Extending machine life with insulated couplings
Machinery manufacturers often use electrically insulated couplings in their equipment to prevent arcing across bearings and other components, Figure 1. The arcs result from electric motor shaft currents caused by:
• Shaft electromotive force (circulating current).
• Shaft magneto-motrice force (local currents).
• Capacitive coupling between winding and the magnetic structure.
• Irregularities in the magnetic circuit that generate electromotive force between the shaft ends.
The problem is that an electric motor, when energized, develops a voltage differential between the shaft ends. In many cases, the voltage forces current to flow from one end of the shaft to its supporting bearing, then through the machine housing and on to the opposite bearing and shaft end.
Although it’s usually preferable to cure the basic cause of a problem, it is not practical to eliminate the voltage differential between shaft ends. This leaves engineers with two options: either provide a preferred path for the current, or interrupt the electrical circuit. To divert the current, they install a grounding brush on the shaft (or sometimes on coupling hubs or spacers); to interrupt the current, they typically insert insulation between mating flanges of the shaft coupling.
Grounding brushes
Installing a grounding brush provides a preferred path for current generated in the shaft, thereby preventing damage to bearings caused by arcing. Grounding brushes for low-speed machines (up to about 3,600 rpm) are relatively simple devices. But they are an expensive solution for high-speed machines because their bristles are often made of silver, and sometimes are gold plated.
A calibrated spring pushes the brush against the shaft, and an adjustable stop controls its travel. Adjusting the travel is very important. If the stop is too far from the shaft, the bristles soon lose contact with the shaft; if the stop is too close, the rod holding the bristles may contact and damage the shaft. In one case, an improperly mounted brush eventually cut a tubular coupling spacer in two.
To avoid problems with grounding brushes, follow a few simple rules. First, don’t economize in selecting a brush — a cheap one can damage the shaft. Second, carefully follow the manufacturer’s instructions for installation and periodic replacement. Make sure the stop is properly adjusted. Letting the bristles wear until they barely make contact causes electrical-discharge milling (EDM) of the shaft through arcing, Figure 2, and subsequent loss of grounding.
Insulated couplings
Though some motor manufacturers insulate motor and generator bearings, their efforts are often nullified by the electrical conductivity of couplings, which transfers the problem to the driven-machine bearings. Rather than insulate the driven-machine bearings, OEMs ask coupling manufacturers to supply units that are electrically insulated.
The degree of electrical conductivity that couplings provide can be:
• Perfect. Flexible metal elements in diaphragm or disk couplings make excellent electrical contact. As a result, the couplings are not affected by shaft current.
• Imperfect. Lubricated couplings make poor contact because the current must flow through a thin film of oil or grease. If arcing occurs due to the current, it causes damage to the torquetransmitting elements.
• None. Insulated couplings and elastomer- element couplings are perfect insulators. Insulated couplings are difficult to manufacture, and need to be carefully handled to avoid damaging the insulation.
Manufacturers insulate couplings by installing a nonconductive disk between the coupling flanges. This also requires insulating the flange bolts, which results in a “busy” design incorporating plastic disks, washers, and bushings.
The insulating components must carry the torque, and this is not an easy task because the materials used are weaker than steel. But the real problem is that the insulating parts must also provide piloting surfaces between the coupling components they connect. These insulating parts are fragile and often become damaged during coupling assembly or disassembly. In one example, the insulation of a large coupling had to be reworked three times before the machine even went into service.
Another concern with electrical insulation is that it increases the coupling’s torsional flexibility, which causes a lower torsional resonant frequency of the connected machines. In some cases, this lower frequency is more conducive to vibration.
Insulation methods
Here are the most common methods for insulating different types of couplings, and the limiting factors associated with such insulation.
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Gear couplings. The flange-to-flange insulation is provided by a disk (usually fiber mat epoxy), and the bolt insulation by tubular bushings and washers under the bolt heads. In the design shown in Figure 3, the insulating bushings provide the piloting between flanges.
Limitations. Conventional couplings usually transmit more than half of the torque through friction between the flanges. But, the friction coefficient of an insulating disk is significantly lower than that of steel, so the bolts and fiber bushings must transmit all of the torque. To transmit the same torque as non-insulated couplings, the number (or diameter) of bolts must be increased. Otherwise, insulated couplings must be derated.
The insulating bushings may deform and sustain damage during coupling disassembly. So you must inspect and replace the damaged bushings before reassembly.
Diaphragm couplings. Flange-toflange insulation is achieved by a composite disk that also provides the piloting between the hub OD and the rest of the coupling, Figure 4. This disk is bonded to the hub with epoxy. The bolt insulation again consists of bushings and washers.
Limitations. As with gear couplings, the disk’s coefficient of friction is low, and the torque must be transmitted through the bolts. Because of interference that exists at the piloting surface, disassembly can break the bond between the insulator and hub. As the insulator OD is precision machined after bonding, this breakage requires reworking the hub (applying and machining a new insulator).
Disk-pack couplings. Instead of flange-to-flange insulation, some manufacturers offer an insulated spacer between the coupling ends, Figure 5. The insulation is provided by a fiber disk that, through small lips, also provides piloting between the two halves of the spacer. As before, bushings and washers insulate the bolts. With this design, the insulated connection can be left intact for the life of the machine.
Limitations. The insulated bolts are located on a small radius with respect to the coupling centerline, so that a given torque creates large forces on the bolts. To maintain the torque capability of such couplings, the spacers must have a large number of bolts at the insulated joint.
The insulation reduces the lateral stiffness of the spacer, which may limit the operating speed of the coupling.
For more on insulated couplings, call Michael Calistrat at 281-437-4656 begin_of_the_skype_highlighting 281-437-4656 end_of_the_skype_highlighting.
For information on consulting services from Michael Calistrat and Associates, circle 313 on the reader service card.
Michael M. Calistrat is a consultant on rotating machinery and owner of Michael Calistrat and Associates, Houston.