Braking Torques

Braking Torques

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).

Limiting Speeds

The brake rotor will have a maximum rotational speed limit due to centrifugal loads on the rotating friction material. However thermal limits from the brake engagement will be lower. At high engagement speeds, the frictional energy that appears as heat cannot be conducted away from the surface quickly enough and the result can be surface burning, high wear and a reduction in the transmitted torque (brake fade).

In practice, limiting speeds are rarely a factor in brake selection, and brakes can be used at 2 pole motor speeds. However catalogue rated torques should be derated for engagement at high speeds and the factor can be around 0.7 in extreme cases.

Friction Energy

Spring applied brakes function by converting the kinetic energy of the machine into heat. For high operating frequencies above about 1 per minute, and where the speed and inertia to be stopped are high, the thermal data of the application should be checked. This is particularly important in applications that are safety sensitive, for example the stopping of a gravity driven load in a hoist, crane or lift.