Incredible but true. Glassband commutator fabrication is tricky business, and the cost of getting it wrong is enormous.
Your first visual sign of a segment pack that is loose on the core may be the risers as shown below (image 1). The shift of the segment pack versus the coils becomes visible, ultimately resulting in complete failure (image 2). The cause can be twofold: Worst is if the commutator was installed incorrectly to the mica wrapped hub and the whole assembly spins on the core. Alternatively, the outer bands may have deteriorated (image 3). If the bands have not been coated, or have been subjected to excessive heat, the resin will burn away, or carbon that has infiltrated burns invisibly from the inside out and the bands give way. In this design, those bands are the only thing holding segments in place.
The lesson? Make sure it’s tight in manufacturing with a retained interference fit to specific standards, and ensure that band inspection is part of your routine maintenance, especially in extreme environmental conditions.
For the record, while I normally post photos of things we build, this is the exception and not one of ours!
