Oven Baking

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

Why is oven baking time such a hot issue?

Time is money
     ...is the short answer.

And it's why the desire to shorten deliveries to customers, even on straight time work, is so strong. Since bake cycle duration can easily be one of the biggest chunks of time in scheduling a project, it seems to be the perfect candidate for cutting.

But there are technical reasons to keep temperatures relatively low and cycle times in place.

Baking is used both to cure materials, and to create an environment which replicates that
that found in operation. Controlling both the temperature and duration of the bake cycle is important to avoid overheating. During bake-off, for example, overheating can result in a reduction in motor efficiency.

Although today's insulation products can withstand higher temperatures, the resin compound requirements for curing must still be met. Further, the modulus of any given material will only allow it to accept a specific amount of thermal soaking. Raising the temperature unfor-tunately cannot speed this process. Fortunately, with the manufacturing software, process improvements and expedited shipping options available today, deliveries can still be improved, putting more of your time (and money) to the bottom line.

“Bake cycle duration is calculated to obtain the greatest differential expansion of the copper segment pack to the steel assembly. This results in the highest molding pressure on the mica, forcing it to its most stable operating situation. Shortening thermal cycles adversely affects commutator stability under rotational stress.”
— From the professional notes of R.F. Sharrow, PE GE Commutator Design Engineer, 36 years