If an excavator sits for a few days and the upper house slowly swings backward down a hill, the standard fix is to tighten the swing brake. But over-tightening the mechanical swing brake is a guaranteed way to destroy the swing motor.
The swing brake on an excavator is a large, spring-applied, hydraulically released multi-disc pack located right on the output shaft of the swing motor. When you pull the swing lever, pilot pressure pushes a piston back, compressing the brake springs and allowing the motor to turn. When you let go of the lever, the pressure drops, and the heavy springs slam the discs together to stop the rotation.
To adjust this brake, there is usually a large locking nut and a set screw on the outside of the brake housing. If a mechanic sets the brake spring preload too tight, the clearance between the steel and friction discs becomes too small. When the operator pulls the swing lever, the hydraulic pressure is high enough to push the piston back, but because the clearance is negligible, the brake discs never fully separate. They drag against each other while the motor is trying to spin.
The swing motor has to work incredibly hard to overcome that dragging friction. Within twenty minutes of continuous swinging, the oil inside the swing motor casing will boil, the motor seals will blow out from the heat, and the motor will seize. The correct way to set a swing brake is to back the adjustment screw completely off until the brake is loose, then slowly turn it in until you feel a slight drag on the discs, and then back it off exactly a quarter or half turn (per the specific OEM manual) to ensure a specific air gap. Never just crank the adjustment nut down until it feels tight. The swing brake is a precision clearance component, not a mechanical lock.
