Archive for September 1st, 2010
A discussion on that post reminded me of this old project: replacing the chain pulleys in the midships chain tensioner on my Tour Easy recumbent.
The problem is that the original pulleys used steel bearings in a plastic race, for reasons that I cannot fathom. They last for a few thousand miles, then get very wobbly and noisy. The solution, as nearly as I can tell, is to replace them with pulleys using cartridge bearings.
This is what one looks like after four years slung below my bike. Surprisingly, the bearings still feel just fine, even though they’re not really sealed against the weather.
Gotcha: the OEM pulleys are not the same OD / number of teeth as pulleys found in rear derailleurs.
Soooo, after a bit of Quality Shop Time, I had these…
This is where you really want an additive machining process, as I turned most of a big slab of aluminum into swarf while extracting each pulley.
The first step is to drill holes around the perimeter where the chain rollers will fit, plus drill out as much of the center bore as possible. Then mill down to the finished thickness across the roller holes and helix-mill the bore to size.
Flip it over and mill the other side to the proper thickness.
Run it through the bandsaw to chop off all the material beyond the outer diameter.
Grab what’s left in the three-jaw chuck and mill around the perimeter to get a nice clean edge.
And then it Just Works. I made another for Mary’s bike, but she said it was too noisy (which is why they used plastic rather than aluminum) and I swapped it for a Terracycle idler.
This is from back in the Bad Old Days before EMC2’s version of G-Code supported loops. You don’t need to see that code, trust me on this.