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Very small Grade 8 or ?

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theoldwizard1

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Is there such a thing as a 1/8" Grade 8 bolt ? I need good shear strength in a small diameter.

How strong are roll pins, in shear ?

Other suggestions ?
 
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joe49

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Allen cap screws are usually 8. 8 is not as good as 5 in shear. Also single or double shear makes a big difference. Double is much stronger.
 
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red92s

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Socket head cap screws that conform to ASTM A574 Have a higher yield, proof, And tensile strength than grade 8 fasteners. A shoulder screw with a 1/8 shoulder will meet that standard,and give you the most possible shear area...you'll just have tiny 4-40 threads.
 
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A_Pmech

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Shear load? Small diameter?

A dowel pin is the usual solution.
 
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theoldwizard1

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A_Pmech said:
Shear load? Small diameter?
Click to expand...
No spec on load (single shear). I would like keep the diameter around 1/8".

A hitch pin would work (like on a trailer hitch receiver), if I could find one small enough.

A dowel pin is the usual solution.
Click to expand...
I always thought steel dowel pins were just for location, not for shear.
 
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theoldwizard1

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red92s said:
Socket head cap screws that conform to ASTM A574 Have a higher yield, proof, And tensile strength than grade 8 fasteners. A shoulder screw with a 1/8 shoulder will meet that standard,and give you the most possible shear area...you'll just have tiny 4-40 threads.
Click to expand...
Thread size is not important as it will not have much tension.
 
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dr_clyde

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Yes, they are used for locating, but a dowel pin would be stronger than a screw in shear for a given diameter for a couple reasons. They are typically heat treated differently and don't have the threads cut out, so there is more material and less stress raisers.

If there are female threads, or if the OP would like to drive it with a socket or something, I would get a high torque socket head cap screw.

http://www.mcmaster.com/#socket-head-cap-screws/=v9ghp8

edit: link doesn't go right to fastener. Click on the High Torque icon on the top row.

Stronger than a grade 8 and available in small sizes.
 
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MatBirch

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Buckgnarly

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joe49 said:
The numbers on charts are what lead some to use Gr 8 over using Gr 5. But Gr 8 fails catastrophically verse Gr 5 deforming. The Gr 8 is brittle and not the best in shear applications.
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I have heard this before, and have to call old wive's tale on this way of thinking....

As long as your bolt is rated for it's use, the 8 will ALWAYS be stronger than the grade 5. You would not choose your bolt based on how it will fail!

An example if you think 8s are weaker (or fail differently) in shear.....Have you ever put a 8 in place of a 5 in a PTO shear pin application? The 8 will not break, but the 5 will shear, saving your equipment. 5s shear just like 8s, but at a lower strength.

I would love to see some real lab testing to show that 5s fail any differently than 8s when they reach failure. Anyone have anything?:headscrat

Edit: there is good reading against the grade 5 being better for shear in the link I posted above....
http://tinelok.com/grade-5-vs-grade-8-fasteners/
 
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dr_clyde

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Buckgnarly said:
I have heard this before, and have to call old wive's tale on this way of thinking....

As long as your bolt is rated for it's use, the 8 will ALWAYS be stronger than the grade 5. You would not choose your bolt based on how it will fail!

An example if you think 8s are weaker (or fail differently) in shear.....Have you ever put a 8 in place of a 5 in a PTO shear pin application? The 8 will not break, but the 5 will shear, saving your equipment. 5s shear just like 8s, but at a lower strength.

I would love to see some real lab testing to show that 5s fail any differently than 8s when they reach failure. Anyone have anything?:headscrat
Click to expand...

I would have to look for the lab results, but I did some lab testing of this very thing.

They do fail differently, but at different strengths. Grade 5 will fail in what is called a ductile failure, meaning the steel stretches until it fails. Grade 8 fails in what is called a brittle failure, meaning it breaks in a more violent sudden failure. This is due to the differences in hardness. Its like freezing taffy. The thawed taffy will stretch until it breaks, and will take less strength to do so then a frozen piece. The frozen taffy will snap suddenly, with little deformation prior to failure. It will also take more strength to break the frozen taffy.

HOWEVER, the failures happen at different points on the stress/strain curve. A grade 5 bolt will fail at a lower point than a grade 8, no matter what the position. This is because the shear strength increases with tensile strength. The increase of hardness of a grade 8 increases its tensile, therefore increasing its shear strength. General rule of thumb for steel is 50% of tensile is equivalent to single shear. Double shear is different than single.

It is generally preferred to have a ductile failure in steel for a couple reasons. Sometimes the deformation can be detected before the failure, allowing replacement or to at least get out of the way of the load. Ductile failures tend to be more gentle, and not as damaging to components.

I would pay more attention to the alloy of a bolt, and select the one best suited to the task, instead of basing these decisions purely on hardness grade. A high alloy bolt might have a tensile strength greater than a standard grade 8, but due to the alloy may still have a ductile failure if it fails.
 
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PETE14

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Interesting thread, with some good info here.

Dr C., could you please briefly define the terms "single shear" and "double shear"? I have not heard of that before. Thanks.
 
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dr_clyde

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Single shear is like having two plates bolted together, and having the plates slide opposite each other. It produces a shear force in one direction on one plane.

Double shear is like having a single plate bolted between two others, and the outer plates pulling opposite the center plate. Kind of like a clevis. It essentially halves the shear force on the pin because the shear is now acting in two places on the pin versus one.
 
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theoldwizard1

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PugetDude said:
Pics of the intended usage would be helpful...
Click to expand...

At a different location.

Plate on top of plate. Long relative to width, about 6"x2". Single through bolt where they overlap. When tension is applied straight along the center line of both plates and directly in line with the bolt, everything is fine (i.e. the bolt is plenty big !)

When the tension is applied off the center line, the plates will rotate, relative to one another, around the bolt. The is insufficient room in the overlap area to install another "full size" bolt. The bolt can handle the main tension load, I just need to prevent the plates from rotating relative to one another.

Looks like I will be using a 1/8" or 3/16" roll pin.
 
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airrj

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PETE14 said:
Interesting thread, with some good info here.

Dr C., could you please briefly define the terms "single shear" and "double shear"? I have not heard of that before. Thanks.
Click to expand...

A visual example. Dr_clyde did a very nice job of explaining it, but pictures are always helpful for me.

Single shear on the top example and double shear on the bottom.

500px-Bolt-in-shear.svg.png
 
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