When a final drive planetary gearbox fails catastrophically, the iron shavings usually destroy the hydraulic travel motor attached to it. Replacing the assembly is a heavy, expensive job. But many mechanics make a critical, machine-killing mistake when installing the new unit: they bolt it on, fill the planetary hub with gear oil, connect the hydraulic hoses, and immediately fire up the machine to test the tracks.
The travel motor sits inside the planetary hub, but it has its own separate hydraulic circuit and an internal case drain that must be constantly flushed with clean hydraulic oil for lubrication and cooling. When you bolt on a brand-new travel motor, the case drain cavity and the motor bearings are completely dry. If you start the engine and drive the track, the main hydraulic pump sends high-pressure oil to the motor, which starts spinning at 2,000 RPM. But because the case drain line is dry and full of air, it takes several seconds for the fluid to push the air out and establish a flow.
During those few seconds, the motor bearings are spinning at high speed with zero lubrication, instantly scoring the internal surfaces and destroying the new $3,000 motor. The correct procedure is critical: before you connect the high-pressure supply lines to the motor, crack open the case drain line fitting at the swivel joint. Crank the engine and let the hydraulic pump idle, pushing oil through the case drain line until clean, air-free oil weeps out of the loose fitting. This ensures the motor is fully primed with lubrication before it ever turns over. Only then should you tighten the fitting and connect the supply hoses.
