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.
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.
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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Design for the application
Severe operating conditions, such as high cycling rates and lubricant contamination, often call for design options that help to extend life. For example, increased machine cycle rates in the packaging and mail handling industries have caused clutch/brake friction materials to wear faster. This led to manufacturers developing longer-lasting ceramic friction material. When machines cycle faster than 100 times per minute, ceramic material triples friction face life. When cycle rates exceed 200 per minute, friction face life increases five fold.
Another problem area involves lubricants. Many operations generate contamination in the form of airborne lubricants -- oil mist from cutting fluids and exposed lubricants. When lubricants reach the friction faces, these faces no longer grip adequately. This problem can be avoided by choosing clutches and brakes with enclosed or partially enclosed housings that exclude such contaminants.
Speeding up installation
The way clutches and brakes are assembled and mounted also influences downtime. For example, they come in both modular and packaged units. Modular versions require machine builders to assemble and install the components (field, rotor, and armature) on their equipment and ensure that they align within tolerances. Packaged units on the other hand are preassembled and prealigned at the factory so they're ready to install in a machine directly from the box. Although packaged units are more expensive, many factories find that they offset this cost by saving a lot of installation and maintenance time. Installers typically apply a packaged unit in 30 min. whereas a modular unit takes about 2 hr.
The most common installation methods are flange mount, shaft mount, and foot mount. Flange-mounted units connect directly between the motor and reducer. To replace one, disconnect it from both the motor and reducer, and install a new one.
Shaft-mounted units that are positioned directly on motor or speed reducer shafts are easy to replace. All that's required is to remove a key, loosen setscrews and wires, and install a new unit. But others operate through jack shafts. In such cases, the shaft and supporting bearings must be moved to get the unit off the shaft. Flange and shaft-mounted units may not be the best choices when designing for reduced downtime, because of the time required for assembly and disassembly.
Foot-mounted units are usually the easiest and fastest to replace. They typically have input and output shafts for coupling to motors and loads, or for mounting pulleys or sprockets. To replace this type of unit, merely loosen the shaft couplings, loosen the fasteners and wires, then remove and replace it with a new unit. Some production facilities that have multiple machines of the same design go a step further to reduce maintenance time, keeping a spare foot-mounted unit in storage, with coupling halves or pulleys already mounted.
Greg Cober is a technical support and training manager, Warner Electric/Dana, South Beloit, Ill.