Date: Tue, 15 Oct 2002 00:27:32 +0100
Reply-To: Clive Smith <clive.harman-smith@NTLWORLD.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Clive Smith <clive.harman-smith@NTLWORLD.COM>
Subject: Re: Torque Wrench
Content-Type: text/plain; charset="iso-8859-1"
Walt,
I have often wondered about this myself, having heard that this
recommendation is (sometimes) made.
There is a problem with long bolts and there are more complex design
criteria when engineers design clamping systems in various engines too.
Before going any further I would say that 'provided you are somewhat careful
and circumspect as you tighten these down, you'll probably have no problems'
.. what do you need to know about though to be circumspect?
The first point that has been made is about the condition of the bolts -
regardless of the torque setting, some manufacturers stipulate that these
must ALWAYS be renewed (some Fiat 1600 and 2000 SOHC and DOHC engines) and
not because they are likely to be corroded or weakened - indeed they may
well be stronger, having been work-hardened to a degree by their prior
tightening.
The reasoning is this: For an engineer to do his sums and guarantee that the
correct (range) of clamping force has been applied, they have veered towards
not just specifying a torque setting, but having attained a specified
torque, then regardless of torque, turning the nuts or bolts a further 90
degrees or so. This is to take the stud or bolt into the plastic range,
beyond it elastic limit. You would have to do the sums with the bolt
material's stress-strain curve to see why, but rest assured that this
technique has been used to ensure that the bolt is not sitting at a stress
point on this curve, that is in any doubt through using a simple torque
setting - where friction under washers, thread lube, lack of stud/block
thread security, manufactuiring tolerances etc. all come into play.
Once the bolt for these systems has been stretched (strained) a specified
amount, one can see why it would be advised to not use it again, although I
have done so succesfully, but not using the manufacturers recommeneded
original clamping procedures (and I'm not going suggest you do or say what I
did to be pretty sure that I wouldn't have problem).
Very long bolts (and remember the engineer can't make these any diameter and
strength he necessarily wants to, he is contrained by other criteria -
diameter for one) that don't have these special torque + turn clamping
settings, do require a quite accurate torque setting, again remembering that
thread friction can vary a lot, as well as other factors.
What the beam Vs click argument is about is probably:
a) A fear that the click type might not be fully serviced and in good
calibrated condition;
b) they might be inaccurately set on the dial - maybe more difficult to
understand the calibrations;
c) dirt or foreign bodies might affect them adversely;
d) they offer little if any prior 'sign' as to when the torque setting is
being approached until they actually click over;
e) the fact that once clicked, it is possible to continue tightening, either
by accident (clumsiness, not knowing its about to click) or deliberately
with no indication of the extent of that 'extra' torque, whereas the beam
type always offer a reading (until you hit the stop of course);
f) maybe a few bad experiences with cheap ones, though I've never seen any.
Both in my opinion are equally prone to misuse, I've used the click type
mainly, and usually set a lower figure first before going on to the full
value, although note that with the torque + turn stipulation, this may
involve a specific 2-step torqueing before the set turn angle. Theoretically
of course, the click type are more accurate and from what I saw in over 10
years of observing fitters building new commercial airliners, never saw a
beam type torque wrench - the click types were returned to stores regularly
to have their calibration checked. Note that although I never new of long
studs being tightened into the plastic region at the time, its a fact that
highly stressed parts of aircraft, wing skins and the like, are allowed to
go into the plastic strain range during extreme and rare loading conditions
without writing-off the whole structure - by design. In crude terms think of
it as gentle work hardening.
If a manufacturer specifies a torque only, follow all the other instructions
re cleanliness, new this that and the other, and provided you are confidant
you can set that final torque accurately - do it by whatever means you have
to hand.
Clive
'88 Syncro Transporter
----- Original Message -----
From: "walt spak" <b20swalt@ATTBI.COM>
To: <vanagon@GERRY.VANAGON.COM>
Sent: Monday, October 14, 2002 11:46 PM
Subject: Re: Torque Wrench
> Dok,
>
> Thanks for your response. I can see that the bolts act as a torsion bar
and
> that they may not all twist the same amount under a give amount of applied
> torque. I still don't see why an accurate clicker set at 22 ft lbs won't
> give you the same 22 ft lbs that a beam wrench will give you.
>
> I tend to agree with the policy to follow the engineers recommendations,
but
> it doesn't cure my curiosity. By the way, I have torque(d) Vanagon head
> bolts with a beam & then applied a clicker only to have it click off right
> away. I have also torque(d) them with a clicker & checked them with a
beam
> only to find that the beam matches the clicker rating with out turning the
> bolt any more. It makes me wonder if the engineer wasn't just prejudice
> towards beam wrenches.
>
> Walt Spak
> Pittsburgh PA
> b20swalt@attbi.com
>
> ----- Original Message -----
> From: "Doktor Tim" <doktortim@ROCKISLAND.COM>
> To: <vanagon@GERRY.VANAGON.COM>
> Sent: Monday, October 14, 2002 5:02 PM
> Subject: Re: Torque Wrench
>
>
> > At 01:12 PM 10/14/2002, you wrote:
> >
> > >Dok,
> > >
> > >Can you give me an explanation as to why a clicker is not accurate
> enough.
> > >You mention the long studs, so I assume that some how affects the
clicker
> > >wrench. If a clicker wrench is calibrated correctly, why wouldn't it
> click
> > >at the proper torque? The torque reading on a beam wrench can change
> just
> > >by moving your head a little.
> >
> > The other issue is the wet sealant under the head nuts. If you use a
> torque
> > wrench you will see that the flexing of the bolts is much like a torsion
> > bar. It makes it impossible to get a clicker to repeat torque at
> consistent
> > turn. You can work the beam wrench more and less near proper torque to
> pull
> > it down to an accurate repeatable position. Mine also incorporates a a
> > "tale-tell" that follows the indicator pin to show the maximum value
> obtained.
> >
> > Despite all than, don't take my word for it. Listen to the engineers who
> > designed it, they say a beam wrench only for this application because
you
> > will get false readings with a clicker. Don't do it their way and you
may
> > assume whatever risk that entails. Do it their way and reduce risk. Pick
> one.
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