RS Technologies


	Torque-Angle Signature Analysis

Overview

The basic torque-angle signature is the starting point for all fastener analysis. It is recorded using a rotary torque-angle transducer and a transient recorder. It shows the rising slope of the applied torque as the fastener is turned. The shape of this signature can reveal much about the assembly as the fastener is tightened. Before analysis can be done, you should be able to identify the Four Zones of Tightening.

Four Zones of Tightening

Zone 1 Rundown: In this zone the fastener is being threaded into the hole prior to contacting the bearing surface. Any prevailing torque due to thread-locking features or deformed threads will also be observed in this zone.

Zone 2 Alignment: This nonlinear zone shows what occurs as the assembly is being brought into alignment and is subject to effects related to drawing the parts together, stress-induced deformations of plating and coatings, and local curface roughness and thread deformations.

Zone 3 Elastic Clamping: This is where the actual tightening is accomplished. This linear zone is a very important characteristic of each bolted joint. This slope can be projected backward to zero torque to locate the elastic origin (see below). If the angle is measured from the elastic origin to the point where torquing was stopped in the elastic clamping zone, the tension is the fastener is directly proportional to that angle of turn. The angle of the slope will be indicative of the amoutn of friction present in the joint; steeper slopes indicate higher levels of friction, flatter slopes indicate lesser levels of friction.

Zone 4 Post Yield: This begins with an inflection point at the end of the elastic clamping zone. Yielding can occur in the bolt or joint assembly as a result of underhead embedment or thread strip in the bolt or mating threads.

Recording the Torque-Angle Signature

The torque angle signature is recorded using a rotary torque-angle transducer and a portable recorder capable of storing enough samples to produce an accurate and readable plot. The input torque of the assembly tool and the angle of fastener rotation is recorded and plotted as shown in the plot below. The first step is to look for evidence of the four zones of tightening. Of prime importance is the slope of the elastic clamping zone which is where the actual tightening is accomplished.

Torque Angle Signature

Analyzing the Torque Angle Signature

In the example above, you can see the first two zones, Rundown and Alignment, curving upward Just past the 50 degree point the curve appears to flatten out as the linear tightening zone is reached. Examine the "flatness" of the linear elastic tightening zone; is it perfectly flat or is there some slight curve to it? If curved there may be some embedment occuring in the joint, the cause of which should be determined. Common sources of embedment are deformation of the parts being clamped, particularly under the head of the fastener, deformation of the threads, or the use of a softer fastener than required for the joint. The presence of gaskets or other soft materials may also account for an embedment curve.

A straight line can be drawn from the linear elastic tightening portion of the torque-angle signature back to zero torque (or the prevailing torque level, if a locknut or similar device is used in the joint). This point is the elastic origin of the tightening angle. Then reapportion the angle of rotation from this point and determine the tightening angle as shown below.

Torque-Angle Curve showing Elastic Origin

In the example above it can be determined that the tightening angle of this joint is approximately 85 degrees. If the angle-tension coefficient is known for this joint, the amount of clamping load present in the joint can be estimated. In this case, it is known that the angle-tension coefficeint is 88.2 lbs. per degree (392.5 N per degree) and thus the estimated clamp load is 7,497 lbs (33,346 N). Note, that the angle-tension coefficient is derived from experimental testing where the actual clamp load is measured.

Summary

There are over 200 factors that determine the integrity of a bolted joint assembly. The Torque-Angle Signature is one powerful means whereby the fastener analyst can find out what is happening when the joint is tightened. Careful examination of the shape and slope of the torque-angle curve can pinpoint potential problems in the joint. For more information, click here to refer to our technical articles page.

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