a lack of grease in the center joint, which allows water and dirt to eat away the seal lips-the two fluids mix. This is an absolute disaster that you must catch early. If high-pressure hydraulic oil leaks down into the gear cases, the gear cases will over-pressurize. The first sign is usually the travel motor axle seals blowing out, dumping thick, milky gear oil all over the tracks.
But it can go the other way, and it's much worse. If the swing gear oil level gets too high, or if the machine sits parked on a steep slope, the thick gear oil can be forced up past the ruined seals and into the hydraulic tank. Hydraulic pumps are designed to pump light, thin oil. When they suck up thick, 80W-90 gear oil, the pump cavitates violently and destroys itself in a matter of minutes.
How do you check this without pulling the machine apart? Look at the hydraulic tank sight glass. Does the oil look unusually thick, or does it have a strange greenish-brown tint? Look at the swing gear oil sight glass. Does it look unusually thin and frothy? And the absolute dead giveaway: pull the dust cap off the center joint grease fitting. If thick, heavy gear oil or milky sludge oozes out of the grease nipple instead of clean grease, the center joint seals are completely gone. Shut the machine down immediately and schedule the joint reseal. Ignoring it will result in replacing the main hydraulic pump, both swing motors, and both travel motors.
