Why Tension, NOT Torque?
Most fasteners are tightened using traditional methods such as manual torque wrenching, pneumatic impact wrenching, or automatic torque cutoff tools. These traditional methods have a well-known drawback – for a given torque there can be large variations in the actual tension produced by these techniques.
About 90% of the torque force applied to a fastener is to overcome friction. The friction factor varies with the amount of scale on the fastener and nut, type of plating, lubrication, presence of rust, the number of times the joint has been tightened and so on. This makes the amount of torque needed to produce a desired tension very difficult to predict without extensive laboratory tests under highly specific conditions. It also makes it difficult, if not impossible, to gauge whether a joint is properly tensioned for a given application of torque.
On the other hand, direct tension indicators respond to fastener tension ONLY, ignoring torque inputs, providing a true measure of joint tightness. Tension is the critical factor in ensuring joint tightness. The best direct tension-indicating methods are based on measuring the strain in fastener shank using the elongation when tensioned.
Proper bolt load is crucial for optimum equipment reliability. The value of torque is commonly used to specify how tight a fastener should be. However, because of the wide range of friction variations, a fastener that is “torqued” to the “right” degree could actually be too loose or even too tight, bringing into question the reliability of the joint and the safety of operations.
Torque versus Direct Tension Indication
Torque ✖ |
Direct Tension Indication ✔ |
| Rotational force | Linear force |
| Approximates joint tightness | Directly measures elongation, tightness |
| Repeated joint tightening changes the amount of torque needed to ensure tightness | Repeated joint tightening does not affect tension measurement |
| Safety can be compromised due to lack of accuracy and reliability | Safety ensured due to reliable, repeatable accurate measurement |
| Influenced by: | Independent of: |
| Fastener and nut size | Fastener and nut size |
| Lubrication | Lubrication |
| Presence of rust | Presence of rust |
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