[Healeys] [SUSPECTED SPAM] Re: torque wrench

[J Armour]

Interesting comment Michael.  Some years ago when visiting a
workshop/factory that sealed porous engine blocks with the  Locktite
product they had a chart on the wall showing the impact of what you mention
as important factors in how much of the torque figure showing on the tool
readout, or click setting;
Absorbed by friction in the dirty or damaged thread?
How much by the out of square nut : washer:clamping surface?
How much to the cheap commercial washers, not hardened washers?
How much due to no lubrication?

The chart showed that with the same torque reading the clamping force and
that is what we are trying to achieve is severely reduced by some of the
conditions and this is hard to quantify but the significant result for me
was with the same torque setting on the tool when the thread was clean and
in good condition the correct washer and grade of nut used with a light oil
lubricant the clamping force was much closer to the value indicated by the
torque reading. But when lubricated with Locktite it was possible to shear
the bolt

This shows how accurate or inaccurate the effect of the  torque reading can
be

Serious engine builders do not use a torque wrench but measure the actual
effect required and that is clamping force plus an allowance for expansion
due to lpad and heat. Therefore they stretch the bolt to a predetermined
figure. In this situation 'more is not necessarily better'

As a side the Australian Standards for Industrial Fasteners will not use
Torque figures foe structual grade bolts. Don¹t ask how I know this. I do
know that all the high strength bolts in a freeway bridge already assembled
and torqued up were required to be removed, replaced and new bolts installed
by the calculated 'turn of nut' measurement system using hardened washers

Joe

From:  Michael Salter <michaelsalter at gmail.com>
Date:  Friday, 16 January 2015 8:53 AM
To:  "healeys at autox.team.net" <healeys at autox.team.net>
Subject:  Re: [Healeys] [SUSPECTED SPAM] Re: torque wrench

Way back in the "olden days" when I was doing my aircraft engine fitter
training we were given a graphic demonstration of just how varied the
clamping force that a nut tightened using an accurately calibrated torque
wrench could be.
Forget the 2 - 5% inaccuracy of the wrench and concentrate on clean, unworn
and correctly formed threads, proper countersinking at the top of stud
threads, sufficient thread below the nut, correct lubrication of the
threads, perfectly straight studs, serrated washers or soft washer
materials, correct tightening sequence etc etc..
When you see how those factors can affect the clamping forces the accuracy
of the torque wrench, which BTW in the services had to be checked daily, you
realize that the torque wrench is only a part of the equation!!

Michael S
BN1 #174
> 


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