SANY SR280RC Main Winch Motor Fracture Analysis

A SANY SR280RC rotary drilling rig working in Yunnan, China suffered a sudden main-winch motor fracture after approximately 930 operating hours. The bore was about 48 m deep and 1 m in diameter. The failure occurred while the drill string was being raised, at approximately 22 m above ground level.

Damage Found During Disassembly

The disassembly revealed several severe failures that occurred at the same instant:

The complete end face of the motor output shaft fractured and separated.
One motor retaining lug fractured at its mounting point.
Three piston ball ends sheared off, while two other piston ball ends showed severe deformation.
Three pistons were forced back into the cylinder block, and one gouged the inner wall.
The center-shaft spherical joint showed no visible wear marks.

SR280RC main winch motor housing, cylinder block, and piston damage

How the Axial-Piston Motor Produces Torque

Pressure oil enters port A or B and passes through the valve plate into the cylinder-block bores. Hydraulic pressure pushes the pistons against the swash plate. The reaction force creates torque around the cylinder-block centerline and turns the output shaft. Reversing the pressure-oil direction reverses shaft rotation.

Axial piston hydraulic motor operating principle diagram

What the Damage Evidence Indicates

The mating surfaces between the cylinder block and valve plate were largely undamaged, and the center-shaft spherical joint showed no damage. These observations indicate that the motor had been operating normally before the event. They also make oil contamination, sustained cavitation, and lubrication loss unlikely as the primary causes.

The sheared piston ball ends and broken retaining lug point to an unusually high torque at the moment of failure. The maximum drive torque available from normal system pressure would not ordinarily be sufficient to cause this pattern of damage, so the report attributed the torque to an external shock load.

The retracted pistons and gouged cylinder-block wall also show that the piston group experienced a high axial force. That force drove the pistons inward, transferred impact through the cylinder block and valve plate, and fractured the weakest axial section at the output-shaft end face.

Probable Failure Sequence

Based on the available evidence, the probable sequence began with drill-string binding or a brief loss of charge oil that allowed the winch motor to overspeed. The resulting rapid deceleration may have sent a severe shock through the reduction gearbox and motor. Torque first overloaded the piston ball ends and retaining lug. At the same time, axial force drove the pistons into the cylinder block and transmitted impact to the output-shaft end face. Because the source report identifies this sequence as an inference, it should not be treated as a proven cause without additional operating data.

Preventive Measures

The source investigation recommended the following measures for affected SR280RC winch systems:

Add a charge-oil check valve to the charge circuit to reduce the risk of oil starvation.
Use the specified ST1505 counterbalance-valve configuration in place of the earlier BVD spool-valve arrangement where the applicable service guidance requires it.
Monitor maximum main-winch pull so abnormal drill-string loads can be identified before they create a damaging shock.

Inspection Takeaway

A fractured winch motor should be evaluated as a system failure rather than an isolated broken component. Inspect the motor, reduction gearbox, charge-oil circuit, counterbalance valve, and winch-load history together. On used rotary drilling rigs, evidence of piston impact, housing fracture, or abnormal winch loading deserves a complete hydraulic and mechanical inspection before the machine returns to service.