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=4qxNec6zfvPcLPXeeXSPAW24Sbohl3Ju1rRL2HmrWn_0TRlkeWajww68QOI0nhDKfw7WJ8fpuni9P6Bb">Sachin_Bapat@ril.com</a> [mailto:<a href="/group/PipingDesign/post?postID=4qxNec6zfvPcLPXeeXSPAW24Sbohl3Ju1rRL2HmrWn_0TRlkeWajww68QOI0nhDKfw7WJ8fpuni9P6Bb">Sachin_Bapat@ril.com</a>] Sent: Thursday, September 26, 2002 4:08 PM To: <a href="/group/PipingDesign/post?postID=6VK9KqH6MuRS_qK_BDJjRmLAWc3Hppz8-S0kmiDWdt6fEQeHzy7nO_SaASNgmqbA-JBx70bNmUs1uddYT4N4f04UPQVGew">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] Received on Thu Sep 26 09:14:00 2002