I spent the rest of my down time thinking about the drive mechanism. The belts and pulleys I want to use are from Maedler North America. I've kept the front half of the pulley layout the same, but I've decided to couple the front and rear sprockets to each other using the longest belt I could find (1440mm!). This will lower the amount of hardware needed to complete the drive. But it will also give me the points from where I can drive the wheels at the two different rates (road and mud).
Here's the design so far:
Here's what the long belt loop, the road gear currently looks like:
The orange dashed lines are idler pulleys that I sourced, but don't have a model for. They would be mounted on jack shafts and would spin freely. The position of the idler all the way on the left would be adjustable along the axis marked by the black line, so the belt can be tightened. This is the "mud" gear:
The two pulley sets floating in space will be connected to the frame with long bolts to make them adjustable, so those belts can be tightened. The belts are all the same length. The long belt and the mud gear belts are both attached to the front wheel sprocket.
The shaft that is driven by the pilots runs through the "Y" frame in the middle and has two pulleys that are engaged independently from each other. Here is the "road mode" drive belt:
And here is the "mud mode" drive belt:
I've been wracking my brain over how to switch between modes. The gears have to spin independently of the shaft they are sitting on, and somehow still engage the shaft on command. Everything I've seen about transmissions uses elaborate bushing and expensive gears. Then I went back to the lawnmower I acquired. It has a shifter with forward, neutral, and reverse.
A quick youtube search on lawnmower transmission brought me to this:
This guy cut open the housing, cleaned all the parts, and then explained how the whole thing is put together. The relevant part is around the 3min mark. The shifter plate spins with the shaft and couples to the output gear using two metal pins that lock into holes drilled into the side of the shifter plate.
I think I can use this same idea to shift between mud and road. I would drill two holes into the side of each drive pulleys and press pins into them that align with the holes in the shifter plate. Then, moving the plate left or right would engage one mode or the other. Here's the gearbox, so far:
I think the shaft probably needs to be a d-shaft and the shifter plate would need a slip-fit d-shaped hole. The plate can't be secured by set screw, since it has to slide back and forth freely. I could probably use a round shaft and grind flats into it in the right places, but the shifter plate would need a set screw low enough to engage the flats.
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