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Bicycle prosthetic

This summer, I got back into bike riding quite a bit. Being in a major city is great, especially when located in an area that’s accommodating to bicyclists through plenty of bike lanes. But I’ve been having an issue, in that both my current prosthetic and my spare are slightly shorter than my other arm. This has caused some imbalances and minor back pain.screenshot-from-2016-09-30-14-55-47

There are actually devices out there geared towards UE amputees, made for riding a motorcycle or bicycle, with a price tag starting at $2,000. At that cost, how could I resist firing up CAD software, and designing my own?

The process began with part selection: A tooling ball was the basis for the ball-and-socket design. This stainless steel part is a spherical ball connected to a shaft, which would have to be bridged to my prosthetic somehow.

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The second part was in creating a locking mechanism. Six ball-nose spring plungers, essentially spring-loaded set screws, surround the center, and rest slightly above the midpoint of the sphere within the socket.

Initially I planned on using a large steel spacer as the substrate for the tooling balls, however I had available a CNC mill to custom mill a solid block of aluminum for both pieces.

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The bike portion consists of a 3-1/8″ x 2″ plate aluminum, one side milled to a 3/8″ depth with a half-cylinder matching the 1″diameter handlebar, and secured to the handlebar with two clamping U-bolts. The other side is CNC milled to a 1.5″-diameter cylinder approximately 7/8″ in length, with a round socket bored out slightly larger than the tooling ball. This socket was also chamfered at a 60° to accommodate the shoulder of the tooling ball.

Six #10-32 threaded holes were tapped at 60° increments about the collar to receive the ball-nose spring plungers, and during assembly the spring plungers were set to equal distances from center such that the tooling ball would be held in with some force.

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For the connecting bracket, another block of aluminum approximately 5-1/4″ x 2-3/4″ x 1-1/2″ thick was CNC milled, then the excess 1/2″ plate used to hold the piece being milled was taken off, and edges were filleted to 100 mils. A 1/2″-20 hole was drilled and tapped to receive a connecting rod, and a 3/8″ hole was added for the tooling ball. Finally, a #10-32 set screw socket was drilled and tapped for holding the tooling ball.

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After initial assembly, the receiver was mounted onto the left handlebar. To adapt the angle bracket to my prosthetic, an old hook was stripped for its 1/2″-20 collar for inserting into the locking socket on the prosthetic. A 1′ length of 1/2″-20 stock was cut down appropriately, and a couple nuts were used to secure this piece rotationally to the bracket and the locking collar.

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Success! Besides trying some new things in CAD and getting extra familiar with operating the machinery my favorite local machine shop had to offer, I also learned a few things about this kind of system:

  • No wrist locking: Before, in using a hook, there was some side-to-side movement restriction which allowed riding with just the hook. Now, it seems that to take my right hand off the handlebar, it’s about as great control as riding with no hands.
  • 1.5-to-3lb ball-nosed spring plungers are the minimum force. Having six of them does lock down the tooling ball nicely, but it could be more secure. I’ll be ordering the 2-5lb version of these eventually.
  • No grabber action. A hook is great for being able to be controlled. This device can’t be used to hold anything, and the cable from the prosthetic isn’t doing anything useful while this contraption is in use.
  • Extra intimidating block of metal: If a guy on a bike with a decent-sized Hossmer Model 6 work hook wasn’t dangerous-looking enough, hopefully the even bigger block of shiny metal will send a message to the occasional bicyclist-averse Philly driver to back off. (I couldn’t help myself.)

All in all, I learned some things, and put together a system that’s working better than what I had before, for about $100 and some hours.

Posted in CAD