| Here's a good read that was recently posted to facebook explaining the difference between torque-to-yield and torque-to-turn (angle method). [ http://www.enginebuildermag.com/2001/02/threaded-fasteners-torque-to-yield-and-torque-to-angle/ ] "
Torque Turn to Tighten
One thing that should be obvious by now is that if we’re going to tighten fasteners to the threshold of yield, we need a better method than measuring resistance to turn. Friction variances could easily get us into trouble. Fortunately there is a method of tightening a fastener that is much more accurate than measuring resistance to turn. It’s called Torque Turn to Tighten (TTT), often referred to as angle turn. With this method, you use a relative low torque to run down and align the fastener (Illustration 5), then rely solely on a measured turn to tighten the fastener to the desired level. What we’ve done has not affected the friction in our fastener, it has taken it out of the equation when it comes to tightening.
For instance, 90 degrees of turn is 90 degrees of turn; old bolt, new bolt, rough threads, new threads, it doesn’t matter. The amount of stretch will be extremely uniform from bolt-to-bolt across the joint. Load scatter is kept to a minimum.
TTT is a far superior method of tightening critical fasteners regardless of whether you tighten them to yield or not. Fastener engineers use sophisticated mathematical models to calculate the amount of turn needed to get a desired load, but what has really fueled the rapid growth in this area is sophisticated electronic equipment. Sensitive electronic load sensing cells coupled with angle encoders using advanced software programs have allowed engineers to test their theories watching run down curves in real time as they tighten fasteners (Illustration 6).
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- Sam |
