Finding the maximum tightening torque for most common types of screws and bolts is an important first step in ensuring the required clamping force. Generating clamping force is the goal of tightening, and it is achieved so the assembled parts will function as intended.
While tightening, or applying torque, to a bolt, the clamping force clamps the parts with a tremendous amount of force that will ideally remain very high and consistent. There are three important factors in understanding and finding the recommended max torque for your bolt: bolt size, bolt grade and friction.
The chart above is from Atlas Copco’s 2019 Industrial Tools and Solutions catalog. Using this will be helpful in understanding the three key factors that go into finding the maximum torque for your bolt.
Bolt size
The size of the bolt is the first factor in finding the recommended maximum torque for your bolt. Looking at the chart above, the thread column refers to the diameter of the bolt in millimeters (mm). An M3 thread bolt is 3 mm in diameter, and similarly an M5 thread bolt is 5 mm. The larger the bolt, the more torque is needed to achieve the necessary clamping force. For the sake of this article, we’ll use an M12 thread to describe bolt size. An M12 bolt is about 12 mm in diameter, or 0.5 in. Understanding the size of your bolt is key to moving on to the next factor, the grade of the bolt.
Bolt grade
Bolts are made up of different steel grades. The grade of the bolt will allow you to determine just how much stress and tensile strength the bolt can handle before breaking. Understanding bolt grades is important in determining the correct tools for tightening. Did you know an M12 grade 10.9 bolt, a common automotive fastener, has an ultimate tensile strength of nearly 20,000 pounds? According to the chart above, the recommended tightening torque for an M12 bolt with a 10.9 bolt grade would be 114.0 Nm. It’s important to note that higher bolt grades require a higher torque to get more clamping force.
Bolt friction
The final key factor is determining the maximum torque for your bolt is friction. Friction will always be present in a bolted joint, as it’s necessary in preventing part loosening in a joint. It must be overcome in two separate places on the bolt: in the threads and under the head of the bolt. For calculating tightening torque, it is important to understand the coefficients of friction, which are equal to 0.125 on our chart above. Similar to bolt grade, higher friction in the joint will require a higher torque to achieve the proper clamping force. According to Atlas Copco’s Tightening Services Laboratory Manager, about 90% of the torque we put into the fasteners goes into overcoming the friction of the bolt. The remaining 10% of the torque applied goes into building clamping force and elastically stretching the bolt.
Finding the recommended maximum torque for your specific bolt is key to achieving the sufficient clamping force and ensuring the assembly will perform as intended. Insufficient torque or incorrect use of lubrication can cause the joints to loosen, while too much-applied torque can result in a broken bolt. To find the recommended torque for your bolt, we suggest finding the bolt size, bolt grade and the friction using the chart pictured above.
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