Torque wrench for shear pin testing 1

[aroundhere]

I always like to test everything if possible. I have a design where I want a pin to shear once a given amount of torque is applied to the shaft. I want the .125 pin to shear when the torque is 120in-lb. I will have to find the correct material to use for the pin because I had to pick a size for the pin before I really knew what the torque would be. I thought the best way to test this would be to buy a beam type torque wrench and try to load it up while keeping an eye on the needle to see what the pin shears at. The 2 parts do have flats for wrenches. Let me know if there is a better way to do this without spending too much $. I found a beam type torque wrench on at mcmaster for $80 that is 0-600in-lb.

Since the pin size and the torque are set, the material of the dowel pin is what I have to determine. I can find a double shear number for 416 SS, but nothing for 316SS or 18-8SS. I would rather go with an off the shelf pin, but if im lucky enough to find a SS pin with the correct properties, im not sure how much the numbers can fluctuate between pins of the same material from different manfs or even the same manfs. This was the reason for the testing. Looks like I need a shear strength of 39,000psi. I typed this out below so I could document it and I figured someone would let me know if my numbers are off.Thanks for any input. I also have a sketch of the 2 parts with the pin installed. The pin has a slight press fit to keep it in place.

120in-lbs/.25R = 480lbs <--pin needs to shear at this force

.125^2 *.7854=.0123sq-in <--.125dia pin area

480lbs/.0123sq-in = 39,000 psi

 

[aroundhere]

Man, I just noticed that torque wrench has 50in-lbs graduations. That may not be enough resolution. They begin to get expensive for more resolution, but we will see.

 

[hydtools]

I would look for a dial-type that has a peak reading indicating needle.

Ted

 

[btrueblood]

I would add to hydtools suggestion that you have a second pair of eyes, or better a video camera, record the dial during torque application. The needle is likely going to vibrate as the pin lets go, and its final position may give a false high error reading.

 

[aroundhere]

The higher accuracy beam type is twice as much as this dial type. Link This one has a max torque pointer. A videa camera is a good idea. I suppose if I ease into it and do multiple tests, I could make this work. Did my torque/force on the pin numbers look ok? My luck, since 416SS dowel strength is too high, the 316SS dowel will not be strong enough. Usually the pin size is selected last. Since we had to get something machined but didnt know the required torque at the time, this one is kind of back asswards. thanks

 

[3DDave]

You are also putting a side-load on the coupling that will greatly influence the results.

They should be called moment wrenches since they produce loads other than torque.

You could just use a 12 inch bar and a 10+ pound spring scale applied at about 90 degrees. Even 5 degrees off of perpendicular makes nearly no difference to the reading; you don't specify a range or tolerance for the measurement.

To get a better result, you'll need a support so that side-loads are not introduced into the coupling.

 

[tbuelna]

aroundhere-

First of all I hope you appreciate just how incredibly difficult it is to get a consistent result when using a shear pin like this as a torque limiting device. You can wide variations in the pin shear point due to things like pin surface finish, relative fit between the pin and shafts, load/cycle conditions the pin is exposed to over its life, etc. The elongation rate of the pin material also matters, since materials like 316 cres will yield quite a bit before they finally shear.

Looking at your sketch, one thing I would suggest is to add some shallow relief grooves to the shaft OD and hub ID at the pin shear plane. The pin will never shear cleanly, and providing some shallow local relief grooves in the mating shaft and hub surfaces will give the material displaced from shearing of the pin someplace to go. Otherwise the displaced pin material will just get smeared into the small space between the mating shaft and hub surfaces causing them to seize together. Also, rather than using a straight cylindrical profile with your shear pin, I'd suggest adding a groove right at the shear plane that has tight tolerances and carefully controlled surface roughness. This arrangement will help isolate factors like stress concentrations due to shaft/hub edge loading, and any pin surface damage created during its operating life, on the shear point of the pin.

Due to variables like torsional stiffness in the mating parts and the difference between the yield and ultimate shear point of your pin, you'll need some method of correlating the input torque to the angular displacements produced in the mating halves of the shaft coupling. And as 3DDave noted, it would be good to install your test article in a fixture that isolates it from forces other than those, like input torque, that you wish to measure.

 

[dhengr]

Aroundhere:
It certainly seems to me that you went about this design problem the wrong way. You need to know the torque you are trying to attain or resist, and the material you will be using, within its spec. limits, then you size the pin or key to fail. The way you’ve done it is bass-ackwards, and forces you to try to find an exact strength of material which likely is a futile effort. Heck... just clamp one shaft down, and put a 3' lever arm on the other shaft, and start loading the lever arm to failure. You will likely pay a premium for a tightly speced. order of pin material which is well within (narrowly within) its normal ASTM spec. Then you will have to test this material for shear, yield and ultimate strength and finally adjust your failure range based on this testing. You will not likely find a perfect material.

 

[aroundhere]

tbue...yes, I was concerned about the pin not shearing clean and potentially galling the parts together. I may need to redesign this coupling. I may want to look research different ways that a shear pin in this configuration (torque) are used. I made plenty of shear pins, but mostly in an axial force scenario. With this design, I would need a hard material that would break cleanly, but still be able to use a punch to tap the pins out after shearing.

 

[aroundhere]

Well, I lucked out. I was able to set up a fixture using an old ball screw so the test would be centered to reduce side load.
I purchased several different material .125 dowels from mcmaster. I will have to dig further to determine the exact materials since they dont publish the info. Even the SS broke clean.
brass 120in-lb
alum 80in-lb
316SS 200in-lb