The 20mm rod was discussed some time ago.
The only reason that I can think of is that with a pipe resting on a flat support, it is highly unlikely that the pipe and the support will be truly parallel. This means that most of the load will be carried on only one edge of the support which will act similar to a knife edge, but not so extreme. The result could be point loading even higher than than with a circular rod support. During pipe expansion the corner will tend to dig into the pipe wall and then release, which will make some fairly horrible noises as well as gouging the pipe wall. Corrosion will make things worse. The rod would make it easier to paint the pipe in place though.
Do I believe the rod is necessary?
No. There are plenty of flat supports that work just fine and have done for years.
It will be interesting to see if anyone has a better explanation or a special case where rods produce a real benefit.
Cheers
Steve McKenzie
-----Original Message-----
From: Sanjay Laturkar [mailto:sanjay.laturkar@proconindia.com]
Sent: Friday, September 27, 2002 1:14 AM
To: <a href="/group/PipingDesign/post?postID=y9Tu6WYajRe_lraxC2Qt39adcUcWvse8g3fSTfhnYLzImUzVzPEnPLCSXg6gn62YYe1hBSF6JA6K482d5aJGhZav">PipingDesign@yahoogroups.com</a>
Subject: [PipingDesign] Re: PTFE plate and other friction issues.
That was very well put Steve!
As an offshoot of the discussion, I find that several organizations have norms requiring welding of a ~20mm dia rod on top of supporting beams of piperacks as well as on pipe sleepers. Theoritically it ensures a point contact for load transfer, if pipe were bare. I am yet to receive a satisfactoray explanation for this practice. Possibility of rust at the interface does not appear to be a good enough reason. On the flip side the local bearing stresses become very high.
I would welcome the group's views on this.
regards
Sanjay Laturkar
Message: 8
Date: Thu, 26 Sep 2002 17:14:57 +1200 From: "Steve McKenzie" <Mechproj@xtra.co.nz> Subject: RE: PTFE plate
Hi Sachin
your query almost answers itself; the "laws of friction" are only an
approximation to what happens in practice. They are useful in many
circumstances, but ignore factors such as heat dissipation, gross
deformation and many other effects.
For example, fast cars have wider tyres for more grip. If the coefficient of
friction was a constant then there would be no point as narrow tyres would
produce the same lateral resistance.
Two pieces of very clean very flat steel will actually bond together,
producing a friction coefficient orders of magnitude higher than the
coefficient normally given, even when the reaction is negative.
A sewing pin resting head down on a block of teflon will have a much lower
coefficient than the same pin point down.
The reasons for the above are fairly self evident, but make the point that
values for friction coefficients should be treated with caution and used
only within the stated range of application .
In the case you state a reasonable load on a 100X100mm teflon block would
squash flat a 1X1mm block to the point where it would fail to support the
reaction.
Cheers
Steve McKenzie
-----Original Message-----
From: <a href="/group/PipingDesign/post?postID=uu5_exA5ksdpt3YZxMShmjDF_NbH5H7K5-zaNVpGAP-0GACBzewM2r0HZNaXAgyS_c5pz3V-StwbWOoKBUca">Sachin_Bapat@ril.com</a> [mailto:<a href="/group/PipingDesign/post?postID=uu5_exA5ksdpt3YZxMShmjDF_NbH5H7K5-zaNVpGAP-0GACBzewM2r0HZNaXAgyS_c5pz3V-StwbWOoKBUca">Sachin_Bapat@ril.com</a>]
Sent: Thursday, September 26, 2002 4:08 PM
To: <a href="/group/PipingDesign/post?postID=y9Tu6WYajRe_lraxC2Qt39adcUcWvse8g3fSTfhnYLzImUzVzPEnPLCSXg6gn62YYe1hBSF6JA6K482d5aJGhZav">PipingDesign@yahoogroups.com</a>
Subject: Re: [PipingDesign] PTFE plate
Importance: High
Graphite plate on SS plate, are good option for reducing friction for temperatures more than 200 deg C , but they have one problem their bonding with metal palte is very sensitive to loading & it often crushes under huge loading so better to check loading at the point of support before using them.
QUERY -If we go through "laws of friction" we find that friction at a point
is independent of area of contact ,so my PTFE plate of dimension 1X1 mm2
should be as effective as 100X 100 mm2 plate , even then the manufactures
mention some specific
dimensions of PTFE plate , why is it so ?
Regards
Sachin
[Non-text portions of this message have been removed]
Texas Flange - a good source for information on industrial flanges, all they
ask is for referrals for designs they help with.
877-610-8924.
www.texasflange.com
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