Bolts

[ris2]

what is the difference between torquing and tensioning in relation to bolts. Is there a particular size or material which needs one in preference to the other.

 

[unclesyd]

Both are referring to the same thing, achieving preload on a fastener. Torquing refers to turning the nut or head of the bolt to achieve a predetermined bolt stress. Tensioning refers to stretching the bolt by some means to achieve a set amount of stretch in order to achieve the same bolt stress. Tensioning usually refers to a stud and nut combination where you have some stud exposed to grip.

 

[tothepoint]

Another consideration in choosing between torquing or tensioning is the required accuracy of the preload. Torque is a very inacurate means of developing precise bolt preload, as the exact amount of friction is often unknown. Bolt tensioning, on the other hand, is very accurate as it relies upon known mechanical principles, and is completely independent of friction. Using the equation d=FL/AE the bolt stretch (d) is determined knowing the active bolt length (L), the desired preload (F), the modulus of elasticity of the bolt (E) and the tensile stress area of the bolt (A). One caveat though. Measuring bolt extension in the field requires skilled craftsmen and proper measuring equipment. Ultrasonic tension indicators are a bit easier to use but are temperature sensitive.

 

[DSB123]

tothepoint,
You may wish to note that bolt tensioning can only provide +/- 30% accuracy of the target bolt load which is not very accurate in the scheme of things.

 

[tothepoint]

DSB123- I think you are confusing torque accuracy with strain measurement accuracy. Torque, at best, is about +/-30% because of a variety of factors (lubrication, calibration, speed of tightening, etc.), measurement of bolt strain is +/-5% accuracy. In applications where bolt preload is critical, bolt strain is the measure used to insure preload.

 

[tothepoint]

Method Accuracy Relative Costs
Feel +/-35% 1
Torque wrench +/-25% 1-1/2
Turn-of-the-nut +/-15% 3
PLI washers +/-10% 7
Bolt elongation +/-3-5% 15
Strain gages +/-1% 20