Re: how much can you torque a bolt

On Nov 2, 12:40 pm, Brian Whatcott <betw...@xxxxxxxxxxxxx> wrote:
On Thu, 01 Nov 2007 15:01:25 -0000, vmpole...@xxxxxxxxx wrote:
... I'm still a
little confused by the concept that the pitch does nothing more than
affect the tensile diameter, after all the pitch is what decides how
much torque gets applied as tensile stress, right?

Thanks,

Viktor Mikhailovich Polesov

The pitch decides how tightening torque converts to stress, in part.
But the efficiency of the conversion has to be less than 50% or the
nut simply unscrews again. Friction is in that sense helpful.

If you meter torque increase with rotation of the wrench, at some
point the straight line flattens somewhat. At that point the screw is
yielding plastically, and cannot provide more stiffness to resist.
Some bolts are specified at that point, a little above or a reasonable
fraction below.

If the friction is high, the bolt shears in torsion before its tensile
yield point.

Brian Whatcott Altus OK

When I was a young engineer I was taught that you can't rely on
friction to help you.

When it comes to a bolted joint design, high friction will increase
the torque require to achieve a desired preload. Low friction will
increase the tensile stress in the bolt for a given torque. Both
extremes must be considered when designing a bolted joint.

For applications where the clamping force is critical you can
eliminate the uncertainty in clamping force associated with friction
by requiring a specific angle of turn, rather than torque.

As Brian states friction will help retain a bolt. However, but if the
bolted joint is critical, e.g. aerospace application or location where
a liberated bolt or nut would cause extensive damage, don't rely on
friction alone to retain the bolt. Options for retaining the bolt can
range from thread compound, lock nuts, to welded, staked or safety
wired nuts.

Dave Parker

.

Relevant Pages

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