A pile-driving contractor reported that a Terex RT 555 55-ton rough terrain crane had developed a progressive hoist fault over a three-week period. When the operator hoisted a heavy load and stopped the hoist function mid-air, the load would begin to drift downward slowly - approximately 4 centimeters over 30 seconds - requiring the operator to "feather" the hoist lever upward continuously to maintain load position. During hoisting operations, the crane's cab display would intermittently illuminate a "Hoist Brake" warning, and the hoist motor's case was noticeably hotter than normal after extended lift cycles.
The technician first understood the RT 555's hoist architecture. The hoist winch uses a "Planetary Hoist Drive" - a hydraulic motor coupled to a planetary gearbox whose output drum winds the hoist rope. Mounted on the hoist motor's output shaft is a "Spring-Applied, Hydraulically Released Brake" - a multi-disc brake that is held engaged by internal springs when hydraulic pressure is absent (holding the load in position) and is released by hydraulic pressure when the operator commands hoisting. When the operator commands hoist, hydraulic pressure enters the brake-release piston, compressing the springs and disengaging the brake, allowing the motor to rotate the drum. When the hoist command ceases, hydraulic pressure is vented, and the springs re-engage the brake, locking the drum.
He verified the symptom by hoisting a moderate test load to mid-height, stopping the hoist function, and monitoring load position over 5 minutes. The load drifted downward by 40 centimeters - far beyond the 1-centimeter allowable limit, confirming the brake was not fully re-engaging when the hoist command ceased. He then connected the crane's diagnostic display and retrieved a "Hoist Brake Pressure Fault" code with a secondary "Hoist Motor Overtemperature" warning.
He removed the hoist motor's brake housing cover and inspected the "Hoist Brake Friction Discs." The disc stack showed severe wear - the friction material on several discs was worn down to the steel backing, and one disc's friction material had broken off in fragments that had migrated into the brake's oil sump. The worn disc stack had reduced the brake's available clamping travel, requiring the brake-release piston to travel further than designed to fully disengage the brake. He then inspected the "Brake-Release Piston Seal" - the hydraulic seal that allows hydraulic pressure to enter the piston chamber and compress the springs. The seal showed visible wear and a portion of the sealing lip had separated, allowing hydraulic oil to leak past the piston - meaning the brake was not being fully released during hoisting, causing the brake to drag, the hoist motor to overheat, and the hoist to lose speed under load.
The root cause was traced to brake-disc wear compounded by seal degradation. The hoist brake's friction discs had worn naturally over extended service (the machine had approximately 7,200 hours), but the worn disc stack had increased the brake-release piston's travel, placing additional stress on the piston seal and accelerating its failure. The failed seal had then allowed brake drag during hoisting, generating additional heat and accelerating disc wear further - a mutually reinforcing failure pattern.
The repair involved replacing the complete hoist brake friction disc set with new discs (along with new separator plates), replacing the brake-release piston seal with a new unit, thoroughly flushing the brake housing to remove all friction-material debris and contaminated oil, and refilling with the specified brake oil. The hoist motor's case was inspected for internal wear (no damage found), and the hoist gearbox's gear oil was replaced. After reassembly and bleeding, the hoist held a moderate test load with no measurable drift over a 10-minute period, the "Hoist Brake" warning did not illuminate during subsequent operation, and the hoist motor's case temperature remained within specification across a full loaded lift cycle. Hoist brake friction-disc inspection was added to the crane's annual inspection checklist.
