Brakes and clutches

July 1, 2002
Brakes and clutches have the potential of saving time, energy, and wear on a system.


Where is a specific brake or clutch type most appropriate? It depends on what it’s being used for.

Bobbi/Nexen: There are four basic application classifications: occasional start or stop, high cyclic start or stop, high inertia start (or stop), and continuous slip.

Andrew/Logan: Brakes provide controlled stops for positioning a machine loading and unloading material or performing an assembly or machining operation. They can also help shut a machine down after so many cycles to inspect tooling and wear items.

John/Deltran PT: Initial application of the correct technology is ultimately the quickest and best way to eliminate productivity issues later on.

Jeff/Placid: Hysteresis brakes produce torque with magnetic fields rather than physical contact, so they really don’t wear out. Another advantage is that slip torque is perfectly smooth, even at near-zero slip rpm. In addition, they dissipate heat well because they are relatively large compared to torque output.

Gary/Magtrol: Hysteresis brakes are ideal for winding strand and web materials including copper wire, fiber-optic cable, textiles, tapes, films, foils, paper, and nylon. They are also ideal load devices for motor performance test dynamometers and as loads for life cycle testing of motors.

Jeff/Placid: But cogging, pulsing output torque is a major disadvantage. In contract, there is never any cogging in magnetic particle brakes and clutches. Although tensioning a very elastic web at near-zero speed isn’t suitable with a magnetic particle unit (due to the slight amount of stick/slip at near zero rpm) slip torque is very smooth. They are more suitable for constant slow to moderate slip speed applications, due to heat dissipation limits — they offer high torque in a small size, so minimal inertia is added to motors, which is good for motor acceleration tests. Low shaft inertia also allows rapid speed changes in winding systems without stretching the web during acceleration. But eventually the particles wear out.

Pat/Stearns: Variable frequency drives (VFDs) have moved into a significant segment of the brake marketplace. A VFD determines rpm, and can ramp loads down to zero rpm. So who needs a brake anymore?

John/Deltran PT: Electronic controls, along with new materials and better manufacturing methods, are keeping clutches and brakes a viable motion control technology for the future.

Pat/Stearns: Also, brakes ensure safety and code compliance. I just heard from a company that makes commercial tumbling equipment for a major trade name. UL has notified them that to they must add a brake to rapidly stop the load to continue displaying their agency marking. That requirement is already in place in similar industries. Another reason to use brakes is for the assurance that a load is secured at zero rpm — either to secure a workpiece, or to ensure that nothing will slip and cause workpiece waste or worse, personal harm.

Jeff/Placid: Hysteresis brakes are not suitable for precisely holding a shaft in position, since torque builds from near zero from a relaxed, stationary position, as the shaft is turned in either direction. Final torque is reached after turning about 5 or 10° (depending on number of poles). Also, there is momentarily extra torque just after this movement. Extra torque does not occur if the shaft stops, and then rotates in the same direction; it only occurs if the shaft is allowed to rotate backward to the relaxed position.

Pat/Stearns: Friction brakes are useful in fulfilling these requirements. Emergency stopping is generally a safety issue, but not always. Perhaps someone has caught clothing or body parts in a rotating piece of equipment. Or perhaps misalignment is developing, or somehow a workpiece has dropped in upside down, or a load shifted. Think of all the e-stops in your airport — lining up the jetway, on baggage handlers and sorters, on people movers, escalators, and lifts ... emergency stops exist for several reasons: for safety, to protect expensive equipment, for convenience, or just in case. Sometimes there is an over-reliance on emergency stopping, and sometimes its use is appropriate.

Friction and wear

What part of a friction brake or clutch goes bad first? When a unit is running properly, friction faces are the first to go. Their chemical makeup is always under scrutiny.

Bobbi/Nexen: For pneumatically activated units, there are three wear items: friction material, air chamber seals, and bearings. The friction material is the normal wear item; seals and bearings wear prematurely when subjected to excess heat, high cyclic rates, and lack of lubrication.

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Andrew/Logan: Lubrication is a key issue in wet clutch applications. Clutches and brakes tend to fail prematurely if an inadequate amount of flow or splash or spray of lubrication is not readily available. Pat/Stearns: There are some unique industries that require metal to metal, oil bath, or ceramic discs.

Greg/Warner Electric: For the extension of wear life, ceramic friction facing is great for high cycle rate applications. This material can triple friction wear life when cycle rates exceed 75 cycles per minute or more, and can provide up to five times the life in 200 cpm applications.

John/Deltran PT: Technology advancement in brakes and clutches tends to be evolutionary. As new materials become available, they can they be applied to improve units.

August/Mayr: Brakes utilizing ceramic fiber friction linings, similar to the materials used on space shuttles, do offer increased friction work and more consistent coefficients of friction.

Jeff/Placid: Friction-type brakes and clutches can be used for controlled slip torque, but friction varies too much for precision applications. Torque changes with slip speed, temperature, humidity, and usage. When high accuracy is not needed, friction units are used due to low cost.

Greg/Warner Electric: But ceramic facing wears at a slower rate and is less affected by frictional heat than standard fiber-based friction facings.

Pat/Stearns: Phenolic resin-based discs are useable in a broader range of industrial applications. We blend and mold our own friction material so we can tweak the chemistry and (just as critically) the process.

Bobbi/Nexen: Designs are improved regularly to provide part geometry for efficient heat dissipation. New component materials are incorporated when they can absorb heat and lessen the amount of friction wear.

August/Mayr: Ball-detent clutches, which utilize rolling instead of sliding friction, can drastically reduce wear and provide significantly longer service life. They eliminate internal splines and similar wear items otherwise necessary to accommodate the axial movement seen within a clutch during a disengagement. This increases service life.

Andrew/Logan: Properly specified clutches and brakes will last for years and years — or for the life of the application. We have applications on machine tools that have operated over 20 years. However, when a clutch or brake does fail, it tends to be from cyclic fatigue, lack of lubrication, or inadequate pressure. Clutches are wear items, and after so many millions of cycles, springs, bearings, friction and steel discs need to be replaced.

Gary/Magtrol: Friction and magnetic particle devices, while they can provide high torque to mass ratios, are by nature prone to wear and large amounts of heat production when used in continuous slip applications. This heat and wear of friction discs and pads (and particle seals in particle brakes) requires more frequent maintenance.


A clutch or brake’s surroundings also determine which is most appropriate.

Greg/Warner Electric: The only type of application we shy away from are those in hazardous environments. Any spark can ignite a vapor or gas, and any friction device can create a spark. Boom! This would apply to any friction-type clutch or brake — ours or anyone’s. We have looked at creating enclosures that will hold some inert gas to operate in, but they aren’t very commercially viable.

Gary/Magtrol: Hysteresis clutches provide absolute control and the smooth application of torque for torque limiting situations such as bottle capping and bolt tightening. The development of torque without friction components, magnetic particles, sliprings, or brushes prevents contamination due to wear particles or leaky seals which makes them ideal for use in food processing and clean room environments.

Greg/Warner Electric: Ceramic-based friction materials were introduced as cycle rate requirements increased over 20 years ago. Prior to that time the cost and availability of materials was a limiting factor. There is no environmental concern with ceramics compared to standard friction material. We get questions on this routinely and provide the material safety data sheets to any customer that requests them.


The torque and speed of a system determine how hefty its brakes and clutches must be.

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Pat/Stearns: Definitions are always good for a spirited discussion. Torque is a measure of rotational force; it can be a retarding braking force or a driving clutch force. A system — not just the component — has to have enough torque to meet the dominant prescribed condition: brake, clutch, tension, limit — whatever. The trick is knowing that condition ahead of time. Whether torque selection is robust or lean goes back to what the design intent is — perhaps a very rapid emergency stop? Or a softer stop that sacrifices the brake to prevent disturbance of a load or overall system?

Jeff/Placid: The torque of hysteresis brakes and clutches is much lower for a given size. Units over about 20 lb-in. are more costly due to large sizes.

Bobbi/Nexen: Air-actuated clutches and brakes are more efficient than electrically actuated units, providing up to six times friction facing life and 40% more torque. They don’t consume energy until cycled, and then use only a small fraction of the energy consumed by an electrical coil.

Andrew/Logan: Designers need to identify what the best and worse case scenario is for their clutch or brake application. What are the maximum rpm, horsepower, available lubrication, and service factor? Does the clutch accommodate the available shaft and key? Will it operate at the required speed? Will it fit within the axial space available? Can it be easily installed and removed in field conditions?

Jeff/Placid: The torque curve of both magnetic particle and hysteresis brakes and clutches shows hysteresis. If torque is plotted versus input current, starting at zero and climbing to 100% input current, and then descending back down to zero, the two lines don’t overlap. Some advice: for best repeatability, always turn up current starting from zero, using the lower line on the hysteresis torque curve to determine torque. Or else, always start at 100% input current and decrease to zero, using the upper line on the hysteresis torque curve to determine torque. Any sequence of torque is possible if the current required for the desired torque is determined experimentally.

Andrew/Logan: Suppliers should get involved as early as possible; they may have simple suggestions that will minimize failure. Regardless of what paper calculations say, field and prototype testing is the way to go. Often, once the problem is identified a simple solution such as an extra lube line or clearance issue can solve it.

Control and integration

How are brakes and clutches made to fulfill their requirements? Getting signals to and from the components is key. How are brakes and clutches offered? Packages and mounting options allow for greater flexibility.

Jeff Pedu/Placid: Closed loop systems automatically eliminate errors due to hysteresis. Accuracy is best with closed loop systems, where actual torque is measured by torque transducers. But this is expensive.

Pat/Stearns: With brake status switches, a switch is installed within the brake to determine whether the brake is set or released. So at zero rpm the brake sets for a holding function. Then when the switch confirms the brake has released, the motor starts. Status switches are a common option; most manufacturers offer them. I don’t know that it has to do with synchronization as much as industry factors today. Example: a crane operator may want to know if a brake is set or released before operating.

Greg/Warner Electric: The key design improvement of productivity in clutches and brakes is to simply reduce the inertia in their system. The lower the inertia being started and stopped, the faster units will engage, and the longer they will live. A second strategy involves using the most efficient control schemes possible, replacing mechanical relays with solid-state devices and controls. Also, use of over-excitation controls to work with clutch/brakes can reduce the electrical portion of engagement time by two thirds; cycle accuracy is also improved.

Andrew/Logan: With a power-applied brake actuated with a limit switch or PLC, the machine will stop in the same position cycle after cycle, saving time and money.

Jeff/Placid: Cogging is pulsing output torque that occurs under certain operating conditions in hysteresis clutches and brakes. When a unit cogs the shaft has preferred positions, resisting movement for about 5-10° (depending on number of poles) and then jumping to the next preferred position. It occurs after the input current is greatly decreased, while the shaft is stopped or moving slowly ... for example, in web tensioning applications where tension must be repeatedly cycled. Cogging continues until manually decogged (by rotating the shaft while lowering the input current from high to low). Or an automatic decogger not reliant on varying input current can reset brakes and clutches in one revolution.

John/Deltran PT: An important area, especially for the protection of expensive equipment, is the development of smart components. These are components that are equipped to monitor their own operation for problems — and even enhance their own performance — through the application of electronics.

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Andrew/Logan: Fluid or air actuated clutches will fail if an inadequate amount of pressure is applied. For example, if an application calls for 80 psi and available pressure to a clutch is only 60 psi, it will tend slip fail prematurely. Pressure switches, which monitor actuation flow to the clutch can be incorporated into the system to prevent premature failure, and signal the machine operator of inadequate flow.

Gary/Magtrol: A matched hysteresis brake provides matched performance characteristics from any number of brakes of like design and rating, making it possible to control a group of brakes from a single sensor as in a multiple pay-off system or open loop application. This assures that the same level of current to each matched brake will result in virtually the same torque from each.

August/Mayr: With the more sophisticated controls in today’s machinery, many designers will rely on them to indicate when a crash or jam occurs. The problem is that the reaction time of these electronic devices is longer than the time it takes inertia present to damage machine components.

Pat/Stearns: A variable frequency drive can be set up to signal a motor to stop rotating while an electromechanical brake rapidly stops and holds the gearbox and load. But brakes have to be wired separately from the drive for proper control of each. Solenoid-style brakes are designed to a typical NEMA voltage range of ±10%. Equipment and system installers accustomed to typical NEMA design B motors may try to wire the coil across the motor windings. But VFDs vary frequency, which doesn’t work with fixed-frequency products. The reverse is also true. Some drive manufacturers have a fixed voltage tap in their power box, which is a convenient arrangement.

Andrew/Logan: Linking clutches together with microprocessors and PLCs enables users to better monitor speed, cycles, air pressure, machine lock-up detection, routine maintenance etc. We’ve found that applications involving this new technology increase the life of the clutch significantly — which lowers the cost of ownership, making clutches a better value.

Gary/Magtrol: Hysteresis permanent magnet units are ideal in applications where electrical power cannot be provided to a brake or clutch coil.

August/Mayr: On critical drives, mechanical torque limiters, which react instantaneously, disconnect inertia thereby preventing damage and associated downtime. Improperly setting these devices can either cause nuisance trips, resulting in premature wear and additional maintenance, or a clutch that does not disengage when a crash or jam does occur.

Meet the experts

John Pieri, Deltran PT of Danaher Corp., Amherst, N.Y.
Expertise: Wrap spring, mechanical slip/drag, hysteresis, permanent magnet, and value-added custom clutches and brakes; spring-set brakes, and friction and multiple disc clutches

Patricia Watson, Rexnord Corp., Stearns Division, Cudahy, Wis.
Expertise: Friction materials, armatureoperated brakes, and safety brakes

Gary Raduns, Magtrol Inc., Buffalo, N.Y.
Expertise: Hysteresis brakes, coil winding and printing applications, large bore brakes

Greg Cober, Warner Electric, South Beloit, Ill.
Expertise: Electromagnetic friction and tooth clutches and brakes, multiple-disk clutches, failsafe brakes, and overrunning clutches and holdbacks, in torque ranges from 5 inch pounds to over 9,400 foot pounds

Jeff Pedu, Placid Industries, Inc., Lake Placid, N.Y.
Expertise: Magnetic particle brakes and clutches, hysteresis brakes, and applications requiring smooth slip torque

Andrew Logan, Logan Clutch Corp., Cleveland, Ohio
Expertise: Fluid and air actuated multiple disc clutches and brakes, with torque capacities from 20 to 3,200 foot pounds

Bobbi Jensen, Nexen Group Inc., Vadnais Heights, Minn.
Expertise: Pneumatic industrial clutches and brakes, servomotor, motor, caliper, drum, tension control, dual plate, NEMA C-flange, linear, and shaft-mounted brakes, friction, dual plate, high capacity, tooth, ball/detent interface, and multi-disc clutches

August Mustardo, Mayr Corp., Waldwick, N.J.
Expertise: Mechanical, pneumatic and electromagnetic torque limiting clutches and spring-applied, electromagnetically released fail-safe brakes

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