The goal was to end up with wheels narrow enough to fit inside goBILDA channel along with the following components:

• Two laser-cut plates for each drop pod (enabling drive mode switching)

• A drive system, in this case plastic chain from goBILDA chosen for its lightweight construction and strength characteristics

• Shaft spacers & clearance for all rotating parts to increase efficiency and reduce unnecessary wear

This left me with about 24mm of usable space, setting the requirements for the custom wheels. 

• A 3D printed core houses two radial bearings for a dead-axled drive

• Laser-cut 1.5mm steel side plates bolt on to the outside using m4 screws, which can be countersunk for even tighter tolerances

  • Since my goal was to interface with the goBILDA infrastructure, these holes are located on a 16mm square around a 14mm hole.

• Each roller is composed of a 2mm hardened steel shaft, two MR52 bearings housed in a steel core, cast 70A polyurethane, and miniature e-clips for shaft retention.

While our FTC team did not have the funding to build the entire octocanum, I was able to make the wheels on my own, and eventually integrate them into an in-channel mecanum drive (without the drop pods & switching capabilities unfortunately). The next iteration will likely avoid casting polyurethane, because this was a massive pain with such small, finicky parts. If you ever need to make custom mecanum wheels, I wrote a profile generator for rollers that you can check out here. 

• The final drop pods need about 15 degrees of rotation

• Actuation should be fast, with little backlash. 

• The entire assembly must fit between the walls of goBILDA channel (43mm total width)

• The output should be non-backdriveable and isolated from input, so that shock loads from terrain do not break the servo, and to reduce strain from continuous servo use.

I'd contemplated using a screw drive for actuation, but it ended up being very difficult to support and orient in a way that would effectively raise and lower the output. Instead, the primary mechanism uses what's called a two-way overrunning clutch. Essentially, four circular pins act as wedges around a hexagonal shaft, all inside a cylindrical housing. As the output attempts to backdrive the shaft, the pins are forced like wedges into the corners between the shaft and housing. The 3D printed input cam has the shape of two parentheses (    ) and when rotated, disengages the pins from their corners and pushes the input shaft.

In the summer of 2022, during my internship at goBILDA, I had the good fortune of being able to build this particular component with physical components. 

In this video, I'm using a high speed, low torque servo with the dual overrunning clutch mechanism coupled to its output. Rotation in both directions works as normal, and I was unable to backdrive the output. (not a scientific test but ehh)