Holding loads with power-off brakes

Sept. 27, 2007
Q. What’s the difference between stopping dynamically and holding a load with power-off brakes? How do I correctly size a power-off brake?

Rocco Dragone

A. With dynamic stopping the brake must absorb the kinetic energy built up by the inertial loads. In such instances the brake transfers that energy, causing heat buildup and wear on the surfaces of the rotating components. With static holding, all rotating components come to a rest and the brake simply holds the load. As a result, there’s little wear and no heat buildup.

Most power-off brake applications typically use the brake to hold equipment in place when the motor or drive is de-energized — as when using the parking brake on your car. There can be some dynamic engagement even in applications that need only a holding brake, and most power-off brakes are designed to absorb that energy. For example, if a brake responds in about 100 msec and motor response time is 20 msec, the brake can be dynamically engaged for 80 msec.

To size a brake for dynamic stopping, first estimate the torque needed to stop the system inertia within the available time. At this point the only known parameter is the load inertia. Later, once you’ve chosen a particular brake, you’ll need to account for the inertia of the brake rotor, friction disc, and hub. So a general rule of thumb is to add 25% to the load inertia to estimate the brake rotor inertia.

The equation that estimates the average dynamic torque needed to dynamically stop the load in an interval of time is:

T = (0.1047 (I w) / t ) – D

where 0.1047 = a factor that converts rad/sec to rpm; I = total system inertia, lb-in.-sec″; w = rotational speed, rpm; t = time to zero (time it takes for the rotating shaft to stop), sec; D = any drag torque in the system aiding the required brake torque, lb-in.; and T = average dynamic torque, lb-in.

Multiply the above results by 1.25 to convert them to static torque. (Most brake manufacturer ratings are static torque, which is typically higher than average dynamic torque.) These and other helpful formulas can be found in Danaher Motion’s Thomson Deltran catalog.

You’ll also need to know how much energy the brake must dissipate. We’ll deal with that topic in next month’s column. — Rocco Dragone

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