Emphasis On Motion Control
MDTmag.com 22 / July/August 2016
spring force, and the use of a proprietary high-coeffi-
cient of friction brake pad.
s; Power: The brake’s 24 VDC required input voltage fell
s; Emergency stopping: The unit’s proprietary friction
brake pad enables it to safely perform more than 500
hundred emergency stops.
s; Radiation protection: The requirement was accom-
plished by expert adjustment of the lead wires.
Part of the requirements finalization process was a cus-tomer/supplier system data exchange which confirmed that
the brake was fully capable of handling a minimum of 500
The prototype system had all the necessary adjustments
other than the lead wire for radiation protection. This did
not interfere with the initial prototyping and was completed the following week. In six weeks, Thomson shipped 20
final systems, to be used in the production of the first 20
mammography systems. The program is now in full production, with the manufacturer shipping about 300 systems per
month. Plans are also underway for modified brake designs
that will meet European power requirements as well. MDT
Selecting Friction Brakes
It may be necessary to consider clutch or brake inertia and
engagement time in calculating load acceleration for some
applications. When the inertia or engagement time of the
clutch or brake initially selected represents more than 10%
of the load inertia or acceleration time, use the inertia-time
equation to solve for acceleration time. Use an inertia value
equal to the sum of the load inertia and the clutch or brake
inertia. Then verify that the sum of the acceleration and
clutch or brake engagement time is still within the required
acceleration time for the application.
Step One: Determine if the application requires a static
(holding) or a dynamic (stopping) brake.
Step Two: For static brake applications, determine the
required static torque to hold the load under worst-case
conditions, including system drag. Skip to Step Five.
Step Three: For dynamic braking applications with a spe-
cific stopping time requirement, first calculate the dynamic
torque (TD) necessary to decelerate the load using the
Where: I = total system inertia, lb-in.-sec2
ω = shaft speed. rpm
t = time to zero, sec
D = load drag, lb-in.
Multiply by 1.25 to convert to static torque. Go to Step
Step Four: For dynamic braking applications that require
the ability to only stall a load, calculate the appropriate
static torque (Ts) using the horsepower-rpm equation:
Ts = (1.25)(63000)(Pk)/ω
Where: P = horsepower, hp
k = service factor
ω = speed, rpm
Step Five: Select a brake model from the manufacturers
catalog with a static torque rating greater than the required
torque. Verify that the selected brake fits into the available
application envelope and mounting configuration.