A heavy construction crew reported that a Kobelco SK350LC-8 excavator made a loud "grinding" noise when swinging. The noise was rhythmic and seemed to come from the swing motor area. The operator also noted that the machine felt "sluggish" when starting to swing, as if it were fighting a resistance.
The technician first checked the swing motor oil level and the brake case drain line. The oil from the drain line was metallic and silvery, indicating internal wear. He suspected the "Swing Parking Brake." This brake is a spring-applied, hydraulically released multi-disc brake located inside the swing motor.
He measured the pilot pressure at the brake release port. The specification is 35-40 bar. The gauge showed 20 bar. This was insufficient to fully compress the brake springs. The technician traced the pilot line back to the main control valve. He found a "Shuttle Valve" (also called a logic valve) in the pilot circuit. This valve selects the higher pressure between the left and right travel pilot lines to release the swing brake (so the swing won't turn unless the machine is anchored or the operator intends it).
The shuttle valve spool was stuck in the middle position by a small piece of silicone sealant (likely from a previous repair). This blocked the pilot flow to the swing brake. The brake was only partially releasing, causing the discs to rub and grind, generating the noise and heat.
The repair involved cleaning the shuttle valve bore, installing a new spool, and replacing the entire set of friction discs in the swing motor (which were burnt). The hydraulic filter was changed, and the system was purged of metal particles. After reassembly, the brake released instantly, and the grinding noise disappeared.
