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Thread: Socket Head Cap Screws vs. Hex Head Bolts

  1. #1
    Associate Engineer
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    Dec 2021
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    Socket Head Cap Screws vs. Hex Head Bolts

    I have an application that is resulting in failed 7/8" hex head bolts. I'm early in the investigation phase, but have read that a SHCS is stronger than the same size hex head bolt. One reason stated is simply because better alloys are used, which gives them higher minimum tensile strength. Others have said that the design makes them stronger, but no explanation behind that claim. Does anyone know if it is true that the design makes them inherently stronger? If so, can you explain why?

  2. #2
    Technical Fellow Kelly_Bramble's Avatar
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    That's a speculative claim "SHCS is stronger than the same size hex head bolt" as bolts are rated by the material and grade.

    https://www.engineersedge.com/hardware/steel_bolts_strength_designation_14060.htm

    A stronger material bolt may or may not solve your problem. You need a deep dive into why that bolt failed.

    In general, when bolts fail either there was a design flaw, manufacturing and/or installation flaw, the bolt has been subject to excessive loading in service or the fatigue life has been exceeded
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

  3. #3
    Associate Engineer
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    Dec 2021
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    Thanks. There is much more to the story than I felt necessary for this post. I realize it's a speculative claim, but wondered if there was engineering evidence to support it, whether a stronger bolt is the solution or not. In this case, the machine ran well for 23 years. At that point, several bolts were discovered to be broken. Removal was difficult due to corrosion, so the plant elected to drill out all 12 holes and replace with stainless steel through bolts. That's when all of the problems started. It is my opinion at this point that fatigue contributed to the failure and they would have been better off replacing all 12 with the same socket head cap screws and rest for another 23 years. Since all 12 holes have been drilled through, their next option is to rotate the assembly 15 degrees and drill/tap new holes and put it back to the original configuration that lasted 23 years.

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