The Experts: The formula for calculating jackbolt torque

Q:  How do I calculate the jackbolt torque (based on a formula)?

A:  Superbolt multi-jackbolt tensioners (MJTs) use small jackbolts to achieve the preload. These work similar to regular bolts except they are preloaded in compression instead of tension. The preload generated by the jackbolts when tightened with a certain torque depends on the friction coefficient of the lubricant, size of the jackbolt and pitch of the jackbolt thread.

Of the three, only the pitch of the jackbolts contributes to the preload. The other two determine the energy lost due to friction. To minimise this loss it is beneficial to use many small jackbolts and a lubricant with a low but stable friction coefficient. For speed of tightening and cost efficiency, fewer but larger jackbolts are desirable, while a higher friction coefficient reduces the risk of self-loosening. Finding the optimal configuration is best left to our experts.

The torque to preload relationship for the specified lubricant is indicated on the drawing or in the Superbolt product brochure. Calculating the required torque for any desired preload is simple because torque and preload are proportional within a wide range. Thus, half the torque results in half the preload.

For special designs and upon request we recommend the tightening torque for your application. When in doubt, please contact your local Nord-Lock office. We’re always glad to help.

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