Internal rather than external heat in clutches and brakes is the usual culprit, because few applications operate in ambient temperatures high enough to cause fade, slippage, and loss of braking efficiency. The heat that causes these symptoms is usually generated when the device operates in a fast-cycling application or with too high an inertial load. When faced with conditions beyond normal range in these areas, the power-transmission system designer must use extra care in product selection and application. The clutch and brake electromagnets (coils) also contribute to the total heat the product experiences, but it is usually small compared with the heat of friction in starting or stopping an inertial load.

A look at thermal capacity

When a clutch engages or a brake stops a load, heat is generated. This heat is mostly from friction of the working elements as they come into contact and reach a static condition after some slippage. Though the heat is normally absorbed by parts of the clutch or brake, the amount that a unit can dissipate depends on the masses and materials of its parts. Thermal capacity is the ability of a given clutch or brake to absorb and dissipate heat without reaching temperatures that would harm its operation.

The amount of heat generated and dissipated by a clutch or brake is a function of load, inertia, speed differential of the load, and number of times the unit must start or stop in a given time. You can calculate thermal capacity from:
TC = 1.7(Wk²)(N/100)²(F)

where:
TC = Thermal capacity, ft-lb/min
Wk² = Inertia seen by the clutch or brake, including its own inertia, lb-ft²
N = Speed differential between friction surfaces at time of engagement, rpm
F = Cycle rate, cycles/min

Be sure the inertia value is that at the clutch or brake.

As any variable in this equation increases or decreases, thermal capacity responds likewise. Therefore, operating problems due to excess heat could result from extreme inertial load, large speed differential, or high cycle rate, as well as from some combinations of more moderate values of these factors.

Heat can cause increased wear as well as fade, particularly in more severe service and load conditions. Most modular clutches, brakes, and clutch-brakes use finned aluminum housings to dissipate heat. In many modular units with open housings, a fan improves cooling. One manufacturer’s line of totally enclosed nonventilated (TENV) modular products includes an internal fan so that axial air movement eliminates hot spots.

Routes to better thermal capacity

If a clutch or brake is too small for the application’s torque requirements, it may seem to perform right — until it exceeds its thermal capacity. One sign is a unit that works well for a while; then its stop or actuation time starts increasing.

To select a clutch or brake with enough thermal capacity for your application without over-engineering, you can:
• Select one designed to provide the inherent thermal capacity.
• Add features like external fans to increase heat-dissipation rate somewhat.
•Modify the system design to reduce the demands on the unit.

When a clutch or brake is replaced with a different one, or when the equipment that it serves is used for a different application, its thermal capacity may also be exceeded. The problem may appear to be simple component failure, but it pays to look further if heat buildup seems to be involved. If the unit can’t dissipate heat quickly enough, a larger unit may be needed. A solution may require close cooperation among the manufacturers of the equipment and the brake or clutch, as well as the end user.

Especially where environmental conditions require a sealed unit, thermal capacity may be a problem. In food plants and similar applications, clutches and brakes are sealed not only to prevent damage from washdown and external contaminants, but also to keep internal elements from migrating into the plant environment. Generally, TENV brakes or clutches required for washdown service must be derated because they reach their thermal capacities sooner than open units. This is especially true on foodprocessing or packaging equipment, which often runs at high cycling rates. Recent design improvements let some new modular TENV units meet or exceed thermal capacity requirements of older open-housed units without derating.

Continue on page 2