Seals always have a certain rate of leakage, which is often unacceptable with acids, lyes, and other processed chemicals. Magnetic couplings eliminate seals by allowing the use of containment shrouds.

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.

Select figure to enlarge.

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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