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.
Troubleshooting clutch/brakes for performance
Many of the same troubleshooting principles for new clutch/brakes also apply to existing installed units. The practice of checking the least expensive repair first is still a good one. Checking to see that the unit is receiving power when it should and that the unit is mechanically able to engage applies to both new and existing units.
Something has changed when a previously operating unit stops working. Sometimes this change is a different motor or a different ratio in the gearbox. Sometimes the unit reached the point at which wear affects performance.
If the clutch/brake has not performed properly since work has been done on it, evaluate how the change might have affected the clutch/brake. Changing a gearbox ratio from 5:1 to 10:1 may not affect the footprint of the gearbox, but it does double the output torque of the gearbox. Such changes that are not readily apparent account for many clutch/brake problems.
Wear is a fact of life for friction devices. When a clutch/brake is worn out, its performance declines. Aclutch/brake that has stopped accurately for millions of cycles begins to stop less accurately; stop times increase inconsistency from one cycle to the next. These changes occur over a period of time. When cycle rates are low, performance decline occurs over weeks; when cycle rates are high-200 per minute, or higher- it will becomes evident in days.
Wear grooves are normal in an electro-magnetic clutch/brake. They result from friction between the metal armature and the metal poles in the rotor or magnet. Pole tracks appear at these contact points (see Photo 1). Pole tracks are normal and should not be machined away. The photo shows pole tracks for a well worn armature. Note that the pole tracks wear more rapidly than the friction material area. This is also normal. When repairing a clutch/brake, replace both friction faces. The two faces wear against each other at the same rate. If only one face is replaced, the new part will wear very aggressively until it matches the wear pattern on the older part.
Factors that degrade clutch/brake performance
A variety of factors adversely affect even properly sized clutch/brakes. Most common among these are improper installation and environmental factors. The most common cause of poor clutch/brake performance is improper mounting. For a clutch/brake to work properly, the friction surfaces must be square and concentric within fairly tight tolerances that vary somewhat by unit size. These tolerances are stated in the installation manuals included with new units. Failure to adhere to these tolerances causes a unit to perform below its ratings. Time spent making sure that a unit is properly installed saves hours of downtime later.
Environmental factors also cause clutch/brake problems. The most difficult situation is airborne lubricant mist. This mist contaminates working surfaces, decreases the coefficient of friction of the working faces, and decreases torque capability. To address this, some models of clutch/brake are enclosed, either completely or partially, so that contamination cannot reach the working portions of the unit.
Enclosed units are also a solution for food industry applications in which pressure hose wash down is part of the cleaning process. Some applications require something beyond enclosing the unit. These cases require special shafts, bearing seals, and coatings to withstand the caustic chemicals of some washdown processes.
Other possible contaminants include chips of metal or plastic, cutting fluids, food products that can fall onto clutch/brake components and cause them to malfunction. In such cases, common sense should prevail. If clutch/brake contamination is likely, a shroud or guard ensures that the unit provides the performance required for continued operation.
Properly selected and properly mounted, a clutch/brake provide millions of cycles of life. Clutches and brakes are fairly simple devices, mounting and installation problems, environmental situations, or changes in the application affect them negatively. Common sense helps in diagnosing most of these difficulties. When additional expertise is required, manufacturers offer toll free technical support to help resolve problems. Most manufacturers also have an experienced sales force capable of providing on-site assistance.