plastic shear screws

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
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On Mon, 25 Mar 2013 07:09:24 -0500, the renowned Karl Townsend

Brass maybe.
http://simple.wikipedia.org/wiki/Tensile_strength
McM have fiberglass/urethane that looks like it might be good eg. 91345A684 if nylon isn't strong enough. Nylon is nasty hygroscopic and I'd worry they might loosen up from expanding and shrinnking.
If you use metal you can always use smaller bolts.
You probably want something with a yield strength relatively close to the ultimate strength for a clean break.
Best regards, Spehro Pefhany
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I'd be worried about nylon et al being too soft. How about aluminum?
Joe Gwinn
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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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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.
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anorton wrote:

How much weight does it need to hold? Instead of using bolts how about something like a push in fastener like you use for automotive trim panels? Or just buy some plastic license bolts and play with cutting them partially till they break with the load you want?
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Thanks for all the suggestions, now I got a better handle on this.
I like the idea of brass, and maybe neck it down in the lathe.
Need a decent guess on what a brass bolt would hold. (By the way the bolt would break in tension NOT shear)
http://simple.wikipedia.org/wiki/Tensile_strength
brass mpa is 250 from web site 1 mpa = 145 psi or 38250 psi
1/4" bolt PI*R*R=3.14*0.125*0.125=0.0833 sq.in
one bolt 38250*0.0833019 lb.
No way in hell, what's wrong with this calc.
Karl
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Try it again. 0.08333 = 1/12.

In tension it will break at the thread root. 3.14*0.1*0.1=0.0314 sq in, 1201 Lbs.
jsw
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On Mon, 25 Mar 2013 11:56:01 -0500, Karl Townsend

Minor diameter of 1/4-20 is 0.1887, so maybe 1/3 of that.
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On Mon, 25 Mar 2013 11:56:01 -0500, Karl Townsend

Three things.
250 MPa = 36300 psi
3.14*0.125*0.125 = .049
And shear strength is less than tensile for most materials. Half to 2/3 of tensile is probably in the ball park for brass. So 900 to 1200 lbs is a better estimate. I'm assuming single shear. For comparison, shear strength of an alloy dowel pin is 7000#.
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I've used 1/4" nylon license plate bolts in breakaway applications. jsw
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