# Making sure brakes don’t break

Oct. 25, 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
Thomson Deltran Product Specialist
Danaher Motion Corp.

A: In last month’s column we ended with the formula used to estimate the torque needed to dynamically stop the load in an interval of time. Once the torque value is determined, multiply the result by 1.25 (25%) to obtain a minimum static torque. The deceleration time plus the brake-engage time must be less than or equal to the desired stopping time.

You’ll need to consider total energy absorption (energy dissipation). You must verify that the brake can dissipate the amount of kinetic energy that it absorbs per cycle for as long as it takes to perform the entire operation. Repeated cycling builds up heat, which the brake must be able to withstand continuously.

Total energy dissipation is typically expressed in units of ft-lb. It is defined as the sum of the dissipated kinetic energy Ek and slip energy Es. Kinetic-energy dissipation Ek is:

Ek = 4.6 10-4 I ω″

where I = total system moment of inertia, lb-in.-sec″, and ω = rotational speed, rpm. Slip-energy dissipation Es is:

Es = 43.6 10 –4 ω D Ts

where Ts = total slip time, seconds, and D = load drag torque in the system, lb-in.

Compare the calculated values of energy-per-cycle and energy-per-minute with the values in the manufacturer’s product data sheets. The calculated values should be equal to or less than the catalog ratings to ensure the brakes have a long lifetime.

It is important to note that all power-off spring-set brakes are designed primarily for use in static-holding systems. Wear factors for friction materials are more critical when the brakes work in the dynamic stopping mode. Applications calling for repeated dynamic stopping may necessitate use of different friction material better suited to high heat and wear situations.

Rocco Dragone is a Thomson Deltran Product Specialist with Danaher Motion Corp. Got a question about motion control or mechatronics? Ask Rocco via e-mail at Editorial@ DanaherMotion.com.

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