So I was pretty confident that my initial guess for a 3/4″ drive shaft would be sufficient. I was wrong.
It turns out wood is HEAVY and with the sprockets mounted 5″ from the bearings you will pull the sprockets down about 1/8″ when you step on the wall (for a 250lb climber). Combined with some bending in the frame this would put us past our allowable total deformation. And the shaft will bow ~1/2″ in the center, not that you will be able to see it but that seems excessive to me. Also it means there is no way you could simply scale this design up if you wanted a 6′ wide wall, which I think would be a nice option.
By increasing the shaft to 1″ the bending at the sprockets will go down to ~1/32″ and 3/16″ at shaft center. Probably an acceptable amount. This led to a new problem: there is now no room for the previous bearings, with the stamped housing, to mount between where the planks run. After shopping around I found some cheap bearings in a chevron mount for around the same price, $12US. Though now the shaft needs to be longer and the gearbox hangs further out.
Changing to a 1″ shaft also makes it very hard to find reasonably priced speed reducers with a hollow output shaft. Going to a solid output shaft will require a coupler (35$) and leave our gearbox hanging waaaayy out, but it should come out a little cheaper than the original gear box. As shown below. I suppose it doesn’t look crazy but I am not excited about it.
If we are changing gear boxes anyway a new possibility opens up. The reason the drive sprocket was so small (below) was to keep torque down on the gearbox, allowing us to use a smaller cheaper gearbox. This choice necessitated the use of an extra idler sprocket next to the drive to spread the chains apart and leave room for the walls internal structure.
But if we go to a bigger, higher torque gearbox and a correspondingly bigger drive sprocket we could skip the extra idler, saving a bit of money, and extra parts and messing around trying to line everything up. Like so:
The catch is that the internal friction in a bigger gearbox might require a larger motor and larger motor driver circuit. Which would then be more expensive. The wall is largely driven by climber weight, so pretty much any motor will work, but it would be nice (but not necessary) if the motor could move the wall without someone on it. I am kind of torn on how to proceed. Any thoughts you might have would be helpful.