Magnetic particle and hysteresis type brakes & clutches are used for precision torque & tension control.
Brakes - When energized, the shaft becomes coupled to the brake’s housing. Typical applications: Brakes can provide adjustable torque for unwinding (payout) of webs, such as wire, foil & paper. Another popular application: Brakes can provide adjustable torque loads to test motors or mechanisms.
Clutches - When energized, the input shaft becomes coupled to the output shaft. Typical applications: Clutches can produce adjustable torque for rewinding webs.
Both produce smooth, adjustable slip torque, controlled by electrical input. Either type can be used successfully in most applications. But, sometimes one type is much better, due to subtle, but important difference in characteristics.
Torque is higher compared to the same size hysteresis brake. A hysteresis brake with a Ø4-1/2 housing diameter produces 15 lb.-inches torque. A similar sized magnetic particle brake produces 70 lb.-inches. But, due to the compact size, heat dissipation capability is limited.
Inertia is much lower compared to a hysteresis brake with same torque rating. For applications with low acceleration, inertia is not important. But, winding systems with indexing motion with fast starts & stops such as high production electric coil winding require fast accelerations. The minimal inertia will not add significantly to desired torque & tension, even during moderately high acceleration.
Although slip torque is very smooth, there is some slight stick-slip at very low RPM. Avoid applications where smoothness is very important at exceptionally low slip RPM (less than about 10 to 30 RPM). Unwinding a web at 10 RPM may be fine if the web in rigid. If the web is very elastic, sometimes even 30 RPM may be too slow, depending on the web path. If the distance from the payout roll to the first nip roll is long, the web acts more elastically, than if the distance is short. As the nip roll pulls the web material, the web stretches and straightens out, then the brake releases, causing the web to contract & droop. This occurs quite quickly, and is observed as a vibration in the material.
Although life is long, avoid constant duty applications with high slip RPM & high torque. The powder particles eventually wear, which causes torque to decrease.
Torque is produced without friction, by interaction of magnetic fields across an air gap. There is no wear and no torque decrease, even at high RPM and high torque. Life is nearly unlimited, except for the ball bearings, which last for years if not abused.
Heat dissipation is superior to magnetic particle brakes & clutches since size is larger for the same torque. Also, the rotating part that produces the torque & converts the mechanical power into heat (that must be dissipated) is exposed more directly to air, for better cooling.
Torque produced by the hysteresis process is perfectly smooth, even at near zero RPM.
But, there are some drawbacks. Cogging is the major characteristic to consider and is often misunderstood. Cogging is pulsing output torque. The hysteresis brake (or clutch) shaft tries to lock into preferred positions (between 10 and 30 positions, proportional to the size). Cogging can occur at any RPM.
Cogging can be avoided by proper electrical input. To avoid cogging, never drastically decrease input voltage and current while there is no slip (Brakes: zero shaft rotation; Clutches: zero difference between input & output RPM). Decrease voltage and current in a ramp vs. shaft rotation. If the electrical current is ramped down, even very slowly while the shaft is not rotating, the brake (or clutch) will cog.
To de-cog, energized the brake (or clutch) to the previous high input or higher. Then, ramp down the electrical input while simultaneously rotating the shaft through about ½ turn or more. (For clutches, rotate the input & output shafts so there is ½ turn slip.) Manually decogging can be quite an inconvenience. Placid Industries patented Cog-Buster decogs the hysteresis brake automatically. A permanent magnet inside an aluminum housing de-magnetizes the internal rotating cup, in the same manner an erasing head on a tape recorder erases magnetic sections on recording tape.
Shaft inertia is much higher, so accelerations must be gradual in winding systems, to avoid stretching or breaking delicate webs & wires.