Controls can be adjusted to increase or decrease clutch/brake engagement times as needed to minimize wear and meet operating requirements. Soft-start controls ease the forces (torque) that make belts and other components slip. The ramped method does it by building up to full torque (100% engagement) more slowly, whereas the reduced voltage method simply reduces the maximum torque. The over-excitation method cuts the engagement time so friction surfaces in the clutch or brake have less time to slip and wear.

Designers select clutches and brakes based on their capabilities, ease of mounting, cost, availability, and even tradition. But an overriding consideration for these components is to keep machines running continuously, reducing downtime and lost production. So in addition to the other selection criteria, designers should select clutch/brake designs for quick replacement, and creatively use their controls to minimize wear and early failures.

Boosting motor productivity

Clutches and brakes are used mainly to rapidly start and stop driven loads, thereby protecting motors from the shock of these starts and stops. They also help to accurately position parts.

When a motor is cycled, it starts to move the load (driven machine or component) from zero or low speed. This requires high current draw that builds up heat in the motor. But high temperatures eventually break down motor insulation and degrade permanent magnets in PM type dc motors.

In high-cycle applications, such as conveyors, motors overheat when they exceed a maximum cycling rate. A good rule of thumb for the maximum rate is 10 to 12 cycles per minute, though some designers prefer a lower number. Where these rates are exceeded, it's usually better to run a motor continuously at its maximum speed and use a clutch or brake to perform the cycling operations. The result will be longer motor life and less current draw.

Besides saving motors from wear and tear, clutch and brake systems can be optimized to minimize wear and downtime of other components too.

Controlling wear

Uptime becomes especially important for manufacturing operations that produce a high value, sometimes exceeding $1,000 per hour or even $10,000. In such an environment, designers can fine tune clutch and brake controls to reduce component wear.

With a modular brake, the machine builder has to assemble and align the magnet and armature, plus a hub and all the other miscellaneous parts shown here. A modular clutch (not shown) adds another part, using a field and rotor in place of the magnet. Packaged units have the same basic components but they come fully assembled and aligned so all the builder has to do is install the assembled unit.

Excessive wear can occur either in the clutch or brake or in other drive components, depending on the clutch/brake engagement time and the load inertia. By adjusting the control, you can achieve an engagement time that not only meets operating requirements, but reduces this wear or more evenly divides it between different components to prolong overall machine life.

Two common scenarios illustrate the point.

In the first case, the strength of a clutch/brake system, i.e. rapid starts and stops, works against long life in other drive train components such as belts, chains, and gears. For example, electromagnetic clutches and brakes commonly engage in 50 to 200 millisec at full voltage (depending on their size). Where the load inertia is high, this can cause V-belts to slip on pulleys, or high stresses to occur in timing belt teeth, chain sprockets, and gears. Both conditions lead to wear.

A soft-engagement (soft-start) control alleviates these wear conditions without making any changes to the machine components. Soft-start controls operate by either reducing the maximum voltage to the clutch, or ramping up the voltage to its maximum value over a longer time. There are disadvantages to both methods. The first method prevents the clutch from transferring its maximum rated torque, generally limiting the torque to about 70% or 80% of the rated value. The second method causes longer engagement times and may increase friction surface wear in the clutch because these surfaces slip for a longer time. These drawbacks are often small when compared with longer life of the entire machine.

One material handling company used this strategy to eliminate short belt life on a conveyor. Because the conveyor cycled at rates approaching 100 per minute, timing belts lasted only 3 months. By changing to soft engagement, the company increased belt life to nearly 18 months with only a slight increase in clutch/brake wear.

In the second case, slow starts and stops (long engagement time) of high inertia loads cause friction surfaces in the clutch or brake to slip excessively, causing them to wear faster. Here, an overexcitation control can improve the wear life. Upon actuation, such a control sends a high voltage spike to the clutch or brake coil to build magnetic flux very rapidly, thereby reducing engagement time an average of one-third. Because the friction surfaces engage for a shorter time, they slip and wear less. The trade-off here is that faster engagement may shift some of the wear back to the other machine components.

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