Re: Torque-to-Yield Meaning Restatement

["Brant M. Miller" <brant@xxxxxxxxxxxxxx>]

All -

I should read my own messages prior to sending them.  The below is a
restatement of my original message of a few hours ago with a couple of
fundamental errors (I claim they are typos :-)) corrected and a bit more
explanation.

David -

Torque-to-yield is a way of tightening bolts that does not depend upon
achieving precise torque values to ensure that the parts being clamped by=

the bolts are held adequately tight.

A typical torque-to-yield procedure might be: (1) torque bolts (or the nu=
ts
on them, depending) to 20 lb-ft, (2) turn bolts an additional 90 deg. =

Basically, step 1 just gets the bolts snug to provide a known starting
point for Step 2.  Step 2 loads (stretches) the bolts beyond their yield
point - the stress level at which they begin to permanently stretch. =

During the early phase of permanent deformation, the bolt will stretch at=

constant bolt tensile force.  (That is, for the ductile materials that
torque-to-yield bolts are made from, the stress/strain curve goes
near-horizontal for a bit.)  Since the clamping force will remain constan=
t
while the bolts are in the range where stress is not dependent on strain,=

it is not necessary to be too precise re the additional rotation in Step =
2.
 It only needs to be enough to get the bolts into permanent yield and not=

so much as to break or weaken them.

Automatic assembly systems often use torque-to-yield.  In a statistical
quality control environment, the probability of achieving correct tightne=
ss
is greater.

When rebuilding by hand, there is some question as to whether or not
torque-to-yield bolts should be replaced with torque-to-tight bolts.

Above, as amended, is probably even clearer than mud.

Cheers

Brant

Date: Mon, 9 Nov 1998 10:17:38 -0800 =