It's very important to get the bolt torque calculation accurate so that your fasteners work well under stress. If you know how to properly tighten M8, M10, or M16 bolts, you can make sure that your joint is secure and avoid costly failure. Let's cover everything regarding torque numbers and application.
What is Torque in Bolts, Preload, and Friction?
It's important to know what torque does in a bolted joint before you start doing maths. Torque turns a bolt so it can be tightened, but it's merely necessary to get the appropriate preload.
Preload is the force that holds your joint in place. However, to achieve the proper preload, you must consider the friction coefficients, which vary significantly depending on the surface conditions and the amount of lubrication.
When you add lubrication, the friction coefficient varies a lot. For normal finishes, the coefficients are usually between 0.12 and 0.18 in dry conditions. When MoS2 paste is used as a lubricant, this drops to between 0.05 and 0.10. This has a direct effect on the bolt tightening torque needed to get your target preload.
When calculating bolt torque, it is essential to consider the accuracy of the torque wrench and the degree of frictional changes. The main idea is:
Adjusted Torque = Maximum Torque ÷ Scatter Factor
The scatter factor takes into account the inaccuracy of the torque wrench (±17% to ±23%).
How to do the maths:
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M30 Grade 12.9 bolt: Maximum torque is 2,280 Nm (with a friction coefficient of 0.125).
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With a scatter of +23%: Mean torque = 2,280 ÷ 1.23 = 1,854 Nm
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Minimum torque = 1,854 × 0.77 = 1,428 Nm
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Set the torque wrench to 1,854 Nm (mean torque)
A full bolt torque chart shows the suggested values for different property classes. The amount of torque needed for M8, M10, and M16 bolts depends on their grade:
M8 bolts: 17 Nm (Grade 8.8), 25 Nm (Grade 10.9), and 29 Nm (Grade 12.9).
M10 bolts: 33 Nm (Grade 8.8), 48 Nm (Grade 10.9), 58 Nm (Grade 12.9)
M16 bolts: 150 Nm (Grade 8.8), 208 Nm (Grade 10.9), and 250 Nm (Grade 12.9)
These numbers are based on normal friction coefficients and need to be changed for
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The kind and quality of the bolt
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The pitch and quality of the thread
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Type of lubrication
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Joint configuration
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Conditions in the environment
Tips for Using It in Real Life
Always use a calibrated torque wrench and follow a set order when applying bolt tightening torque. When tightening multi-bolt joints, do it in stages, usually starting with 50% of the final torque and then going up to 100% in a cross-pattern.
Keep in mind that torque values are not set in stone. Tension-indicating washers or ultrasonic bolt monitors may be needed to check the actual preload in important situations.
Get Help from Professionals
To calculate the right bolt torque, you need to know what your individual application needs. We offer full technical support at BigBoltNut to help you choose the proper fasteners and optimise the best torque settings for your projects.
Download Full Torque Chart PDF to get detailed torque specs for all of our fasteners, including those in metric and imperial sizes and all standard grades and materials.