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Tips & Tricks: Torquing Bolts Correctly


A big challenge in assembling bolted joints is verifying that the required bolt preload has actually been achieved. There is a big difference in achieving the torque specified and achieving the preload specified. And in every joint design, it is the bolt preload which will determine the strength and life of the joint.

In this Tip & Trick we explain the most common torquing methods available, along with their limitations. We also highlight some off-the-shelf products which can be used in a variety of applications to improve the repeatability and accuracy of bolted joints.

1. Torque Control Tightening
As can been seen by study of the chart below, a fundamental problem with relying on torque values is because the majority of the torque is used to overcome friction (usually between 85% and 95% of the applied torque). So even slight variations in the frictional conditions can lead to large changes in the bolt preload.
piechart
2. Angle Control Tightening
Also known as turn of the nut method, the nut is done up to snug-tight, and then the nut is turned a set angle past this point up to or beyond the elastic range of the material. This method often requires experimental determination of the required angle, also the fastener can only sustain a limited number of re-applications before it fails.
3. Yield controlled tightening
Can be used in critical applications (such as cylinder head bolts), this method allows very accurate preloads by detecting the yield point of the fastener under the combined tension and torsion. However the cost of the tools involved to detect the change in slope of the bolt’s stress/strain curve when approaching the yield point means that use of this method is expensive and therefore relatively uncommon.
4. Bolt stretch method
The accuracy is significantly improved when direct bolt-elongation measurements are taken. For smaller bolts, this can be achieved using ultrasonics or strain-gauges, and requires specialized technicians. For bolts larger than M20, need hydraulic tensioning devices to reach the required torque. A small amount of preload reduction however does occur when the pressure is removed as the nut elastically deforms under the load.
5. Heat Tightening
Heat tightening utilises the thermal expansion characteristics of the bolt. The bolt is heated and expands: the nut is indexed (using the angle of turn method) and the system allowed to cool. As the bolt contracts it creates a preload in the joint. This is not a widely used method as the process is slow, and is generally used only on very large bolts.
6. Use of tension indicating methods
There are a wide number of ways bolt tension can be indirectly measured, from load indicating washers to fasteners that measure bolt extension. Here is a brief list of some off-the-shelf products which you may or may not already know about.
rotabolta) Rotabolt

http://www.jameswalker.biz/videos/1-RotaBolt-Overview

squirterb) Load Indicating Washers

http://www.appliedbolting.com/squirters.html

smartboltd) SmartBolts
http://www.smartbolts.com/dti-smartbolts%C2%AE/
hydraulice) Hydraulic Tensioners

SKF Bolt Tightening Handbook.pdf

Cf) SuperBolts

http://www.nord-lock.com/products/superbolt/

7. Change in bolt length
And don’t forget measuring the change in length of the bolt with verniers or similar. Shigley rates this method as being very accurate where possible.
In summary, actually achieving the preload desired is not an easy task. If possible design joints with a larger number of smaller fasteners, as this will be much easier to develop the required bolt torques on installation. This is especially true for fasteners greater than 12mm (1/2 inch) in diameter, which may require torque multipliers etc to achieve the required preloads.
Material sourced from lots of places including:

http://www.boltscience.com/pages/tighten.htm
SKF Bolt Tightening Handbook
"Mechanical Engineering Design", Shigley 1st Metric Edition

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