Torqueing with crow's foot socket

[DePenning, Charles [EPM/MTN]]

I believe that you are incorrect, but you can do a little experiment to find
out why for yourself.  Take one torque wrench (beam type desireable for this
experiment- real-time readings are possible with this one), one crow's foot
and a bolt & nut and something to torque said bolt and nut against.  Attach
the crow's foot with opening pointing along the axis of the torque wrench
axis.  Torque the bolt down to a nice number, say 100 ft*lb.  Now, turn the
crow's foot 90 degrees from where it was, and loosen the bolt.
Theoretically the loosening torque will be less than the tightening torque,
due to the fact that with the opening of the crow's foot parallel with the
torque wrench's axis, there is a slightly longer moment arm that the force
(of your arm) is acting trough on the bolt.  Slightly more is the important
phrase here.  There is a lot of variability here, in the coefficient of
friction of the bolt/nut interfaces, as well as the inaccuracies of the
torque wrench you are using, which is usually in the 5% of reading range for
most torque wrenches.  So, just from the torque wrench error in reading for
that 100 ft*lb torque, a 5% inaccuracy will give a 5 ft*lb bias error, which
will probably be way more than the difference in changing the crow's foot
orientation. But the theory of turning the crow's foot so that the opening
of it is 90 degrees from the axis of the torque wrench is the industry
accepted method for obtaining the most accurate torque measurements
possible.

Charley 

Message: 3
Date: Tue, 6 Apr 2004 09:39:22 -0500
From: "Flanner, Chris" <cflanner at aosmith.com>
Subject: Torqueing knock sensor/crow's foot
To: <quattro at audifans.com>
Message-ID:
	<BF5CF60FC3A2344988D6DFBDEFB2AC43033618 at whqex01.aosmith.org>
Content-Type: text/plain;	charset="us-ascii"

Here's how I see it . . .
 
Changes in the location of the crow's foot relative to the socket connection
of the torque wrench (I'll call it the "head") has no effect on the reading
as long as the head and the crow's foot apply torque in a parallel axis.  No
matter where the torque is applied by the crow's foot (or socket or
whatever) the equal and opposite torque is seen at any desired point in any
parallel axis including the torque wrench's head where it is converted to an
indication.  Only the force on the torque wrench handle and the equal and
opposite reaction force at the fastener change when the position of the
crow's foot changes.  This, of course, ignores the effect of friction on
torque readings but that's another issue.
 
Am I wrong?




Charles DePenning
Test & Evaluation
Fisher Controls Int'l LLC
641-754-2708 (Tel)
641-754-2277 (Fax)
charles.depenning at emersonprocess.com