A hoist brake that failed to hold a load. The hoist lifted a 500 kg load at 0.5 m/s. The brake was a spring-applied, electrically released disc brake, rated at 100 N·m. When the motor stopped, the load dropped 50 mm before the brake grabbed. The customer thought the brake was undersized. It was rated for the load. The issue was the brake was sized for holding torque, not dynamic braking torque. This is about hoist brake sizing and the difference between static and dynamic requirements.
The holding torque
The brake must hold the load when stationary. The required holding torque:
T_hold = m · g · r_drum / η
Where m is the load (kg), g is 9.81, r_drum is the drum radius (m), and η is the gearbox efficiency. For 500 kg, drum radius 0.1 m, gearbox ratio 50:1 (reflected to motor), efficiency 0.8: T_hold = 500 × 9.81 × 0.1 / (50 × 0.8) = 490.5 / 40 = 12.3 N·m. The 100 N·m brake has 8x margin. It holds the load fine when stationary.
The dynamic braking torque
When the load is moving and the motor stops, the brake must decelerate the load before it slips. The kinetic energy must be absorbed by the brake. The dynamic torque required to stop in a given distance:
T_dyn = T_hold + (m · v²) / (2 · s · ω_motor)
Where v is the speed (m/s), s is the stopping distance (m), and ω_motor is the motor speed (rad/s). For v=0.5 m/s, s=0.05 m (50 mm drop), motor speed 1500 RPM = 157 rad/s: T_dyn = 12.3 + (500 × 0.25) / (2 × 0.05 × 157 × 50/1)… wait, let me recalculate. The reflected inertia to the motor: J_load = m × r² / ratio² = 500 × 0.01 / 2500 = 0.002 kg·m². The motor rotor inertia is about 0.0005 kg·m². Total J = 0.0025 kg·m². The deceleration needed to stop in 50 mm at 0.5 m/s: α = v² / (2s × ratio) = 0.25 / (2 × 0.05 × 50) = 0.05 rad/s². Wait, that’s wrong. Linear deceleration: a = v²/(2s) = 0.25/0.1 = 2.5 m/s². Angular deceleration at the motor: α = a × ratio / r = 2.5 × 50 / 0.1 = 1250 rad/s². T_dyn = J × α = 0.0025 × 1250 = 3.1 N·m. That’s small. But the brake must also overcome the holding torque during deceleration. Total: 12.3 + 3.1 = 15.4 N·m. The 100 N·m brake is plenty.
So why did the load drop 50 mm? Because the brake didn’t apply instantly. The spring-applied brake takes about 0.1 seconds to fully engage (the armature has to travel 1 mm). During that 0.1 seconds, the load accelerates under gravity. The distance dropped: s = 0.5 × g × t² = 0.5 × 9.81 × 0.01 = 0.049 m = 49 mm. That matches the observed 50 mm drop. The brake wasn’t undersized — it was slow to engage.
What was changed
1. Used a fail-safe brake with zero backlash. The original brake had a 1 mm air gap (armature to disc). The gap meant the brake took 0.1 seconds to apply. The gap was reduced to 0.3 mm. The application time dropped to 0.03 seconds. The load dropped 5 mm instead of 50 mm. Acceptable for the hoist application.
2. Added a load-holding valve. For vertical loads, a mechanical load-holding valve (counterbalance valve) in the hydraulic or pneumatic circuit holds the load immediately. The valve closes when flow stops. The load can’t drop. For electric hoists, a motor brake with zero clearance is the equivalent. The counterbalance valve is the standard for hydraulic hoists.
3. Specified a safety factor of 2 on dynamic torque. The 100 N·m brake was 8x over the holding torque. But the dynamic application time was the issue. For critical hoists, the brake should engage in under 0.05 seconds. This requires a small air gap, high-force springs, and a direct-acting design. The brake cost 2x more but the drop distance is under 5 mm.
The brake safety standard
Hoist brakes must meet EN 60204-1 or similar: the brake must hold 1.5x the rated load without slipping. For a 500 kg load, the brake must hold 750 kg. The 100 N·m brake holds 4000 kg (reflected). The safety factor is 8x. The brake is safe. The drop distance is a comfort issue, not a safety issue. For people-lifting hoists, the drop distance must be under 50 mm. For material hoists, 200 mm is acceptable.
The brake sizing I use: 2x safety factor on holding torque, but check the application time. The dropping load wasn’t an undersized brake — it was a 1 mm air gap that took 0.1 seconds to close. Reduce the gap to 0.3 mm for fast engagement. For critical hoists, add a mechanical load-holding valve. Safety factor on torque is standard; application time is often overlooked.
