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Thread: Caulculating Required Bolt Clamp Load

  1. #1
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    Caulculating Required Bolt Clamp Load

    Dear Experts, 1st post!

    I would like some help in determining what clamp load is required for a bolted joint between a gearbox flange and sprocket. Please see attached photographs.

    Sprocket.JPGGearbox.JPG

    The reason for this is that I have been asked to review our bolt torques which previously had been compiled from a combination of experience and those used in similar industries and also by referencing generic tables on the internet.

    I am also told that when using Nord-Lock washer the torque should be increased to achieve the same clamp load due to the increased friction caused by the washer. I have also received conflicting advice regarding thread lock. Some people saying it acts as a lubricant and others that it increases friction?

    I can get tests done on the bolts to show the relationship between torque and clamp load to see how much I should increase the torque by when used in conjunction with Nord-Lock washers or with the application of thread lock.

    What I don't know is whether the torque values we have been using are correct in the first place and so would like to apply a bit more theory and calculation to substantiate these values. I believe the first step would be to determine what clamp load is needed to hold the joint in place and then to calculate the size and number of bolts required to resist the shear force?

    Any help greatly appreciated.

    Many thanks
    Last edited by technicalstudio; 06-30-2011 at 07:50 AM.

  2. #2
    Administrator Kelly Bramble's Avatar
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    First, you need to determine the maximum torque loading required by the gear-spocket in operation. Also determine if there will be any loading imparted perpendicular to the sprocket gear that will impart additional axial tension loading on the bolts.

    Are you using grade 8 bolts?

    Once you know the loading that the bolts need to absorb, then you can compare the current design to loading requirements to see if you need to add or reduce bolt shear capability.

    You will need to know bolt pre-load to baseline the fasteners relative to max. (peak) torque requirements of the sprocket.

    http://www.engineersedge.com/calcula...alculation.htm

  3. #3
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    Kelly, thank you for your prompt reply.

    Firstly, in answer to your questions, I know the output torque of the gearbox's we use and bolts used to bolt the sprocket to the gearbox are all grade 12.9 socket head cap screws.

    What I want to know is how to calculate what clamp load is required to hold the joint in place. From that I should be able to determine the torque needed to achieve this clamp load and from test I can establish what torque increase is required with the addition of the Nord-lock washers or thread-lock.

    Kind regards

  4. #4
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    I am pretty much with Kelly. Given that it looks to be only a chain sprocket, the shear on the bolts would be the primary consideration. Looking at those pics, it appears unlikely that there would be any appreciable load perpendicular to the sprocket center-line, so friction between the two mating surfaces should not be an issue.

    If instead of a sprocket it had been a driven wheel in contact with another surface (ground etc) then load perpendicular to the center-line would have some relevance.

    I think the tightening torque figures would come down to general recommendations for the bolt diameter, bolt material and the Nord-lock washer. As I see it, the load appears all rotational and thus, all pretty much handled by the shear of the bolts and the suitable thickness of the flanges. If very high rotational loads then maybe ream the holes and used fitted bolts, maybe even dowels too.

    Can't help with the thread lock debate, but if water is involved in the operating conditions, probably not a good idea to be considering it anyway.

    Dave
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  5. #5
    Administrator Kelly Bramble's Avatar
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    Quote Originally Posted by technicalstudio View Post
    Kelly, thank you for your prompt reply.

    Firstly, in answer to your questions, I know the output torque of the gearbox's we use and bolts used to bolt the sprocket to the gearbox are all grade 12.9 socket head cap screws.

    What I want to know is how to calculate what clamp load is required to hold the joint in place. From that I should be able to determine the torque needed to achieve this clamp load and from test I can establish what torque increase is required with the addition of the Nord-lock washers or thread-lock.

    Kind regards
    Mostly, I am not asking questions... Did you look at my link? For another simpler clamp load calculator see: http://www.engineersedge.com/calcula...orque_calc.htm. Generally, for 12.9 bolts loaded in pure shear, the proper torque is 80 to 90 percent peak applied loading. Don't ask where I got this.... don't remember.

    Also, see: http://www.engineersedge.com/fastener_thread_menu.shtml

    Once you determine the shear loading applied to each bolt you can then determine the average shear stress applied to each bolt. --> http://www.engineersedge.com/materia...ear-stress.htm

    Here's a question - why Nord-lock? Doesn't seem a good ideal to me as your bolts are really hard and the Nord-lock ridges are unlikely to bite into the bolt to prevent loosening during cyclic loading. I would review the application and maybe consider a flex-lock nut or a castle plastic lock nut for anti-loosening.

  6. #6
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    The sprocket and gearbox pictured form the drive assembly of crawler tracks used on large heavy (90T) semi-static crushing and screening machines. These are generally operating in harsh mining and quarry conditions. We actually use thread-lock on every thread regardless of whether or not Nord-Lock washers are used. Nord-Lock washers are designed for use with grade 12.9 bolts and we use them as security given the shock and vibration conditions created by these machines.

    To my understanding there is no standard torque setting for a given bolt and that values vary depending on application, operating conditions and other various parameters that are factored in. The values we work to at present have been found by taking the average values from a number of charts found on the net and as I said learnt from experience and of those used on similar applications.

    The bolt grade, size, thread pitch, number of bolts and PCD are all pre-determined by our gearbox manufacturers. They do not specify a torque value?

    From our torque tables we specify that a grade 12.9, M24 x 3 dry bolt should be torqued to 910 lb.ft? Knowing the above and the output torque of the gearbox it must be possible to verify this value via calculation rather than to assume it be correct?

    Look forward to your replies.

  7. #7
    Administrator Kelly Bramble's Avatar
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    Quote Originally Posted by technicalstudio View Post
    Knowing the above and the output torque of the gearbox it must be possible to verify this value via calculation rather than to assume it be correct?

    Look forward to your replies.
    YES!!!! Again, did you see my links? Do you know how to calculate the shear loading applied to the bolts via the gear-spocket?

    http://www.engineersedge.com/calcula...orque_calc.htm

    http://www.engineersedge.com/calcula...alculation.htm

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    Yes Kelly I did see your links thank you.

    As I see it the first link is telling me what clamp load is achieved for a given torque or size of bolt and vice-versa.

    However, I want to know what clamp load is required for this joint not what torque is need or what bolt size to use to achieve it.

  9. #9
    Administrator Kelly Bramble's Avatar
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    Why do you need the "clamp load is required"? Isn't the torque loading imposed by the spocket-gear being absorbed by the bolts and tight fitting clearance holes?

    Ok, to give you an answer to your question... I going to assume that you want the clamp load to be great enough for a friction that can withstand movement due to the forces of the gear-spocket.

    Basic friction equation, Coulombs Law of Friction.

    F = uR, where u = coeffiecient of friction between the two metals, R is the clamp force and F is the force or friction loading.

    Using the equation for a "Flat Clutch Plate" Uniform Pressure Theory, which I think should adequate... See attachment

    Where:

    T = Torque
    u = Coefficient of friction of your materials
    R = Load given by bolt torque or clamp force if you prefer.
    Do = OD
    Di = ID

    I think that the clamp load force should be greater than the force imparted by the sprocket-gear.
    Attached Files Attached Files

  10. #10
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    At the risk of sounding repetitive, I can't see why the friction load is an important consideration. There must be something horribly wrong if the bolts are not taking all of the drive load. The amount of assistance gained in friction between the mating parts would be negligible, surely? If the bolts are not handling it, then maybe you need to key the two mating parts.

    Dave
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    Insanity: "doing the same thing over and over again and expecting different results." Albert Einstein

  11. #11
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    Kelly, could you please take a look the following link in answer to your question "Why do you need the clamp load" -

    Please see paragraph "Bolt Preload" and "1. Failure of the bolt to provide sufficient clamp force" here http://www.boltscience.com/pages/bc1.htm

    PnkertonD,

    Can I ask, If you were asked "What torque setting should be used for these bolts?" how would you answer this?

    I have asked the bolt manufacturer for guidance and was told it is up to your engineers to tell you. (So it's bounced back to me). I have searched the internet and found various different charts. There are no industry standard values for a given size or grade of bolt. It is application dependant. Further to this, how do I check these values without just assuming them to be correct?

    If you consider the bolts connecting a wheel to a hub on say HGV, various manufacturers will give different torque values. Why, because each vehicle, size of wheel, PCD and number of bolts vary. They know bolts will not fail in shear but how have they arrived at the torque settings? The torque is set according to the required clamp force.

    Thanks for all your help (and patience)

  12. #12
    Administrator Kelly Bramble's Avatar
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    Quote Originally Posted by technicalstudio View Post
    Kelly, could you please take a look the following link in answer to your question "Why do you need the clamp load" -

    Please see paragraph "Bolt Preload" and "1. Failure of the bolt to provide sufficient clamp force" here http://www.boltscience.com/pages/bc1.htm
    OK, and .....

  13. #13
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    and so what clamp force do I need???

  14. #14
    Administrator Kelly Bramble's Avatar
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    Confused

    Quote Originally Posted by technicalstudio View Post
    and so what clamp force do I need???
    With all due respect technicalstudio, sufficient information has been communicated on this forum for YOU to calculate the values that you need

    The first question YOU need to answer is which feature of the flange design is absorbing the loading? Is it the bolts in shear? or is it the friction between the flanges generated by the static friction? Or a combination or bolt shear and flange friction?

    I would calculate it for all scenarios not that hard..

    I will not do the math for you....

    As far as "various manufacturers will give different torque values", an engineer or equivalent has taken in all the considerations we have discussed and/or made a few assumptions.

    The torque values on these charts where probably determined by assuming that shear is the principle and only applied loading scenario. Shear only loading assumptions and designs for bolts is a relatively standard approach. Generally, torque charts are based on the proof load which is usually somewhere between 60 to 85 percent of the bolt Tensile Strength as the base line for applied clamp force calculations. Lubricated and dry threads should be considered in the torque design. The size and number of bolts on a shear flange is determined by the torque applied to the flange and a FOS (factor of safety) applied to the bolts to ensure that the bolts, flanges and associated hardware is of sufficient strength ti absorb the loading. The FOS is typically 3 or greater.

    If a bolt is subjected to considerable axial loading, static or fatigue (cyclic) then the math changes and the bolt design, torque and clamping force must be adjusted to ensure some percentage or tensile strength is not exceeded yes, this more complicated.

    Conclusion and questions:

    Every thing I have seen looks like shear is the principle loading on the bolts. Is it?

    Do you know the difference between shear and axial tension loading?

  15. #15
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    Quote Originally Posted by technicalstudio View Post
    Can I ask, If you were asked "What torque setting should be used for these bolts?" how would you answer this?
    Not to sound too flippant, but there is an old saying in the greasy-hands and skinned knuckles world when it comes to tightening nuts on bolt, "keep turning until it strips, then back it off a quarter turn."

    I really think Kelly has gone over this in more detail than I would bother to have done, and it is up to you from here. Bear in mind that the tensile strength of the bolt is not the deciding factor on tightening torque, it is actually the failure point of the threads.

    I really think you need to get past this friction between the plates for driving the sprocket. It is a non-issue, but then, I have been wrong before, just check with Bruiser.

    Dave
    Generally, I will not give you the answer to your question, but I will guide you into discovering how to solve this yourself.
    Insanity: "doing the same thing over and over again and expecting different results." Albert Einstein

  16. #16
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    100% with Dave on this.
    You don't design for friction to do the job, only shear in the bolts.
    The clamping torque has to be sufficient to prevent loosening of the bolts together with a locking agent .

  17. #17
    Project Engineer CCR5600Design's Avatar
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    My response is not based on my engineering experience, but rather on my engine building experience.

    Previously in this thread, variables such as lock washers, thread locking compound and the like were discussed. It is my experience that as a bolt is tightened, it stretches. This stretch can be physically measured with a simple dial caliper.

    Here's my suggestion:

    1)Construct a mockup of the installation for ONE bolt using the same bolt, flange material and flange thickness. (If the actual installation is into a blind hole, you will need to make the hole in the mockup a through hole to access the threaded end of the bolt for measurement.
    2) Using the formulae links Kelly and Dave supplied earlier, determine the clamping load required for this application.
    3) Based on information from the bolt manufacturer, determine the amount the bolt should stretch under the required clamping load.
    4) Measure the length of an un installed bolt. Record this length.
    5) Add to this measurement the amount of stretch required to achieve the desired clamping load. This is the desired installed length of the bolt.
    6) Using the desired/required thread locking combinations (threadlocker, lockwashers, etc) install the bolt in the mockup. Doing this will compensate for the thread locker and/or washers in the equation.
    7)Gradually tighten the bolt with a beam style torque wrench until the desired installed bolt length is is achieved.
    8) Record the torque value required to achieve the desired installed bolt length and use this value for the installation in your application.

    I hope this helped.


    Ron
    Last edited by CCR5600Design; 07-01-2011 at 02:50 PM.

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