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Sources of errors in a Torque Wrench

  

Define the problem:

How do we measure the tension in bolts? Torque wrench, special washer, ultrasonic bolt gage, deflection meter, laboratory load cell, ..., etc.

Why measure tensions in bolts using expensive bolt gages ($20,000+) or other instruments? Why not just use a torque wrench?  

Torque wrenches:

Torque wrenches do not measure tension, they measure torque. The torque reading is converted to a tension estimate, which is good enough for most applications. But, sometimes:

  1. You have to use special bolts that are pre-tensioned nearer to their limits. Bolts tensioned near their limits cannot afford much error.
  2. You may need to verify your design.
  3. You may need to test the actual field installation of the bolts to make sure they are being properly installed. 
  4. Flange bolts may allow a toxic fluid to leak. It is not unusual for flange bolts to vary in tension even more than +-50%. As you tighten one bolt you are loosening others.

Torque wrenches are usually safe if all you need is plus or minus 30% in bolt tension. Some engineers will say they are good enough for plus or minus 20%. Flange bolt tensions can vary even more, no matter what bolting sequence is used. See actual case study.

 

Error sources:

1.      Static and Dynamic friction. As the bolt is tensioned, two surfaces are sliding against each other. The dynamic friction coefficient determines the force on the torque wrench while sliding occurs. But, as the operator gets near the target tension, he may stop and restart by repositioning the wrench. This allows the much higher static friction to cause the operator to think the bolt has already reached the target torque.

2.      Surface irregularities. At the microscopic level, surface irregularities cause changes in the friction coefficient. The changes may be uniform or more pronounced at certain torque positions.

3.      Lubricants. Lubricants are generally used to get more consistent readings. Again, the surface irregularities at the microscopic level cause lubricants to help some irregularities better than others. A more consistent reading (more precision) may also be more consistently wrong. Repeatability does not mean accuracy.

4.     Operator mechanics. The operator may not keep the torque wrench at a perfect 90 degrees to the surface during torqueing.

5.     Bolt threads. At the microscopic level the threads do not load up evenly. The same problem with surface irregularities applies to each thread that is in contact with its mating surface.

6.      Calibration. Each bolt/nut pair is different. Unless each bolt/nut combination is individually calibrated using the same lubricant, the repeatability of the final tension varies.

7.      Side load tension. If the bolt has any side load tension, the tension profile will not be uniform. One side of the bolt will be in more tension than the other side. 

... many more. Some engineers claim over 70 factors that affect torque. See: http://www.rstechltd.com   

See SPIRALOCK Corp. notes for additional confirmation and further errors.  Additionally the reader should visit the Twining Labs and Bolt Science websites.

How do you measure tension?

1.      Laboratory. Attach the bolt/nut combination to a calibrated load cell and see what the actual tension is. This can give the average stress but not the maximum stress on any section of the bolt.

2.      Ultrasonic bolt gage. Ultrasonic Strain Gage: Send a sound wave down the bolt and measure the time it takes to return (time of flight). As the bolt goes under tension, the speed of the sound wave decreases and the bolt gets longer (see animation). Both of these effects cause the time of flight to increase. This increase in time can be converted to an increase in tension. This also can give the average stress but not the maximum stress on any section of the bolt. But, you need a reliable and easy to use ultrasonic bolt gage. Bolt gages have been around for over 20 years (see comparison table). They have been so difficult to use reliably, that few engineers have the experience. SureBolt's new technology dramatically improves reliability and ease of use. See SureBolt animation.

3.      Strain gages and other techniques.  

 

For more information, see the main SureBoltTM navigation screen.   

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Last Modified: September 27, 2011