Fault Phenomenon: SANY SR285 rotary drilling rig when suspending a heavy load, the main winch wire rope automatically slides down (load dropping).

Troubleshooting Process:
– Checked brake: main winch brake pads worn excessively.
– Checked brake valve: counterbalance valve internal leakage or stuck.
– Checked hydraulic motor: motor internal leakage.
– Checked wire rope: wire rope spooling tangled.

Root Cause: The main winch counterbalance valve spool is stuck partially open by debris, unable to lock the high-pressure oil, causing cylinder movement before the brake engages.

Required Parts: Clean or replace main winch counterbalance valve assembly.