Brakes produce heat when the brake is energized and the shaft is rotating.
The faster the slip RPM, and the higher the torque, the hotter the brake will run. If the slip heat dissipation rating is exceeded, the brake will overheat and fail. The brake must be physically large enough to not overheat.
Calculate heat input:
HEAT (watts) = TORQUE (lb.-in.) X SLIP RPM X 0.012
For web handling applications,
HEAT (watts) = WEB TENSION (lbs.) x WEB SPEED (feet/minute) / 44