RE: cylinder studs: torque or length?

[Ben Barkow <dr.ben@xxxxxxxxx>]

> From: "Minor, Bob" <Bminor@xxxxxxxx>
> Subject:
>
> What you have to realize is that a torque spec, if properly developed,
> is derived by calculation to achieve a prescribed amount of stretch. 
> The
> stretch is what actually provides the clamping force through tension 
> and
> compression. If the variables you mention affect the final clamping
> force to the degree that you surmise, they would affect the torque
> wrench method every bit as much as the degree method.   Where the 
> degree
> method is superior with regard to achieving a more precise clamping
> force, is that it is not affected by factors like lubrication and
> surface finish of the parts.

I promise to post nothing more on the subject after this... for a while.

If you can't locate the wispy smeary indefinite location of that 
insubstantially light 15 ft-lb initial torque pressing down some 
slightly rubbery-at-first gasket stuff and 5 iffy joints in anything 
smaller than a 60 degree arc of uncertainty, it just doesn't matter how 
many times you stand on a chair and holler over and over again 
"stretch, properly derived from calculation stretch, final clamping 
force stretch... physics is smart...." - you just can't be torquing 
accurately. You can always make the nominal motions, but you can't be 
doing it accurately.

This calls for a little experiment although a bit subtle how to conduct 
it to compare the variability of a 15 ft-lb head torque setting as 
compared to a 35 ft-lb setting.

Ben
Toronto

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