Welcome to our Bolt Torque and Axial Tension Calculator, a tool designed to help you estimate the correct torque for fastening bolts, screws, and nuts. On this page we generically use the term “bolts” but this also applies to screws and nuts. When you tighten a bolt, the torque you apply plays a critical role in achieving the desired clamping force and ensuring the fastener remains secure under load. Understanding how bolts behave when torqued is essential for achieving the proper fit and preventing failure. A bolt, when tightened, stretches slightly, behaving somewhat like a spring under tension. This stretching is key to generating the clamping force that holds the components together. However, this behaviour can be influenced by several factors, including the friction present in the threads and at the bearing surface under the bolt head.
Friction in both the threads and under the bearing surface of the bolt head can pose significant challenges when applying torque. As you turn the bolt, friction resists the rotation, which means the torque you apply doesn’t entirely go into stretching the bolt. Instead, a large portion of it is absorbed by the frictional forces at play. This can lead to inaccurate results, making it difficult to achieve the correct clamping force. The friction in the threads, for example, varies depending on factors like lubrication, surface coatings, and thread geometry. Similarly, friction under the bolt head can cause variations in the force applied to the fastener, depending on the surface condition of the material being clamped and the smoothness of the bolt head itself. These inconsistencies make it challenging to rely solely on torque as an indicator of the bolt’s true tension or clamping force.
This is where our Bolt Torque and Axial Tension Calculator comes in. By entering relevant details such as the bolt size, material, and the type of lubrication used, you can get a suggested torque value that compensates for the friction and ensures the bolt is torqued correctly. Whether you’re working on automotive repairs, machinery maintenance, or structural assemblies, the calculator ensures that your bolts are tightened to the right specification, preventing issues such as over-tightening, under-tightening, or premature failure of the fasteners.
It must be remembered however, that torque tightening is not an exact science, and the resultant stretch developed in the bolts can vary extensively from predicted to actual. In applications where structural integrity and safety is critical, we suggest you only use this calculator as a guide and consult with experts in the field for the proper guidance.
The formula for calculating axial tension is:
F = \frac {\sigma . \pi . {d_{minor}}^2}{4}
where:
F = Axial tension (N)
σ = 75% of the yield strength of the bolt material (MPa)
dminor = Minor diameter of the bolt (mm)
[Used to determine the stressed area of the bolt]
The formula for calculating torque is:
T = F . k . d_{nominal}
where:
T = Suggested tightening torque (Nm)
dnominal = Nominal diameter of the bolt (mm)
[Outside diameter of the bolt e.g. for M8 use 8]
k = Torque coefficient (from 0 to 1)
[This factors in friction for the threads and bolt head]
Typical values:
- Dry steel threads: 0.20 to 0.30
- Lubricated steel threads: 0.15 to 0.25
- Galvanised threads: 0.20 to 0.25
- Low friction oil: 0.10 to 0.20
Disclaimer: The information on this page is provided in good faith. Ezee Calc assumes no responsibility or liability for any errors or omissions in the content of this site. Information contained in this site is provided on an “as is” basis with no guarantee of completeness, accuracy or usefulness.