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gearcutter (Industrial)
13 Jan 08 20:42
I've been trying to get my head around lubrication regimes related to gears and I need some help in trying to figure out what the numbers mean.
 I understand why it's important that a design ensures that Lambda is = to or greater than 1. What I don't understand is this; I have accumulated a fairly large range of different designs which I have reversed engineered from various OEM examples to use as a guide for new designs. When calculating Lambda from these examples around 90% of them show Lambda to be well under 1, sometimes as low as 0.2. Yet none of the examples I have chosen showed signs of damaging surface fatigue other than the usual polished type of finish. By polished I mean that the flanks are shiny and no evidence of the original grinding marks remain. All the examples used were case hardened & finish ground.
 AGMA 925 suggests anything less than Lambda 0.4 is classified as regime I or boundary. At a pitch line velocity of around 10 m/s and Lambda of less than 0.4 AGMA 925 suggests there is a greater than 80% risk of wear.
 My question is: How would you define the wear in this case and what effect does it have on the overall longevity of the sufaces?
 I’m guessing that the wear is related to initial asperity contact and could be beneficial over the longer term assuming flash temperatures have been kept low enough to avoid scuffing or scoring.
marctorrance (Mechanical)
24 Apr 08 17:21
How are you working out the film thickness?  Are you using the Dowson and Higginson equation?  Two important parameters (apart from speed) are temperature and alpha value (pressure-viscosity coefficient).  You also need the contact pressure, which you can work out from Hertz theory.

If that is all OK, it is possible that you are overestimating the roughness (so decreasing the lambda ratio).  Generally in gears the roughness decreases during running-in, and so the published roughness is actually higher than the roughness in service.
 
coatingengineer (Chemical)
26 Apr 08 1:08

Hi.

Have you defined your failure mode?  

For example, is it pitting, especially micro-pitting?  Aside from lowering contact stress and increasing material strength, your lubricant film thickness can be the most important parameter.   

If it's adhesive wear, it should normally subside over time.  Minor imperfections ('asperity') can be removed, thereby increasing the area of contact.

Nevertheless, wear can be inevitable, based on your boundary-lubricant conditions.  

Polishing wear can be reduced using additives less chemically active.  Scuff wear can be reduced simply by running half-load several hours, to lower the surface roughness.

Tell me more, and I'll try to point you in the right direction.

For now, try this link:

<a href="http://www.industrialcoatingsworld.com/Low-Friction-Coatings/index.html">Low Friction Coatings</a>

Best wishes.
 

William Gunnar
http://www.IndustrialCoatingsWorld.com
 

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