Understanding & Applying Proper Torque for Metric Fasteners

Torque Explained

Tightening a nut onto a bolt causes the bolt to stretch slightly. Much like a spring, the bolt resists this stretching, and its tendency to return to its natural state creates clamping action between, say a cylinder head and a manifold, or two pieces of sheet-metal housing.

It is critical to the component’s operation that the amount of tension created holds the parts together strongly enough to prevent their separation by outside forces such as the machine’s vibration, the load stress generated during operation, gasket creep, temperature fluctuations, and more. Too much torque, however, can stretch the fastener too much, to the point where it chips, breaks, or yields. Bolts and screws are rated by their “proof load” – how much tension they can withstand before they fail. As a rule of thumb, torque a “clamp load” (also known as “preload”) 75 percent to 90 percent of the proof load is optimal.

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Torque Relationship Chart

 

Using the Chart

The torque required to achieve optimal preloads is a function of several factors, most notably the bolt’s diameter, its surface treatment, and its strength grade or class.

The wider the bolt, the more torque required to stretch it and create clamping tension. Similarly, higher-grade bolts are stronger and more resistant to stretching, so additional torque is needed. Finally, the bolt’s material and any plating and lubricants applied to it will determine how much friction must be overcome in order to tighten it. As you can see from the chart, dry, untreated bolts require more torque than lubricated ones do in order to achieve the same clamp load. Zinc-plated bolts fall somewhere in between.

The chart uses the accepted equation T = K x D x P to provide a guideline for applying the correct torque to most sizes and grades of metric bolts. In the equation T is the tightening torque, converted to foot-pounds; K is the coefficient of friction, as determined by thread pitch and depth, bolt composition, and surface treatment; D is the bolt’s nominal diameter in millimeters; P is the clamp load (calculated at 75 percent of yield).

Simply find your bolt’s grade and surface treatment along the top row of the chart and its diameter down the left column. Where the row and column intersect, you will find the recommended tightening torque.

Caution

Mr. Metric provides this chart and the accompanying information as good-faith advice to its customers. All material herein is advisory only. Mr. Metric has made every effort to ensure the information is accurate. In providing this information, we have made a determined effort to ensure its accuracy. We cannot be held liable for any damage incurred as a result of using this information, as torque is only an indirect indication of tension and in-proper tightening can lead to injury.

Threaded fasteners are used to create tension and, therefore, clamping power. Measuring these forces is an inexact process for several reasons:

  • Torque is an indirect way to gauge tension.
  • The amount and type of lubrication varies from bolt to bolt and lot to lot.
  • Friction coefficients can never be precisely measured. This chart uses the generally accepted K-factor values of 20 for plain finished bolts, 0.22 for zinc plated bolts, and 0.10 for lubricated bolts.
  • The torque settings in the chart are calculated for general applications.

For these reasons, if your specific process or product calls for a different torque value, use that recommendation, rather than the ones provided in the chart.

Maintenance

Check the torque of all bolts periodically, as several factors can relax their tension over time. When replacing bolts, use the same or higher class (new shear bolts should be exactly the same class as the originals). Tighten all replacements to the same torque as the original, and tighten all companion bolts in the component simultaneously using a tightening sequence to mitigate the effects of elastic interaction.

Still unsure which torque is right for your job? Contact us to speak with a representative for more information.

About the author

Mr. Metric is the spokesperson for www.mrmetric.com, an online store for metric fasteners. Get $6.00 flat shipping with no minimum orders, plus advice and best practices for using metric fasteners in any application.

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