This summer, I started bicycling a bit more. But I’ve been having an issue: both my current prosthetic and my spare are slightly shorter than my other arm (by choice). This has caused some imbalances and minor back pain while biking.
There are actually devices out there geared towards upper extremity amputees, made for riding a motorcycle or bicycle. But how could I resist the experience of designing my own?
The process began with part selection: A tooling ball was the basis for a ball-and-socket design. This stainless steel part is a spherical ball connected to a shaft, which would have to be bridged to my prosthesis.
The second part involved creating a locking mechanism to securely hold the prosthesis. Six ball-nose spring plungers, which are set screws with a spring-loaded ball head, surround the center. These rest slightly above the midpoint of the sphere within the socket, allowing the tooling ball to be held securely in place.
Initially I had 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.
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° intervals 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.
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.
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.
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, there is only slightly more control compare to 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, until hitting a pothole. I installed the 2-5lb version of these, and even then they could be tighter.
- 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.
By the end of producing the device, I had improved on my machining know-how, and put together a system that’s working better than what I had before. Up next: college consuming all of my free time, followed by an automotive version of this device.