Load slipping after brake application indicates insufficient holding torque or unstable friction. Common causes include worn or glazed linings, oil/grease contamination, incorrect air gap/clearance, weak or broken springs (in spring-applied brakes), or misalignment reducing effective contact area. On drum brakes, poor shoe adjustment or uneven contact can reduce torque. On disc brakes, rotor runout or surface damage can cause inconsistent clamp force.
System-level causes also matter. If the brake is engaged before the VFD has stabilized the load at zero speed, the load may move as torque transitions. Gearbox or coupling issues can mimic brake slip, so confirm whether the brake is actually holding the shaft.
Corrective actions: inspect lining thickness and condition, replace contaminated friction materials, verify spring force and actuator stroke, and adjust clearance to OEM spec. Then perform a controlled holding test at rated load. For critical lifts, ensure the brake selection includes adequate safety factor and verified friction material performance over temperature and wear range.