stribeck curve over velocity --> speed (rev/s)

[Simunaut]

Hello everybody, I'm searching for a method to calculate the heat release in a hydrodynamic plain bearing. In fact I have a formula for it but I need the friction coefficient, like we have it in the stribeck curve. The mixed friction operation is of particular interest, since for pure hydrodynamics I also have data. I even found an empirical model for the velocity/friction coefficient relation of a lubricated material pair. The question is, if it can directly applicated to a plain bearing problem . Does anybody have some experience with this ?

 

[romke]

the friction coefficient in the Stribeck curve is not directly related to the speed of the shaft in the bearing but to the (dynamic viscosity*speed)/(stress on the bearing).

since you have data for the pure hydrodynamic part of the curve, you might be able to calculate the friction coefficient that occurs at the minimum of the curve.

if so, you will know the minimum friction coefficient and from the material combination (shaft/bearing) the maximum friction coefficient may be obtained by assuming there is dry friction when the speed of the shaft is 0.

thus, you end up with a minimum and maximum coefficient and from that at least the maximum amount of heat that dissipates could be calculated.

depending on the application the heat release in the non hydrodynamic part of the curve may not be that important - normally only at start up or stop down the operation will be in that region. much more heat will be released during hydrodynamic operation at higher speeds - due to fluid friction.

that may well be a appreciable amount - a temperature rise of about 80 deg C in oil temperature when passing through the bearing is not uncommon in the type of engines found in trucks. if you know the flow rate through a bearing you could calculate the influx of heat into the oil.

heat dissipation may be a problem at slow running speed, but the underlying problem then is in fact contact between bearing and shaft. when running continuously in that regimen the bearing may eventually fail due to the heat generated, but just trying to get rid of the heat through cooling will not prevent failure, because the root cause is contact and the resulting wear particles.

 

[Aled87]

simunanut, whats the name of the empirical model that you have found for sliding friction? i would like to check it out for help on my final year project which is based on engine friction,

many thanks

 

[Simunaut]

Thank you much for the Answer, romke. This sounds reasonable, I'll have to check if the order of friction losses calculated from such a simple assumption will be realistic. In fact, we do have measurements on Temperature drop over the bearing. From this drop and lubricant flow the heat transported away by lubricant can be calculated. The Problem about this is, that the heat transportation through conduction in the actual shaft can not be estimated this way (it adds additional cooling). So the whole heat generated in the bearing is leaving two ways.
The friction on the non-hydradynamic part is absolutely importand, since on full load the particular device is working 70% of the time in mixed friction conditions. At least that was what a simulation showed.

Since the heat release shall be used in a thermohydraulic model, this heat flux is of particular interest.

Now about the paper I found. I looked over it again and found that it is obviosly not suitable for plain bearings (It's called "Mixed lubricated line Contacts" by Irinel Cosmin Faraon). From my understanding of the term "line contacts" this is more like what we have for example between two gear wheels. But maybe that's what you are looking for, Aled87 ?
So I looked for more fitting ones and found these two:

The Stribeck Curve: Experimental Results and Theoretical Prediction // J. Tribol. -- October 2006 -- Volume 128, Issue 4, 789 (6 pages)

Calculation of a Stribeck curve of a journal bearing
D. Bartel, L. Deters

I'll order copies from my local library, will take some time but it's very cheap (I'm living in Germany, so no idea if such a library service exists in other countries).

 

[romke]

If you live in Germany you might also take a look into this book:

"Lagerung und Schmierung von Verbrennungsmotoren" (Affenzeller, Glaeser, Springer-Verlag, Berlin).

If gives detailed info on the calculation of bearings and might well be of use to you (assuming that you can read german).