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

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


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
--
"it's the network..." "The Journey is the reward"
Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

In article , Spehro Pefhany
wrote:

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


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.

I'd be worried about nylon et al being too soft. How about aluminum?

Joe Gwinn

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.

--
Cats, coffee, chocolate...vices to live by
Please don't feed the trolls. Killfile and ignore them so they will go away.

"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.

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*0.0833=3019 lb.

No way in hell, what's wrong with this calc.

Karl

"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

I've used 1/4" nylon license plate bolts in breakaway applications.
jsw

"Karl Townsend" wrote in message
news
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

Try it again. 0.08333 = 1/12.

one bolt
38250*0.0833=3019 lb.

No way in hell, what's wrong with this calc.

Karl

In tension it will break at the thread root.
3.14*0.1*0.1=0.0314 sq in, 1201 Lbs.

jsw

On Mon, 25 Mar 2013 11:56:01 -0500, Karl Townsend
wrote:

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*0.0833=3019 lb.

No way in hell, what's wrong with this calc.

Karl

Minor diameter of 1/4-20 is 0.1887, so maybe 1/3 of that.

On Mon, 25 Mar 2013 11:56:01 -0500, Karl Townsend
wrote:

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*0.0833=3019 lb.

No way in hell, what's wrong with this calc.

Karl

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#.

--
Ned Simmons

anorton wrote:
"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.


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?

--
Steve W.