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Nick Long - n/a
18-Apr-06, 08:26 AM
index.php?t=getfile&id=194&private=0



Here we have an example of the famous spark erosion that plagues racing hovercraft. Its quite unusual to find it this far south of the Manchester Ship Canal, but there it is, unmistakeable in all its glory, having eaten away the shaft underneath a bearing.



This would not even have been discovered if I hadn't noticed the inner race was turning on the shaft and opened it up for a look.



Nick

tonybroad - n/a
18-Apr-06, 12:45 PM
here we go again - i reckon that's fretting Nick not spark erosion (cue posting from Anne Scrimshaw)i've seen enough of it the last 4 years



mail me a hi-res pic and i'll get both our tribology experts from the Jost Institute of Tribotechnology at the university to give a 2nd opinions and settle this debate once and for all



with great respect,



Tony

jar2 - n/a
18-Apr-06, 12:57 PM
Nick,



I'm no expert, but that doesn't look like spark erosion to me - it should look like this:



index.php?t=getfile&id=195&private=0



It would have characteristic distinctive irregular tracks.



The place you should see it should be in the bearing grooves or balls/rollers not the shaft. To get a spark there has to be a gap between two conductive surfaces (no direct electrical connection) - that cannot happen between the inner bearing shell and the shaft as they are in constant contact.



It looks to me like the inner shell is rotating on the shaft and picking up on the surface causing pits and wear. If the inner shell is rotating then either the bearing has partially seized or it's a poor fit in the first place. What type of bearings are you using and in what condition was the bearing you removed? Are they rated for the rotational speed and temperature gradient of the engine? The temperature gradient can be critical if the inner shell expansion rate is faster than the outer and there is a small bearing ball/roller clearance. This can cause tightening of the bearing and the inner shell can then start to rotate on the shaft until the temperature equalises - a few cycles like that and the shaft starts to look like yours!