Magnetic Assemblies

Magnetic Coupling

A magnetic coupling is a device that is capable of transmitting force through space without physical contact.  Attractive and repulsive magnetic forces are harnessed to perform work in either a linear or rotary fashion.  In its simplest form, a magnetic coupling is comprised of two components: a driver and a follower.
The driver is the portion of the mechanism connected to the prime mover (motor).  Through magnetic interaction, the follower reacts to the motion of the driver, resulting in a non-contact transmission of mechanical energy. This non-contact power transmission has multiple benefits, including but not limited to:
1.  Isolation of components
o   minimizes or eliminates mechanical vibrations through magnetic damping
o   allows for the insertion of a mechanical barrier between the driver and follower to separate environments and allow operation under pressure differentials
2.  Highly tolerant of misalignment between the prime mover and load
o   Axial
o   Radial
o   Angular
3.  Speed variation and regulation between prime mover and load
1, Classifications of Magnetic Couplings
Three classifications of magnetic couplings are available, depending on the intended use of the device:
Class 1 - Synchronous
Class 2 - Eddy Current
Class 3 - Hysteresis

Synchronous (Class 1)

As the name implies, this coupling is a synchronous version that inherently results in a 1:1 relationship between the motion of the driver and follower. As taught in grade schools, like magnetic poles (North-North and South-South) repel each other while opposite poles (North-South) attract. Synchronous couplings exploit these “attractive” and “repulsive” characteristics to produce motion. By placing an array of alternating pole permanent magnets (N-S-N-S) on the driver and an equivalent array of alternating pole permanent magnets on the follower, a “coupled” magnetic circuit is produced with each North and South pole in the driver linked to each, respective, South and North pole of the follower. As the driver moves with respect to the follower, the magnet poles start to overlap one and other, leading to a “push-pull” effect, and, consequent motion. The magnitude of the resultant force depends not only on the amount of overlap, but also on the chosen magnetic material’s characteristics and separation distance between the driver and follower. At some displacement, however, the peak force producing capabilities of the coupling are achieved.  Displacement beyond this point results in a decoupling. This decoupling manifests itself as a ratcheting action resulting from like magnetic poles is the driver and follower repelling each other. Unlike its mechanical equivalent, however, the decoupling does not, generally, lead to permanent damage; and synchronization is reinitiated at the next magnetic pole coupling point.

Eddy Current (Class 2)

This coupling is an asynchronous version that relies on a speed mismatch between the driver and follower to produce a force.  An array of alternating pole permanent magnets (N-S-N-S) is placed on either the driver or follower, and an electrically conductive material (typically aluminum or copper) is placed on the mating component.  When the driver is translated with respect to the follower, an electrical current is induced in the conductive material, which results in a magnetic field that opposes the permanent magnets and “couples” the two components.  Amperes Law governs the relationship between the induced electric and resultant magnetic fields.

The magnitude of the resultant force is directly linked to:

1.  Speed differential between the two components
2.  Magnetic material characteristics
3.  Resistivity of the conductive media
4.  Separation distance between the driver and follower. 

Unlike the Synchronous Coupling (Class 1), this asynchronous version is a “lossy” device and prone to Ohmic loss heating resultant from the induced electric fields.

Hysteresis (Class 3)

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.

*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 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.  The details of these configurations are found below.

Torque Couplings

•   Coaxial – these types of magnetic couplings are configured so that one member of the coupling is fully nested within the ID of the second member.  The two components share a common axis about which both rotate.

o   Axial misalignment – Very tolerant.

  1.In fact, it can easily be designed to accommodate very large axial misalignment if required.

  2.Radial misalignment – Tolerant.

  a.The amount of tolerance is based on the spacing between the driver and follower.  The larger the spacing, the greater the tolerance to radial misalignment.  Large radial offsets in closely spaced coupling may lead to excessive radial loads on bearings.

  3.Angular misalignment – Tolerant

   a.The amount of tolerance is based on the spacing between the driver and follower.  The larger the spacing, the greater the tolerance to angular misalignment.

o   Face to Face – these types of magnetic couplings are configured so that the magnetic flux is transferred about the flat end faces of  the cylindrical assemblies.  The two components are attracted to one and other axially, and typically require additional thrust bearing support for proper integration.

   1.Axial misalignment – Mildly Tolerant.

   a.The amount of torque transmission is directly proportional to the axial spacing and number of magnets utilized in the design.  Small variations in air gap may lead to large changes in torque

  2.Radial misalignment – Highly Tolerant.

  3.Angular misalignment – Tolerant

  a.Due to the relationship between torque output and axial spacing, high angular misalignments may lead to unexpected reductions in torque

Linear Couplings

•   Tubular – these types of magnetic couplings are configured so that one member of the coupling is fully nested within the ID of the second member.  The two components share a common axis about which both translate.

o   Axial misalignment – Tolerant.

  1.Inherently, linear couplings align axially.  As such, any misalignment will lead to the driver pulling the follower into position.

  2.Radial misalignment – Tolerant.

   a.The amount of tolerance is based on the spacing between the driver and follower.  The larger the spacing, the greater the tolerance to radial misalignment.  Large radial offsets in closely spaced coupling may lead to excessive radial loads on bearings or shafts.
  3.Angular misalignment – Tolerant

  a.The amount of tolerance is based on the spacing between the driver and follower.  The larger the spacing, the greater the tolerance to angular misalignment.

o   Planar – these types of magnetic couplings are configured so that the magnetic flux is transferred about the flat end faces of the magnetic assembly.  The two components are attracted to one and other and typically require additional thrust bearing support for proper integration.

  1.Planar (direction of motion) misalignment – Tolerant.

   a.Inherently, linear couplings align axially.  As such, any misalignment will lead to the driver pulling the follower into position.

 2.Planar (perpendicular to direction of motion) misalignment – Very tolerant

  a.Designs can be produced to constrain 2-DOF if required

 3.Angular misalignment –Tolerant

  a.The amount of angular misalignment depends on the air gap between the two members

Help me designs

1. What type of coupling is required?
1. Linear
2. Torque
2. What topology are you considering?
1. Face-to-face (torque coupling)
2. Coaxial (torque coupling)
3. Tubular (linear coupling)
4. Planar (linear coupling)
3. How much force or torque would you like to transmit?
4. What Class of coupling are you considering?
1. Class I - Synchronous
2. Class II - Eddy Current
3. Class III - Hysteresis
5. What speed will the coupling be traveling? (velocity or RPM)
6. Is a barrier between the driver and follower required? If so, what pressure differential would you like the design to accommodate?
7. Across what temperature range will this coupling operate?
8. Are there corrosive elements or fluids that need to be taken into consideration? If so, what type are they?
9. Geometric requirements:
1. Driver
1. Shaft size
2. Mounting type
1. Set screw and key
2. Compression (threaded shaft end)
3. Taper Lock (not available on all sizes)
3. Max. OD
4. Max. Length
2. Follower
1. Shaft size
2. Mounting Type
1. Set Screw and key
2. Compression (threaded shaft end)
3. Taper Lock (not available on all sizes)
3. Max. Length
10. Bearing support (radial and axial) is typically provided outside of the coupling system, but can be accommodated in the design. Does bearing support need to be designed into the coupling?
11. Is dynamic balancing required (for rotary systems)?

Materials

1. Magnetic materials – Application dependent. Typically based on thermal and corrosion resistance requirements

2. Electrically Conductive materials – Typically based on cost and size constraints

3. Driver and Follower Structure – Application dependent.  Typically based on corrosion resistance and cost constraints.

4. Barrier – Typically based on pressure and speed requirements

Help me designs

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?

 Eddy current 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 resulting in non-linearity (see accompanying torque curves). In hysteresis drag devices, theoretically, the work done per revolution is constant and independent of time resulting in the torque being 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.

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) an overload leads to a ratcheting effect as like poles repel one another.  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.