High Tensile Sprocket Bolts

[Tom Line]

Fun times. Our 2010 soccer bot was 8 wheel drive. The AM sprockets were small enough that we had to use spacers for the chain to clear the wheel hub.

We have sheared off close to 50 bolts. Grade 8, partial thread. We know they are tight, because we were tightening them so MUCH that we started bending the sprockets and compressing the plastic of the plaction wheel.

Simply put, the combination of AM sprockets, with AM spacers, on AM plaction wheels, can't hold up over time. The 10-32 bolts will sheer every time if you're putting serious loading on them. This is on a 4 inch wheel with 24(ish) teeth on the sprocket. 2 cims per side, run through tough boxes. A 3:2 sprocket ratio from the tough box to wheels.

AM could address this by putting locking indentations on both the plaction and the sprocket, and then putting bumps on the spacer so that when the three are clamped together, the bumps/buttons take the shear.

Bolts stink in shear. Tension is their game.

This year, we went with bigger sprockets so that we could get rid of the AM spacers. We never sheared a single bolt. Lesson learned.

[Chris is me]

Originally Posted by Tom Line

This year, we went with bigger sprockets so that we could get rid of the AM spacers. We never sheared a single bolt. Lesson learned.

We just used it as an excuse to switch to live axle. Works well so far.

[AdamHeard]

Spacing off the sprocket will increase the likelihood of breaking bolts. We've never sheared 3 bolt patterns of the same diameter in situations with higher loading, but the sprocket was aligned to the shaft and there was no gap between the items being fastened.

For teams that keep failing the full 6 bolt pattern, have you tried replacing 2-4 bolts with shear pins?

A while ago I noticed the holes in Am's sprockets are .206", which is a bit big for a #10. This slop (and not accounting for it by aligning the sprocket on the shaft) is further weakening the interface.

[Trent B]

Inherently by using spaces you are cantilevering the bolts which extends the amount of shear stress it is feeling. I would go with adams earlier part suggestion of a hex shaped nut that can screw onto the bolt in fit in the placation wheel holes, thus the bolts will be rigidly held relative to the wheel hub and have extra support (via the nut) for the period they extend beyond the surface of the plaction wheel.

Shearing is greatly related to the amplitude of stress, if you can see concentric semi circles that all seem to be surrounding one point on an edge of the sheared face and expanding as they cross the face it was likely fatigue failure.

Steel bolts, if below their fatigue limit should never fail due to fatigue, if you notice a fatigue pattern (you can find pictures online) you need to reduce the load you are putting on the bolts through some means.