By Zak Khan

Hysteresis refers to a specific magnetic phenomenon. Certain magnetic materials, called ferromagnetic materials, have a tendency to remain magnetized to some degree even after the magnetic field they are subjected to is removed. This “memory of magnetic history” is referred to as hysteresis. This also means a magnetic field in the opposite direction must be used to drive the magnetism down in ferromagnetic materials back to zero.

These properties are useful in couplings because couplings ideally should engage and disengage whatever they are holding together smoothly and consistently. Additionally, permanent magnet couplings work without contact wear and have far lower friction. In a hysteresis coupling, one side is a ring or disc shaped piece made from a permanent magnetic material that is ferromagnetic and thus able to undergo hysteresis. The other side of the coupling is then made of an electrical conductor back with iron, again usually in a disc or ring shape. The ability of the coupling to “remember” magnetic fields and because they start one way, reverse, and then reverse again in a cycle means that hysteresis couplings “slide” smoothly, that is, they can take up motion in the system, and heat up less than other designs.

Hysteresis couplings are useful when an application requires a constant moment that needs to be maintained over a wide range of revolutions. Additionally, unlike some other couplings, hysteresis couplings do not “ratchet” when they exceed their synchronous force threshold. Instead, they still operate smoothly. They are also quieter than some other coupling designs.

There are some drawbacks to hysteresis couplings. Materials that exhibit hysteresis come in limited sizes. Because ferromagnetic materials can lose their magnetic properties above certain temperatures, certain operating conditions in high heat settings may not be suitable for hysteresis couplings. Engineers considering hysteresis couplings should consider these variables in their systems. Manufacturers provide relevant operating conditions and product specifications for their coupling designs.

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