A construction crew reported that a Komatsu PC200-8M0 excavator's upper structure was swinging much slower than normal, and a "whining" noise was heard from the swing motor area. The machine had over 8,000 hours of operation.
The technician tested the swing speed (should be 12-14 seconds for a 90-degree turn). It took 22 seconds. He checked the pilot pressure to the swing brake release valve, which was normal (35 bar). He then connected a flow meter to the swing motor supply line. The flow was adequate, but the pressure drop across the motor was higher than spec.
He removed the swing motor (a piston-type motor) and disassembled it. He found that the "Piston Shoes" (the slippers that ride on the swash plate) had severe fretting wear. The hydraulic oil had not been changed recently and was contaminated with fine silica dust. This dust acted as an abrasive, wearing down the bronze shoes.
The wear increased the internal leakage (slippage) inside the motor. Instead of pushing the cylinder block to rotate, the oil leaked past the shoes into the case drain. This caused the speed loss. The "whining" noise was caused by "cavitation" (air bubbles collapsing) because the worn shoes created turbulence in the suction chamber.
The repair involved replacing the entire rotating group (pistons, shoes, cylinder block, and swash plate) with a genuine Komatsu rebuild kit. The hydraulic tank was drained and flushed. The swing gearbox oil (gear oil) was also changed, as the swing bearing seal had weeped slightly. After reassembly, the swing speed returned to spec, and the whining noise ceased.
