Clutches and brakes

Clutches and brakes couple, decouple, accelerate, and decelerate rotating machine components and maintain them at proper speed. The functions of each are so similar that their rolls are often interchangeable. Whether the clutch/brakes are mechanically, electrically, or fluid power actuated, they are considered mechanical devices because they transmit mechanical power.

Clutches and brakes are generally used in rotary motion applications. The clutch or brake must be designed to convert mechanical energy absorbed during relative motion or slippage into heat energy. It must survive both mechanical and thermal steady-state and shock loads imposed by the system during the operational cycle without damage.

The first step in selecting a clutch or brake is to determine the configuration required, depending on performance characteristics.

After selecting a type to use, the required size must be determined. Selection charts are available for this purpose.

A typical chart is based on the horsepower and speed of the system, which should be the shaft speed at the clutch or brake, not motor speed.

For example, the clutch needed to drive a shaft powered by a 1-hp
motor rotating at 1750 rpm is Model A. If the shaft speed at the clutch is a 2:1 reduction, clutch speed would be 875 rpm, and Model B would be selected.

The relationship between system horsepower and speed is given by the following equation for determining the dynamic torque capability required from the clutch or brake:

D = 5252 x hp x SF

_____________

N

where:

D = dynamic torque, lb-ft

hp = horsepower

SF = service factor (ranges from 1-10)

N = speed of clutch/brake, rpm

As rpm decreases, torque increases. For this reason, the best location for a clutch or brake is on the high-speed shaft of a machine.

Speed should not be too low; below 300 rpm is not recommended for friction-driven units. At low speeds, burnishing or mating wear of the friction faces does not occur, and torque capacity may be reduced dramatically.

When a clutch or brake must be operated at very low speeds, it might be necessary to oversize the unit. In these cases, it is normally sufficient to use a unit with a static torque rating two times the calculated dynamic torque requirement.

In a basic oil-shear drive, torque is transmitted through shearing of an oil film between two disks. As the rotating input disk moves toward the stationary output disk, the oil shearing action forces the output disk to begin rotating. There is no friction material-to-metal contact until input and output disk speeds are nearly equal. Then the oil film breaks down, allowing full static engagement. Wear is greatly reduced by the oil film, which lubricates while transmitting most of the dynamic torque of engagement.