Magnetic Hysteresis Couplings

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Magnetic Hysteresis Couplings – Hysteresis

As a hybrid of the Class 1 and Class 2 technologies, this coupling is typically used in an asynchronous fashion as a force limiter, but can be utilized in a synchronous state. An array of alternating pole permanent magnets (N-S-N-S) is placed on either the driver or follower, and an easily magnetized/demagnetized material known as Hysterloy is placed on the mating component. At rest, the permanent magnet array is designed to magnetize the Hysterloy, resulting in a synchronously coupled magnetic circuit*. Should these forces suffice for the application, this coupling will operate in a synchronous state. Magnetic Hysteresis Couplings
*The volumetric force density can be orders of magnitude lower than the Class 1 coupling due to the magnetic characteristics of the Hysterloy.

However, should the prime mover induce forces in excess of this synchronized operating state, the driver decouples from and begins to move with respect to the follower. This motion causes the Hysterloy to cycle through its magnetization loop (magnetize-demagnetize-magnetize) via the permanent magnets on the mating component which are now translating with respect to it. Like the Class 2 eddy current coupling, the magnetic field from the permanent magnets is being utilized and converted. However, unlike the eddy current coupling where the energy from the magnetic field is converted to a flowing electrical current (and heat), the cyclical progression around the Hysterloy’s magnetization loop (hysteresis loop) utilizes the magnetic energy to convert the magnetization state of the Hysterloy material from a North pole to a South pole. As a result of this variant on the energy conversion mechanism, hysteresis coupling are much less prone to (although not completely excluded from) Ohmic heating.

Unlike the fully synchronous coupling which experiences a “ratcheting effect” when it exceeds its synchronous force threshold, this coupling continues to operate smoothly at asynchronous speeds while maintaining the force threshold. This is accomplished without the Ohmic heating inherent to the Class 2 coupling. Consequently, this Class 3 coupling provides a synchronous solution that can be decoupled and operated in an asynchronous state.

Magnetic Hysteresis Couplings are capable of transmitting forces both linearly and rotationally. Consequently, in addition to selecting the Class of coupling required (synchronous, eddy current, or hysteresis), the coupling type also needs to be specified. Two Types of couplings exist:
• Type 1 – Torque
• Type 2 – Linear

As their names imply, torque couplings (Type I) are used to transmit forces rotationally while linear couplings (Type II) are used to transmit forces linearly. As one might expect, each coupling Type also has a variety of geometric topologies that can be utilized to meet the design intent.

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. planar linear magnetic coupler

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.