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.
...
|