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How does preloading help reduce stresses in bolts?

  1. Feb 14, 2013 #1
    Hi guys:

    today I learned in class that if you have two members that you wish to joint together, that each of them will experience a force away from each other. A bolt is there to prevent them from moving apart.

    however, that's the obvious part, what get my confused is that, how exactly does preloading (tightening up the bolts) help reduce the tension force in the bolts? If i do a 3rd law from the applied force and trace it all the way down the bolt, forces exert on the bolt should still be the same be there preloading or not! if anything it should have higher stress!

    I am not seeing it visually, please help~~

    Last edited by a moderator: Feb 14, 2013
  2. jcsd
  3. Feb 15, 2013 #2


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    Have a look at the Wikipedia page for bolted joint. The diagram in the Theory section is a simple analogy.

    On an injection moulding machine we had M16 bolts failing after about 9 months. They retained the injection unit casting so they were fully loaded every cycle (10 seconds). It turned out fatigue was the cause as they were done up "tight" with an allen key. Not preloaded correctly with a torque wrench. We haven't had one fail in 4 years since replacing all the bolts and correctly preloading them.

    I can't find an S-n curve for bolts but this page has one for aluminium.
  4. Feb 15, 2013 #3


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    hihiip201: You are correct. The bolt tensile stress will always be higher with preload, when you apply the applied tensile load, not lower (assuming the bolted plates do not gap).

    The advantage of having adequate bolt preload (although it sometimes causes a much higher bolt stress) is, the bolt stress does not fluctuate nearly as much, when the applied load fluctuates, thus usually increasing fatigue life span of the bolt.
    Last edited: Feb 15, 2013
  5. Feb 15, 2013 #4
    i thought so too, but this article confuses me


    I think what ^ they are saying is that before you exceed the preload, all you have is the preload, but after that would just be whatever forces you exert on the two members?

    please correct me if I am wrong.

    and thank your for your reply.
  6. Feb 16, 2013 #5


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    Perhaps this might help.
    It might be easier to comprehend if they had put a very stiff second spring BETWEEN the support plate and the bracket, and then preloading the bolt, putting the second spring in compression.
    You could then see that a tensile force of the bracket would act to reduce the compression of the second spring by the same amount as the applied force on the bracket. The bolt tension would then still be at the preload value, which previously was the compression of the second spring, but is now the applied force + a lessor compression value of the second spring.
    Does that make any sense?
  7. Feb 17, 2013 #6

    i was trying to picture it with a regular spring, then I noticed you said very stiff, are you trying to model the distortion of the members material with this spring? (youngs modulus = big k)
  8. Feb 17, 2013 #7

    Im going to try to describe what I think the principle is, please correct me if I am wrong.

    so essentially when we preload in the picture that's shown in the website, there's the spring force pushing the bolt upward on the top side of the support plate, on the other hand, there's an reaction force exerted on the bolt by the compressed member/very stiff spring on the bottom side of the support plate (in the bracket).

    say if this tensile force is 2 N, and we increase the weight of the bracket for 1N, the force of the very stiff spring will decrease, but its contribution + the weight will still be 2N and hence the spring force on top will still be 2N until more than 2N weight in increased in the bracket?
  9. Feb 23, 2013 #8


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    sorry to not get back to you sooner if you are still there.

    You quite right have the picture.
    Stiff spring goes in between the bracket and support. Model of the modulus of the material.

    Take it from the other viewpoint.
    Attach the support and bracket together with a nut and bolt but do not tighten.
    Place a weight W on the bracket. You can see that the bolt has to withstand the full force of the weight W. The top spring should read W.
    Now, tighten the bolt just until the bracket and support are touching but no more.
    Top spring should still read W.

    The next step is to remove the weight W. Since there is less force on the bracket, the top spring can now relax a little bit, but only up to the point where stress/strain of the bracket and support material equals the force of this top spring. The top spring should read just under W. The bolt is holding the materials under compression by a small amount just under W.

    So, if you have a loose joint and keep adding/removing W, the tensile force on the bolt fluctuates from 0 to W. With preloading, the bolt will experience a fluctuation from only just under W to W.
  10. Feb 26, 2013 #9

    it makes sense to think about it that way...

    but i wanna ask, that little relax distance of the spring, will it be extended into the members? or more to the head? or both?

    What I meant to ask is what is the deformation look like in the material before and after we release W?

    it would seem that if we are using the exmaple of the website, there will already be deformation on the support plate before W was released.

  11. Feb 26, 2013 #10

    also, i can see that as the load increases (but still smaller than preload), the deformation of the stiff spring decrease so the downward spring force decrease, hence "compensate" for the increase of load . But how do we know for certain(mathematically) that the sum of these forces must still be the preload?

    i feel like there's something real obvious here im not seeing.
  12. Feb 28, 2013 #11
    In my textbook it is very confusing.

    it says:

    P is the external loading
    Fi is the preload
    Pb is the portion of P that's taken by bolt
    Pm is the portion of P that's taken by member.

    if P is exerted on the members, then isn't Pm just P? I mean at the end all your forces have to be balanced by your bolts anyway isn't it? compressing your member with bolts by preload will not, and should not reduce the force that the bolt has to take, or the members.

    Im so stuck...
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