Tightening to Yield
Bolts tightened to yield provide consistently higher preloads from smaller diameter bolts. The reduced fastener stiffness reduces the fatigue loading to which the bolt is subjected under repeated external load reversals, e.g. cylinder heads and connecting rods.
In theory, a bolt tightened to its yield point will provide the strongest and most fatigue-resistant joint possible, within the physical limitations of the bolt material and manufacturing process.
The downside of this method is the cost of the sophisticated equipment necessary to determine when the bolt goes into yield.
Torque Tension Calculator
For further information and guidance on establishing the correct tightening torque for a fastener, see Norbar’s web based calculator, http://www.norbar.com/Home/Torque-Tension-Calculator
When Torque Doesn’t Equal Tight
As we have established, it is the tension in a fastener rather than the torque that is the critical factor. Torque is an indirect means of establishing tension and in a correctly engineered joint and with a controlled tightening process, it is a satisfactory method under the majority of circumstances.
However, in joints that are highly critical due to safety or the cost and implications of machine down-time, a more direct means of establishing tension is needed. Various methods exist including several types of load indicating bolts or washers. However, one of the most versatile methods is to measure the extension of the bolt due to the tightening process using ultrasound and this is exactly what Norbar’s USM-3 does. For full details of this instrument please visit the Norbar website