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.

Options for repairing broken commutator risers

If the risers are broken at the brush track or only slightly beyond, your options are severely limited.


Inserting new risers is not a viable option, since the heat required to braze the new riser in place would quickly burn and destroy the segment mica between the bars. Soldering is also not recommended, due to the likelihood of contamination of the segment insulation. In addition, solder will rarely withstand the operating temperature requirements of the unit.

T.I.G. welding extensions is not an option due to the small amount of riser material remaining attached to the comm. In this situation, there are typically two possible options remaining for repairing the problem:

Refilling the commutator - involves reusing the steel core, but manufacturing new copper, risers, and insulation. Reinsulating the commutator - involves reusing the steel core, and the copper bars (on large units), replacing the risers and insulation. Note: reinsulation is not typically an option for glassband commutators, which we will address in a future issue of Motor Fax.

If the break is further along the riser, the repair is much less complicated.

T.I.G. welding extensions is a good solution to this problem, assuming that the commutator is otherwise in good condition and that there is still substantial brush life left on the unit. To effectively perform this repair, the risers need to be thoroughly cleaned, removing all carbon contamination.

New risers should be fabricated from the same material used in the original unit. This is typically half hard, oxygen-free copper, in thicknesses ranging from 0.020" to 0.125". Riser extensions should be bent prior to installation, allowing 0.002" in width and 0.312" in depth greater than coil sizes for easy coil installation.

For recurring problems with cracked risers due to vibration, you may want to consider adding a row of lashing to help minimize this effect.

Shown below are some of the more common riser styles, to help in your identification.

How much damage can flux contamination cause?

Enough to kill a commutator.

When soldering coils into the risers of a commutator, the worst potential consequence is flux contamination, especially in solid riser comms. The same flux that cleans the copper to permit the solder to adhere uniformly to the risers creates a potential for contamination that can destroy a commutator. Flux contamination can occur during the soldering process when flux and/or excess solder seeps away from the riser slots and finds its way to the underside of the commutator. Because the flux and solder are conductive, they render the mica insulation useless. When the insulation can no longer prevent electricity from jumping from bar to bar, the commutator shorts out. Once the mica insulation is contaminated, almost nothing can be done to restore its insulating properties. When the commutator shorts out due to flux contamination, there are usually only two options: Reinsulate (replacing all mica insulation throughout the commutator) or rebuild.

When inspecting a commutator for flux contamination, look for discoloration of the copper bars. The discoloration may take several forms. Streaks of solder and flux are silver in color; carbon residue where arcing has occurred is black; contaminated copper bars can take on a darker, mottled and greenish coppery hue, or the contaminated area may just appear lighter than others. Compare the rear dovetail to the front, since the front will be free of contamination. As always, when taking a comm apart, be sure to band it tightly and bake it before attempting to remove the v-rings.

There are several ways to reduce the risk of flux contamination during the soldering process:

  • While soldering, angle the comm so that flux and solder run away from, and not toward, the bottom of the comm.
  • Use flux and solder sparingly.
  • Flux and solder small sections of the comm at a time.
  • Use a rosin-core solder to reduce the need for flux, or a flux- based solder so the solder contains the flux.

If the commutator is contaminated, some solvents will clear minor contamination, but call your ICC representative at any time for advice on how to proceed.

If you have any technical questions don't hesitate to call, we can talk you though the troubleshooting process. 865-983-7444