An electric brake is a friction brake whose engagement is controlled electrically — typically a spring clamps a friction disc against an armature plate, and energising a coil pulls the armature back to release the brake. You see this exact arrangement on the rear of almost every servo motor used in robotic arms, including the Fanuc M-710iC and ABB IRB 6700. Cut the coil voltage and the spring re-clamps in 20–80 ms, holding the shaft against gravity or runaway loads. That fail-safe behaviour is why every elevator, crane hoist, and vertical-axis CNC depends on one.
Electric Brake Interactive Calculator
Vary clamp force, friction, radius, friction faces, and air gap to see brake holding torque and pull-in margin.
Equation Used
This calculator estimates the static holding torque of a spring-applied electric brake. The clamp force from the springs is multiplied by the friction coefficient, the mean friction radius, and the number of active friction faces. The air-gap output compares the entered gap with the article's typical 0.6 mm pull-in limit.
- Uniform clamp load across the active friction faces.
- Mean friction radius is supplied directly by the user.
- Pull-in margin uses the article's typical 0.6 mm maximum air-gap threshold.
- Worn torque uses the article's midrange 70% torque retention after lining wear.
Operating Principle of the Electric Brake
The mechanism is mechanically simple, but the details decide whether it lasts 10 million cycles or burns out in a year. A coil sits inside a steel housing. A friction disc — usually a sintered or organic lining bonded to a steel hub — slides on a splined motor shaft. Behind it sits an armature plate held off the disc by 6–12 compression springs. Cut power to the coil and the springs slam the armature into the disc, clamping it against a fixed wear plate. Energise the coil with the rated DC voltage (commonly 24 V, 90 V, or 103 V DC) and the magnetic flux pulls the armature back across an air gap of around 0.2–0.4 mm, freeing the shaft.
That air gap is the single most important number to watch. As the friction lining wears, the gap grows. Past about 0.6 mm on a typical industrial brake the coil can no longer pull the armature in cleanly, and you get drag — the brake stays partially engaged, the motor pulls extra current, the lining cooks, and the gap grows faster. This is the classic failure mode and it kills brakes long before the friction material itself is consumed. Most quality brakes, like the Kendrion 76 series or Mayr ROBA-stop, give you a wear-adjustment screw or auto-wear-compensating springs to bring the gap back into spec.
Residual magnetism is the other gotcha. After the coil de-energises, a small amount of flux can stick the armature against the pole face for a few extra milliseconds, delaying brake engagement. On a vertical-axis servo holding a 50 kg payload, those milliseconds let the load drop 5–10 mm before the brake catches. Quality brakes use a thin non-magnetic shim (typically 0.1 mm brass or stainless) to break that residual hold and guarantee a clean release of the armature when power drops.
Key Components
- Brake coil: A wound copper solenoid moulded into a steel pole housing, usually rated 24 V or 103 V DC with an inrush current of 0.5–3 A depending on size. When energised it generates the flux that pulls the armature plate back against the springs. Insulation class F (155 °C) is the practical minimum for industrial use.
- Armature plate: A flat steel disc, typically 4–8 mm thick, that travels axially across the air gap. The face that contacts the friction lining must be flat to within 0.05 mm or you get uneven wear and chatter on engagement. It is the single moving part during a brake cycle.
- Friction disc (rotor): A splined hub carrying a sintered or organic friction lining, bonded or riveted on both faces. Coefficient of friction sits between 0.35 and 0.45. The lining is consumable — typical service life is 5–20 million cycles depending on inertia per stop.
- Compression springs: Six to twelve helical springs that supply the clamping force when the coil is de-energised. They produce 60–80% of nominal torque even after the lining wears 1 mm, which is why spring-applied brakes are classed as fail-safe.
- Wear plate (mounting flange): The fixed steel surface the friction disc clamps against, machined to the same flatness spec as the armature. Often integral to the motor end-bell on servo brakes, or a separate bolted ring on industrial gearmotor brakes.
- Manual release lever: An optional hand lever or pull-cable that mechanically retracts the armature without coil power. Required by code on elevator and theatre-rigging brakes so a stuck car or batten can be lowered during a power outage.
- Anti-residual shim: A 0.1–0.2 mm non-magnetic spacer (brass, stainless, or epoxy) bonded to the armature face. It prevents the armature sticking to the pole face after de-energisation, keeping engagement time under 30 ms on a properly built servo brake.
Who Uses the Electric Brake
Electric brakes show up wherever a motor shaft has to hold a load when power drops — gravity loads, runaway inertia, or safety-critical positioning. The category splits into two big families: spring-applied power-off brakes (the fail-safe type described above) and power-on brakes (which engage when energised, used in tensioning and clutching applications). Industrial servo and gearmotor brakes are nearly all the spring-applied type because code, common sense, and insurance underwriters all demand the load stays put when the contactor opens.
- Industrial robotics: Fanuc, KUKA, and ABB six-axis robots fit a Kendrion or Mayr power-off brake on every joint motor — typically axes 2 and 3 carry the largest brakes because they hold the arm against gravity when the controller estops.
- Elevators and lifts: Otis Gen2 and KONE MonoSpace machine-room-less elevators use a dual-circuit drum brake on the gearless traction motor, with two independent coils and spring sets so a single coil failure cannot drop the car.
- Overhead cranes and hoists: Demag DR-Pro wire-rope hoists fit a spring-applied disc brake directly on the motor's non-drive end shaft, sized to 175% of motor rated torque per FEM 9.683.
- Wind turbines: Yaw and pitch drives on Vestas V90 and Siemens SWT-3.6 turbines use multi-disc spring-applied brakes from Stromag or Antec, holding the nacelle yaw position against gusts up to 35 m/s when the yaw motors are off.
- Theatrical rigging: JR Clancy PowerLift and ETC Prodigy hoists in venues like the Sydney Opera House use double-redundant spring-applied brakes — one on each end of the gearbox — because a falling batten over a stage is unacceptable.
- CNC machine tools: Vertical-axis ball-screw drives on Haas VF series and DMG Mori NHX machining centres fit an integral servo brake on the Z-axis motor to prevent the spindle head dropping when the drive de-energises.
- Medical imaging: Siemens and GE C-arm fluoroscopy units use small 24 V DC power-off brakes on each rotational axis so the radiographer can position the arm by hand and have it stay put when released.
The Formula Behind the Electric Brake
The core sizing question is always the same — how much torque does the brake need to produce to stop and hold the load. The clamping force from the springs, multiplied by the friction coefficient, multiplied by the effective friction radius, gives you the static holding torque. At the low end of the typical operating range — say a small 0.4 Nm servo brake on a robot wrist — the friction radius is tiny and clamping force dominates the design. At the high end — a 5,000 Nm crane hoist brake — you need multiple discs in series because a single-disc geometry would have to grow impractically large. The sweet spot for most industrial gearmotor brakes sits between 10 and 200 Nm with a single friction disc and six springs.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Tbrake | Static holding torque produced by the brake | N·m | lb·ft |
| n | Number of friction surfaces (1 for single-disc, 2 for double-faced disc) | — | — |
| μ | Coefficient of friction between lining and steel (typically 0.35–0.45) | — | — |
| Fspring | Total axial clamping force from all springs combined | N | lbf |
| rm | Mean friction radius — average of inner and outer lining radius | m | ft |
Worked Example: Electric Brake in a vertical-axis pick-and-place gantry brake
A packaging line integrator in Parma is sizing the holding brake for the vertical Z-axis of a 3-axis pick-and-place gantry handling 8 kg cheese wheels. The Z-axis uses a 16 mm pitch ball screw driven by a 750 W Mitsubishi HG-KR servo with an integral spring-applied brake. The total reflected load on the motor shaft (payload plus carriage plus screw inertia) demands a holding torque of 2.4 Nm with a 1.5× safety factor. The brake has 2 friction surfaces (double-faced disc), μ = 0.4, mean friction radius rm = 22 mm, and they need to verify the catalogue rating of 4.0 Nm covers them at the nominal, low-wear, and end-of-life conditions.
Given
- n = 2 surfaces
- μ = 0.4 —
- Fspring,nom = 230 N
- rm = 0.022 m
- Trequired = 2.4 N·m
Solution
Step 1 — compute nominal holding torque with fresh springs at full preload, 230 N total clamping force:
That clears the 2.4 Nm requirement with a margin of 1.69× — the published catalogue rating of 4.0 Nm matches our calculation, so the brake stops the loaded carriage cleanly and holds it in place when the contactor opens.
Step 2 — at the low end of normal operation, springs settle in after the first few thousand cycles and lose roughly 8% of preload. Fspring drops to about 212 N:
Still 1.55× the requirement. The carriage holds rock-solid and you would not see any droop on a dial indicator clamped to the head.
Step 3 — at end-of-life, the lining has worn 0.8 mm, the air gap has grown, and spring preload has fallen further to roughly 180 N. The friction coefficient also drifts down to about 0.35 as the lining glazes:
This is the moment to trigger maintenance — you are at 1.15× the requirement, below the 1.5× safety factor the integrator specified. Push past this point and a sudden estop with a full cheese wheel on the gripper can let the load creep 2–3 mm before the brake fully clamps, which is enough to mash a wheel against the conveyor edge.
Result
The brake produces 4. 05 Nm of holding torque at nominal condition, comfortably above the 2.4 Nm required. In practice that means the Z-axis carriage stops within 15–25 ms of the estop signal and a dial indicator on the gripper reads zero droop under the 8 kg cheese wheel. Across the operating range the torque falls from 4.05 Nm fresh, to 3.73 Nm after spring settling, down to 2.77 Nm at end-of-life — the sweet spot is the first 70% of lining life where torque stays above 3.5 Nm. If you measure less than 3.5 Nm on a freshly installed brake, the most likely causes are: (1) air gap set above 0.4 mm at install so the armature never fully releases the springs, (2) oil contamination on the friction lining cutting μ from 0.4 to below 0.2 — common when a leaking gearbox seal sits directly above the brake, or (3) wrong coil voltage applied during commissioning, leaving the brake partially energised and chewing the lining within hours.
Electric Brake vs Alternatives
The spring-applied electromagnetic friction brake is the default choice for fail-safe motor braking, but it is not the only option. Permanent-magnet brakes give cleaner release but cost more, and dynamic (regenerative) braking handles deceleration without any friction parts but cannot hold a static load. Pick the wrong type and you either over-spend or end up with a load that drops on power loss.
| Property | Spring-applied EM brake | Permanent-magnet brake | Dynamic (regenerative) braking |
|---|---|---|---|
| Holding torque range | 0.3 to 5,000 N·m | 0.1 to 200 N·m | Zero static holding torque |
| Engagement time (power-off) | 20–80 ms | 5–20 ms | N/A — does not engage at zero speed |
| Fail-safe on power loss | Yes — springs apply | Yes — magnets apply | No — load coasts or falls |
| Service life (cycles) | 5–20 million | 10–50 million | Effectively unlimited (no friction wear) |
| Maintenance interval | Air gap check every 1–2M cycles | Largely maintenance-free | None — purely electrical |
| Relative cost (per N·m) | Low — baseline | 2–4× baseline | Built into drive — near zero added cost |
| Typical application | Servo holding, hoists, elevators | High-cycle robotics, medical | Conveyor stops, EV regen, traction |
Frequently Asked Questions About Electric Brake
This is almost always the air gap creeping above the release threshold as the lining wears. Most servo brakes spec a fresh gap around 0.2 mm, with a maximum allowable gap of roughly 0.5–0.6 mm. Once you cross that limit, the coil flux can no longer fully retract the armature, the brake drags, and the drive sees torque pulses that look like a holding fault.
Pop the brake cover, slide a feeler gauge between the armature and the pole face with the coil energised, and compare to the manufacturer spec. If you are above the wear limit, either back-adjust the wear screws (Mayr, Kendrion, Stromag all provide them) or replace the brake module — most servo brakes are not user-rebuildable.
You can, but you will burn through the lining fast. Spring-applied brakes are sized primarily for static holding plus emergency stops — typically rated for a maximum of 1,000 to 10,000 dynamic engagements per million cycles. Use one as a service brake for normal stops and the lining wears out in months instead of years.
The rule of thumb: if the brake sees more than one dynamic stop every 10 cycles, you have specified the wrong tool. Use regenerative braking through the drive for normal decelerations and reserve the friction brake for holding and estop only.
1:1 is the bare minimum and only works for purely horizontal applications with no inertia. For vertical-axis or hoisting duty, the standard is 1.75× to 2.5× motor rated torque, and elevator/crane codes (FEM 9.683, EN 81-20) explicitly require at least 1.75× the static load torque on each independent brake circuit.
The reason is not the static load itself — it is the dynamic peak when the brake clamps onto a still-rotating shaft. Add the inertia-deceleration torque to the load-holding torque and you frequently see 2× the motor nameplate before you have any safety margin.
Hot coils have higher resistance — typically 30–40% more at 130 °C than at 20 °C — which drops the current and the magnetic pull. If the supply voltage is already at the low end of tolerance, the hot coil cannot generate enough flux to fully retract the armature, and you get partial drag.
Check the actual DC voltage at the coil terminals while the brake is hot. If it sags below 90% of rated voltage, you either need a stiffer supply, a forced-cooling fan on the brake, or a brake rectifier with overexcitation (initially boosting voltage to 1.5× rated for the first 100 ms to guarantee fast release before settling to holding voltage).
Dual, every time, and almost every safety code mandates it. EN 81-20 for passenger elevators, ASME B30.16 for hoists, and most theatrical rigging standards require two independent brake circuits — separate coils, separate springs, separate friction surfaces — sized so either one alone can hold the rated load.
The probability of a single brake failure during the design life is small but not zero. The probability of two independent brakes failing simultaneously is the product of the two — effectively negligible. That is the entire safety case behind dual redundancy, and it is non-negotiable above people.
Some click is normal — the armature is a steel disc slamming into another steel surface under spring force, and you cannot make that silent. But a sharp bang loud enough to startle someone usually means the air gap is too large, the armature is travelling too far before contact, and the impact velocity is excessive.
The fix is the same as for the holding-fault issue: check and reset the air gap to the manufacturer's fresh-install spec. A correctly gapped brake makes a soft thud, not a bang. Persistently loud engagement also accelerates lining cracking and bond failure, so do not ignore it.
Static torque is what the brake holds when the shaft is stopped. Dynamic torque is what it produces while the shaft is still spinning during engagement, and it is typically 60–75% of the static value because the friction coefficient drops once you transition from static to kinetic friction.
Size your stopping calculations (energy, time-to-stop) against the dynamic value. Size your holding calculations (gravity load, runaway prevention) against the static value. Mixing them up is one of the most common sizing mistakes — you end up with a brake that holds fine but cannot stop a moving load in the required distance.
References & Further Reading
- Wikipedia contributors. Electromagnetic brake. Wikipedia
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