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Confirm Formula Please (total load on load cell)

  1. Apr 17, 2015 #1
    I am using the following formula to determine the lb load against the bottom of a load cell from tightening the bottom anchor bolt nut (which anchor bolt goes through the center of the load cell structure and anchors to a bottom base plate) to a certain torque force, plus adding a live static load from above, creating a total load on the load cell.
    Will this formula give me the correct total load ?

    FOMULA: T=(K)(D)(P)
    T= Torque in inch lbs (divide by 12 to get ft lbs )
    K= Coefficient of friction ( nut to dry bolt threads) ( range of .35 to .40 from standard table )
    D= Bolt diameter in inches
    P= Bolt's desired tensile load in lbs ( generally 75% of yield strength )
    Then add to P the static live load to get the TOTAL LOAD applied to the load cell.

    I tried to upload a sketch of above and this system would not accept it ( to large). <<Mentor note: Personal identifying information removed>>
    Thank you very much!
    Last edited by a moderator: Apr 17, 2015
  2. jcsd
  3. Apr 17, 2015 #2

    jack action

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    Science Advisor
    Gold Member

    It depends.

    If you torque your bolt first to create the preload and then add the static load, the load cell will read the highest of the preload OR the static load.

    If you put the static load first and set the preload with the static load on the load cell, the load cell will read the preload PLUS the static load. And if you take the static load off, the preload on the bolt will increase by an amount equivalent to the static load, maybe leading to failure.

    I came to this conclusion by modeling your load cell as a spring. If you preload it with, say, 100 lb, it will deflect by a certain amount. Adding 50 lb on the spring will not deflect it more because whatever deflection increase would the static load impose, it would also remove the preload; so everything will balance out and stay constant. If you add 150 lb, the spring will deflect more, completely releasing the preload, such that the spring will only «feel» the 150 lb.

    If you put the load on it first, say 50 lb, the spring will deflect. By adding the preload to the bolt, say 100 lb, the spring will deflect even more, to an equivalent 150 lb. If you remove the static load, then the spring will push 150 lb against the bolt.
  4. Apr 20, 2015 #3
    Thanks for your reply.
    I understand your concerns. Lets see if this helps.
    Once the bottom nut is tightened or torqued and the preload is set (and before any static live load is imposed from the top). The load cell is then calibrated to zero by it's indicator brain box. Then the live load from above is imposed and the display window, after pressing the appropriate button, shows the live weight only on the display window. So at that point the load cell is only reading the live load applied after is was re-calibrated to zero, after the preload was imposed.
    Assuming that is all good, I am really double checking if the formula I used to calculate the preload was a correct one to use.
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