Extension Banding and why it's CRITICAL

Check out this image of bad banding versus good banding.  See all the gaps and holes? 

Those tiny little slits between the banding and the steel put this commutator (on the left) in a vulnerable position. The fine particles of carbon that come off the brushes find their way into any little gap or hole.  Enough contamination will lead to a short or even worse, a failure.  To get the most life out of your commutators check the banding frequently and if needed replace the banding.  

ICC is more than happy to help. Call or email anytime if you have questions. We can walk you through what to look for and even guide you through the process, we'll even tell you exactly what products we recommend using.

When should you dip a commutator in varnish?

There are several good reasons to make sure that you never subject a v-ring commutator to varnish or VPI, all of which are critical to the unit's operation.

Commutators are designed with gaps throughout (see Fig. 1). This allows for differential expansion and contraction of the various materials in operation, and results in successful operation over many years. If varnish is introduced into these gaps, the commutator can no longer perform as designed and the varnish can cause three distinct problems:

Overheating: When dipped with varnish, these gaps are filled, which inhibits cooling and can often result in overheating in specific areas of the commutator.

Imbalance: Uneven distribution of the varnish may result in imbalance of the armature. For example, if the unit is dried horizontally, the varnish will pool to one side, and within the confines of the commutator, it may never entirely cure. This material can then result in the overheating noted above, but also in imbalance in operation.

Shorting: In addition to the problems noted above,exposing the commutator to any impurities in the varnish can also result in failure due to shorting bars. Though most repair facilities keep their varnish as clean as possible, minimal impurities which would not affect coils, will potentially bridge the small spaces between commutator bars.

What should you do if you receive a commutator that has been dipped?
Depending on the severity, the comm may indeed need to be refilled. However, if after having banded the unit and taking it apart, you discover that the varnish deposits are minimal and contained mainly to the dovetail area, you may be able to simply clean the dovetails and replace the v-rings. Sanding or taking a very light skim cut should do this effectively.

For tips on v-ring replacement, see Motor Fax "Replacing V-Rings", or call us for information.

How do you avoid loosening a comm when you take it apart? And why is it important?

It's important because a loose commutator will cause endless amounts of problems in operation. High bars, chipped brushes, and poor commutation make your job of checking for commutator tightness an important part of routine maintenance. 

How is tightness built into a commutator?
In manufacturing, the copper and mica segments are assembled in a circular form. The resulting "segment pack" is checked for skew and angle. A steel ring which has been machined to the rough OD of the commutator, is then compressed over the segment pack in a press. The tonnage used will vary depending on the size of the comm, but can easily range from 10 tons to 90 tons. The resulting compression provides the commutator tightness which all following procedures are designed to maintain. In some instances, the commutator may be banded to obtain additional stability.

What keeps a v-ring commutator tight?
The dovetail angles that are cut into the segment pack are the foundation for keeping the commutator tight. With the compression ring still in place, the segment pack is assembled to the steel caps and hub, and insulated with mica v-rings. The caps seat against the internal angle (typically 30°) and retain the compression after a series of thermal closings under torque and tonnage. When the compression ring is removed, this assembly keeps the commutator tight. In effect, we have created a spring, which is held tight by the pressure exerted internally.

How do you take a comm apart without releasing the tightness?
Since the steel caps are keeping the commutator tight, removing the outboard cap will immediately release this pressure, loosening the comm. It is almost impossible to get the compression back in the segment pack once it has been released. Before pulling a commutator cap, always band the brush track, preferably with a resiglass tape, applied hot and cured (see banding material specifications for temperatures). You should be applying the tape at 300-400 psi, covering approximately 3/4 of the brush surface, and building it up to about 1/4" per side. To finish, wrap 8-10 wraps over your tucking loop to ensure that the banding stays in place. Alternatively (though not preferred), a steel ring machined to between 0.010" and 0.020" smaller than the brush diameter (depending on the size of the unit) can be heated and applied over a nomex sleeve covering the brush track. The ring should be first measured cold before installation, and then measured again once cooled after installation to ensure that sufficient fit has been obtained. With banding or a ring in place, the cap can now be safely removed, and the v-rings replaced or other maintenance performed.

Checking for tightness.
During routine maintenance, bolt and nut v-ring commutators can be checked for tightness by applying a specific torque to the bolts, or to the nut with a spanner wrench. Typically, torque values are approximately 50% of the maximum rating for the bolt grade and size. If you have questions regarding a specific unit, call us for a recommendation

When is reinsulating a comm an option?

Usually if it has inserted risers, is a v-ring style commutator, and uses over 1000 pounds of copper. 

There are, of course exceptions to this. If the commutator doesn't use quite that much copper, but it has a very thick or very wide copper bar, reinsulating may still be a great cost saving alternative.

What is replaced in a reinsulated commutator?
Typically, reinsulation refers to the replacement of segment mica between the bars, and replacement of the mica v-rings and mica tube. It should also include new risers, and any 
peripherals like lashing or wrapped caps.

Why would you want to reinsulate instead of refill?
Cost. Reinsulating a commutator typically doesn't save any labor hours, since each bar has to be cleaned thoroughly before re-use. However, if the comm uses a significant amount of copper, the material savings can quickly amount to several thousand dollars. On a smaller comm, however the material savings would be quickly outweighed by the disadvantages of reinsulating.

When shouldn't you reinsulate a commutator?
When there is limited brush life left, and when the copper is damaged in some way.
If the copper has been overheated and annealed, it will no longer be re-usable. If the dovetails have been bent or cracked, a refill will be necessary. Finally, if there is insufficient flat on the interior of the comm to allow for removal of copper for reassembly, reinsulation would not be

How can you tell if reinsulating is possible?
For thorough inspection and final determination, the commutator will have to be entirely disassembled. While an external inspection will show the condition of the bars and the amount of brush life left, inspecting the dovetails and copper hardness requires individual bar inspection.

Can you reinsulate a solid riser comm?
Not typically, and not cost effectively. Solid riser comms are slotted, and the slots would need to be plugged to withstand the new compression for reassembly and subsequent machining. In addition, in reinsulating, in order to maintain brush diameter, mica thickness is increased. On a solid riser comm, this would also increase the riser diameter, and would change the slot depth and position.

What about glassbound commutators?
Glassbound comms are a totally different design. Since the copper shrinks to a mica wrapped hub, any additional removal of material would adversely affect the fit. In addition, once grooved for banding, the copper material remaining under the band is very thin, and unlikely to withstand compression.

Having trouble with banded commutators?

Glassband commutators were first designed by General Electric in 1960 for their redesigned line of 580 and 8000 frame machines, and later for their MD800 Armored Motors. Although performance of these commutators is good, end users and motor repair shops sometimes give them less than favorable reviews because of the difficulty in field repair of the units. With v-ring commutators, bolts can be tightened and vrings replaced, but glassbound commutators are designed to be virtually maintenance-free. In addition, the glassband comms are significantly lighter in weight than their v-ring equivalents, and under normal duty requirements, routinely out-last v-ring units by 50%.

However, how many motors do you see operating under anything but "normal duty requirements?" The problem arises when end-users in high contamination environments end up having to replace bands on a far too regular basis.

The only restraining force on glassband comms are the res-iglass bands. This material is made from high tensile glass yarns laid parallel and bonded with fully catalyzed thermosetting resins. The glassband commutator is set with an interference fit of approximately 0.030" to a mica wrapped and cured steel hub. The retained interference fit, measured by the growth of the segment pack, should be, at minimum, 0.015". The inside of the segment pack is bored smooth, specific to the cured mica-wrapped hub. Glassband grooves are cut after the segment pack is assembled to the core. Finally, the commutator is banded to an OEM specified number of wraps at 500 lbs PSI. It is then baked for several hours at approximately 300 degrees. The bands are applied multi-stage to ensure maximum strength.

Most repair shops will apply a coating of Viton® to the bands prior to the commutator going into operation; this step should not be omitted. If the bands have not been coated, carbon can creep behind the band, causing burning from the inside out. Though this might not be seen in a visual inspection, it can easily result in the failure of the unit.

What you should consider before converting from a glassband to V-ring comm:

Converting a glassband comm to a v-ring is an alternative to dealing with ongoing problems. But there are several things to consider when presenting this as an option to your customer:

A v-ring style commutator will be significantly heavier than the glassband it replaces. The v-ring design requires not only the addition of a hefty steel part including a spool and two caps, but also a substantially wider (and therefore heavier) copper bar.

As a result, performance may be affected and the weight gain should be measured against the motor as a whole and the application for which it has been designed.

V-ring comms are more familiar, but they do require ongoing maintenance such as checking for  tightness. However, if your glassband is being rebanded once a year, v-ring tightening is going to be a welcome change.

Converting to a v-ring will mean a substantial cost outlay (typically 3x) compared to the cost of a refill. However, when weighing the cost of contant band replacement, the capital expenditure associated with the conversion will often make sense.

Conversion to a v-ring is a major redesign and requires both engineering and production experience from your commutator manufacturer. Ensure that you are being asked to approve conversion drawings, and ask for references.

Your commutator supplier should be able to help you explain this alternative by providing you with support materials and technical references. If this is a topic which applies to one of your customers, let us know, we'll be glad to help.