Would it be possible to add a little defoaming agent?  It doesn't take much, just a tiny fraction of a percent.  I believe there are compounds which work for oil like others do for water solutions.  I wouldn't be surprised if the same agents, simethicones, work for both oil and water.

Don't know if you've solved your problem yet, but I'll pitch in.  I suspect that your problem is not really foaming, but insufficient dwell time to deaerate the oil.

A simple way of separating the air and oil is running the oil over an angled screen above the oil level in the reservoir.

Is the reservoir separate from the gearbox sump?  If so when the scavenge oil is returned to the reservoir it can be directed against a reservoir wall above the oil level.  This can actually be a surprisingly effective method of separation.

Consider installing a perforated screen around the zone of action so that a quiescent zone is created away from the gears. Turbulence is high within the mesh and low away from the gears.

Thanks for the suggestions.  We noticed that we were pressurizing the gearbox, so we installed an atmospheric vent with a demister filter at a high point.  We haven't run since I originated this thread, but I'll let you know how it works out.  The reservoir is separate from the sump, about 30' below.  We may use your separation ideas if our filter doesn't work.

there are a few things to notice:

1: do not ever put an additional defoaming agent into an ATF - eevry ATF already contains the optimum amount and putting in more would increase the problem

2: it seems that not defoaming but deaeration is the problem (foam is airbubbles encapsulated in a thin oil film above the fluid level). You have airbubbles in the fluid itself and these need some time to rise to the surface...time that obviously is not available. the best way ro deal with this kind of problem is to avoid airbubbles being formed. this can be achieved by making sure that the return oil does not "splash"into the reservoir but is introduced gently. A baffeplate might therefore help, another solution would be to introduce the returned oil under the reservoir level in stead of just letting it fall back.

I have a problem with air in gear box oil causing high temperatures.  I want to use the angled screen suggested by enirwin to deareate the oil.  Could you please help by telling me what size and how many square feet of screen are needed? Thank you.

Hi there.
well we had at my place simular problem and we did <see above> screen solution.
First we made a cylinder shape "mash/filter/catch".we made mash <like 1/8"diam. per hole> that was going below the oil level ~2". conecting outgoing stream with that mash reduced air bubbles in system ab to 85%.
if you need more info how and what to do on this idea just ask.
wish you luck.

~B~

Thanks boke. I think I understand how the screen is set up, but would like to get a sketch. If you could send me one at paul.mureiko@dsm.com or to discuss further.

Once again thank you.

pmureiko,

I have seen a couple of methods of screening.  One is having an angled screen (about 30 degrees from horizontal) above the oil level and introducing the return oil to the screen tangentially at the botttom of the screen.  The oil falls thru the screen into the reservoir.  The other method is to have the angled screen partially submerged in the reservoir, isolating the pump suction line from the return line.  You will probably have some trial and error as there is not a lot of information for sizing the mesh.  A 60-mesh w/ .009 inch dia wire screen might be a good place to start.  Good Luck.

A late update.  We were pressurizing the gearbox, so we installed an atmospheric vent with a demister filter.  Air entrainment cleared up right away.

Larry