The benefits of a "Torque to Yield Tightening" strategy
The importance of selecting the right tightening method
One of the major problems with the use of bolted joints with regard to achieving an accurate preload is the precision of the bolt tightening method selected. Insufficient preload caused by an inaccurate tightening method is a frequent cause of bolted joint failure. It is important for the designer to appreciate the features and characteristics of the main methods employed to tighten bolts.
Torque to yield tightening
“Torque to yield tightening” is a method which is tightening a fastener so that a high preload is achieved by tightening up the yield point of the fastener material. In fact “torque” is related to the preload in the fastener which is directly related to the clamp load in the joint as “angle or rotation” is related to the deflection or the elongation of the fastener. In most cases, when we tighten a bolt, we try to tighten it as close as possible to is yield point. This gives the highest clamp load available in the joint from that bolt without permanently stretching it. So it can be used again and again.
How close we can actually get to that point in production will depend on how well we can control the assembly process. The goal of the most common used tightening strategies (torque controlled or pre torque and angle controlled) is to stay within the elastic area of the bolt. This means that if the bolt is disassembled, it resumes the same length than before the tightening operation. In the elastic region, this relationship is very linear; An increase in preload will give a proportional increase in deflection and if we remove the load, the material will return to its original size, just like a spring. But as we continue to apply more load, at some point this relationship becomes non-linear. We’ll get proportionally more deflection with a relatively small increase in preload and if we remove the load, the material won’t return to its original size.
The yield point of a material is typically defined as the point where 0.2% permanent deflection occurs. That means, if we apply enough load to stretch a material to its yield point, when we remove the preload it will be 0.2% longer. As we continue to apply more load the material will continue to deflect until it finally fails.
Yield point tightening consist in applying a tension in the bolt just beyond the elastic area whilst maintaining bolt reliability. After disassembly, the bolt is slightly longer than its original length. This may seem contradictory to the theory explained above stipulating that the ideal clamping force is achieved at the yield point of the bolt. The load at the point however is a bit smaller than the tensile strength of the bolt in case it is loaded in pure axial tension. The answer lies in the fact that the torque applied during the fastening process will create torsional stress that will limit the required axial preload.
To do this consistently requires special equipment that monitors the tightening process. Basically as the tightening is being completed the equipment monitors the torque verses angle of rotation of the fastener. When it deviates from a specified gradient by a certain amount the tool stops the tightening process. The deviation from a specified gradient indicates that the fastener material as yielded. This “torque to yield” method is sometimes also called yield controlled tightening or joint controlled tightening.
The trend in recent years is to tighten to or just beyond the yield point of the bolt, which results in a more accurate amount of clamp load being generated in the bolted joint. Or, in other words, the variation in clamp load is more accurate if we apply the torque to yield method because its technique relies on the “strength” characteristics of the bolt. This is totally different of a tightening process that is based on friction control (“tighten to torque” tightening method). It’s very important to e.g. engine builders to control friction variables to their best ability to ensure even load across the joint. As an example race engine builders routinely use studs with hardened washers for mains and heads. The hardened washer gives a very uniform surface for the nut to turn against and keeps friction variances low. As the strength of a bolt is more controllable than its friction parameters the results in clamp load variances will amount to ¼ whilst tightening to yield compared to clamp load variances whilst tightening to torque.
The designer of bolted joints is responsible for guaranteeing a minimum of clamp load that is required to maintain a durable bolted joint under various loads. Wide variances in tightening results will show the same variances in their corresponding clamp loads, which results in the choice of the bolt or screw to be adjusted accordingly. The wider the clamp load variance, the larger the bolt/screw needs to be. Consequently, designers opt more and more for tightening processes with minimal variances to ensure a better predictable durability of the bolted joint. Hence yield-controlled and pre torque & angle tightening being the only tightening methods resulting in relatively constant clamp loads when deployed in “high volume” applications (better than 10% of the total variance).
Very accurate preloads can be achieved by this method by minimizing the influence of friction and its scatter. The method has its roots in a skilled workman’s “sense of feel” on a wrench which allowed him to detect the yield point of the fastener with reasonable precision. Today specific programmable electronic torque/angle wrenches can measure the tightening angle from the snug point or threshold torque. The angle is measured by an angle sensor or electronic gyroscope build-in in the wrench. The inbuilt readings memory enables measurements to be statistically evaluated. Tightening curves can be analyzed using the software via the integrated tightening-curve system. Thanks to a special measuring process, it is possible to display the yield point.
With the electronic equivalent of this method, a DC tool control system is used which is sensitive to the torque gradient of the bolt being tightened. Rapid detection of the change in slope of this gradient indicates the yield point has been reached and stops the tightening process. This is achieved by incorporating sensors to read torque and angle during the tightening process. Since angle of rotation and torque are both measured by the control system, permissible values can be used to detect fasteners which lie outside their specification (having too low a yield for example). Once you implement this strategy you are going to see torques all over the place because you are stopping at the yield point. Remember, friction plays a big part in this! Strain washers could be used to determine the amount of strain on the bolt when we shut off on yield. Important is that the operators on the plant floor will have to get used to the idea that the torques will be “all over the place” when we apply the yield controlled strategy and be not consistent at all.
yield-controlled tightening ensures the highest amount of clamp load without the risk of “over” tightening.