Re: Flow around lip seals

From: <Gordon.Reddek@Alcan.com>
Date: Thu Feb 05 2004 - 17:51:00 EST

Steve,

I have always assumed two mechanisms are at play here, firstly surface tension and secondly the hydrodynamic forces generated in a fluid bearing. Surface tension normally pulls a fluid into a crack and that force is large. Cracks in welds are detected for example by wetting the one side of the weld with diesel and waiting for the diesel to appear on the other side. Diesel is a very penetrating fluid. I think surface tension will ensure that the crack wets when the bearing is standing still. Once rotation starts that thin lubrication layer will act like the film in fluid bearing and a pressure will develop in the area where the film is thinnest, pushing the seal away and creating another area where the film is thinnest where the same will happen again. The net result being that the seal is always pushed away from the shaft surface. I expect that the lubricating layer gets replaced by liquid being drawn in by surface tension.
One way to test this theory would be to coat shaft and seal with something that has negative surface tension and prevents surface tension pulling the lubricant into the crack. If the seal fails...Voila.

Gordon

"Steve McKenzie" <mechproj@xtra.co.nz> 05/02/2004 06:03 PM
Please respond to PipingDesign

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• Message from "Steve McKenzie" <mechproj@xtra.co.nz> on Thu, 05 Feb 2004 08:03:40 -0000 ----- To: <a href="/group/PipingDesign/post?postID=_dKwv1OQIgTCbEOatjHeZrIJjl73YU5a2iieD2pm0sah61bs6U6SETUTuNYWlDltyKUKS_TDA9jDH45BN0p7AWCX2k4">PipingDesign@yahoogroups.com</a> Subject:
  [PipingDesign] Flow around lip seals
  Gents I have spent part of the day wondering about why lip (oil) seals dont run dry at the seal face and burn out. At first thought the rubber band action should squeeze the oil out of the sealing band. It was suggested that the seal taper on the oil side helps feed oil towards the sealing band. I have half convinced myself that this is true at low Reynolds numbers (due to viscous rotation) but would like some proof. If it is true, then a rotating shaft running in a coned bearing should tend to pump oil out the small end of the cone. Provided the Reynolds number is low enough to stop centrifugal force throwing it the other way. I could test it mathematically, but always seem to stuff up vector diagrams and the like. Can it be done simply by CFD? Is there any way I could CFD model this? Its only interest. In case you're uncertain, I am trying to bludge off someone to check this for free. The independent variables would be diameter, speed, cone angle,clearance at outlet, density and viscosity. The output would be axial pumping rate and direction. I have read the "state of the art" on the net, but the bottom line seems to be that the mechanism is unclear.

Cheers

Steve

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Received on Thu Feb 05 17:51:00 2004