Clutches and brakes with an electromagnetic interface offer definite advantages, including accurate engaging and clean releasing. Besides being quick, they also run cooler and cleaner, with the convenience and controllability of electrical components. Here we take a look at the four general friction and non-friction types, how they work, and when they’re most appropriate.

Magnetic particle

When load torque is less than output torque, a magnetic particle clutch drives without slip. When load torque is increased, the clutch will slip smoothly at the torque level set by the coil input current.
Select figure to enlarge.

In magnetic particle brakes, an output disc (attached to the output shaft) sits untouched inside a housing. Remaining empty space within the housing is filled with magnetic shavings or powder that remains free-flowing until acted on by a magnetic field radiating from a stationary coil, embedded in the housing. When the coil is energized with dc power, the powder solidifies into chains along magnetic field lines, fixing the disc to the housing, and stopping the load. Only a few common materials show enough magnetism to do this effectively; these include iron, nickel, and chromium.

Less common magnetic particle clutches work in the same way; however, the stainless steel powder fills an empty space between a cup-shaped input rotor and an output shaft armature. A dc-energized coil in the housing locks these input and output devices together by magnetically exciting the powder between them; current induced in the rotor determines magnetic field strength and the amount of particle bonding, which in turn determines the amount of torque transmitted.

Magnetic particle clutches and brakes are infinitely adjustable making them particularly useful in tensioning and positioning applications where continuous changes of speed are required. Because they wear so little, these clutches and brakes are appropriate for synchronous and otherwise heat-inducing slip operation.

Hysteresis

In these clutches and brakes, hysteresis losses transmit constant torque for a given current. Used mostly in fractional power applications, they exhibit almost no wear. Brake units consist of a fixed magnetic pole assembly and a moving drag cup, which constitutes a rotor. The rotor is suspended by shaft bearings into a close-tolerance groove in the assembly; current applied to a coil in the pole structure creates a magnetic field in the groove. As the rotor turns, its magnetic particles do a constant flip-flopping in an attempt to stay magnetically aligned with the groove’s field. Braking resistance results from the hysteresis heat losses resulting from the molecular friction in the pole and rotor.

A coil on the pole assembly generates a magnetic field in the assembly and drag cup. Hysteresis losses in the cup cause the flux to change more slowly than through the assembly, which transmits smooth torque through the drag cup.
Select figure to enlarge.

Though a slight eddy current effect is always present, full rated torque is independent of slip speed, the relative speed between rotor and pole assembly. During normal operation the rotor’s magnetic orientation is constantly realigned by its rotation and by coil current changes; this dynamic operation results in smooth transitions between torque levels for coil power adjustments. However, it is possible to set up a jumping pole condition on the brake rotor which results in pulsating cogging torque, also called torque ripple. It occurs when input current is greatly reduced and shaft rpm is low. Happily, this inherent hysteresis brake characteristic can usually be avoided or effectively controlled. Some units even feature automatic decogging.

Unlike brakes, hysteresis clutches have pole assembles that are free to move when driven. When magnetized, the drag cup’s specific hysteresis behavior supplies clutch linkage; hysteresis losses in the cup cause the flux to change more slowly through the cup than through the pole assembly. In this way, constant, smooth torque is transmitted through the drag cup as it is forced to rotate in the pole assembly groove.

In applications where electrical power can’t be supplied to a clutch or brake’s coil, permanent magnets can provide hysteresis braking. Permanent magnets are of hard magnetic materials with domains that stay in an aligned orientation, even in magnetic fields. By manually moving the magnets, the amount of magnetism acting on a brake’s output rotor can be adjusted. Also, because there are no electrical connections a brake unit can be used as a clutch. In this case, the pole assembly drives the rotor, and torque is transmitted through the magnetic air gap. These brakes and clutches are best suited to applications with fixed torques.

Continue on page 2