When an operator slews a 20-ton excavator house and lets go of the joystick, the machine must stop immediately. The swing brake clamps down, and the massive momentum of the house is absorbed by the brake plates. But what happens to the hydraulic oil inside the swing motor during that sudden stop? That is governed by the anti-cavitation check valve, and if it fails, it destroys the swing motor in hours.
When the brake locks, the rotating group inside the motor wants to keep spinning due to inertia. This creates a massive vacuum (cavitation) on the trailing side of the pistons, and an extreme pressure spike on the leading side. To prevent the pistons from exploding or cavitating, the swing motor has an anti-cavitation check valve. The moment a vacuum occurs, the check valve pops open, sucking oil from the tank or case drain to fill the void, while a cross-port relief valve bleeds off the pressure spike.
If this check valve sticks closed-usually from varnish or debris-the motor cavitates violently every time it stops. The operator will hear a distinct, deep "growl" or "groan" from the swing gear as microscopic vapor bubbles collapse and tear the bronze valve plate away from the piston shoes. If the check valve sticks open, the swing motor will never hold pressure, and the house will freewheel past the brake engagement point, swinging wildly. If your swing motor sounds like it's grinding rocks on the stop, do not keep running it. Pull the check valve, clean the spool, and lap the seat before the cavitation eats the rotating group alive.
