Quote:
Originally Posted by Trent B
I guess where my confusion lies is if you have a physically longer spacer (eg one that goes into the holes on the wheel as well) would that offset some of the shear force and thus make the bolt more resistant to bending than it would be with a short spacer?
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I think you mean a spacer that actually enters the holes in the wheel or sprocket? In that case, the larger surface contacting the spacer ends would help by distributing forces over a larger area. My previous comments assumed the spacer was on the surface of the wheel & sprocket. But a larger diameter spacer would help even more.
Quote:
Originally Posted by Rob Stehlik
I know bolts are quite weak in shear, but I figured 6 of them on a 4" wheel would be plenty strong.
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Well, yes, they are. But that's not the only force here. The real issue is the cyclic shear these fasteners are subjected to. If the robot only went forward (= no cyclic load), and never saw any collisions (= no shock load), the fasteners would last a long time. But shock loads, reversing loads, and reversing shock loads
all take their toll, especially if the fasteners are even slightly loose.
Pins would be better, because they don't have stress-concentrating threads, but if they're even a little loose - and in plastic, everything is loose - they'll eventually cause a failure too, but more likely by damaging the plastic and not by failing like the screws. That can be mitigated by sleeves to help distribute the load across a larger area of plastic.
(The compressive strength of plastic is a few thousand Pounds per Square Inch (PSI). That of metal is tens of thousands of PSI. If you put a 500 pound shock load on a 1/4" inch square (0.0625 sq in)of plastic, that's 8000 PSI.)