Bolt torque chart: the Atlas Copco guide

A bolt torque chart provides the maximum tightening torque required for the most common bolt types. Finding the correct value is the first step towards any efficient operation: this is the only way of ensuring the right joint force without putting yourself at risk of assembly issues involving high bolts, damaged threads, stripped bolts and others.

The following chart appears in the 2019 Atlas Copco Industrial Tools and Solutions catalog. It gathers the three required variables to understand and determine the maximum recommended torque for your bolt: bolt size, bolt grade and friction. Another key factor is to learn how the pieces accommodate when joined, i.e., whether the joint is rigid or flexible, helping you to correctly apply the joint force to the process.

When looking at the bolt torque chart, the thread column indicates the bolt diameter in millimeters (mm). An M3 threaded bolt has a 3 mm diameter and, similarly, an M5 threaded bolt has a 5 mm diameter.

The larger the bolt, the more torque is needed to achieve the required joint force.

This article will use an M12 thread to describe the bolt size. An M12 bolt has a diameter of about 12 mm, or 0.5 in.

Understanding the bolt size is essential for proceeding to the next factor, the bolt grade.

Bolts are built from different grades of steel, defining the degree of tension and tensile force the bolt can endure before snapping. Learning these is critical to determining the correct tightening tools.

Did you know that an M12 bolt with a 10.9 grade can withstand a maximum tensile force of about 20.000 pounds? As the chart illustrates, the recommended tightening torque for an M12 bolt with a 10.9 bolt grade would be 114.0 Nm.

Remember that higher bolt grades require a higher torque to achieve greater joint force.

Friction is ever-present in a bolted joint since it is required to prevent the part from loosening. This must be dealt with in two separate spots on the bolt: the threads and under the bolt head.

When it comes to calculating the tightening torque, understanding the friction coefficients, which are equal to 0.125 in our table above, is key. Just like the bolt grade, a higher joint friction will require a higher torque to achieve the right tightening force.

About 90% of the torque applied will overcome the bolt friction, according to the Atlas Copco Tightening Services Laboratory Manager. The other 10% goes into the tightening force and elastically stretches the bolt.

Insufficient torque or misusing lubrication can lead to loosening joints, whereas too much torque can cause a broken bolt. For this reason, using the bolt torque chart above is a must.

These problems, when they arise, can negatively affect productivity, generate waste and rework, drive up production costs, jeopardize the tightening performance and, consequently, the brand's own reputation.

Our Product Expert Paulo Vieira explains in the following video why these situations may occur and the steps needed to avoid process breakdowns.