Brakes produce heat when the brake is energized and the shaft is rotating.
Clutches produce heat when the clutch is energized and the input and output shafts rotate at different RPM's.
The faster the slip RPM, and the higher the torque, the hotter the brake or clutch will run. If the slip heat dissipation rating is exceeded, the unit will overheat and fail. The model must be physically large enough to not overheat.
Calculate heat input:
HEAT (watts) = TORQUE (lb.-in.) X SLIP RPM X 0.012
For brakes, SLIP RPM = RPM
For clutches, SLIP RPM = (Input RPM - Output RPM)