I've got two of them, first one was a first-issue, second one is a third issue.
My first issue is one of the very few mechanisms I've ever seen that scared me. The reason is that it's like a diesel, really, and that driveshaft thing is just rolling along for laughs. But there is one set of gears on the wheel/driveshaft of each truck, so that the trucks MUST stay in step with each other or they start fighting each other through the driveshaft.
I've popped the universals off the end of the crankshaft on mine TWICE, and each time it was reason for near heart-failure. So in the repair process, I've observed that while there's a lot of 'slop' in that design, you can still screw it up royally, get that truck gear moved over a tooth or two in relationship to the worm and suddenly you've tightened up the drivetrain and the universals, instead of happily riding along, are now under tension with each other. I VERY CAREFULLY checked the twist limits on both trucks against each other, centered both of them, then repaired that connection and it held. I'm also very careful to run it very slowly, and remember at all times that you can never run a Shay too slow. Top speed around 8mph.
But if you pull the motor off, or a truck off, and simply reassemble it, you may not realize what you've accidentally done to put that driveshaft under stress. If you do, sooner or later, something in that driveshaft is going to 'give'. So if you have one that is running nicely, be happy, and don't, whatever you do, take the motor or trucks apart unless you carefully record the exact position of every rotating part you mess with. There's not another mechanism like this on the planet that I know of. The only other one that drives me as nuts are old Atlas classics that develop razor noise for no good reason after reassembly and I finally have to resort to the Beardon Blockectomy.
I really love my Shays, I really do, but I still fear for my sanity when I run them, and I never take the trucks off or the motor off without seriously debating if it's worth it. And anybody knows me that I'm recklessly fearless when it comes to messing with mechanisms. Not this one.
If I was redesigning it, I'd make one of those sliding driveshaft joints a round tube-in-tube so that there was no way this kind of tension could happen, and only one truck would spin the shaft at the cylinders. If the trucks got out of step with each other, no harm, no foul. I just wonder how many of these have to go back to Atlas for repairs, and what process they use to reset the trucks vs. the shafts.
