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How can the shear strength of a steel rod be improved? The application I am thinking of is rods used to make bolts where we want the bolt to resist being sheared off after it is attached.

Some of the possibilities I see are forging the rod in some way rather than rolling it. Rolling the threads, as opposed to cutting them, and by heat treatment and tempering. How can I quantify such treatments on shear strength? Are there are other methods not mentioned?

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    $\begingroup$ I mean...the obvious answer is to make it thicker. I suppose you could use a different thread geometry. One with more rounded corners and less sharp corners to relieves stresses but that is getting super specialized. What is this bolt being used for anyways? Isn't it being used to clamp two pieces of material together? The friction from that is mostly what is supposed to hold them together. $\endgroup$
    – DKNguyen
    Commented Aug 14, 2022 at 22:54
  • $\begingroup$ either get a better bolt (you aren't about to start manufacturing bolts better than the bolt people) or change your design so bolts aren't in shear, which is not what you should use them for. $\endgroup$
    – Tiger Guy
    Commented Aug 15, 2022 at 4:49
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    $\begingroup$ Change the design so the bolt is in double shear. Bolts are designed to be used in shear - especially in expensive drivetrains that can get blocked. SO much cheaper to replace a shear bolt compared to a shaft, both in time and money. @TigerGuy $\endgroup$
    – Solar Mike
    Commented Aug 15, 2022 at 7:52
  • $\begingroup$ @SolarMike But what about an application where you don't actually ever want the bolt to shear? And I thought they were shear pins? Not shear bolts? $\endgroup$
    – DKNguyen
    Commented Aug 17, 2022 at 1:52
  • $\begingroup$ @SolarMike But what about an application where you don't actually ever want the bolt to shear? And I thought they were shear pins? Not shear bolts? To be fair, I keep hearing conflicting opinions. The only thing anyone seems to be able to agree on is that the best way is to make your parts interlock so the bolts $\endgroup$
    – DKNguyen
    Commented Aug 17, 2022 at 5:03

2 Answers 2

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You can buy standard bolts made to meet different shear stresses without failure, right off the shelf. High-strength bolts are sold by ASTM Grade numbers (3, 5, 7, etc.) where the bolt head has little bumps pressed into it- 3 bumps for a grade 3 bolt, 5 bumps for a grade 5, and so on.

All the info you need to specify bolt strength can be found on-line.

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Very high strength fasteners are made by heat-treating and may also be carburized. Cold work is used for lower strengths; the strength is not uniform through the crossection and ductility is reduced. Heat-treated fasteners can have rolled threads. The type of forging or drawing does not make much difference after heat-treatment. Best to use commercial products as listed by @ Niels

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