Chuck Yung
EASA Senior Technical Support Specialist
Eddy-current couplings (coupling/clutch/brake), while slowly being replaced by variable-frequency drive (VFD) technology, are still in use and will require repairs (Figure 1). As these devices become less common, the expertise pool is also shrinking. On the positive side, their specialized nature ensures that those familiar with these units will be in demand for some time.
An eddy-current coupling allows an electric motor to drive a load where variable speed is needed. The coupling is comprised of a spider and drum, concentric to one another, with multiple bearings and separate shafts (Figure 2). One or more DC coils surround the drum. When DC voltage is applied to the coil, the drum and spider become magnetically coupled. As the DC current is increased, the magnetic field becomes stronger and slip decreases. At a lower DC current applied to the coil, the magnetic field weakens, and the output speed decreases. Of course, the applied load also affects the slip. Under no-load conditions, very little current is needed to couple the drum and spider, so they rotate at the same speed.
While their rugged simplicity made these clutches popular, there are some pitfalls to be wary of. Most have to do with the unique construction features of an eddy-current coupling. Problems can occur in:
- Lubrication
- Concentricity
- Flux isolation
- Sealing
- Corrosion
Lubrication
Access to and lubrication of the pilot bearing of eddy-current couplings is only possible when the machine is stopped (Figure 3). A grease port is drilled through the shaft length, and there is a grease fitting on the shaft, usually near the bearing housing. Since the grease fitting is mounted on the shaft, the pilot bearing can only be lubricated when the machine is stationary. Consequently, of the four to five bearings in a clutch, the pilot bearing is the least likely to be relubricated.
The pilot bearing is usually a roller bearing, while the other bearings are each ball bearings. Since roller bearings require more frequent lubrication than ball bearings, the roller bearing is the most likely to fail.
Concentricity (shaft deflection)
The spider and drum share the same axis of rotation and two common bearings, so the concentricity of all bearing supports and rotating parts is especially important.
When the pilot bearing housing, deep in the end of the drum, is repaired, its concentricity is critical to proper operation of the clutch. If the pilot bearing is not concentric to the drum, the spider-drum interface will cam (tolerance is < 1 mil).
Shaft runout, as normally checked, may be perfect. However, there is only one way to detect if the pilot bearing housing is not concentric. Place a dial indicator (Figure 4) on the output (drum) shaft and rotate the input (spider) shaft. If there is indicator deflection, the pilot bearing housing is not concentric to the axis of rotation. Although the pilot bearing will only cause deflection on the output (spider) end, it is good practice to check deflection on both ends of the assembled clutch.
Flux isolation
For most designs, the drum is built from two carbon steel pieces separated by a non-ferrous band (Figure 5). In those designs, the spider is generally cast as one piece.
Another design (Figure 6) uses a different construction method. The spider is pieced together from several ferrous castings, which are separated by non-ferrous braces. In order to provide good flux separation, the sections of the spider must be magnetically isolated from one another. Manufacturers use non-magnetic stainless steel for those braces.
When cracks appear in these braces, as they often do, it is imperative that the repairer replace them only with non-magnetic stainless steel. Ferrous replacement braces would bridge the spider sections, compromising the flux path and weakening the clutch. Slip would increase and the operator would try to compensate by increasing the DC current. This would generate more heat and shorten the insulation life of the coil.
Sealing
The construction of the clutch rotating elements is such that a “pilot bearing” shown in Figure 3 is located inside the blind end of the drum. Because this pilot bearing is essentially in the bottom of a barrel, access is awkward. Manufacturers normally provide three tapped holes beneath the bearing so that push-out bolts can be used to press the bearing out. If the clutch is water-cooled, these bolt holes are plugged using setscrews that must be sealed with sealant or Teflon tape.
When the setscrews are removed to push the bearing out, it is easy to forget to replace them. When the setscrews are replaced, sealant is required to prevent circulating water from flushing out the grease.
The end brackets and their retaining bolts must also be sealed to prevent leaks. The normal practice is to seal the rabbet fit using silicone or similar product, plus copper washers under each bolt head. Copper washers should be replaced each time the clutch is repaired. Failure to do so increases the chance of leaks.
Leads from the clutch coils are normally sealed using silicone or a potting compound. Depending on the clutch design, the coils may be sealed in a copper “can,” taped and varnished, or integral to the clutch spider assembly. Some designs capture the coil(s) behind a removable casting in the frame itself. That design must also be sealed to prevent excess water ingress.
The Eaton Dynamatic coupling and Elmagco brakes require water flow at slightly over atmospheric pressure. If too much water is introduced, the water level inside the clutch rises and flushes grease from the bearings. Emulsified grease is an indication that the water volume was too high.
The coil is also usually secured by clamping pressure when the removable casting is installed. It is good practice to place rubber or neoprene pads on both sides of the coil to cushion it.
Corrosion
Cooling water also may lead to corrosion problems, especially when the clutch is idled without precautions. If water is left in the clutch, rust will seize the spider and drum together. An important aspect of clutch repair is to use suitable epoxy paints to coat the internal parts. Paint quality should be the best available. Sand-blasting to bare metal, followed by application of a good zinc primer and epoxy paint, is required to prevent rust.
When the interior of an eddy-current coupling is rusty, scale removal may be accomplished by continuous flushing of the cooling system using one products from these manufacturers:
- Calgon Company
- Oakite Products Company
- Ohio Valley Checmical Corporation
Repair of rotating drum
Cooling water circulating through the drum is essential to prevent the drum from overheating and warping. When an operator forgets to start the water before starting the clutch, the flood of cold water (when the water supply is turned on) may warp the drum so badly that it seizes to the spider.
Treated freshwater is recommended for cooling. The use of saltwater for cooling can reduce the life of the eddy-current brake from 15 years to three years or less.
As with an electric motor, the air gap is important. If the spider or drum has been severely damaged, increasing the air gap will increase the slip. Increased slip may be a problem for the user (less production); it also generates more heat in the drum. A 50% increase in the air gap dimension can result in a 70% reduction in braking torque. Rusting converts iron/steel to iron oxide, which no longer carries flux.
In some cases, it is possible to build up the outside diameter (OD) of the spider by welding and then machining it to restore the correct air gap.
Note: The drum inside diameter (ID) was originally copper-plated to resist corrosion, so any remaining copper must be removed before attempting to weld the drum bore.
The thin wall of the drum makes welding and machining difficult, so repair may be impractical. If a replacement drum is to be fabricated, it is important to know that it must be fabricated from two pieces of steel tubing separated by a non-magnetic portion (Figure 5). This magnetic separation is critical to the performance of the clutch. If another vendor has incorrectly repaired the drum or manufactured a replacement from a single piece of material without providing the non-ferrous separation, the drum and clutch will operate at a much higher temperature and the speed control will not function properly.
Welding a new drum requires considerable expertise and fitting. The non-magnetic portion of the drum should be welded to the magnetic tubing before final machining to size. The entire drum should be machined during one operation to ensure concentricity.
Storage of Eddy-Current Couplings or Brakes
If a water-cooled eddy-current clutch/brake is to be placed in storage, an important step is to blow the clutch out with compressed air immediately after testing. The shaft should be rotated monthly to redistribute lubricant in the bearings and to prevent brinelling of the bearings. When testing a clutch in the service center (unless a load is to be applied), it is not necessary to use water-cooling. The slip will be negligible under no-load conditions.
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