When an excavator throws a final drive-either the hydraulic motor leaks or the planetary gears explode-there is a strong temptation to replace only the broken side. The other side seems fine, and buying two is a massive hit to the budget. This is one of the most expensive mistakes you can make on an excavator. You must always replace final drives and travel motors in matched pairs.
Over thousands of hours, the internal hydraulic motors wear down. The piston plate and the valve plate develop microscopic grooves, which lowers the volumetric efficiency of the motor. An old, worn motor simply cannot move as much oil as fast as a brand-new motor.
If you put a new motor on the left track and leave the old, worn motor on the right track, the left track will spin faster. The machine will drift hard to the right, and the operator will have to constantly hold the left joystick to compensate, fighting the machine all day. Worse, if the machine has an auto-track straight-travel valve, the ECU will try to equalize the flow to keep the machine straight. It will end up starving the new, fast motor of pressure to match the slow, weak motor, bogging the machine down and generating immense heat. The only way to balance the system is to have two motors with identical internal wear profiles. If you're going to spend the money on one, spend it on both, or send the good motor to a hydraulic rebuilder to have its volumetric flow matched to the new unit.
