Fastener Torque Part 2 – How To REALLY Measure Tension

Tension, Not Torque Matters: How to Measure Bolt Tension

In our last post, we discussed how measuring torque is an inaccurate way of getting the right bolt tension. In fact, a brand new bolt will require +/-40% torque to achieve the same tension as another new bolt out of the same box. Since tension holds a bolt in place, getting it right is critical: too tight and the bolt can break; too weak and the bolt can loosen. So if a torque chart isn’t worth the paper it’s written on, how do we measure tension?

Read why torque is an inaccurate measure of tension in Part 1 of this paper.

Ultrasonic Testing
Ultrasonic Bolt Tension Monitors (Extensometers) allow the bolt installer to ultrasonically measure the actual load, stress, and elongation produced by tightening a threaded fastener of any material. Laboratory as well as hand-held portable sized units are available. Though expensive, ultrasonic testers are the only accurate way to measure tension accurately post-installation. They are also ideal for in-service monitoring and difficult to reach bolts.

Hydraulic Bolt Tension Calibrator
These hydraulic load cells measure tension independently of torque. These devices are used pre-installation to verify torque/tension relationships in order to determine an accurate torque. For example, look at a torque chart for an Inconel 625, 1/4 ”-20 dry (unlubricated) bolt, the torque should be 8.2 ft-lb. Using a hydraulic bolt tension calibrator you test a bolt at this torque only to find that it doesn’t give you the required tension of 84 ksi; due to your conditions you are only getting a tension of 78 ksi. Now you use the calibrator to learn what torque you will need to apply to that bolt in order to give you the needed tension of 84ksi.

Here is a video that shows how a bolt is tested pre-installation in order to determine the torque required to get the necessary tension.

DTI (Direct Tension Indicator) – DTIs are now offered by Special Fasteners China.!
A DTI is a specialty load indicating washer that the ASTM describes as “capable of indicating the achievement of a specified minimum bolt tension in a structural bolt and are intended for installation under either a bolt head or a hardened washer”. As a fastener is tightened, the arch-like protrusions are compressed and the change in distance between the base of the DTI washer and the bolt. This distance correlates to a value of tensile force induced into the fastener. By inserting a tapered feeler gauge between the protrusions to the bolt shank you can verify the distance and hence the tension by comparing the value to manufacturer’s instructions.