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Grade 8.8 bolt strength

  1. Oct 29, 2011 #1
    hi,

    great forum.

    i have searched the internet for 2 days to figure out the following, but with no luck.

    question,
    i am going to remove my diesel engine which weighs 800lbs and bolt it to an engine stand.

    i will be using 4 hex bolts with dimensions, 3 1/4'' long, just over 1/2'' thick, 13 threads per inch, 1 1/4'' thread pitch, marking 8.8 on head.

    i am trying to figure out if 4 of these bolts can hold the 800lb engine???

    i assume this method of strength is shear strength??

    can someone tell me what one of the above afformentioned hex bolt can hold ??

    thanks in advance...
     
  2. jcsd
  3. Oct 29, 2011 #2

    Stephen Tashi

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  4. Oct 30, 2011 #3
    That could be a dangerous assumption. Are you also guessing the weight of the engine?

    Not possible to provide meaningful answers without knowing the design of the stand.
     
  5. Oct 30, 2011 #4
    the weight of the engine is 800 lbs. the engine stand is rated to hold a 2000 lb engine.

    after some research, the hex bolt is;

    8.8 metric class, 8.8 mark on head, steel material,

    tensile strength = 800 N/mm squared, 116000 psi

    yield strength = 640 N/mm squared, 93000 psi

    thanks again.
     
  6. Oct 30, 2011 #5

    Ranger Mike

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    i have pulled big block engines out of cars with two 3/8 bolts and an chain hooked to an old farm tractor with hydraulic forks..think i was about ten years old... was my younger not know better years..i would remove as many parts *** possible once placed on the engine stand...my opinion..
     
  7. Oct 31, 2011 #6

    Stephen Tashi

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    Maybe if we keep this thread alive, a real mechanical engineer (or student thereof) will answer. I'm not one. The engine will probably get bolted to the engine stand before the theoretical discussion reaches a conclusion. Nevertheless, the engineering aspects would be interesting to discuss.

    This link says that for fasteners, an empirical guide to shear strength is 60% of "minimum tensile strength". http://www.portlandbolt.com/faqs/bolt-shear-strength-considerations Is "minimum tensile strength" the same as "yield strength"?

    The way the typical engine stand works is that there are 4 bolt holes on one end of the engine block where the transmission is normally attached. A vertical plate on the stand is bolted to the engine block by 4 bolts into those holes and the engine is held only by it's attachment to the plate. So the length of the engine block is going to matter. The block is going to be like a cantilever beam, only supported at one end. (There should be plenty of exercises in mechanical engineering texts about such beams!) I think a long engine block will exert more tension on the bolts than a short one of the same weight.
     
  8. Nov 1, 2011 #7
    I would recommend no more than 60% of yield strength in tension and 30% of yield strength in shear. It is better to design to a fraction of yield (metal deforms permanently but does not break) versus tensile (failure) strength.

    I would flush your arrangement through all the formulas in:

    http://www.roymech.co.uk/Useful_Tables/Screws/Bolted_Joint.html

    Also consider bearing stress and shear tear out for the plates the bolts go into. Let me know if you need these formulas.
     
    Last edited: Nov 1, 2011
  9. Nov 4, 2011 #8
    Probably your question is bit vague. What kind forces, moment are expected on the bolt. If it is just axial force, that is encountered by the bolt, then you asre good to go even with smaller bolt dimension.

    Any by the way what is the length of thread engagement

    For a less detailed calculation, I would suggest the calculators in the below link

    http://www.tribology-abc.com/sub9.htm
     
  10. Nov 4, 2011 #9
    Bruca 003,
    Your question is a bit vague. A picture would be a great help.

    Please state how far apart are the bolts and the overall size of the engine(for CG).

    It is not a good idea to calculate the direct shear stress only; bending stress also needs to be calculated ( with the possibility of torsion also).

    If the bolts are to be under pure tension only, the bolts are more than enough to hold the engine.

    I hope this helps.
     
  11. Nov 7, 2011 #10

    nvn

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    No. "Tensile strength" is an unclear, abbreviated term meaning tensile ultimate strength, not tensile yield strength.

    bruca003: In post 1, are you saying your bolts are M12 x 1.75? Or are they M14 x 2? (The number after the "x" is thread pitch, in mm.) Measure the diameter of your bolts accurately in mm. And measure the axial length of, say, ten or fifteen threads, in mm. Let us know.

    Assuming your engine and engine stand are the same as for this http://forum.ih8mud.com/attachment.php?attachmentid=407438", then it appears the vertical bolt spacing is h1 = 164 mm, and it appears the distance from the bolts midpoint to the engine center of gravity (CG) is a1 = 360 mm (or less).

    Using h1 = 164 mm, a1 = 360 mm, and yield factor of safety FSy = 1.70, it currently appears M12, property class 8.8 bolts will work, assuming you torque them to 42.0 N*m, and assuming you have a thread engagement length (Le) of at least 16 or 18 mm. However, please let us know if your h1, a1, and Le dimensions are different, because it can make a big difference in the stress results.

    As saurya08 mentioned in post 9, what is the vertical spacing between your upper and lower bolts (h1)? And what is the horizontal distance from the bolts midpoint to the engine CG (a1)? Also, what is the upper bolt thread engagement length (Le) in the tapped holes? What is the tapped hole material? And are the threads of the upper tapped holes greased, or dry?
     
    Last edited by a moderator: Apr 26, 2017
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