Hi Cyberjack, sorry I missed your post on the failure of SB Diplomat plastic gears.
Gears are usually made from thermoplastic, most often nylon, and they do age in the presence of moisture, ozone, photochemical smog, and various other noxious conditions. The modest temperatures likely in the Diplomat drum drive gear should produce very little effect. Hence I suspect the problem here is a combination of the design of the gear, and given the date of manufacture, the likely absence of reinforcing fibres in the plastic. You will notice that the broken Diplomat gear shown above, has not changed colour noticeably: it is still a rather pristine white, which suggests the material may be in good condition.
My guess is that if the design had provided either a greater radial thickness of plastic around the hub, or the fibre reinforcement normally used in plastic gears for the past 20 years, there would not have been a problem. Note that I am not certain fibre reinforcement was not used, I am merely speculating that this might have been the case, given the date of manufacture.
I may be missing something, but I don't see why SB could not have had the gear mould remachined as soon as the problem became known. It looks as if all that would have been required is a very simple lathe or milling/boring operation on the mould. Instead they provided what must have been very expensive aluminium gears as a service fix. Mind you, the Diplomat may not have been a sales success, so long-term solutions may not have been a priority.
