By: Motion System Design|
Mostly, electric clutches and brakes run for hundreds of thousands of cycles with little watching. Overlook a few of their basic needs, however, and these normally unassuming devices can suddenly protest
Though clutches and brakes differ in purpose and application, for troubleshooting and problem-solving purposes their operating principles are similar. We focus here on positive-action units that are either electrically or mechanically actuated (“spring-set”), which includes brakes with On-Off action and Start-Stop clutches.
Most problems in the field show up as overheating, torque loss, or coil failure. Unless you dig until you find the culprit, you’ll probably keep replacing failed parts that result from the problem, without resolving the problem. Here are some tongue-in-cheek guidelines to help you.
Fail to read instructions
It has become a cliché: “When all else fails, read the instructions.” The fact: Clutches and brakes are precision devices with close tolerances. For troublefree service, they may have special assembly, installation, or adjustment requirements you must not overlook. When replacing a unit, even with one that looks the same, don’t just pull the old unit and slap in the new one. Instead, “before all else fails, read the instructions!”
Likewise, some sizes or designs have procedures for adjusting the air gap as it increases because of wear, or procedures for replacing components as they exceed wear limits. If you fail to follow all the maintenance and adjustment procedures in the instructions, you may not ruin your clutch or brake immediately, but you can be sure you’ve shortened its life.
Most electric brakes and clutches are installed in a direct-drive configuration, and alignment is just as important as it is with a motor. If bearings and shafts are not aligned, extra stresses and vibrations that develop can turn into major problems. Most industrial clutches and brakes are similar in design and have a small air gap between armature and magnet body. The effect of any misalignment is magnified, particularly on clutches, and can cause vibration and torque loss.
When you install or replace clutches or brakes, be aware that there are NEMA specifications on how they fit to motors or gearboxes, governing tolerance ranges for things such as register and bolt circles. Brake and clutch manufacturers set their design criteria to be compatible with this “window,” and proper alignment is normally the equipment manufacturer’s responsibility. You can incur trouble when mounting a new brake or clutch on used equipment if you don’t assure that motors or drives remain within NEMA tolerances. If the motor end bearings are worn, any shaft wobble will be exaggerated farther from the motor where the clutch or brake may be mounted — and what is really a motor problem may first show up as clutch or brake failure.
To avoid trouble, measure shaft runout both axially and radially, and check it against NEMA tolerances. These are usually around 0.004 in. radially and 0.002 in. axially, but they vary some by motor size. Also, check motor-shaft end float. Most equipment can tolerate float up to about 0.020 in. Most manufacturers’ instruction sheets include allowable indicator runouts for mating equipment.
Overhung loads can cause trouble. Particularly on clutch applications with a sprocket or sheave, there is a limit to the side force or tension you can apply without overloading the bearing and deflecting the output shaft.
If a unit has a retaining bracket, don’t bolt it down tightly or weld it in place unless you want premature failure. The retaining tab is mainly to prevent rotation when the clutch is applied, and it is necessary only to pin it in place so it is somewhat free-floating. Bolting it tightly can create hub-and-shaft misalignment and bearing prestress. This can lead to quick clutch failure and even drive damage.
We’re back at “read the instructions,” but repair or replacement of a clutch or brake presents many opportunities for misassembly.
Setscrews are a simple but often-overlooked example, if calls from the field are an indication. You’ll find them on driven hubs of components that mount to motor shafts. Depending on component size, components may be keyed only or keyed and secured by setscrews, especially on larger units. Fail to read about them on the instruction sheet if you want failure in the future.
Some brakes and clutches come with features such as special sealing and plated components for washdown applications, such as those in the food industry. Replacing plated fasteners or other components with unplated items will accelerate rust and cause early failure. Finally, replacing components such as friction discs with nonoriginal parts may drastically change operating characteristics. Use components not designed and tested with your specific brake or clutch and you may compromise performance. A replacement friction disc may look like and fit like the original, but it could have a much different friction coefficient and shorter life, for example.
Shake and break
Two types of vibration can trouble your clutch or brake. Operating vibrations from uneven loads, poor alignment, or misassembly will obviously destroy a unit if left unchecked. These phenomena are often cumulative, growing larger as they cause wear or loosen mountings. More subtle are environmental vibrations, caused by other adjacent or mobile equipment. Particularly when machinery having clutches or brakes with ball or roller bearings is stored or inactive for a long time, these vibrations and the bearings’ static loads may cause “false brinelling.” Result: Tiny dents in the bearing raceway. They will cause vibration and wear as rolling elements pass over them when the clutch goes back in service. The easy solution: Periodically turn the shafts on which the units mount — maybe once a month. It can improve the health of motor and line-shaft bearings, too.
Forget about storage
For a really bewildering and embarrassing experience, neglect any care of clutch and brake units waiting to replace spent compatriots or to go on your new products. A poorly protected clutch or brake waiting in the tool crib or storage room can get hurt. Avoid on-shelf vibration. Also, a well sealed and stored clutch or brake can be kept indefinitely. Normal factory packaging should be enough for about three months’ storage in an area without environmental control. For longer storage, a unit should be sealed in an airtight container with a desiccant to entrap moisture that may remain in the package, thus preventing rust. Superficial rust won’t hurt operation, but if rust is heavy enough to cause some armature deterioration, it can cause uneven pull and reduce the unit’s torque. Also if, because of rust particles, the unit does not seat properly, it could drag and generate excess heat.
Give it the wrong voltage
Torque depends on the coefficient of friction of the plates and also on the amount of pull force between the armature plate itself and the magnet body and coil assembly. Pull is a function of the coil’s specific voltage rating. Decreasing coil voltage reduces clutch torque so, where full torque is required, be sure full rated line voltage is supplied. When reduced torque is necessary, you can control a unit by means of an adjustablevoltage rectifier for the coils.
A more damaging mistake than applying the wrong value of voltage, though an easy one to make, is to apply ac voltage across a dc coil. Though most brake and clutch coils operate on dc, the manufacturer can supply them with a rectifier that lets them use ac line input. In some applications, a dc coil may operate (inefficiently) on ac for a short time before it fails. That can throw troubleshooting efforts off the track by making the failure look like an equipment fault.
Size it wrong
This misguideline applies less to the original equipment designer; more to the equipment repair specialist; and even more to the equipment redesigner and rebuilder. They can meet problems when a clutch or brake is replaced or when the equipment on which it serves is used for an application that differs from its original.
If the unit is too small for the torque requirement, it may work for a while before exceeding thermal capacity. Though the problem may seem like simple component failure, look further if a clutch or brake appears to lack thermal capacity. If it can’t dissipate heat quickly enough, you may need a larger unit. A solution may require close cooperation among makers of the equipment and of the brake or clutch, as well as the user.
You can also drive a brake or clutch beyond thermal capacity by failing to size it for high cycling. And heat buildup caused by any combination of high inertial load, rapid operation, and friction may be compounded by high ambient temperature. Make no mistake: Failure will result if the unit’s thermal capacity is exceeded.
Most units are designed to burnish or run-in quickly in normal running. Others may be factory preburnished or available with optional preburnishing. Unless clutches and brakes are burnished properly, they may not deliver rated torque.
On standard brakes and clutches, inner and outer poles may extend a few thousandths of an inch past the frictionlining surface. This is normal, and the excess material will generally wear down properly after a short period of normal use. However, take care on lowspeed applications. If a clutch is burnished at low speed, say 200 rpm instead of 1,800 rpm, and the armature is not hardened, it could gall or “tear.” If the clutch or brake is not properly burnished, the friction material may not seat properly, and then the unit will never deliver rated torque.
When a brake is used on a PC or PLCcontrolled application, the load may be electronically driven to a stop before a brake sets, so it sets at zero speed and does not burnish at all. Many adjustablespeed motor drives have dynamic braking in which, during a slowdown or stop, the motor becomes a generator, putting power back into the utility lines. The “generator” load itself acts as a brake.
Use a friction clutch as a slip clutch
Friction clutches described here are not designed to slip or “feather” a load as does a slip clutch, which is intended for soft starting or downline equipment protection. Instead, they are designed to produce the minimum torque needed to keep the input and output shaft locked together in On-Off fashion. The more they slip, the faster they wear and the more heat they produce, because of the excess friction. Avoid this by sizing the unit for maximum torque needed to drive the output. Then apply a service factor, generally 1.5 to 2, or maybe even 3, depending on the application.
Forget the work environment
When lubricant or other material that can change the friction coefficient gets between friction surfaces, up to 75% of torque can be lost. It’s easy to notice large-volume contaminants — big gobs or clumps — but some are more subtle, such as lubricant from a chain drive. Near equipment that gives off contaminants, install a brake or clutch shroud.
Environmental and service conditions that can cause trouble for brakes and clutches include exposure to:
• Wetness or dampness, like steam.
• Gritty dust, like grinding fines.
• Oil mist.
• Salty air.
• Chemical fumes.
Also, poor ventilation for a brake or clutch can raise its temperature and bring on trouble. If the unit is overshrouded or crammed into a tight cabinet, for example, it may suffer needlessly.
Another caution is the opposite of what you might expect: In early operation, do not blow out excess dust that accumulates. It helps maintain the right torque and improve any necessary burnishing. Sometimes you can be too clean.
Don’t recheck before you call
Often, you can blow a simple oversight into a much bigger production. In one case, a noise problem on some equipment brought a call to the clutch manufacturer. After a service call involving several people and many hours of downtime, the problem was traced to the installer’s failure to tighten setscrews — as described in the instruction sheet. By one estimate, 20% of trouble calls could be avoided by just reviewing instructions.
If you really want such problems, of course, misfile the instructions. Better yet, have no file for such items. Make sure they get lost or scrapped quickly, preferably even before installing the unit or storing it. For special fun, throw away the packaging before storing so you can’t tell readily what unit you have.
James D. Klann is Field Service Manager, Stearns Div., Rexnord Corp., Milwaukee.