The problem is that there is also a rotational component in the bolt itself-
witness a bolt which is rusted into place, you can twist the head right off of
it, but the threads don't move. tThe diameter of the bolt/screw takes up a
certain amount of the rotation. That's why measuring the stretch makes the
torque reading moot. Once you've measured the stretch at a certain # of flats
(or degrees rotation) the same amount of rotation applied to the same bolt will
generate the same APPLIED preload force.
Mark Haynes
Simon Favre wrote:
> Mark gave us an excellent explanation of why bolt stretch is the best
> technique for critical fasteners. Unfortunately, this seems to only be
> applied to rod bolts, and even then only to the ones bought from
> aftermarket suppliers, or provided with aftermarket rods.
>
> What about the main bearing caps? Aluminum block motors tend to use
> studs to avoid wear on the block threads, and iron block motors tend to
> use bolts going into blind holes. Head bolts are often the same way.
>
> OK, bear with me on this. If the actual requirement is to have a certain
> preload on the fastener, and that preload can be converted to a measure
> of stretch, then given the thread pitch, isn't it possible to convert to
> degrees of rotation? Is anybody tightening blind fasteners by degrees of
> rotation? By my calculations (off the top of my head), a stretch of
> 0.0069" on a 3/8-24 bolt would require just about 60 degrees of rotation.
>
> I can see on large fasteners (6" diameter bolt!) where this would be
> fairly easy. Is the problem with doing this on small fasteners the
> increased precision required in turning an exact number of degrees
> repeatably? Is there currently a device to enable such a precise rotation
> on a nut or bolt?
>
> Mark Palmer wrote:
> ...
> > On the other hand ... torque is a pretty awful way of estimating preload.
> > There are just too many variables -- principly, the coefficient of friction
> > in the threads and under the head of the fastener. If you could somehow
> > magically measure actual preload (force), you would find that you could
> > torque 10 fasteners to the exact same torque and have widely differing
> > preloads. Not a good thing on a rod bolt or bearing cap.
> >
> > As an aside (this whole thing's an aside, actually) ... in critical
> > large-scale fasteners, like you find in civil work and heavy industry, the
> > preferred tightening method (if you can't measure stretch) is the
> > "turn-of-the-nut" method. Tighten to snug, then continue to tighten to a
> > specific degree of rotation of the nut. or bolt. This is actually more
> > accurate than torque, in terms of reaching target preload. Even civil
> > engineers regard torque as the least reliable method.
> ...
|