Tension vs torque

[ChavoLC2]

Was reading where GM, John Deere are starting to use DTI and HADTI(high accuracy) smart bolts for some applications. Would be nice when they come out with head bolts as well as others that you can read true tension within 3% especially if torque can be off 30% ...



Torque vs. Direct Tension Indicators
Tightening Bolts – Bolts are often tightened by applying torque to the head or nut, which causes the bolt to stretch. The elongation results in bolt tension or pre-load, which is the force that holds the joint together. Torque is relatively easy to measure with a torque-wrench, so it is the most frequently used indicator of bolt tension.

Unfortunately, a torque-wrench does not measure bolt tension accurately, usually only about +/- 30%, because it does not take friction into account. The friction depends on bolt, nut, washer-material, plating, surface smoothness, machining accuracy, degree of lubrication and the number of times a bolt has been tightened. Fastener manufacturers often provide information for determining torque requirements for tightening various bolts, accounting for friction and other effects. However, in field applications, this information is often not available, practical or administered poorly.

 

[turbows6]

Most bolts that have a specification given for stretch for example on a Turbo buick wavelock 2000 rod bolt, run a cleanup tap though the big end of the rod, apply ARP grey grease and torque to 55 foot #s like insructed and when I checked the rod bolts with a stretch gauge they matched the stretch specification given by ARP almost exactly. I think an accurate torque wrench is perfectly acceptable if you follow instructions and use proper precautions like making sure the threads are clean and torqued to the proper amount for the lubricant (light oil or grey grease as instructed).