Continuing the tour of micro Sawppy beta 1 (MSB1) from the ground up, the rear steering knuckles are attached to the suspension bogie. In Sawppy V1, the bogie’s attachment joint to suspension rocker had a mechanical design oversight: there were no mechanical limiters to rotation angle. I didn’t realize it was even a problem until Sawppy over-rotated during a climbing demo and flipped the bogie assembly, pointing mid and rear wheels skyward. Not only does it render Sawppy immobile, but it also puts a ton of stress on the wiring harness. I designed and installed a crude limiter to prevent wire breakage, but solving the problem more elegantly was on my to-do list. MSB1 represented the first attempt.
The joint itself is an adaptation of Sawppy V1’s design. A pair of bearings are embedded in the printed joint. On Sawppy a length of 8mm metal shaft is used as rotational axle, and the bearings are held in place with E-clips cut into the shaft. I wanted to move away from those clips so here I’m trying Ameer’s idea of using fasteners directly. In the case of MSB1, M3 screws.
A washer is used as spacer to give the bogie a little room to rotate without rubbing against the rocker arm, this aspect was the same as on Sawppy V1. (After I took this picture I realized the washer is shown installed on the wrong side, they’re supposed to be on the far side of the camera.)
I was wary of bearings damaging the M3 thread, but hoped the rover is light weight enough that it would not become a huge issue. That turned out to be secondary to a bigger problem: this design is very sensitive to how tightly the fastener was torqued down. In Sawppy V1 this wasn’t a problem because the e-clips stay in their slots. But as this screw is tightened, it presses down along bearing center axis. Too much pressure, and the bearing could no longer rotate smoothly. Too little pressure, and the joint rattles. This is a problem shared with a few other designs like the 4tronix rover, which mentioned this specific issue in their online assembly instructions.
I put that on my list of new problems to think about, and moved on to the old problem of over-rotation. I 3D-printed several ideas to interrupt rotation in some way and eventually settled on this design:
A little C-shaped piece of plastic that can clip into place over the M3 screw. One end goes over the head of the screw, and the other end covers the nut on the opposite side. This keeps its center portion in place between the rocker and bogie suspension components, preventing over-rotation. This draft is bulkier than it needs to be, I will thin it down for future revisions so it looks less like a big wart on this bogie’s connection to suspension rocker.