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