Helical Mesh Lubrication, Flow Rate Calc. Methods??

[ewillia13]

I've been searching on and off for the last 6 months for established methods for determining the minimum and recommended flow rate to lubricate and cool a high speed helical gear mesh and have found very little. And the methods I have found have produced a broad range of flow rates.

This is a high horsepower high pitch line velocity gear set (3000hp and 25000 ft/min) so I understand that methods for the average lower pitch line velocity gear sets will not be as reliable. I have already determined the delivery method and the ratio of lubrication to cooling. Looked at scuffing and wear risk, know minimum film thickness needed, oil temperature rise and so on. I just need to determine the amount of flow for lubrication.

The methods I have seen consider things such as face width, heat generated (mesh efficiency), EHD theory, and tooth surface roughness.

Any recommendations or references would be greatly appreciated. Thanks

 

[tbuelna]

ewillia13,

Your gear mesh oil flow rate will be based solely on cooling requirements. The oil mass flow rate is a function of the oil specific heat value, it's inlet temperature, and the temperature rise it achieves across the mesh. Most lube oil systems I've worked with usually end up with a temperature rise of around 40 to 50 degF.

With high pitch line velocities, a good approach is to jet about 25% of the flow into the mesh and the other 75% of flow at the mesh exit. The oil jets should all be oriented radially to the gearshaft axis and be kept as close as possible to the gear OD. The jet orifices sizes and discharge pressure can be established by a time/motion study of how far the radial oil jet penetrates into each passing tooth space at the max PLV. Jet penetration to the root is ideal, but anything past the pitch diameter is probably acceptable.

Keep the oil flow rates to a minimum, since high PLV gears tend to incur high oil windage losses.

Good luck.
Terry

 

[ewillia13]

Terry,

Thanks for the advice, this is very helpful.

One more thing, After some research and reading through an AGMA paper on the topic I am considering separating the cooling and lubrication flows as you said but instead of spraying the cooling flow into the out-mesh, spraying it on the disk/web of the gears. An AGMA paper claims this has a significant effect on the gear tooth center temperature and requires less overall flow. Any thoughts on this approach?

Thanks again for the help

 

[tbuelna]

ewillia13,

The heat load is generated at the tooth flank surface, so this is where the gear temps will be highest. With carburized gears, it is also critical to keep this case hardened surface's temperature below it's temper limit. Other wise the material will begin to de-temper and lose strength. Some high performance carburizing grade gear steels (like 9310) are tempered as low as 300degF.

I can see where spraying the backside of the gear rim might be beneficial in one respect. It would allow the oil to maintain contact with the gear body for a longer period of time, thus improving the heat transfer. Oil jetted into the tooth spaces is immediately flung off, especially with high PLV's. But the back surface of the rim would also be at a lower temperature than the gear tooth flank surfaces, which would not be beneficial for heat transfer. So overall, I'd stick with jetting oil radially into the tooth spaces.

NASA technical reports server has some excellent reference material on oil jetting and high speed gears:

http://ntrs.nasa.gov/search.jsp

"Lubrication and cooling for high speed gears", by Townsend, D. P.

"Study of lubricant jet flow phenomena in spur gears", by Akin, L. S.; Townsend, D. P.; Mross, J. J.

Hope that helps.
Terry