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Thread: Screw Torque VS attachment material

  1. #1
    Associate Engineer
    Join Date
    Dec 2015
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    Screw Torque VS attachment material

    Have looked into "Torque" requirements for fasteners for the last week...It seems that there are many tables, charts and graphs that describing torque(s) for various size fasteners and some go as far as giving data for the various material that the fasteners are made from.
    My question is "does this data include what the fasteners are being screwed in to also? or just characteristics of the fastener itself such as; tinsel strength, Axial tension, plating, friction coefficient...etc..? "
    I'm currently looking for the torque requirements of a M6 X 10 Stainless Steel(A2-70) SHCS the catch is this particular screw is being inserted into Aluminum (6061), G-10 (fiber resin material), DuraStone 203 (cellulose material) all requiring a different torque (from experience).
    Now most charts state that a M6 X 10 needs to be torqued to around 7 Nm but the majority don't state what material "if any" that their screwing the fastener into.

    I know how and done the 10 piece test up to stripping then cut back 75 to 80% to get a "safe" torque rating to use, but what I'm looking for is the actual equation that I would need when looking only at the properties of the components involved.
    I know there are many characteristic(variables) for the material itself(attachment material not screw material), consistent conductibility, the thread type, the condition of the tap, is lube being used...ect...
    but still if all the variables are known there still must be a core formula requiring particular variables.

    If anyone could direct me to material just short of a degree in Mach Eng....I would be grateful.

    Or any web sites that cover both material of the fastener and attachment material would be great also.

    Thanks guys

  2. #2
    Lead Engineer
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    Aug 2013
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    Houston TX USA
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    Published torque limits are generally based upon the failure of the minimum thread diameter of the screw under the combined stresses of tension and torque that the screw experiences during its installation. As far as the strength of the mating part is concerned, the failure of this item is based upon the shearing of the internal thread of an equal strength material of appropriate thread engagement. As a simple, and quick, determination of the required minimum thread length of a matching part of the same material I have generally just looked up the thickness of the standard nut of the same strength classification as the screw/bolt.

    A general determination of the required internal thread engagement of an alternative metal material can be made using the ratio of the shear strength of the new material relative to that of the standard nut; and this should work for machine screws since they are generally not forged in the manner of high strength bolting nuts. You notice that I have focused upon the internal thread length ratio rather than a torque ratio, this is because it is generally assumed that the strength of the mating internal thread will always be designed to take advantage of the full strength of the screw.

    Accepting all of the variables you have listed that go into a published screw torque, , for metals, ie aluminum, the adjusted torque could be based upon the same ratio basis given above; but, for your composite materials, that is not necessarily the case because these materials probably do not have the same shearing modes and characteristics as metals.

    Long story, finally short, depending upon the criticality and safety requirements of your application screw joints' design, I would suggest that you are better served by using your own current test method for determining the appropriate torques for the composite materials at the thread engagement lengths that you desire/require for your application(s) as long as the reduced screw grip load at the lower torque is acceptable.

  3. #3
    Associate Engineer
    Join Date
    Dec 2015
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    Thank you very much for your reply...sometimes its the trees that keep one from seeing the forest...by looking at the "thread' as you mentioned...it doesn't matter if the threads are located in tapped plate or a nut as long as the threads and material are the same...so for dissimilar metals it's the material with the weakest link (generally) that would determine max. torque...and as for the composites I've gotten that same reply form the Manufacturing Engineers of the composite material as to "trail and error" testing to match my application as being the best avenue to take here.

    Thanks again.

  4. #4
    Technical Fellow Kelly_Bramble's Avatar
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    Tell me and I forget. Teach me and I remember. Involve me and I learn.

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