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Tension vs. Torque Explained (sort of…)

Question: Can someone explain how tension and torque relates to bolted connections?

Answer: Sure, we’ll try our best. The relationship between tension and torque should be looked at cautiously, since it is very difficult to indicate the range of conditions expected to be experienced by a fastener. Torque is simply a measure of the twisting force required to spin the nut up along the threads of a bolt, whereas tension is the stretch or elongation of a bolt that provides the clamping force of a joint. Bolts are designed to stretch just a tiny bit, and this elongation is what clamps the joint together. Torque is a very indirect indication of tension, as many factors can affect this relationship, such as surface texture, rust, oil, debris, thread series and material type just to name a few. Virtually all the torque/tension tables that have been developed, including ours, are based on the following formula:

T = (K D P)/12

  • T = Torque (ft-lbs)
  • D = Nominal Diameter (inches)
  • P = Desired Clamp Load Tension (lbs)
  • K = Torque Coefficient (dimensionless)

The value of K is a dimensionless torque coefficient that encompasses variables such as those listed above, as well as the most significant variable, friction. The value of K can range from 0.10 for a well lubricated/waxed assembly, to over 0.30 for one that is dirty or rusty. The values we used when calculating our values are:

  • 0.10 = Waxed/Lubricated
  • 0.20 = Plain, as received condition, slightly oily
  • 0.25 = Hot-Dip Galvanized

The appropriate torque value to use in a specific application is best obtained by using a calibrated torque wrench and a Skidmore-Wilhelm load indicating device to equate actual torque to the desired tension. For ASTM A325 and A490 structural bolts The Research Council on Structural Connections (RCSC) recommends:

The pre-installation verification procedures specified in Section 7 shall be performed daily for the calibration of the installation wrench. Torque values determined from tables or from equations that claim to relate torque to pretension without verification shall not be used.

(RCSC Specifications, June 2004, pg. 62, 8.2.2)

An alternative and more accurate method for assuring proper tension would be to use a direct tension indicator or DTI. These are available for use with ASTM A325 and A490 structural bolts and are engineered to compress at the proper tension, assuring the installer that the proper clamp load is achieved. Hopefully, this short introduction to bolt connections helps address some of the confusion surrounding this issue.

Download Estimated Torque Values

Posted by Dane McKinnon

Phone: (503) 219-6991 Email: danem@portlandbolt.com
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The below content is submitted by readers and has not been researched or checked for accuracy. It is not endorsed in any way by Portland Bolt.

23 Responses to “Tension vs. Torque Explained (sort of…)”

  1. Bryan Carr says:

    Should you torque a bolt to a recommended torque even when using helical spring lock washers?

  2. Adam Oakley says:

    Bryan Carr » The formula from Industrial Fastener Institute (IFI) “Design of Bolted Connections” (M-64), used to determine values for our toque chart states it is meant for, “… steel bolts in their as-received condition.” Although, IFI goes on to state there many factors (surface texture, material hardness, thread series) that only have a “modest effect” on the torque-tension relationship. The primary influence on torque calculations is coefficient of friction. Again, the only “correct” way to establish an accurate torque value is to test the fastener in their actual joint application.

  3. Galliou says:

    Is the “Bolt Torque Chart” applicable for anchor bolts?

  4. Adam Oakley says:

    Galliou » It is hard to say if these calculations would be very accurate for anchor bolts. The biggest uncontrolled variable when determining torque is friction. Since the surface between the bolt assembly and the joint surface in the field can vary greatly it is difficult, if not impossible to accurately estimate torque for anchor bolts. One option would be to consider load indicating washers. This washer will give consistent tension values regardless of environment.

  5. Hevii Guy says:

    Direct Tension Indicationg washers provide a false sense of security: They cannot indicate whether the joint has been overtightended. Neither do they provide a means of checking load migration after initial installation.
    Skidmore-Wilhelm machines provide an accurate indication of torque to load but ONLY on the exact fastener being measured under the the conditions experienced in the device AT THAT TIME. Once the tested wrench is used in the field, myriad friction factors and load interactions mean that all bets are off; one has no way of knowing what the actual load will be – it’s still a “guessing” exercise unless load is verified AFTER the bolt has been tightened and then again, after all of the other bolts have been tightned (to compensate for load transfer)

  6. Mike Girard says:

    Just a minor point of clarification: Torque is not energy. It is a twisting force.

    • Jonathan Waltner says:

      @Mike Girard – You are absolutely right. The content of the post has been updated. Thanks for catching that.

  7. Charles says:

    Hevii Guy,

    There is no such thing as an overtightened connection, until you break the bolt in tension. RCSC – Sec. 9.2.4, states: “A pretension that is greater than that specified in Table 8.1 shall not be cause for rejection”

  8. Manoj V K says:

    The same above mentioned formulae are used for calculating torque for countersunk screws or any other multiplication factor available…???

    • Dane McKinnon says:

      @Manoj – The torque formula would differ from the one listed because screwing into tapped holes usually creates a longer thread engagement which can affect the friction coefficient. An engineer would be the best person to consult regarding this.

  9. Manoj V K says:

    @Dane McKinnon -Thanks

  10. Lily Well says:

    For 5/8″ Step bolts, of SAE grade 2, what torque value should we be specifying during installation. This is for a transmission line tower.

    • Dane McKinnon says:

      @Lily – That is a difficult question to answer. We have torque values on our website, but every application is a bit different, so there is no one all encompassing torque value that works for all applications. Controlled lab testing is the only sure fire way of determining your specific torque values. http://www.portlandbolt.com/technicalinformation/bolt-torque-chart.html

  11. Jegan says:

    Hi, I need the Torque report format, Could you provide format(sample)? Until we dont have the format.Thanks

    • Dane McKinnon says:

      @Jegan- I am unsure what you are requesting. You can download the torque chart as a pdf, or you can print what you see on the screen.

  12. Alex says:

    P = Desired Clamp Load Tension (lbs), is this the value given in kips on the RCSC code, if so how to convert to lbs

    • Dane McKinnon says:

      @Alex- Kips=1,000lbs. So 51kips=51,000lbs of tension.

  13. Duncan says:

    Any guidance for 3/4″ lag bolts in SYP timber? Specifically a railing base plate with lags into stringer below (perpendicular to grain), with concerns about torque loading that twists off the lag and how much to reduce this torque to avoid lag damage? Do you have maximum allowable torques for lags?

    • Dane McKinnon says:

      @Duncan – Apologies, but we do not have any engineers on staff, so cannot comment on application-specific questions like this. I am not aware of any allowable torque values for lag bolts.

  14. Jared says:

    Is there a torque chart for different sizes of the ASTM 1554 grade 36 anchor rods?

    • Dane McKinnon says:

      @Jared – These torque charts are calculated for use in steel to steel connections. Anchor bolts are not typically torqued to a specific requirement, as the AISC says that snug tight is acceptable.

  15. ersin says:

    I used M10 10.9 allen head screw on the aluminium rail. for assembly of bus seats..under rail there is sheet. ı searched a lot. everybody is saying aluminium reduce the torque value.
    ı found 33.9 Nm and 20Nm. difference is too big.
    whats your idea?

    • Dane McKinnon says:

      @Ersin – Thank you for your question, but our area of expertise is with carbon and alloy steel structural applications, we don’t have any information regarding aluminum Apologies.

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