Fastener with a calibrated shear break point?

[Boothby171]

I'm looking for a fastener or pin that can be purchased with a known shear fracture point. I say fracture instead of failure because yielding is not enough. I need it to break into two pieces. I need a relatively accurate fracture value (+/- 5%), as it is for an indusatrial robotic application.

Amybody ever come across on of these?

Thanks in advance,

Steve

 

[unclesyd]

Here is one type bolt and some sources. Whether you will get precision is the big question.

http://gbcutter.com/Bolts.html

 

[haggis]

Steve,

Sounds like you are talking about a shear pin similar to what is used on shear flanges on conveyor drives. Why not machine a few?

If you know the point where you want it to break, depending on the steel you are using, and knowing it's shear strength, you can figure out the required square area of the pin to shear at a given force (in psi) and you can machine an undercut on the pin to the required diameter.

Haggis

 

[yates]

I am afraid you are searching for a very rare beast. A carefully necked tensile fuse-bolt can be designed and made to get within plus or minus 5 percent with a lot of hard work, but in shear, never. The difference between two distinct shear tool blocks for a shear test will already give you more than 5 percent error on the same bolt or piece of bar. This is because the shear resistance is very dependent upon the radius of the guillotine cutting edge and the horizontal play of the blade in its guide. Also, what material are you talking about ? cold work and strain hardening effects can make the rupture strength dependent upon load rate or simply the surface area bearing on the bolt circumference. I can give you a stunning demonstation anyday of a titanium rod which has greater than a 5% decrease in shear strength after being painted, quite simply because the circular guillotine contacts a half-circumference in one case but only a point load in the other.

 

[Boothby171]

Thanks, Unclesyd.

I'm amazed at haw hard it appears to be to find SOURCES of these things.

I've trid Avibank, DrivLok, "GlobalSpec," "DirectIndustry," and GooGle--all near-misses, and no direct hits!

:grumblemu

--Steve

 

[Boothby171]

Thanks, all.

It's that "hard work" thing that I'm trying to avoid. It's not me that's going to be doing the machining and the testing--it's my client. They have no budget for any of that; they just want to buy something with a known shear capacity.

Specifically, it's a locking shot-pin. They're having a hard time with their control system (atypical for them--I expect that they'll figure it out and fix it soon enough), and have damaged the surrouinding weldment when they tried to drive through the shot pin (after failing to retract it!).

I know my original design works, at least! But now they have changed the spec, given us the change order (all is good), and this new thing is what they want.

...and the search continues.

 

[ornerynorsk]

Garden tillers, snow blowers, and many other types of outdoor power equipment are equipped with shear bolts. A sketchy starting point, but just a thought.

 

[MintJulep]

What is the load? How flexible are you on diameter?

As noted by Yates, you are looking for something that is difficult to achieve - with the type of materials that exhibit properties normally desirable for mechanical fastners.

You need something with very little ductility, and very brittle. Think about things other than metal.

Glass, or a plastic of some type perhaps.

How about a "pre-sheared" part glued back together?

 

[Boothby171]

MJ,

That's pretty much the direction we're heading.

We'd take the shot pin, cut it in half (so to speak), tap internal threads in the mating faces, polish the faces (and oil them upon reassembly), and use a calibrated stud.

And we've even started discussing ceramic threaded rods for the stud, with the thought that brittler=better.

The issue then becomes predictability (but also will we see a purely shear load across the stud, or will it also see tension as the pin reacts about its far point when the shear load is imposed on it).

Right now, we're making sure our client really wants this...

 

[griffengm]

boothby,
What about using a preloaded detent ball or series of balls in place of the shot pin? The variable spring pressure allows for some adjustment.
Seems to me that something like this is used on end effectors to allow alignment compliance. I have used them on screw machine tools to allow movement in a plane in an otherwise rigid toolholder.
Just a thought.

Griffy

 

[Boothby171]

Griffy,

I've thought about that, but the actuator for the shot pin doesn't easily lend itself to that solution.


...yet...

Maybe I'll think some more about that!

Thanks.

(and, to repeat: thanks to everyone for their help in this)

--Steve

 

[MintJulep]

Boothby,

Threads introduce variabilty into the part. If the tension forces allow it consider a cylindrical shear pin and using an adhesive on the mating bore to hold things together.

 

[Boothby171]

Sorry,

..and neck down the part of the stud that winds up in the shear plane. And then polish all the tool marks out of it.

 

[griffengm]

Boothby,
What powers the shot pin? Hydraulic? Electric? I'm guessing not air so what about an air cylinder? Has the advantage of variable stroke and compressibility.

Griff

 

[Boothby171]

Griff,

It's a pneumatically actuated shot pin. The axis of the actuator is orthogonal to the direction of the shearing force.

I'm not allowed to play any sensor or control tricks--the whole reason we're exploring the shearing pin is because the client is having difficulties with their automation (sensors & controls!) vendor getting the interlocks right.

Otherwise, I'd just drill a central/axial hole, put in a strain gauge and a adjustable threshhold latching control box, and be done with it.