3D printed plastic will be a huge part of building Sawppy the Rover, but 3D printed plastic has some very definite limits. The chassis construction technique article covers use of aluminum extrusion beams for large fixed elements, this post discusses the steel parts involved in moving elements: ball bearings, steel shaft, and e-clip holding them all together.
Every part of Sawppy’s body that requires a rotation motion will have some variant of this assembly. The bearings are commodity “608” type ball bearings. High quality capable bearings are expensive, but mechanical demands for a little rover model is quite modest so they don’t need to be very good. In fact, bearings that fail QA for their primary purpose might work just fine here. Which is good, because such rejects are how some of the lowest priced vendors on Amazon and eBay get their inventory.
These ball bearings can take load both along their radius and along their axis. What they can’t take very well is a torsional load. This can be mitigated by using a pair of bearings side by side to balance the load. The wider apart the pair are, the better they can resist twisting.
Inside this pair of bearings is an 8mm steel shaft cut to length as needed by the design. When the shaft is used to transfer torque, detents are cut into the shaft so we can install couplers with set screws. And when the shaft needs to stay in a certain place relative to the bearings, grooves are cut into the shaft so retainer clips can be installed to keep shaft in location.
This general pattern is used in the following locations on Sawppy the Rover’s body:
- Driving: Each wheel has one of these assemblies so the driving servo motor does not have to bear rover’s weight, it only has to drive.
- Steering: Each corner has one of these assemblies so the steering servo motor does not bear the rover’s weight or physical loads from wheel motion, the servo only has to steer.
- Suspension: Each wheel bogie pivots on one of these assemblies at the end of a rocker, and each rocker pivots on a similar assembly where it joins the body.