Surface speed vs. Oil viscosity = wedge 2

[mayt4u]

Does anyone know the relationship between the speed of a shaft verse the oil viscosity to determine the minimum thickness of the wedge that would be created.

Background: We have a 500kg rotor that is turned via a VFD. The operators have been taken up to one hour to run this motor up to full rpm. (3660) We think this is doing undo wear on the plain bearing and causing premature failure. We have changed to oil from ISO 32 to ISO 68 in hopes that this would help, it didn't. Prehaps the damage was already too far gone. The question is what minimum speed that this thing could run at without loosing the wedge. Shaft size is about 2.5"

Thanks in advance

 

[byrdj]

Not what you are asking for, but in the same thinking;
for steam turbines requiring slow rolls, the recomendations is cool the lube oil (<90F) for speeds below 1000rpm, warm oil >100F prior to exceeding 3000rpm, with ~120F for operation at 3600.

For smaller drive turbines, the norm is quick roll (<minute) to a minimun speed (~2200rpm) then accelerated as needed up to maximun 6000rpm

 

[electricpete]

another piece of info required to answer your question would be the approx rotor weight.

Then you can use a chart like that shown in Neale's Tribology Handbook figure 7.1.

It gives minimum viscosity (at operating temperature) as a function of linear speed (rotational speed * 2*pi*r) and bearing pressure loading (often half of rotor weight divided by bearing area)

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[Tmoose]

The question is a good start, but.........
Shaft journal roughness, runout, roundness, and taper, and the matching condition of the bearing can profoundly change how thick an oil film is needed. Like, if conditions are bad enough, full speed is not enough.
The right EP oil additives can help survive short term.

What's the oil delivery method? I sort of recall Those shaft riding ring "pumps" have some minimum speed, too.

My dad's old FORD tractor has SEVERELY worn rod journals. I'm guessing way over 0.010 inch diametral clearance. It is silent at any running speed, but clatters like Savannha BigWitch going from 0 to 5 rpm, or vice versa at shut down.

 

[electricpete]

Here is a calculation that shows minimum speed would be around 60 rpm:

http://home.comcast.net/~electricpete/eng-tips/MinViscositySleeve.pdf

You'll note there are several assumptions mentioned in my file. It also makes use of the Neale chart listed here:

http://home.comcast.net/~electricpete/eng-tips/MinViscositySleeveChart.ppt

Yes, Tmoose is certainly right there are assumptions built into these charts and it's very important to understand them and view all results with due skepticism (and double check with multiple sources).

No doubt there is an assumption of typical surface roughness. If the bearing or journal is damaged, the calc is worthless.

I looked at Stakowiak's Engineering Tribology page 215 and found a very interesting thing about sleeve bearing misalignment. Apparently it does NOT substantially change the minimum film thickness that occurs (which is suprising since to me misalignment seems like an additional loading and load decreases min film thickness). But what it does is force the load to be carried on a smaller area of the beairng and increases the psi loading seen by the babbit, so the babbit will be closer to its limits (or beyond it's limits).

I don't know if irregularities in taper and roundness act the same but I would suspect they do.


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[electricpete]

Whoops, I misread the chart and used the 500 psi line instead of the 700 psi line. OK, bump it up to 70 rpm minimum to compensate. (the calc is probably not terribly exact anyway).

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[electricpete]

I should backtrack on the comment about misalignment. Stakowiak is assuming misalignment and a constant radial load on the beairng. I think Stakowiak is saying the beairng can tolerate misalignment at the same radial loading without any decrease in the resulting minimum film thickness. But if the misalignment causes a radial reaction force, then clearly we have to increase the radial loading in the calcualtion which would decrease the resulting min film thickness. I'm guessing it might change his conclusion if the bearing created a reaction moment (vs reaction radial force) as well.




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[electricpete]

Out of curiosity, what is the reason for the slow ramp-up? Trying to evenly warm a rotor?

I would throw at least a factor of 2 conservatism into the calc to allow for unknowns and tell them to get to 150 rpm quickly. I would think they could evenly warm at 150rpm just as good as at 10 rpm without seeing excessive vibrations.

If there are other reasons and they are just slowly ramping all the way through the speed range, then I would wonder if they are spending excess time crossing resonances which can can also beat up the bearings (among other things).

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[electricpete]

Cooling the lube oil for the start - that was a good idea also.

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[electricpete]

The operating range identified in the Neale graph is within the linear range (left side) where increase in viscosity results in a proportionate decrease in min operating speed. Decreasing oil temperature by 10C should almost double the viscosity (vi=0.95) which should almost halve the min oerating speed.

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