I had a Kitchen Aid of the Hobart era for many years and abused it
much, mostly grinding meat and making sausage, but also with 20 loaf
batches of whole grain breads while there were 3 kids at home. There
was a plastic/nylon toothed gear in the planetary head that would
fail after much abuse, and I disassembled and replaced it several
times before the bearings on the motor went out and the motor shaft
was damaged beyond practical repair. The disassembly was not
terribly difficult, with getting the head gently pried apart from the
housing being the most difficult step. The gear is held in place by
a small pin that was friction fit and once the pin was removed the
old gear came off and the new one was installed in a few
minutes. The symptoms were that the motor would run but the beater
would not move.
I still have one of those gears if someone wants it - send me an
e-mail with an address and I will mail it on.
Ken in Juneau