"Karl Townsend" wrote in message
...
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

For a shear bolt you want something that is weak but brittle. In other words
you do not want it to deform or stretch elastically before it breaks. I
would go with the other suggestions here of turning a groove in a high grade
bolt just below the head. Someone suggested smaller bolts, but those would
stretch more before breaking. Still, we could be over engineering this and
all these solutions might work. It depends on how delicate is the part
being protected.