A general construction contractor reported that a Komatsu PC490-11 excavator chattered (jumped erratically) when slewing, and when the swing lever was released, the upper structure continued to coast for a long distance before stopping. Furthermore, the machine would slowly drift when parked on a slope.
The technician first checked the pilot pressure and the main swing relief valve, both of which were within specification. He then focused on the "Swing Motor Mechanical Brake." This excavator uses a spring-applied, hydraulically released multi-disc brake. He disconnected the brake release hose and tested the pilot pressure to the brake cylinder; it was 35 bar, sufficient to release the brake.
He disassembled the swing motor's "Brake Control Valve Block." He found the "Brake Relief Valve" (a small cartridge valve that protects the brake circuit from pressure spikes when the swing is stopped suddenly) was stuck in the open position. The internal poppet had worn a groove into its seat. Because this valve was permanently bypassing, the brake release pressure was bleeding off too quickly during operation. This caused the brake discs to drag continuously against the housing, glazing the friction material. The glazed discs could not hold the machine on a slope, and during slewing, the intermittent dragging caused the violent chattering.
The repair involved replacing the brake relief valve cartridge and the entire set of multi-disc brake plates, as the glazing could not be machined out. The swing motor case drain line was flushed to remove burnt friction material. After reassembly, the swing stopped crisply within 1 meter of lever release, and the drift on slopes was eliminated.
