Troubleshooting clutches and brakes

Follow these guidelines when you suspect problems.

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The use of clutches and brakes to control motion--be it an assembly line, bottling facility, or an industrial bakery--can result directly from the need to minimize downtime and increase productivity. Standard industrial electric motors cannot provide long life in high cycle rate applications nor can they provide consistent accurate positioning. Adding a clutch/brake to the drive system alleviates the shock of rapid starts and stops. It also extends motor life and efficiency and enables accurate positioning of materials for efficient, productive operations.

While applications for electrically operated clutch/brakes vary greatly, one fact remains constant--if the clutch/brake components do not operate correctly, the application will have a hard time operating efficiently. To successfully operate and maintain clutches brakes, consider the following factors.

Proper sizing and selection minimizes wear

For a clutch/brake to operate consistently and wear normally, it must be sized to handle the intended application. When designing a clutch/brake system, specify the correct size clutch/brake on the basis of accurate speed, load, and inertia calculations. Undersizing a clutch/brake results in aggressive wear that causes premature unit failure. The extra inertia an oversized clutch/brake adds to an application may adversely affect other drive train components. If you are not sure about sizing and selection, your clutch/brake supplier will be glad to offer assistance.

Basic versus packaged products

Many clutch/brake manufacturers make two families of clutches and brakes--basic and packaged. You assemble and install into your machine a unit from the basic product family; the manufacturer pre-assembles the packaged product. Some customers prefer basic products because they cost less and provide some flexibility in mating to special mounting brackets or output devices. While not very complex, installing a basic product does require alignment and assembly skill and the time to do the work. Concentricity of bolt circles to shaft, squareness of mounting for the two working components, and angular misalignment are critical issues when assembling clutch/brakes.

Packaged clutch/brakes were introduced over 30 years ago to reduce the installation time and to eliminate common assembly errors. Because these units are pre-assembled at the factory, they already meet alignment tolerances. Many customers find that using a packaged clutch/brake actually saves money during machine assembly compared with the cost of purchasing and the labor cost of assembling a basic product for the same application.

Packaged clutch/brakes reduce maintenance time because they are ready to be used right out of the box. Installing a packaged product requires only the installing a key, setting the set screws,attaching the wires, and aligning the mating machine components. In today's industrial environment, machine downtime can result in lost production costs in excess of $1,000 per hour. The reduced time required to install a packaged clutch/brake easily covers the increased purchase price of a packaged unit. That is why nearly 80 percent of new applications use packaged clutch/brakes rather than the basic equivalent.

A packaged product line parallels the basic product line. Thus, an existing basic clutch/brake can often be replaced with a packaged clutch/brake with little or no machine redesign. The packaged product line offers shaft or foot mounted units and units designed for use with C-Face motors and reducers. Your clutch/brake supplier will gladly assist in selecting the best packaged product to use in upgrading your application.

Troubleshooting clutches and brakes

Clutches and brakes are fairly simple devices. Determining the cause of poor performance or failure, therefore, is fairly simple. Problem solving is often quickest when users recognize that the clutch/brake malfunction may be a symptom of a problem rather than the source.

When evaluating the improper performance of a clutch/brake, begin with the simplest and least expensive elements. Ask the simple question: "Has it ever worked?" This is the first step in a flow chart that helps to focus quickly on the possible sources of clutch/brake problems.

If the unit is newly installed and has never run, check its assembly or installation. Using the least- expensive-to-most-expensive philosophy, a maintenance technician should first check for a blown or missing fuse, disconnected or damaged wires, and improperly set torque controls. Too often, expensive clutch/brakes are replaced only to find that a 25-cent fuse caused the unit not to work. Wires can work loose from their connectors, a new switch may not install in the same way as the old, or the torque pot on a control may prevent enough power from reaching the clutch/brake. Finally, check coil resistance using a standard volt/ohm meter.

If power is reaching a good clutch/brake coil but the unit still fails to function, then check mechanical conditions that arise from improper installation or assembly. One of the most common problems is an air gap that is too large. Electro-magnetic clutch/brakes require the two friction surfaces to be in close proximity. When powered, the magnetic force created by the coil causes the friction faces to clamp together. If the air gap is too large, magnetism cannot pull the pieces together. Once the air gap has been set, it should never require readjustment. Better models of clutch/brakes automatically adjust the air gap to compensate for wear throughout the life of the unit. Other mechanical points that enhance performance include a proper key in the keyway and a burr free splined hub.

Burnishing

Finally, for new clutch/brakes there is the issue of burnishing--the action of wearing two surfaces against each other to achieve intimate planar contact. For a new clutch/brake, the normal flatness tolerance for the friction faces is a few thousandths of an inch. Therefore, the friction faces will not make full face contact when initially installed and new units exhibit less than full rated torque. To address this, some vendor's pre-burnish packaged products at the factory, especially failsafe products. None of the basic product is pre-burnished. Proper burnishing can require as few as a dozen or as many as several hundred cycles. The difference is in the amount of work being done at the friction faces during each cycle. A high speed, high inertia load burnishes quickly, a low inertiaload takes longer. A new unburnished unit provide from 60 to 70 percent of rated torque when first engaged. Burnishing brings this up to rated torque.

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