8/12/2019 Magnetic Couplings FAQ 2012

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1. Can coupling be used without a pressure barrier? YES. The pressure barrier is only required

if a pressure differential is expected between the two components or if the areas must be sealed

2. What is the torque / speed behavior for hysteresis and eddy current couplings? Eddycurrent devices have a straight-line torque-speed characteristic, with the torque increasing linearly

with the speed by virtue of the fact that

the induced currents vary in magnitude

with the speed of the rotating field. The

eddy current torque coupling converts

energy into heat in the conductive device.

If the heat is not dissipated, it will increase

the temperature of the conductive device

and cause the drop off of the torque. Inhysteresis drag devices, the work done

per revolution is constant and independent

of time. So torque is constant vs rotational speed. If the hysteresis component is conductive, it will

also generate eddy current torque. As shown in the accompanying graph, the eddy current torque is

proportional to rotational speed. So the torque curve of hysteresis coupling can be shown below.

3. Is there a speed limit to these couplings? The maximum speed of a coupling is very difficult to

quantify. This depends on the design and application. Mechanically, the system can be designed

and manufactured to handle virtually any speed (balancing and banding). Eddy currents induced in

conductive media are typically the limiting factor in any magnetic coupling. These eddy currents lead

to reduced force transmission and heating that may rival the efficiency of induction heaters in very

high velocity applications.

4. Are there size limitations for these devices? Virtually any size and shape can be produced.

Torque couplings ranging from the mNm range to the kNm range have been designed and

manufactured. Linear couplings from the mN to the kN range have also been designed and

manufactured.

5. Do magnetic couplings self support or float? Magnetics is a tricky topic that lends itself to ideas

of levitation. Unfortunately, magnetic fields are never perfectly balanced and will always cause

assemblies to shift to the lowest energy state. Consequently, all magnetic couplings require full

mechanical support through radial and thrust bearings.

6. What happens if I exceed the force/torque rating of my coupling? Luckily, due to the non-

contact nature of these devices, they are inherently tolerant of overloading. For synchronous devices

(Class I), and overload leads to a ratcheting effect as like poles repel one and other. If the vibration

does not cause damage to the system, the device will either recouple (torque couplings) or can

be reset (linear couplings) once the load is returned to normal. For eddy current devices (Class

II), permanent damage may occur as a result of eddy current heating. The amount of damage and

heating is proportional to the amount of time the overload is active. For short durations, the system

will simply recouple.

Magnetic Couplings

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