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Permanent magnet, permanent power transmission

Aug. 1, 2004
When frequent disengagement wears too heavily on mechanical couplings, magnetic designs transmit power without the physical contact that leads to deterioration.

Magnetic couplings transmit high torque with no physical contact, and they disengage smoothly when load gets too high. Generally speaking, the couplings use magnetic power in three different ways; here we’ll discuss the two types (hysteresis and synchronous designs) that don’t require separate controls.

Hysteresis couplings utilize high-strength permanent magnets on one hub (with flux intensity B x field intensity H = 28 or more) and lightly magnetized hysteresis material to serve as the magnetic surface on the opposing hub (usually with BH = 5 or less). This provides a smooth disengagement and maintains the preset torque level at output, but keeps the torque value relatively low. In contrast, synchronous couplings utilize opposing high-strength permanent magnets on both hubs. This doesn’t allow for disengagements as smooth as those accomplished by hysteresis units, but the much stronger magnetic field delivers more torque in the same size coupling.

The third way to transmit loads magnetically is with Eddie currents. As mentioned, the separate electronic controls required by Eddie-current couplings do complicate designs. Some setups use separate coils and controls to induce magnetism; others rely on the currents generated by the motion of one hub rotating inside another. While these couplings are effective, hysteresis and synchronous couplings use permanent magnets for driving power that is always on.

Synchronous units

The operation of a synchronous coupling is similar to that of a synchronous motor. The couplings consist of two multiplepole magnetic cylinders. The poles on one hub line up with the opposing poles on the other hub — north to south. This creates very strong magnetic fields, which force the hubs to rotate in sync. If torque load becomes high enough to overcome the magnetic field, the magnets slip and try to align themselves with the next set of opposite poles, giving the mechanical feel that is generally associated with a synchronous motor.

Torque is produced by the magnetic field across the air gap between the two cylinders, so a reduced air gap increases torque carrying exponentially. Similarly, a widened air gap quickly diminishes the torque rating. On some flat disc-style synchronous couplings, changing the air gap also has an effect on the axial loading of the shafts. Loading increases when the air gap is reduced. For this reason, proper bearing support should be taken into account.

Generally speaking, the coupling style does not determine the type of magnet, rather the application does. Because neodymium iron/boron (Nd FeB) magnets deliver more flux per volume than samarium cobalt (SmCo), they’re naturally the first choice. However, as applications grow harsher and temperature becomes an issue, SmCo magnets become more suitable. A general rule of thumb: For applications less than 240°F, NdFeB magnets are best. Above that, SmCo magnets can handle temperatures to 600°F. Because the operation mechanism is magnetic, transmission of rotational movement is exact.

Two synchronous types

The two synchronous coupling styles operate the same way, and can used in all the same applications: transmission of torque through a barrier, torque limitation, quick disconnect, and high offset. Their difference is in their magnet layout.

Cylindrical designs can transmit over 8,000 lb-in. of torque. These synchronous barrier-type couplings (which can be hermetically sealed) slip when overloaded, thus reducing the risk to connected pumps or other equipment. The couplings are designed as two parts that nest one within the other, producing a magnetic field across the air gap between them. Magnets are placed around the outer diameter of a smaller hub, and around the inner diameter of a larger hub. The smaller hub is then inserted inside the larger hub. The outer member is typically attached to the driving motor, while the inner member is usually attached to the driven system.

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This design provides complete axial stability so high torque requirements do not pose a problem. In fact, there is almost no limit to the amount of torque this style of coupling can transmit. The result is an extremely smooth, constant torque. Also, if the coupling transmits motion to inside a tank, an indent piece can be mounted to the system frame between driver and follower to keep fluid from the outside environment.

Disc designs. If space is tight and a nesting-cup design takes up too much space, a disctype magnetic coupling is sometimes more appropriate. The coupling’s two plates are flat and have magnets mounted on the inner faces. Unlike nesting- cup styles, disc couplings don’t need a special indented interface if they’re transmitting torque into a tank. Flat discstyle couplings are constructed with magnets attached to flat plates that face each other with an air gap (and sometimes a wall barrier) in between. This design is typically used where the simplicity of a flat barrier is desired, or in lower-torque applications. That’s because torque is limited by the amount of axial loading that occurs in higher-torque applications.

On the plus side, misalignment tolerance is increased, and magnetic attraction between the two members tends to be powerful. Following an overload, these couplings also act as clutches to disengage and slip (even when transmitting through a barrier) to reduce the risk of damage to connected equipment.

Hysteresis units

Recall that hysteresis is the lagging of magnetic induction behind a changing magnetic field. As a hysteresis coupling’s strong permanent magnets pass over its lining, they magnetize and de-magnetize it. This hysteresis process is what gives this coupling the smooth disengagement and consistent torque (over the entire motor speed range) at the coupling output. Hysteresis units feel like a smooth friction clutch, but are much more accurate — with no wearing friction. Hysteresis magnetic couplings have the same general construction as synchronous cylindrical designs, with two parts that nest without touching for maintenance-free operation.

In case of overload, hysteresis magnetic couplings become clutches, disengaging without the abrasion and jerks that limit the wear life of conventional mechanical-type couplings. Following an overload, the torque transmitted remains at the set torque level, which holds the tension.

Tension-control designs. These permanent-magnet hysteresis couplings transmit torque and work well in tension-control applications — even under constant slipping, as in bottle cap tightening or packaging machine tension control. With one half locked down, the other half always operates under a consistent and smooth tension. As there is no friction to cause wear and tear, hysteresis clutches maintain their accuracy much longer than any other type of tension control device. Winding and unwinding applications also benefit from the design.

Quick comparison

Permanent-magnet couplings transmit torque by magnetic forces between internal and external rotors. As sealing elements in pumps and agitators, they guarantee a hermetic separation of drive and driven side, serving as reliable seals to prevent serious leakage. This is especially helpful where critical liquids like aggressive acids and lye are present.

Most permanent-magnet magnetic couplings consist of three components:

• An external rotor, usually fixed on the drive shaft (motor side)

• An internal rotor, usually fixed on the driven shaft (pump side)

• A containment shroud, flange-mounted to the (pump) housing.

Hysteresis magnetic couplings work by slippage or lagging. The drive hub starts out by accelerating before the driven rotor. After a certain time, the driven rotor may or may not run synchronously. In some cases the rotor is in continuous slippage; this means hysteresis magnetic couplings generate more heat than the synchronous magnetic couplings.

In synchronous magnetic couplings the speed of the outer drive hub and the inner rotor are the same; there is no real slippage during the operation. However, there is a twisting angle between the rotors during the start-up of the motor and later during the permanent operation. To know the exact twisting angle during the acceleration of the drive (within 1.5 sec up to 1,600 rpm, for example) we must know the mass moments of inertia of the driven and the drive. Then we can calculate the load on the coupling and the twisting angle between the rotors. The approximate twisting angle after the start-up phase during the permanent operation can then be calculated.

Andrew Svabas, Product Manager of KTR Corp., Michigan City, Ind.

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