For rolling element bearings that are not lubricated by a forced lubrication system, there are a few methods available to the user. Oil mist has been presented as the best method of lubricating bearings. It is the best and has some advantages.
As an automatic system, there is little operator action required other than to insure the resevoir has oil in it. By maintaining positive pressure in the bearing housing, contaminants from the atmosphere, moisture and dust, cannot enter the bearing housing. This assumes that high quality bearing isolators have been installed, as they should be for all installations.
(The wet sump method is the most cost effective method of rolling element bearing lubrication)
Oil mist can also save some energy by reducing the drag of the bearings through the oil. The downside is that oil mist systems are expensive to install, especially as retrofits to existing units. Oil mist can be installed as a supplement to a wet sump.
The most cost effective method of rolling element bearing lubrication is the wet sump method. Install high quality bearing isolators to minimize contamination. Also install a drain sight glass that will give a visual indication of contamination so the operator can drain water from the bearing housing or detect particulates that indicate more serious issues with the bearings. Utilizing a synthetic oil appropriate for the application will enable extending oil changes to 6 months, a year, or even longer. Consult your oil provider for recommendations.
Operators do have to check oil levels on their rounds to ensure that sufficient oil is available for the oil rings or flingers. Keeping the oil level below the bearings saves significant power by not forcing the bearings to plow through the oil and generate heat. Flingers are becoming the preferred method of oil distribution due to being positively driven by the shaft, but oil rings also work well if properly set up, including making sure the pump shaft is level. Following this guideline will provide bearing reliability that is comparable to oil mist at considerably less initial investment.
Magnetic bearing isolators
Vertical shafts present special difficulties as oil is slippery and gravity works to move it along the shaft and out of the bearing housing. There are magnetic bearing isolators that provide a more positive seal and minimize the leakage. Oil mist is better for vertical applications as there is no oil level to be maintained for bearing lubrication. Wet sump designs must have an oil pump to move the liquid from the sump to the bearings. The oil pump designs I have experienced work well from the factory, but are very difficult to repair while maintaining the tolerances necessary for efficient operation.
For more information on lubrication, PIP REIE686/API RP686, PIP REIE686A and API 614 are excellent sources for lubrication information. PIP stands for Process Industry Practices and is a consortium (non-profit) the strives to develop best practices for a variety of industries. Visit pip.org and take a tour. API is the American Petroleum Institute and is well known.