Hex Shaft Wear Experiences

Technical Technical Discussion
1

We did a post mortem on our AM Toughboxes today after 2 regionals and the championships. We were surprised by the wear on the gear that mates with output hex shaft. The hex shaft worked the gear over pretty well - the robustness of the system was reduced by the gear material selection and the generous pilot hole drilled for the broaching operation, but still it was a surprise.

The gear did not fail, but it is a cautionary example for us. We have been thinking about using hex drives for our wheels, in a west coast configuration, and are reconsidering the material and geometry choices for the wheels given our experience with the AM transmission’s hex drive - a system with minimal lateral stress as compared to the wheels.

We would be very interested to hear from teams who have used hex broached drive wheels in aluminum or softer materials - specifically if they have issues with the hex shaft deforming the wheel hole and if it is something that they design around. Wider hubs, steel keeper plates on the face of the wheel, clamping the wheel using the keeper screw in the end of the axle and a collar on the hex shaft, living with the wear, and swapping the wheels all seem to be possibilities, but perhaps there are simpler solutions - or it really isn’t an issue within some design constraints?

Inputs?

2

With 7075 hex shafts, and 6061 wheels with about 5/8" maybe length-through-bore, we’ve experienced no wear issues. The key is to make sure your broached hole and hex shaft are a rather close fit. I’ve noticed some of the AndyMark ones are a little loose, but I would not be concerned about them rounding out.

3

I believe some of the issues that occur with the Toughbox and other mass produced hex shaft mechanisms, is the fact that they are mass produced. There is a tolerance in which the parts are made, some will be further from perfect then others.

4

We used live axles with cantelevered wheels last year using 1/2" hex aluminum. It worked out very well. The wheels were AMs with hex bore and some omni’s with the hex adapter. The sprockets on the wheel axles were nylon and the problems we had with wear was on the sprocket teeth due to inadequate tension in the chain. We had no wear in the hex parts at all.

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