If you could make a precise rigid coupling for a joint between a can motor and a gear box would it be a mistake? This is on a brass ho steam engine. Motor would be nailed down.
Depends first on whether there is any misalignment between the axis of the motor and the input to the gearbox. Tubing is often used to facilitate relatively smooth accommodation, whereas a rigid shaft might need universals at each end and perhaps a slip joint to permit slight length change to run quietly. I have seen pinned rubber discs used in a kind of giubo arrangement (Alco Models BP20) to allow rigid shafts without splines or Cardan joints.
It would be a disaster. Truck moves in every direction. Floating shaft transfers power and adjusts for this motion.
Trucks on a steam engine?
An approach that was used in the past was a long, fairly stiff wire from a tender-mounted motor through or under the cab and firebox to a gearbox at one of the driver axles. This was the only practical way to motor small power like Civil War-era 4-4-0s for many years. It could be surprising how unobtrusive this could be if the wire were painted black in its visible length.
A similar thin wire drive might be an answer to replacing “tubing” without having to go entirely to a rigid shaft.
I wound up using a piece of the insulation from a 14 ga. stranded wire. Using wire strippers I cut a piece about 3/8 long and heated it slightly with a torch while it was pushed up a scribe. Made for a tight push on fit and so far is working fine. Ordered a coupling kit somewhat like athern diesels use for the next one.
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