Hi Davis
I think you can trust a good fitter a little more than your table suggests. If we are talking about the main cooling water pump on a PWR, I would not, in a pink fit, rely on feel. Nor would I use torque. Bolt extension and gasket compression is the appropriate method in this application.The gasket compression is used as a crosscheck on the bolt extension.
For some light relief, I used to know a WW2 test pilot who spent a good deal of time at Rolls Royce engine department. He told me that torque wrenches were not allowed and tommy bars had to be used. A different length tommy bar for each bolt type. Reportedly, this was the outcome of a series of tests. I have not been able to get a confirmation on this anecdote and the storyteller is now deceased. Perhaps someone may be able to part the mists of time.
I suggest you have a go at flogging up a few joints and measure the initial release torque as a check. You may be surprised. You will find the angular displacement per blow (torque) and the bolt "ring" (tension) are useful indicators to your progress. This goes back to first seating the bolt, and then applying a given angular displacement. You should be able to work the required angular displacement out for a given flange/gasket combination. Remember statically indeterminate problems? If you have the required tension and the bolt pitch it is pretty easy.
Two years ago I put my first ball mill together. The 7m diameter endplates (27 T each) were bolted on with about 2" bolts. I followed the book and had them all torqued up to spec with a Hytorc hydraulic torque wrench. My mill fitter who had installed about 50 mills told me he had done this, and had flogged up the ones that needed a bit more. None have broken and now they use a rattle gun for routine tightening!
The point is, critical applications excluded, you had better make sure that the bolts can stand being tightened by a fitter with a Crescent, Stillsons and/or a flogging hammer, because thats what will eventually happen.
I guess I am recommending the judgement of a fitter over a single torque wrench reading. For critical applications this is obviously not enough and the more accurate, expensive and time consuming methods must be used.
Cheers
Steve McK
> I think my opinion is that whenever possible, use some kind of gasket load
control OTHER THAN OPERATOR FEEL. Accepting the fact that gasket load
control is not required or practical in all cases (perhaps a plant air line,
or water line), if one does make the decision that the compression on the
gasket must be controlled beyond the "feel" of the operator or mechanic, it
is a safe assumption that just about ANY mechanical means is more accurate.
Let's face it, that's what tools, technology and instrumentation are for:
they enhance the ability of the human body, unaided, to achieve better
results. This thought process is re-inforced by data from John Bickford's
EPRI white paper entitled:
>
> "GOOD BOLTING PRACTICES: A Reference Manual for Nuclear Power Plant and
Maintenance Personnel" He describes the results of a study on just this very
subject. He shows the results of measurements of "typical preload accuracies
for various tightening methods..." I have reproduced a selection below:
>
> PROCEDURE COMMENT
PRELOAD ACCURACY (%)
> Torque Calibrated torque wrench using friction
> factors from the literature
> -Hard Joint
+/- 20 to 40%
> -Soft Joint
+/- 30 to 70%
>
> Operator Feel For Blts less than 1"
+/- 100 to 200%
>
>
>
> >>> "Steve McKenzie" <Mechproj@xtra.co.nz> 03/25/02 03:29AM >>>
> Ralph and I must have started work in nearby workshops.
>
> For heat exchanger bonnets flogging spanners and sledge hammers were
almost
> always used. I believe they give better results than torque wrenches
because
> the flogger (yes) can see how much the nut turns with each blow. You can
buy
> spanners with a hexagon on one end and an anvil on the other. They sell
> well. With a torque wrench, one tends to concentrate on the dial too much.
> To make a tight joint, bolt tension is the desired result. It is too easy
to
> believe that tension is the direct proportional result of torque. This is
> not true. A damaged thread, dirt, dust or a change in frictional
coefficient
> (lubricant) can cause significant departures. My opinion is that feel is
far
> more important than a gauge reading, unless the gauge is one that measures
> bolt extension. For many structural steel bolts, the rule is to snug it
down
> (tighten it till it seats hard) and then tension it by turning the nut a
> certain angle, say 1/8 turn, hopefully more.
> If a bolt is properly snugged (seated) then an additional angular turn
will
> produce a known extension and therefore hopefully a known tension in a
known
> bolt. I prefer this method, although it is obviously useless when soft
> gaskets are used. With critical gaskets I tighten to the "crush" value
from
> the gasket manufacturer, but reseat according to the manufacturers tension
> values, normally set by bolt extension or pretension. Heated bolts are
very
> slow for assembly unless on ideal assemblies such as a turbine casing. The
> only way I know to be absolutely sure of producing a known tension is to
use
> necked bolts which are designed to stretch in a certain area. These are
> fairly unusual and expensive.
>
> For general purpose joints, I leave it to the fitters. They know what
works
> and save hours of senseless calculations.
> However a series of tables showing bolt torques for various flange/gasket/
> bolt lube combinations would be very useful. This site would be a good
place
> to provide such information. I do not believe that operating pressure
should
> be included as this is included in the flange rating. A required tension
per
> bolt would be invaluable. If someone else has done it, lets "borrow" it.
>
> I use superbolts in tight spaces such as the joint in a mill girth gear.
> They are handy but very slow, and I still like to check the bolt extension
> where possible (often not). For those who do not know, a superbolt has
> around six small bolts threaded through the main bolt head and bearing on
a
> hardened washer. Tightening the small bolts tensions the main bolt.
>
> Obviously "special" joints such as those requiring belville washers and
the
> like will still require individual attention.However it would be useful to
> have a set of tables dealing with the other 98%.
>
> Cheers
>
> Steve McK
>
>
>
>
>
>
>
> ----- Original Message -----
> From: "SARE, RALPH H." <SARERH@YANPET.SABIC.com>
> To: <PipingDesign@yahoogroups.com>
> Sent: Monday, March 25, 2002 6:57 PM
> Subject: [PipingDesign] hand wrench and sledge hammer
>
>
> > ok. lets take it a little further. whats the max. torque that can be
> > obtained using a torque wrench - about a thousand foot-lbs. if you pull
a
> > 100 lb at the end, the hand wrench extension will be 10 ft - good enough
> for
> > a 1 inch dia bolt. double the bolt dia and you needed at least 3x the
> torque
> > that a 1 inch bolt can handle (torque req't increases by the third power
> of
> > its dia) requiring too long an extension. the torque values are fine but
> > achieving it with large dia bolt is another story (heat exchanger
channel,
> > rotating equipment casing, etc). use of hydraulic torque or tensioner is
> > also not the answer since it too has some severe limitations ( antoher
> mthod
> > i remember is the use of torch to heat up the bolt).
> >
> > hand wrench & sledgehammer method although crude, sometimes work quite
> > reasonably well. by the way, what happen if someone use a machine bolt
> > rather than a stud bolts in a flange joint? is it permissible?
> >
> > -----Original Message-----
> > From: Geoff Stone DD&D Australia [mailto:blenrayaust@yahoo.co.uk]
> >
> > Where it gets dodgy is when the bolts are dirty, galvanised or from an
> > unknown
> > source. Also 316 ss bolts may give you a problem if you dont use "never
> > sieze"
> > and galling occurs.
> >
> >
> >
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> >
> >
> >
>
>
>
>
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Received on Tue Mar 26 02:24:00 2002
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