In article ,
Karl Townsend wrote:

Last year, I was air blast spraying the orchard, got out of position
when turning and caught the spayer head on a three inch tree branch.
Broke the head right off the machine. Imovable object meets
iresistable force.

OK, I've been rebuilding this for the last several days. I made the
mating surface from the air fan to the head two flat plates with eight
1/4" bolts holes around the outside of the duct. I want this to be the
weak spot this time. Steel, even grade 1, is too much, I guess.
Page 3042 of Mcmaster Carr has these plastic bolts
http://www.mcmaster.com/#catalog/119/3042/=m142xv

Could anybody give me an estmate of the relative strength of nylon,
polycarbonate, and PTFE, to grade 1 steel in 1/4" bolts? other
considerations on best shear bolts? Other material choices?

Karl

I used a lot of nylon hardware back in my high-voltage days.

I'd notch steel bolts, with the notch in position to line up with the
mating surface. Plastics have a lot of yield before they go, and fatigue
badly. Chuck them up in the lathe, try out a few different depths and
root profiles, pick the one you like. Cheap and adjustable, and IMHO
less likely to fail when you don't want them to.

A less well equipped guy might hacksaw, file or use a cutoff disk in a
grinder to cut partway through them, but you are pretty well equipped
and can therefore get a nice repeatable result from a lathe. You can
even calculate a theoretical shear strength based on the cross-sectional
area and see how it matches up in practice.

I might also safety wire them on to avoid (most) potential of puncturing
expensive tractor tires with the broken ones falling out.

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