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