Fastener Torque Part 2 – How To REALLY Measure Tension

Tension, Not Torque Matters: How to Accurately Measure Bolt Tension In our previous discussion, we explored why relying solely on torque measurements is an unreliable method for achieving proper bolt tension. Surprisingly, even brand-new bolts from the same batch can require up to ±40% variance in torque to produce identical tension levels. Since the tension in a bolt is what ensures its reliability—too much and it risks breaking, too little and it might loosen—it's clear that accuracy matters. So, if torque charts aren't the solution, how do we precisely measure tension? Read more about why torque is an inaccurate measure of tension in Part 1 of this series. --- ### Ultrasonic Testing Ultrasonic Bolt Tension Monitors, often referred to as extensometers, provide an ultrasonic means of determining the actual load, stress, and elongation generated when tightening any type of threaded fastener. Both laboratory-grade and portable handheld models are available. While they come with a higher price tag, these ultrasonic tools remain the most precise method for measuring tension after installation. They're especially useful for hard-to-reach bolts and ongoing in-service monitoring. --- ### Hydraulic Bolt Tension Calibrator Unlike torque methods, hydraulic load cells independently measure tension. These devices are typically used before installation to verify the relationship between torque and tension, allowing installers to adjust their approach for optimal results. For instance, if a torque chart suggests that an Inconel 625, ¼”-20 dry bolt requires 8.2 ft-lbs of torque to achieve 84 ksi of tension, but testing reveals that it only reaches 78 ksi at that torque level, a hydraulic calibrator helps determine the exact torque needed to reach the desired tension of 84 ksi. Here's a video demonstrating how this process works during pre-installation testing: [Insert Video Link]. --- ### DTI (Direct Tension Indicator) – Now Available from Extreme Bolt! A Direct Tension Indicator (DTI) is a specialized load-indicating washer designed by the ASTM to confirm the attainment of a specified minimum bolt tension in structural applications. As the fastener is tightened, the arched protrusions compress, altering the distance between the DTI washer's base and the bolt shank. This change in distance corresponds directly to the tensile force applied to the fastener. By using a tapered feeler gauge to measure this distance and comparing it against the manufacturer's guidelines, you can ensure the correct tension has been achieved. Extreme Bolt now offers DTIs, making this technology accessible to more professionals who prioritize precision in their work. --- By leveraging advanced tools like ultrasonic monitors, hydraulic calibrators, and DTIs, we can finally move beyond the limitations of torque charts and ensure that every bolt is installed with the perfect amount of tension. Precision matters, and modern solutions make it possible to achieve that goal every time.

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