Understanding Torque and Tension Relationship


Fastener assemblies are simple machines but they are often used in very complex structures.  When working in situations where joint failure is catastrophic making assumptions about how well they are working is not an acceptable method of operation.  In these cases liability and refastening costs are so high that it is essential to understand the hows and whys of fastening with bolts and nuts to answer torque and tension relationship.

It is recognized we measure torque when tightening threaded fasteners only because measuring bolt tension, the more important quantity, is much more difficult. The torque and tension relationship created from the resulting bolt elongation is most commonly described by T = F*K*D where T is torque, K is the nut or friction factor (nut factor for this article), D is the bolt diameter and F is the bolt tension or preload. Less common is a discussion of how this equation was derived, in particular the origin of the nut factor. Because this single variable K determines the critical torque-tension relationship, the factors that cause its value to vary about 300% across a range of common applications are not clear. Examining the sources and relative sensitively of these factors is the subject of this column.

Here are some other variables that can affect torque:
·         Hardness and strength of the fasteners
  • Lubricants
  • Types of washers
  • Thread condition
  • Surface characteristics of the parts being assembled (plating, coating, hardness or smoothness)
  • Improperly maintained or underpowered power tools
  • Operator “feel”

It's all About Clamping Force
As you turn the nut onto the bolt, it causes the threads to press tightly against each other. As tightening continues, the desired clamping force is achieved — the nut pulling in one direction, the bolt resisting from the other. Naturally, when the bolt and nut are this tight, resistance to further turning is very high. Your torque wrench will tell you this but it is not “torque” that determines bolt and nut tightness. It is "tension." Tough torque and tension relationship right there. Tension is an indication that the length of the bolt shaft is being stretched. . . actually getting longer. You can’t see it with the naked eye, but that’s what’s happening.

If Nuts Could Talk
Torque wrenches provide only part of the information you need about fastener assemblies.  They tell you how much resistance they are getting from further turning of the nut.  What does that resistance mean?  Is the nut saying "Okay, every one of my threads is in tight contact with those on the bolt and I'm creating a tight, secure joint." Or is it saying "The rust on this bolt is catching on my threads making it hard to turn.”  The dial indications may be the same for both situations so a fixed amount of torque may not always produce the same tension.


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