high speed gears

[Kevinator]

I am in the idea generating phase of my gear design. Does anyone see a problem with lubricating high speed gears (at 20000 rpm) with an oil bath? I know oil injectors are the norm but I am just curious why thats so.

Thanks in advance for any advice

 

[MikeMilburn]

Aireation possibly, just a guess....Mike

 

[butelja]

Foaming is very likely, as already mentioned. Also, if you can cure the foaming then windage losses (fluid drag) may become a big factor and cause overheating and low efficiency.

 

You don't mention the diameters but at that speed I would guess centrifugal force would throw off any oil before it got to the mesh. Anything over 4000 ft/min should ideally be force fed.

 

[shopguy]

It is my understanding that one problem with an oil bath on a high-speed gearbox has to do with the fluid dynamics of the oil. As the gear tooth reaches full engagement with it’s mating space, the only place for the oil to exit is out the sides at the root. What may amount to an urban engineering legend has it that if the gear is turning fast enough and the face of the gear is wide enough, the exit velocity of the oil becomes supersonic. I suspect that before you get close to that situation, the fluid dynamics will overcome the mechanics of the transmission and instead of a transmission, you end up with a self-contained oil pump.

 

We usually look at the rubbing velocity at the pitch line. Anything over 20,000 fpm is considered high speed. covert your 20k rpm to see how fast the teeth are coming into and out of mesh. Churning loses become a problem along with dynamic affects in the load zone. I suspect the biggest problem will be in the support bearings. They die a quick death at that speed if the design isn't just right. (ask the turbine bunch)

 

[Kevinator]

Thanks for all the responses! You all are too kind.

Kevinator