Shear Pins

[SPYDERLK]

Well - I thought ASTM grade10.9 was equal to SAE grade 8. Looked at my torque tables and now I'm not so sure. Have to see if there is a cross-reference table somewhere.

You are correct as to metal strength according to about every chart I can find. Ive yet to see a composite side by side comparison with discussion. McMaster Carr has a chart showing comparability in metal strength between grades. 5 = M8.8, 8 = 10.9, 9 = 12.9.

For comparing torque on similar sized bolts you must consider the root area. This will vary with both the bolt diameter and its thread pitch(which affects root area, and the mechanical advantage of the helix). A small difference in root factored against difference in pitch will require some orderly thinking to judge whether the torque figures indeed make sense.​

 

[oosik]

AH - - "orderly thinking". Hmmmm - - that's probably why I've never worried about it too much. Orderly thinking goes against my retirement plans and can make the head hurt.

 

[sixdogs]

Well - I thought ASTM grade10.9 was equal to SAE grade 8. Looked at my torque tables and now I'm not so sure. Have to see if there is a cross-reference table somewhere.

I'm pretty sure 12.9 is grade 8 or higher and 10.9 is grade 5 but not sure of that one.

 

[shui5612]

"..a regular bolt"?
"Make sure (it's) of a smaller diameter"?

Don't take this the wrong way, but this is some of the worst advice I've ever read on the internet.

By regular bolt I mean any bolt you can find in the shed with a nut that fits in the threads.

And by smaller diameter I mean the following:

If the shear pin was 5/16ths of an inch thick, almost "any" bolt that is 3/16ths can be used temporarily to finish the job.

While you may not agree with my advice, I have never had an issue with any equipment because a bolt was too strong and not a "designed" shear pin.

Bolts are pretty weak when it comes to shear strength, a bolts grade primarily determines how strong it is in resisting stretching the threads and the amount of force it can hold (torque).

For example, a grade 8 bolt, bolted to hold two pieces of metal holds together; the majority of the strength of the component comes from the friction that the bolt creates between the surface areas of the pieces. Not the bolt itself.

If you think that using a "normal" bolt will cause catastrophic equipment failure, you are wrong.

I've used normal grade 3-5 bolts in my blower without any issue even if it is designed to run "grade 5 shear pins".

Of course a grade 8 bolt will most likely bend instead of breaking off cleanly ( from my experience), it is why I'm suggesting that the person uses a smaller diameter, which in 97% of cases will be a weaker bolt, irrelevant to the grade.
I.e. a 1/2inch thick grade 3 bolt will have a higher shear strength than a 1/4 inch grade 5.

 

[Texas Fella]

"While you may not agree with my advice, I have never had an issue with any equipment because a bolt was too strong and not a "designed" shear pin."

In my career I have seen multiple (and expensive) failures caused by people using a "regular" bolt in place of a shear bolt/pin. Not saying that you could not get by doing it, but I would not recommend it. Never understood why people were so worried about buying a $2 bolt and nut. As a side note, if you are breaking a lot of them over and over, perhaps consider changing operating techniques.

 

[shui5612]

In my career I have seen multiple (and expensive) failures caused by people using a "regular" bolt in place of a shear bolt/pin. Not saying that you could not get by doing it, but I would not recommend it.

While I never said I'd recommend it, I meant it mostly as an emergency/backup scenario.

While I agree that if you take the same size bolt I.e. 1/2inch and install a grade 8 instead of a grade 3, it may result in significant damage, a grade 5 may result in no damage even if the implement was meant for grade 3. (There is always a risk of damage).

It's why I recommend downsizing the bolt which will minimize the risk pretty significantly.
I.e. instead of using a 1/2inch shear pin grade 3, installing a 3/8ths grade 5-8 bolts shear strength remains comparable therefore provides roughly the same level of protection.

All I'm saying is that I personally agree with you @Texas Fella that it's "the best option" to run shear pins, that using a bolt wont hurt especially in the event of an emergency.
If anyone is curious as to looking further into my argument, there are many bolt shear strength charts out there and you can see that smaller diameter bolts and pins can withstand a LOT less force than larger ones IRREGARDLESS of bolt grade. The grading compares the bolts of the same diameter which are significantly stronger in grade 8 than grade 3.
But a grade 8 bolt 1/4inch diameter can withstand a tenth or fifth of a larger diameter bolt such as 1/2inch grade 3-5.