Sawppy Field Repair: Servo Coupler

Every time I encounter problems at a public Sawppy appearance, it is a lesson. It might be a lesson in how I might be able to improve Sawppy’s design, or it might be a lesson in how to better prepare a kit for field repairs a.k.a. Sawppy First Aid Kit. When a fuse burnt out at Caltech Science for March 2019, that was easy to address: add replacement fuses to the kit.

The broken servo coupler encountered at SCaLE 17x was a little tougher. It wasn’t enough to keep an extra 3D-printed coupler handy, because a little block of plastic isn’t very useful by itself. It requires a heat set insert to accommodate a set screw holding against an 8mm shaft, and it requires four more screws to fasten against the servo horn. Fiddling with a lot of tiny screws would not be very practical when trying to make field repairs away from my workbench. This is especially true when I have a literal ‘field’ to deal with: on grassy grounds where a lost screw is very difficult to recover.

Sawppy servo coupler FRU

The answer to minimizing chance of lost screw is to prepare a replacement assembly consisting of:

  • 3D-printed coupler
  • Heat-set insert already installed.
  • Set screw already installed inside the heat-set insert.
  • Servo horn that comes with a serial bus servo.
  • 4 small screws already fastening the coupler to the servo horn.

With such an assembly ready to go, the only fastener I risk losing during replacement is the screw holding the servo horn to the servo output spline.

The first downside of this approach is that I only have as many servo horns as serial bus servos. Keeping a replacement unit ready in Sawppy’s field repair kit means I have an extra serial bus servo with no horn to use it. Perhaps I could purchase extra servo horns, or maybe this is just a hint I should also have an extra replacement servo in the bag.

The second downside of this approach is that, in case of steering couplers, a replacement would not be identical. Steering trim would be slightly different with a replacement coupler due to natural variation from unit to unit. When I perform this replacement, I would also have to adjust Sawppy’s steering trim for proper operation. But in an emergency? It’ll probably be close enough.

(Cross-posted to

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