BIZOMAT MAKEOS · Power Transmission
Spring-Applied Holding Brake Sizing (Stop Time, Energy and Holding)
Will this spring-applied brake stop and hold the load in time, and can it absorb the stopping energy at this number of stops per hour?
Check of a spring-applied (fail-safe) motor brake: kinetic energy of the inertia at the motor shaft, the net decelerating torque with the load either driving through the stop (lowering, overhauling) or helping it, slip time and total stop time with the engagement delay, stopping angle, friction energy per stop and per hour against the brake’s permissible values, the brake torque needed for the required stop time, and the holding factor on the static load torque.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Speed at the brake | rpm | Speed of the brake shaft when braking starts (usually motor speed). |
| Total inertia at the brake shaft | kg·m² | Motor, brake and load inertia referred to the brake shaft. |
| Load torque at the brake shaft | N·m | Static load torque at the brake shaft (for a hoist, the load torque through the gearing). |
| Load during the stop | choice | Whether the load keeps driving during the stop (overhauling) or helps to stop it. |
| Brake torque (dynamic rating) | N·m | Dynamic braking torque of the brake (from the maker). |
| Engagement delay | s | Time from switching off until the brake torque builds up. |
| Required total stop time | s | Longest acceptable time from signal to standstill. |
| Holding factor required | Ratio of brake torque to static load torque you require for holding. | |
| Permissible energy per stop | kJ | Largest friction energy per stop for the brake (from the maker). |
| Stops per hour | Number of stops per hour. | |
| Permissible energy per hour | kJ | Largest friction energy per hour for the brake (from the maker). |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Speed at the brake
- What it is
- The speed of the brake shaft at the moment braking starts, usually the motor speed.
- Units and format
- Enter a number in rpm: revolutions per minute. The form accepts 0.000001 rpm to 100000 rpm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1450 rpm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Stop time rises in proportion to speed; energy per stop rises with its square.
- Get it right
- Use the speed at the brake, not at the load if there is gearing between them.
What this means: Total inertia at the brake shaft
- What it is
- The total rotating and moving inertia seen at the brake shaft: motor rotor, brake disc, couplings and the load referred through the gearing.
- Units and format
- Enter a number in kg·m²: kilogram square metres (mass moment of inertia). The form accepts 0.000001 kg·m² to 100000 kg·m². Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.05 kg·m² as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Slip time and energy per stop are directly proportional to inertia.
- Get it right
- Load inertia behind a reduction ratio i is divided by i² when referred to the brake shaft.
What this means: Load torque at the brake shaft
- What it is
- The steady torque the load puts on the brake shaft, such as a hanging load acting through the gearing.
- Units and format
- Enter a number in N·m: newton-metres. The form accepts 0 N·m to 10000000 N·m. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 20 N·m as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- When the load drives through the stop it lengthens the stop; it also sets the holding factor.
- Get it right
- Refer the load torque to the brake shaft through the gearing; zero means no static load to hold.
What this means: Load during the stop
- What it is
- Whether the load keeps pushing the shaft round during the stop, as when lowering, or helps to stop it, as when lifting or with friction.
- Units and format
- Choose one of 2 options from the list: Load drives through the stop (lowering, downhill); Load helps the stop (lifting, friction).
- Example
- The form opens with “Load drives through the stop (lowering, downhill)” selected as a worked example. Choose the option that matches your case.
- Effect on the result
- A load that drives through the stop subtracts from the brake torque; one that helps adds to it.
- Get it right
- For a hoist, check the lowering direction: it is the harder case.
What this means: Brake torque (dynamic rating)
- What it is
- The braking torque the brake produces while slipping, its dynamic rating from the brake maker.
- Units and format
- Enter a number in N·m: newton-metres. The form accepts 0.000001 N·m to 10000000 N·m. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 60 N·m as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- More brake torque shortens the stop; it also sets the holding factor.
- Get it right
- Use the dynamic (slipping) torque, not the higher static holding torque some makers quote.
What this means: Engagement delay
- What it is
- The time from switching off until the brake torque builds up, during which the load keeps moving at full speed.
- Units and format
- Enter a number in s: seconds. The form accepts 0 s to 5 s. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.05 s as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Adds directly to the stop time and stopping angle.
- Get it right
- Enter seconds; makers often quote it in milliseconds. It depends on how the coil is switched.
What this means: Required total stop time
- What it is
- The longest acceptable time from the stop signal to standstill, including the engagement delay.
- Units and format
- Enter a number in s: seconds. The form accepts 0.000001 s to 600 s. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.5 s as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- It does not change the stop time, only the check and the torque needed to meet it.
- Get it right
- It must be longer than the engagement delay; take it from the risk assessment or machine standard.
What this means: Holding factor required
- What it is
- How many times the static load torque you require the brake torque to be, so the load is held securely at rest.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 5. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- It does not change the holding factor, only the holding check.
- Get it right
- Take it from the applicable standard or your design rule for the machine.
What this means: Permissible energy per stop
- What it is
- The largest friction energy the brake can absorb in a single stop without damage, from the brake maker.
- Units and format
- Enter a number in kJ: kilojoules. The form accepts 0.000001 kJ to 100000 kJ. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 5 kJ as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- It does not change the energy, only the per-stop check.
- Get it right
- Enter kJ, not J; catalogues often list it in joules.
What this means: Stops per hour
- What it is
- How many braking stops the brake makes in an hour of operation.
- Units and format
- Enter a plain number with no unit. The form accepts 0 to 10000. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 20 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Energy per hour is directly proportional to stops per hour.
- Get it right
- Count dynamic stops only; engaging an already stopped shaft absorbs no friction energy.
What this means: Permissible energy per hour
- What it is
- The largest friction energy the brake can absorb per hour without overheating, from the brake maker.
- Units and format
- Enter a number in kJ: kilojoules. The form accepts 0.000001 kJ to 10000000 kJ. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 100 kJ as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- It does not change the energy, only the per-hour check.
- Get it right
- Enter kJ; check the maker’s figure is per hour and not per minute.
Method and formulas
- ω = 2π n ÷ 60; net torque = T_b − T_L (overhauling) or T_b + T_L (assisting).
- Slip time t_b = J ω ÷ T_net; total stop time = t_d + t_b; stopping angle = ω t_d + ω t_b ÷ 2.
- Friction energy per stop W = T_b ω t_b ÷ 2 (= J ω² ÷ 2 × T_b ÷ T_net); per hour = W × stops.
- Torque for the required stop time = J ω ÷ (t_req − t_d) ± T_L; holding factor = T_b ÷ T_L.
- Example: 60 N·m stops 0.05 kg·m² from 1450 rpm in 0.2398 s, absorbing 0.86462 kJ per stop.
Limitations
- Constant brake torque; the dynamic torque falls as the lining heats and wears.
- Speed is taken as constant during the engagement delay; an overhauling load speeds up a little.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Choose a motor brake for a hoist, conveyor or axis
- Check a brake for more stops per hour
- Estimate how far a load travels after an emergency stop
Who it is for
- Design engineers
- Maintenance engineers
- Power transmission engineers
What you get
- Net decelerating torque and stop time
- Stopping angle
- Energy per stop and per hour, torque for the required time and holding check
Interpreting the result
- For hoists the overhauling case governs: the net torque is the brake torque less the load torque.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 2455–2459. Topic reference; no source tables or recommended settings are reproduced.
Learn the Concepts Behind This Tool
These BIZOMAT Library readings explain the method this tool uses, with worked examples.
Method explained here · BIZOMAT original book
Motor brakes, holding and proof
In Clutches and Brakes by the Numbers
What it covers: How to size a spring-applied motor brake for stop time, distance, energy and holding factor, why a hoist tool differs on a slipway, and how to prove and record a brake.
Method explained here · BIZOMAT original book
Eddy currents, brakes and holding
In Permanent Magnets in Machines by the Numbers
What it covers: How moving magnets induce eddy currents and drag, eddy-current speed control on a gravity conveyor, permanent-magnet holding brakes on a vertical servo axis, and stopping energy limits.
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