Fault Phenomenon: CAT rotary drilling rig rotary head operates only in a single mode (low speed/high torque or high speed/low torque), unable to continuously vary.

Troubleshooting Process:
– Checked variable mechanism: motor swashplate actuating piston seized.
– Checked control oil pressure: insufficient pilot control pressure.
– Checked feedback rod: mechanical feedback rod detached.
– Checked oil quality: severely contaminated hydraulic fluid.

Root Cause: The swashplate actuator inside the rotary head variable motor is stuck at the limit position due to metal debris in the oil, unable to adjust displacement based on load.

Required Parts: Disassemble and repair rotary head variable motor or replace assembly.