An excavator was brought in with a catastrophic failure. The operator swung the house, heard a loud bang, and 50 gallons of hydraulic oil sprayed all over the side of the building. The mechanical shaft seal on the swing motor had blown completely out of its housing.
The shop replaced the swing motor, assuming the seal had aged and failed. Two days later, the new motor blew its seal identically. We traced the hydraulic schematic. Inside a piston motor, a small amount of oil intentionally bypasses the pistons to lubricate the internal bearings. This "case drain" oil must be routed back to the tank through a dedicated, low-pressure return line.
We disconnected the case drain hose at the tank and blew compressed air through it. It was completely plugged with a chunk of Teflon seal debris. With the drain blocked, the internal bypass oil had nowhere to go. The pressure inside the motor housing spiked from a normal 50 PSI to over 3,000 PSI. The shaft seal-designed to hold 50 PSI-was hydraulically extruded out of its groove, blowing the motor apart from the inside out. Clearing the case drain line and installing a 200-psi burst disc on the motor housing prevented the third explosion.
