When an excavator loses power to one track-say, the left track moves slowly while the right track spins freely-most mechanics immediately assume the left travel motor or final drive planetary is failing. They start pricing out a $6,000 replacement motor. But before you condemn the motor, you must test the center swivel joint.
The swivel joint (rotary joint) is the heavily armored manifold at the center of the undercarriage that allows high-pressure hydraulic oil to travel from the upper frame (which swings) down to the travel motors (which don't). Inside the joint are a series of precision-machined grooves and O-rings that keep the left travel, right travel, and return lines completely isolated from each other as the house rotates.
Over thousands of hours, the internal O-rings can wear grooves into the aluminum spool. When this happens, high-pressure oil from the right track circuit can bypass internally across the spool and bleed directly into the left track circuit. When the operator pulls the left travel lever, the pump sends oil to the left motor, but the oil is escaping into the right track circuit through the worn swivel joint. The left track barely moves because it's starved of pressure. To diagnose this, you must cap off the hydraulic lines at the travel motors and do a pressure test at the swivel joint. If you block the flow at the motor and the pressure still won't build, the travel motor is fine-the swivel joint is leaking internally. Rebuilding a swivel joint costs a few hundred dollars in seals and machining; replacing a travel motor costs thousands.
