Nut factor calculation of target torque

The Nut Factor Calculation of Target Torque provides a simple formula for calculating target torque. The formula is described in ASME PCC-1–2022, Appendix K. This guide is based on that description.

The nut factor calculation of target torque formula is:

    (SI Units)

        T = K D F / 1000

    (U.S. Customary Units)

        T = K D F / 12

    D = nominal diameter of the bolt, mm (in.)

    F = target bolt load, N (lb)

    K = nut factor (see below)

    T = Target Torque, N·m (ft-lb)

Calculate torque in seconds.

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Screenshot of a target torque calculation in the Flange Bolt Chart app.

The Nut Factor

The Nut Factor, denoted as K, is a dimensionless constant derived from experiments and closely linked to the friction coefficient. In many ambient temperature applications, K is considered to be approximately equal to the friction coefficient plus an additional 0.04. For instance, friction coefficients of 0.14 and 0.10 would roughly equate to nut factors of 0.18 and 0.14, respectively.

Small variations in nut factor significantly impact resulting bolt load

Various sources provide tables of experimentally determined nut factors, but it's crucial to ensure their suitability for the specific application in question. In industrial settings, such as those involving SA-193 low-alloy steel bolts for pressure vessels and piping, the nut factor generally ranges from 0.16 to 0.23 at room temperature. It's important to recognize how small variations in the nut factor can significantly impact the resulting bolt load. For example, altering the nut factor from 0.1 to 0.3 leads to a 200% change in torque, not just 20%. Also, insufficient lubrication can greatly increase the variability in the achieved bolt load.

[A]ltering the nut factor from 0.1 to 0.3 leads to a 200% change in torque, not just 20%.

Recent studies have indicated that the nut factor is influenced by the material of the bolt, its diameter, and the temperature during assembly. These elements are significant and should be taken into account when selecting a nut factor or anti-seize compound. Users are advised to either consult test results for similar bolt and anti-seize specifications or conduct their own nut factor trials, considering their specific conditions, bolt size, and material.

Nut factor trials

Conducting nut factor trials can be relatively straightforward. It involves tightening a bolt to a specific torque and then measuring the resulting bolt load using methods like calibrated ultrasonic measurement, a calibrated load cell, or by observing the pressure increase in a hydraulic tensioner. Additionally, the maximum temperature indicated by an anti-seize product's manufacturer may not be a reliable predictor of its effectiveness in facilitating joint disassembly after being subjected to high temperatures. Thus, seeking test results on similar materials and operating conditions is recommended to guide the selection of the most appropriate product for a particular application.

Additional information

Additional information on the nut factor calculation and other target torque formulas can be found in the Handbook of Bolts and Bolted Joints. Detailed formulas can be found in Chapter 3, while additional theoretical and experimental information and equations can be found in Chapters 12 and 32.