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

Sponsored Recommendations

Diaphragm Pump Technology Drives Industrial Washers

Jan. 23, 2025
Discover high-performance pumps and systems built to handle various gases, liquids, and chemicals with precision.

Harmonic Drive Actuators with Integrated Drive Technology

Jan. 17, 2025
Discover the future of motion control.In this video, we explore how integrated drive technology (IDT) from Harmonic Drive is revolutionizing the precision mo...

7 factors to Consider When Choosing the Right Gear Technology

Jan. 17, 2025
Choosing a drive involves several design factors that depend greatly on the task at hand. This top 7 list will guide you, whether your task requires precise and exact movements...

What are the Benefits of Actuators with Integrated Servo Drives?

Jan. 17, 2025
Actuators with Integrated Servo Drive Technology (IDT) simplify cable management, control hardware, and commissioning while achieving outstanding performance in a compact size...

Voice your opinion!

To join the conversation, and become an exclusive member of Machine Design, create an account today!