Applications

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.

Continue on page 2