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) 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?