Aussie Sawppy - Hex Drive

A project log for Aussie Sawppy

An antipodean sibling

teamsgTeamSG 04/21/2020 at 22:444 Comments

OK. So COVID-19 has obviously crashed any significant outdoor fun for a while now. We had plans for Sawppy excursions to interesting terrain that were victims of Western Australia's sudden inter-regional travel restrictions. Such is life...

So, the rover upgrades have now gone a bit deeper than they might have done in happier times.

Let's talk shaft couplings.

We've had a fair bit of trouble keeping the adapter / shaft joints tight on the drive and the steer servo motors as well as the steer knuckles. I think Roger's original design is sound and not an uncommon solution, but the materials crash the party.

Metal shaft with detent, metal grub screw, metal threaded insert all surrounded by printed plastic. That plastic's job is to resist concentrated stresses along multiple axes. Plastics tend to deform over time under steady stress and I think that's where the frequently loose shaft joints come from.


I'll throw a rider in here and say that we're using bog-standard PLA across the board on the rover. More exotic plastics may be tougher / more robust and so less problematic.


Anyways, constant mucking with loose shaft connectors arising from Sawppy driving around the house got me pondering. Then @Quinn Morley and his monster rover build showed me the answer.

Hex profile drive shafts! The hex profile means that the motor torque for driving and steering is transmitted over a *significantly* larger surface area of plastic. We're yet to do any serious torture testing, but early indications are all good for a sustainable solution.

Some details...

If you do the calculations it turns out that a 7mm hex shape across the flats is a touch over 8mm across the points - almost a perfect replacement profile for the round 8mm drive and steer shafts. No need for new bearings!

Finding 7mm hex shaped rod was a firstly a challenge and secondly expensive, so we just bought four extra-long 7mm Allen keys to cut up instead.

We had to change to proper circlips (8mm ID) because the original E-clips don't work on a hex profile.
I hacked up the OnShape designs for the servo and wheel adaptors to put a hex instead of round hole in the centre. A bit of dimensional experimentation yielded a snug interference fit on the shaft.

Same mod for the rover's corner steering knuckles - hex hole substitution. There is also now an extruded a cable path through the new knuckles, in lieu of the surface recess in the standard design.

End result is that now the grub screw mostly provides a locating force along the shaft's axis. The interference fits means there's actually not much of that locating force required.

Nothing's coming loose so far :)

Sweet as, bruh'!


Roger wrote 04/22/2020 at 20:58 point

Modifying allen keys is a great idea. I expect they're a stronger grade of metal than generic hex profile stock.

I recognize this mechanism as a weakness in my Sawppy 1.0 design, and it's great to see different rover builders explore alternate approaches.

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TeamSG wrote 04/23/2020 at 00:40 point

7mm is an odd size for off-the-shelf rod / bar stock - so Allen keys were the obvious choice. and yep, the Allen keys are great quality steel.

In terms of mechanical design, the shaft coupling arrangement is the rover's #1 weakness, but the list is blank after that. Anything else I've modified has been about aesthetics, not functionality.

Sawppy 1.0 is solid.

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Quinn Morley wrote 04/22/2020 at 17:45 point

This reminds me, I have a dozen more hex bars to cut! Glad it is working well.

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TeamSG wrote 04/23/2020 at 00:22 point

There's always something else to do. When you think you're at 80% done, you're barely half way there courtesy of latter stage project scope creep.

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