[TR] torque wrench question

[John Innis]

The answer is, it depends on the TYPE of torque wrench.  A "clicker" or
digital load cell will always read correctly.  But a beam style wrench can
be fooled depending on exactly ho your cheater bar attaches.  This is
because a clicker torque wrench is measuring torque against a spring inside
the bar.  You pre load the spring by selecting the torque you want then no
matter how much force you apply to the handle it still clicks at the right
torque.  A Beam style wrench works different.  It is measuring the about of
twist in the socket end of the beam.  It is essentially comparing the
amount of force on the socket to the the force on the handle.  It iwll only
read correctly if the socket and handle are the length it expects them to
be.  This is why the handle on these often has a pivot in the middle.  The
wrench will ONLY read correctly if the handle is balanced on this pivot
(not touching the bean at either end.  Now, you attach a cheater bar such
that it can ONLY contact the wrench at that pivot and no where else, it
will still read correctly.  If your cheater bar makes contact with anywhere
else on the bar, game over.

On Sat, Aug 11, 2018 at 8:16 PM, Randall <tr3driver at ca.rr.com> wrote:

> > The wrench will
> > measure the same torque whether I choke up 6" or use a
> > cheater bar (yes, I
> > do to get that 105ftlb).
>
> If the wrench reads the same, then why bother with the cheater bar?
>
> Answer is, it doesn't read the same.  The wrench responds to the torque it
> sees, which is force applied to the handle times the length of the handle.
> Your cheater bar increases the length, so the same force at the end of the
> cheater bar results in more torque at the fastener and a higher reading on
> the torque wrench.
>
> But, when the cheater (extension) is between the wrench and the fastener,
> the reading doesn't change even though the torque is greater.  That's why
> you need a correction factor, based on the length of the extension vs the
> length of the torque wrench.
>
> Maybe a thought experiment would help.  Let's say you have a little bitty
> torque wrench, only 1' long and it only reads to 30 ftlb (for 30 lbf on the
> handle).  Now let's say there is a 6' extension between it and the
> fastener;
> and you still apply 30 lbf to the handle.  Do you really think it will
> still
> be only 30 ftlb at the fastener?  Or that the torque wrench is magically
> going to read 210 ftlb? (30 lbf times 7')
>
> Now imagine you use a 7' long wrench instead, and pull on it with 30 lbf.
> How much torque does that apply?
>
> -- Randall
>
>
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