Monday, July 25, 2011

Custom bevel gear box

I took an unexpected break from fabrication this past week.  We live in Minneapolis, Minnesota, in the northern US, a state that borders Canada (thanks to all of the international readers I've had so far).  Summers are usually warm, but not too hot.  The past week was a record heat wave of the decade.  We had the same temperatures as Texas for most of the week.  I wasn't motivated to work on the boat dripping with sweat in the garage.  Fortunately, the weather is now back to normal.

I have been doing more prep work on some of the remaining panels.  However, the biggest update is that the gear box parts were complete, and I now have them assembled.  The gearbox is similar to the recent custom boxes Rick has made.  He uses pillow blocks for bearing mounts, and this allows for easy manual machining in his garage shop.  I don't even have a drill press, so I knew I would be sending out these parts to a local job shop with CNC capability, and I designed a similar box with flange bearings.  It requires some precision holes for the bearings, but most shops should be able to manage.  The motivation is to get a box that should be able to hold up during a sprint, and that is fairly light weight, and has an efficient gear ratio without need for a chain to reduce further.  The gear ratio is 4:1, higher than any stock gearbox available.  It weighs in at 5.4 lbs including crank arms.

The housing is 4 x 6 x 1/4" aluminum extrusion.  I had a small spacer block welded in to provide a gap between the two output shaft bearings.  Everything is held in place with stainless dowel pins and aluminum shaft collars.  The crank shaft is 17 mm (this is the largest flange bearing size I could find) with taper square ends for standard bike cranks.  The output shaft is 3/8" stainless.  I left it long because this end will telescope inside the drive shaft tube to allow for leg length adjustments.

We had some machining hiccups and I also needed to fine tune the fit a little between a few pieces using sand paper, but overall the result looks really good.  I think it should hold up really well.

Total time up to this point (just for the gearbox assembly): 5 hours.


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  3. Did you ever finish this? Can we get a bill of materials and instructions? Or can we just buy some finished ones from you?

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