Spring applied brakes have catalogue rated torques. These are nominal figures and may require a small amount of running in before full torque is achieved. In particular a holding torque application without any dynamic friction may not achieve the rated torque.
The required brake torque M (Nm) to stop an inertia J (kgm2) from speed n (r/min) in t (seconds) can be approximated with the formula:
M = (JN ÷ 9.55t)
This is only an approximation as there are factors that affect the delivery of torque:
- There are short delay times whilst the energy in the coil decays and the armature moves. These are significant when the required stopping times are short.
- Friction in the machine works with the brake and shortens stopping times
- Gravity loads can either assist the brake or, as is more common, work against the brake and increase stopping times.
Some brake designs have torques that are manually adjustable, usually by rotating a nut that compresses the inner set of springs. Also, brakes can have variable torques by factory setting of the springs. Typical ranges are -50 to +10% with adjuster nuts and -50% to +50% with factory setting (the highest torques suiting holding duties with infrequent dynamic stops).