EHDL is a preferred lubrication regime for rolling element bearings. The alternative would be boundary lubrication where more wear and stress occurs.
I reviewed one of my tribology handbooks and saw an interesting historical development of EHDL theory. In the old days using only Hertzian contact (the elasto part of EHDL) they predicted lives much shorter than in practice. Even when adding hydrodynamic film effects (the HDL part of EHDL), the large force/area pressure loads caused prediction of films much smaller than roughness and hence boundary lubrication with prediction of very short life. It remained a mystery for awhile. Only when they added in the dramatic variation of viscosity with pressure did they get good results. So EHDL has 3 elements: 1- hertzian contact deformation, 2 - hydrodynamic theory, 3 - pressure-viscosity coefficient. That was a long story but I wanted to make the point based on the above, it appears your FEA with hertz theory in absence of other factors probably would likely predict much too low of a life. (maybe it would be some value to have a lower bound on the life?)
To stay in the EHDL regime and avoid boundary lubricaiton, you would need to exceed the minimimum viscosity at operating temperature as shown on Figure 1 of page 4 here:
The chart is based on mineral oil. If using synthetic you also need to look at p/v coefficient (varies widely for synth but not for petro).
Also the chart would be based on typically smooth bearing surfaces. If your shaft or housing are rough you'll need to boost the viscosity for that. Not sure how to easily quantify that.
In general it is not difficult to meet the viscosity requirement unless you expect to have a temperature problem. Higher temperature causes lower viscosity. Increased base oil viscosity causes lower operating-temperature viscosity. Bottom line is for high linear speed application you may not be able to find any suitable lubrication that will give a reasonable temperature within the safe range of the lubricant (as a conservative rule - less then 210F).
18000rpm
divide by 60
300 rotations/second
multiply by 2Pi
1885 Radians/second = w
Linear speed = w*R = 80 m/sec
R = w*R/w = 0.04m = 40mm
D = 80mm
From the chart that looks like just over 3 cSt at operating temperature.
From figure 2 it looks like VG22 or even lower would work. As long as that is low enough to keep you from heating above 100C or so.
NO. I did that wrong. I'll leave it there as an example to myself what not to do. You need to determine your minimum speed and figure your viscosity from there.
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