A servo-motor holding brake prevents unwanted motion of a stopped load when the drive is deactivated. Manufacturer instructions distinguish this holding function from stopping a moving load; the selected calculator therefore reports only ideal static gravity torque.
A servo brake's rated torque is the easy number. The numbers that decide whether it actually holds are the air gap, the coil voltage at the terminals, and how the friction lining has aged. Specify the rating, then verify the failure modes.
"A servo brake holds a stationary shaft. It does not stop a moving one. Every time the brake is asked to dissipate kinetic energy instead of just clamping, you spend lining life that was sized for ten million dry holds, not ten emergency stops." — Robbie Dickson, FIRGELLI Automations founder and former Rolls-Royce, BMW, and Ford engineer
Servo Holding-Brake Ideal Static-Torque Reference
Explore ideal static gravity torque through a selected reduction without selecting or rating a brake.
Equation Used
- The load is stationary and gravity acts vertically.
- The drum radius is perpendicular to the suspended force.
- The selected reduction is ideal and positive.
- Brake selection, dynamics, efficiency, thermal behavior, wear, controls and safety are excluded.
The selected multiplier is transparent arithmetic, not a prescribed design or safety factor.
Spring-applied holding topology
Spring-applied brakes clamp when de-energized and release electromagnetically. The animation shows the friction members moving symmetrically onto and away from a disk fixed to the motor shaft, followed by an abstract ratio block, drum and suspended load.
The ratio block is not a dimensional gearbox reconstruction, and the animation does not simulate stopping or falling.
What the calculator determines
Select suspended mass, drum radius, an ideal reduction G and a transparent reference multiplier M. The calculator reports gravitational force, load-drum torque, ideal motor/brake-shaft torque and the multiplied reference torque.
The former efficiency direction, catalog-step selection and 50 N·m utilization result are removed.
Ideal static gravity relation
F = mg, Tload = F r, Tshaft,ideal = Tload/G, and Tref = M Tshaft,ideal, with r converted from millimetres to metres.
Selected-input arithmetic
For 20 kg, 50 mm radius, G = 5 and M = 1.5, force is 196.133 N, drum torque 9.807 N·m, ideal brake-shaft torque 1.961 N·m and multiplied reference torque 2.942 N·m.
Model boundary
The selected multiplier is not a prescribed safety factor and the reference torque is not a brake rating. Real selection requires the manufacturer's project-planning method and consideration of transmission direction and efficiency, temperature, brake characteristic values, wear, air gap, voltage, control sequence, diagnostics, permissible emergency stops, redundancy and applicable safety requirements.
What usually goes wrong with a servo brake?
- Air gap drift from lining wear. As the friction disc thins, the gap grows past 0.4 mm. The coil can no longer pull the armature in cleanly — you see doubled release current, release chatter, or the brake never opens and the motor trips on overcurrent.
- Residual magnetism with too-tight gap. If shims are over-removed, the gap closes below the spec window and residual magnetism in the armature drags the disc after de-energisation. A vertical axis sags a few millimetres per minute.
- Coil keep-alive voltage from the drive. Some drives leave a holding voltage on the brake line for fast re-release. The armature sits 50% pulled-in, normal force is halved, and the load drifts overnight.
- Friction lining contamination. Ballscrew grease mist on the lining drops the effective coefficient of friction by 30 to 50%. The rated torque is no longer what the brake actually delivers.
- Thermal contraction at overnight shutdown. As the machine cools, the ballscrew shrinks axially and the lining contracts 5 to 10%. A brake sized at the calculated minimum loses its margin and the head drops.
- Mounting flange distortion. Uneven bolt torque on the motor end-bell warps the reaction face by 0.1 to 0.2 mm. The armature clamps unevenly, the disc squeals at release, and release current rises.
How should you test a servo brake before trusting it?
- Measure coil release current against the datasheet. Doubled current means an enlarged air gap from lining wear or flange distortion. The brake will still hold today, but it is on the way out.
- Feeler-gauge the air gap through the inspection slots. Compare against spec (typically 0.2 to 0.3 mm). If it exceeds spec by more than 0.1 mm, pull a shim or replace the rotor.
- Measure voltage at the brake terminals during release, not at the supply. Long cable runs with undersized conductors can drop 2 to 3 V at 1 A coil current. Below 22 V on a nominal 24 V coil, you risk incomplete release.
- Run an overnight drift test under nominal load. Park the axis, power off the drive, and measure position at the head with a dial indicator 12 hours later. Drift above 0.1 mm on a properly sized brake means the brake never fully engaged, the lining is contaminated, or the gap has drifted.
- Cycle the brake under controlled deceleration, not E-stop. Verify the drive performs the decel and the brake only clamps the stopped shaft. Counting actual dynamic stops over a service interval predicts lining-replacement cadence.
Questions
Can this choose the next catalog brake size?
No.
Can a holding brake stop a moving load?
This model does not calculate that; manufacturer instructions may restrict normal use to holding a stopped motor.
Is M a certified safety factor?
No.
References & Further Reading
- Wikipedia contributors. Electromagnetic brake. Wikipedia
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