Sorry about the long dissertation, but I have a chicken vs. egg type question about air entrainment.

We are experiencing air entrainment on a newly modified bearing lubrication system. Component has journal bearings on either end (I/B and O/B). The new oil system is comprised of a single oil reservoir which is the low point of the system. A gear type oil pump draws oil from the reservoir and supplies it (at a low pressure around 7-8 psig) to the bottom of each bearing. The bearing housings drain to the oil reservoir. Bearings also have slinger rings for start-up and coast down.

At any oil level higher than ¼" above the oil ring, air starts to appear at the inlet to the OB bearing siteglass after about 1 hour of operation. The oil level in the inboard housing begins to decline shortly after this, until it goes out of sight low on the siteglass.

We estimate that the pump has increased oil flow to the bearing 5-10 fold over ring-only lubrication. I suspect that the oil is now picking up air in the housing; however I'm unsure of what the exact mechanism is. Some historical data on similar systems says that aeration happens when oil discharged from the bearing impinges on other spinning parts in the reservoir and becomes aerated. It seems that the oil in this fine spray would lose the excess air in the housing head space, which is vented. Alternately, could oil splashing into the reservoir cause air to be incorporated into the oil volume and then be carried in to the drain of the housing.

Notably we have run the system at several different levels. We experienced entrainment at starting oil levels 3/8" above the ring, but not at ¼" above the ring; even after 5 hours of operation. Why would phenomonon stop abruptly?

I can't say I understand your whole scenario. But oil rings are known to operate erratically when oil level is not within range. Perhaps at the high level they are bouncing around and causing foaming.

Here is an excerpt from the EASA Technical Manual that suggests above bottom of the ring (i.d) is the max oil level.

quote:

"Oil level. If oil level information is available from the
manufacturer, follow it. If not, as general guideline, the oil
level should be approximately 1/4" to 3/8" (6 to 9 mm) above
the inside of the bottom of the oil ring (Figure 3). Too low a
stationary level means the oil level is dangerously low when
some of the oil is in play (in the bearing, dripping down the
inside of the chamber, etc.) Too high an oil level means
increased friction between oil and rings. The rings will turn
slower, supplying less oil to the bearing."

On all our machines with oil rings there is a viewing port for oil rings. If y ou have this, it should be easy to check whether oil ring operation becomes erratic when the level is increased.

Another possibility (less likely), when you increase oil level in the bearings are you possibly transfering oil from the sump which decreases oil leve in sump so the pump loses suction and draws air?

This message has been edited. Last edited by: electricpete,

Thanks for the thoughts E'Pete.

We are seeing air entrainment with oil level in the normal range for ring lubrication (exactly 3/8" above the ring). Also adding oil to the housing only increases pump suction level. There is no indication of oil starvation or cavitation.

What I think we have is a marginal housing design that allows too much surface agitation and spray with inadequate air removal time and/or venting of the oil system. I was looking to find out what would most effectively reduce air intake (though it's likely impossible without increasing the size of the housing)

There is very little literature available on the exact causes of air entrainment, and the concept is a bit of a "hard-sell" to management here because no one can quite visualize what's going on. Also, I can't prove anything can be improved untill I do a significant mod and then run under conditions that formerly caused foaming.




But again I've typed up a long dissertation. Sorry.

It sounds like oil pump starvation is ruled out.

Have you ruled out that erratic oil ring operation at high oil levels is causing the agitation? It seems credible to me, since 3/8" is the upper limit in the reference above and you don't have the problem at lower levels. What other possible explanations are there for why it only occurs at higher levels? (I assume there are no rotating parts in the bearing housing other than shaft above oil level and oil ring... I can't see anything other than oil ring motion to explain it) Have you been able to observe the oil rings through a viewing port?

I agree that it is intriguing that the phenomonon stops rather abruptly between 1/4" avove the ring and 3/8". We ran at up to 5/8" above on the same housing before the forced oil mod, with no problems; though at much lower oil delivery rates. We also have three other "identical" components that are able to run at higher oil levels without the air entrainment issue. There is a shaft mounted disk in the same housing (overspeed protection) and a ball bearing for thrust.

One thought on the lower oil level not causing the problem is that is gives more margin before the disk is contacted.

A second thought I had (grasping at straws) was that at this lower oil level more of the oil may hit the walls of the housing and therefore reduce the overall force of the oil hitting the surface of the oil; therefore less air would be mixed in. This is part of what prompted my original questions. The fine oil mist hitting the walls may also help drive off some the excess air. I don't have a "warm" feeling about any of these possibilities, though.

This message has been edited. Last edited by: VTT,

I haven't heard you say that you have ruled out erratic operation of the oil ring. By the term "erratic", I mean that the oil ring is bouncing around wildly and bouncing off the top of the oil. I have seen it happen. There are many things that contribute to whether oil ring is stable or erratic, but oil level is one of them. Do you have a small viewing port above the bearing to observe whether the oil ring is jumping around?

I can't completely rule out eratic oil ring operation and likely this could contribute to the problems. I have neglected the rings in general because I have one component that has never experienced air entrainment and another that performs OK over a fairly wide oil level range. Then I have two that "foam" at fairly low levels. All have the same design rings and all the rings were inspected when the modification was installed.