Tension Control: Mag Particle Clutch/Brakes Provide Smooth, Clean Torque Control

Published: December 01, 2008, By By Greg Cober, Tension Specialist, Altra Industrial Motion

There are many web tensioning and other manufacturing processes that require adjustable, smooth torque while maintaining a clean, dust-free environment. For these converting processes, magnetic particle clutches and brakes are an excellent solution.

Magnetic particle clutches and brakes used for tension control or torque limiting are electro-magnetically actuated and consist of input and output members with a cavity between them. Within that cavity are magnetic particles (very small metal particles). An electro-magnetic coil is built into the input to the unit.

When DC power is applied to the coil it creates magnetism. That magnetism causes 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 by the coil. Full power causes the particles to group together as a solid mass, low power allows the particles to slip against each other.

There are several key positive features of magnetic particle designs. 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 magnetic particles designs. Processing of electronic or optical products can similarly benefit from this dust-free feature.

In addition, the ability to engage across a wide range of input power makes the 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 the particles to slip against each other eliminates this behavior, even down to single-digit RPMs.

The primary constraint on magnetic particle units is that they must be sized to handle both the torque and heat dissipation for their application. Since 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 x 63025/RPM

In its simplest form, heat can be calculated as Watts = 0.0118 x torque x slip RPM.

Selecting a unit that meets both criteria will ensure 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.

Greg Cober serves as tension control specialist as well as training manager for the Warner Electric div. of Altra Industrial Motion. In 2008, he marked his 30th year in the industry. Greg has worked in two different sales territories in upper New England and in the Chicago metro area. He also served as product manager for vehicular products during the 1980s. He has more than 20 articles published in industry trade magazines in the past ten years. Contact him at 815-389 6423, This email address is being protected from spambots. You need JavaScript enabled to view it..