Bolt preloading & elongation

Mechanical Engineering, Manufacturing, Design Reference ... ... Nuts, Screws, Set Screws, Thread Stress Area, Hex Bolt Identification .... Stress, Strain, Hookes Law,

The calculator is VERY good!

I would suggest you introduce as well the ISO strength classes if possible.

I totally agree with the comment on uncertainty when tightening is used but since in small threads up to M30 tightening via torquing is still the most used technology I think it is positive to mention the torque-angle procedure which reduces drastically even if not totally the uncertainty level.

You deserve a second GA.

Thanks for the compliment, Nick . I've taken note of your comment and have made the "Assembly by Turn-of Nut" visible in the newest upload.

Here's the link : Hevï Bolting Calculations ver 8.1

With all due respect, I don't quite agree with your statement that "the torque-angle procedure which reduces drastically even if not totally the uncertainty level". Whereas this might be true in a controlled environment such as on a modern assembly line, this is is hardly the case in the outside world. The biggest problem with Angle of Turn is knowing at which point to mark the Nut before commencing turning. If this point isn't correct, the fastener may end up being either too tight or too loose

The phrase "snug tight" used by so many Engineers in the Civil field makes me laugh uncontrollably and cringe at the same time. It's very obvious that one person's "snug tight" could be quite different than another's. Understandably, there isn't much control with any method based on subjective "feel", including this one .

Furthermore, if the joint is sprung or if it's compressible or if the nut/bolt isn't perpendicular to the flange or if the spotface is damaged/burred or a number of other variables, an unknown degree of turn may be expended simply to try to compress the joint before actually loading the bolt. Hence, the final load could be quite unpredictable.

Thank you for the respect! What I understand by the torque-angle procedure is the following one which I analysed with a modern torque wrench able to "measure" and "display" as well torque as angles:

- the bolt/nut is tightened to a predefined torque which has to be at the beginning of the quasi linear assembly behaviour

- from this point on the bolt/nut is further turned with an angle which corresponds to the wished preload.

It is no "feeling" in the torquing level (human feeling has an uncertainty of up to 40..50%!!!) and the angle measurement can be started at the same level so that in fact the operator has only to stop at the prescribed angle which is displayed by the tool. The angle is measured by the integration of the rotating speed signal supplied by a Coriolis gyroscope. The angle errors I had were around 0.5 to 1% of end value. The torque measurement was within 1 to 1.5% end value.

The advantage of this procedure is that the uncertainty related to the friction which is a stochastic value is reduced to the level of the predefined torque. Since this torque is much lower than the final one the uncertainty is reduced about with the ratio of the two values.

I developed years ago such a tool (different sizes for torques from 100 to 1000 Nm) and the procedure which gave very good results. Unfortunately for different reasons I was not in the position to produce the tool in big series.

Nick,

I am familiar with the concept of such tools and congratulate you on developing technology such as this. You must have many happy Users where the tools are used in modern production facilities . However, outside of environments such as these, the impressive results are unlikely to be experienced.

"the bolt/nut is tightened to a predefined torque..." This is the crux of the problem!

The initial minimal bolt load developed by this "predefined torque" may be repeatable on an assembly line where fasteners and flanged configurations are new and conditions can be controlled. Unfortunately, the same cannot be said for joints in the "real world" (my world of power plants, refineries, offshore platforms, mines, steel mills etc..). For the reasons outlined in my previous post, it is wrong to think that the predefined torque has had the correct effect on the joint so that you can begin the angle-measurement phase. Unless verifying by independent means, one can only assume that it has. Hence, there are still certain components of "feel", "guess" "and "hope" involved with this method

I totally agree, there is a difference between "small & middle" and "big" bolting.

I am more familiar with the 1st category and you with the second. The problems are similar but the solutions have to be different. With respect to the predefined torque threshold it came out that the value can have quite large tolerance limits since it is the zone where the assembly goes through the 1st compression zone and the final results are not very much changed even if the value has variations up to 25%.

I will never say that this approach is more precise than tensioning. I only claim that in the range where torquing is the most used technology (cost, productivity, aso) this approach can lead to a lower uncertainty level thus increasing the assembly reliability.

In mechanical joints i.e. with good surfaces and plane contacts it can be assumed that contact occurs at about 10...15% of the final torque. In structural assemblies the full contact occurs at about 50% full load. So that in structural assemblies the possible uncertainty reduction will only be around 50% but can be around 80% in the other case. You are more active in the "structural" assembly domain so that combined with the bolt dimension (usually ≥ M24) it is the domain where tensioning becomes optimal if cost and results are considered.

In fact I developed (but was not able-as mentioned in my previous mail- to spread it) a technique which compensated the variations at every assembly condition. The final angle in the display was continuously monitored by the measurements done during the tightening. I tested it and it worked. I checked the results via US measurements and the correlation was pretty good.