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Frequently Asked Questions
What is the difference between a stopping brake and a holding brake?
One absorbs energy; the other resists a torque. They are sized on different numbers and confusing them is the most common specification error.
A stopping brake decelerates a moving mass. The energy it must absorb is fixed by the inertia and the square of the speed, and it all becomes heat in the friction faces within seconds. The limiting factor is usually thermal capacity, especially with frequent stops.
A holding brake keeps a stationary load stationary. There is no motion, so no energy is dissipated and no heat is generated - but the brake must produce enough static torque to resist whatever is trying to turn the shaft, indefinitely, and often with a safety factor set by a standard.
Many applications need both, in which case the brake must satisfy the thermal requirement of stopping and the static torque requirement of holding. Sizing on one alone gives a brake that either overheats or creeps.
How is the energy per stop calculated?
From the inertia being stopped and the speed it is stopped from - and the speed term dominates because it is squared.
The kinetic energy of a rotating mass depends on its moment of inertia and on the square of its angular velocity. Double the speed and the energy the brake must absorb goes up four times. That is why an application that seems modest can present a very large thermal load if it runs fast.
The inertia must include everything the brake decelerates, referred to the brake shaft - the driven machine, the gearing, the couplings and any load. Where a brake sits on a high-speed shaft ahead of a reducer, the reflected inertia of the load is divided by the square of the ratio, which is often why brakes are fitted there.
Multiply the energy per stop by stops per hour to get the continuous thermal load, and compare it with the brake's rating. This calculation is the selection; torque alone is not.
Should the brake fail applied or fail released?
Applied, wherever a loss of power could allow something dangerous to move - and that decision drives the whole brake choice.
A power-applied brake needs energy to brake. If the supply fails, the brake releases. That is acceptable on a machine where coasting to a stop is harmless, and it suits cycling duty where the brake is applied briefly and often.
A spring-applied brake holds by spring force and needs energy to release. Lose power and it engages. On a hoist, a lift, an inclined conveyor or a vertical axis, that is the only acceptable behaviour, because the alternative is a suspended load falling.
The question to ask is simply: if the power disappears right now, what does this machine do? If the answer involves anything falling, running back or continuing to move dangerously, the brake must be spring-applied.
Many machines carry both - a service brake for stopping and a fail-safe brake for holding.
Where should a brake be fitted in the drive train?
On the high-speed shaft for economy, on the low-speed shaft for safety - and on safety-critical hoisting duty, often both.
A brake on the motor or input shaft sees a fraction of the torque, because torque is multiplied by the gearbox downstream. It can therefore be much smaller and cheaper, and the reflected load inertia is reduced by the square of the ratio, easing the thermal duty too.
But it only controls the load through the gearbox. If a coupling, a shaft or a gear tooth fails between the brake and the load, the brake is no longer connected to what it is supposed to hold.
A brake on the output or drum shaft acts directly on the load and remains effective whatever fails upstream. It must be far larger for the same holding effect.
Lifting applications frequently require a brake on the high-speed shaft for normal duty and a second, independent brake acting on the load for safety - and the governing standard usually specifies this rather than leaving it to judgement.
What friction materials are used and does it matter?
Organic, semi-metallic and sintered materials, and the choice affects torque consistency, temperature tolerance and wear rate.
Organic linings give smooth engagement and quiet operation with modest cost, and suit moderate duty. Their coefficient of friction falls as they get hot - fade - which matters on repeated hard stops.
Semi-metallic materials tolerate more heat and hold their friction coefficient better at temperature, at the cost of more noise and more wear on the mating surface.
Sintered metallic linings handle the highest temperatures and energies, and are used on heavy or high-cycle duty. They are aggressive on the disc or drum and unforgiving of misalignment.
Contamination matters more than grade. Any oil or grease on a dry friction face collapses its torque capacity immediately, and the usual sources are an adjacent leaking bearing seal or over-lubrication during maintenance. A brake that has suddenly lost torque is usually contaminated rather than worn.
How is brake wear monitored?
By lining thickness and by actuator travel, and the second is often the easier measurement.
As the friction material wears, the actuator has to move further to bring the faces into contact. On many brakes there is a visible indicator - a wear scale, a pin, or simply the position of a lever or armature - and increasing travel is a direct proxy for wear.
That matters beyond replacement timing. On an electromagnetic brake, increasing air gap weakens the magnetic pull and slows engagement, so wear degrades performance before it exhausts the lining. Some designs are self-adjusting; those that are not need periodic adjustment to a specified gap.
On safety-critical brakes, wear checking is normally a scheduled inspection with recorded results rather than a look-when-convenient task, and the governing standard may specify the interval.
A brake whose stopping distance has increased is reporting wear, contamination or a lost adjustment, and all three warrant investigation rather than acceptance.
Do industrial brakes need a manual release?
On holding and fail-safe brakes, almost always - because a brake that engages on power loss also traps the load.
A spring-applied brake holds when there is no power. That is exactly what is wanted for safety, and exactly what prevents recovery: if a hoist stops with a load suspended and the supply is gone, nothing can be lowered until the brake can be released.
A manual release - a lever, a hand knob or a release bolt arrangement - lets the brake be opened deliberately so the load can be lowered under control, recovering the machine.
It has to be designed so it cannot be left engaged accidentally, since a brake propped open is no brake at all. Many designs are spring-returning or require continuous manual force for that reason.
On passenger lifting equipment the arrangements are prescribed by standard. Check what the governing standard requires rather than treating manual release as an optional convenience.