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Motion System Design

Magnetic particle clutches, brakes offer clean torque control

Many web tensioning processes and other converting applications require smooth and adjustable torque while maintaining a clean and dust-free environment. For these processes, magnetic particle clutches and brakes are often a suitable solution. These clutches and brakes used for tension control or torque limiting are electromagnetically actuated and consist of input and output members with a cavity between them. Within that cavity are magnetic particles, which are basically very small metallic particles. An electromagnetic coil is built into the input to the unit.

When dc power is applied to the coil, it creates magnetism — thereby causing the magnetic particles to group together within the cavity and thus connect the input and output. The strength of that connection is based on the amount of power applied to the coil and therefore the amount of magnetism generated. For example, full power causes particles to group together as a solid mass, while low power allows particles to slip against each other.

Several features of magnetic particle designs make them a wise choice in web processing applications. Because the particles are contained within the cavity, there are no wear particles as might exist in a clutch or brake with a friction design. Processes for medical products such as sterile bandages or pharmaceuticals benefit from the clean operation of these magnetic particles designs. Processing of electronic or optical products can also benefit from this dust-free feature.

What's more, the ability to engage across a wide range of input power makes magnetic particle designs excellent for applications such as tension control or torque limiting where a less than fully locked-up condition is an advantage. Frictional units can suffer from a stick-slip behavior when low engagement force and low speed are applied. In magnetic particle clutches and brakes, the ability of particles to slip against each other eliminates this behavior, even down to single-digit rpm's.

The primary constraint on magnetic particle units is that they must be sized to handle both the torque and heat dissipation of their application. Because many are used in tension applications where a constant slip is occurring, heat can be a constant feature and excessive heat can degrade the magnetic capacity of the particles.

Sizing a unit is a relatively simple process: Torque is determined as it would be for any product: T = hp × 63,025 rpm. In its simplest form, heat can be calculated as Watts = 0.0118 × torque × slip rpm. Selecting a unit that meets both criteria ensures a long performance life. Beyond that, selection is simply a matter of selecting the appropriate voltage of coil and unit bore size for the application.

For more information, contact Greg Cober, tension specialist at Warner Electric, an Altra Industrial Motion company, (815) 389-6423.

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