Author: Michael Khonsari, E.R. Booser

With improvements in oil formulations, oil mist lubrication systems are becoming increasingly popular in a variety of applications where only limited oil feed supply is required. Examples include bearings of electric motors, pumps and compressors in many oil refineries and petrochemical plants¹; gears, cams, chains and sliding surfaces of machines in steel and paper mills; construction equipment and an increasing range of other industrial applications.²Compact mist supply units are also available for use with electric motors, machine tool spindles and similar local applications.

Oil mist systems present an attractive alternative both to grease at low-to-moderate speeds and to circulating oil systems for high speeds and high temperatures. Properly installed oil mist systems offer the following advantages:

• No oil changes and reduced maintenance requirements
• Reduced lubricant consumption (up to 70 percent compared to sump lubrication)
• Lower friction and reduced bearing temperatures
• Mist delivery pressure blocks entrance of contaminants
• Less wear and increased life of machine elements
• Lower capital costs

Newer air/oil systems are used in steel mills as alternatives to oil mist.³ To minimize loss of stray mist to the surrounding environment, liquid oil is injected from a positive displacement pump directly into the air stream. The oil, injected at time intervals, is then propelled by the air stream along the feed line as droplets in a spiral motion to enter a bearing as an oil spray.

Mist Supply 
Oil mist is an aerosol mixture of very small oil droplets (one to five microns) suspended in air with the appearance of smoke. This mist is generated by passing compressed air through a venturi or vortex to siphon oil from a small central reservoir (Figure 1).⁴

Figure 1. Basic Oil Mist Generators

Pressure of this inlet air is regulated to properly deliver the oil. Droplets larger than about five to seven microns are not easily transported by the air stream, therefore, they are typically intercepted by a baffle for return to the reservoir.

Figure 2. Components of a Typical Oil Mist System

Figure 2 illustrates the main components of a typical oil mist system.² It includes a pressure-controlled, filtered air supply, heaters to stabilize the air and reservoir oil temperatures at 130° to 170°F, a mist generator and distribution piping or tubing.

The initial dry mist generated then flows at 15 to 20 feet/second at distances up to 1,000 feet or more through pipes, tubing and hoses for delivery from headers commonly maintained at the pressure of a 20-inch water column (0.7 psi, 5 kPa).⁶ If the mist stream becomes turbulent above about 24 feet/second, mist droplets strike wall surfaces hard enough to stick and prematurely drop out of the mist stream.

The first 50 feet of a delivery header should be sloped back to the mist generator to return any droplets dropping out of the mist stream. Any longer feed manifolds and auxiliary headers should then be sloped either to drains or back to the generator to avoid low spots where trapped oil would interfere with mist flow.

Mist Delivery 
Orifice fittings to meter mist supply to individual machine elements involve one of the three types of classifiers in Figure 3.

Figure 3. Classifier Fittings to Agglomerate Fine
Oil Particles in Dry Mist to Larger Droplets in
Wet Mist at Lubrication Points

A mist fitting consists of a simple metering orifice for delivering a fine wet spray with minimum condensation. As this fine spray then encounters rolling-motion elements – such as in ball or roller bearings, gears, chain or cams – the fine oil particles are agglomerated by the turbulence action and the larger wet droplets deposit as lubricating films. The mist is commonly fed into the bearing housing on one side of the row of balls or rollers and is discharged from the opposite side (Figure 4).

Figure 4. Representative Mist Flow Pattern for
Ball and Roller Bearings

Spray and condensing fittings are used for sliding-motion elements. The major difference between the two types is how long mist particles are maintained at high velocity under turbulent flow conditions to promote agglomeration of the fine oil particles in the dry mist feed. To lubricate sliding surfaces, journal bearings and the like, the resulting wet spray then runs down adjacent surfaces in arrangements such as those shown in Figure 5.

Figure 5. Mist Lubrication of Plain Bearings

Oil Requirements 
Table 1 provides a guide for the quantity of oil normally fed to various machine elements for moderate service requirements.⁴

This information pertains to horizontal shafts with unidirectional load, bearings in any position with oil retained by seals, or porous metal or other self-lubricated bearings. Feed rates in Table 1 should be doubled for heavy-duty service, oscillating bearings, unsealed bearings subject to shock load with constantly shifting load zone, preloaded bearings and nonhorizontal shafts.⁵ Special considerations are needed from related experience or supplier information for surface speeds over 600 feet/minute.

Lube system suppliers, machinery builders and lubricant suppliers should also be consulted for appropriate feed rates and lubricant specifications because requirements vary for individual installations and may double for speeds, loads and temperatures above those in conventional factory applications.

Two examples drawn from experience under mild operating conditions illustrate the small amount of oil sometimes needed by bearings. A ball bearing in a 10-horsepower electric motor ran satisfactorily for several weeks at 3,600 rpm with a single drop of SAE 10 oil. In another case, the 9-inch journal of a wick-oiled railroad locomotive propulsion motor ran without failure for one hour at 600 rpm after the lubricating wick was removed. Small porous metal bushings and oil-impregnated polymer bearings are often used for the life of appliances with no additional lubrication. In general, oil volumes given in Table 1 provide suitable excess for reliable operation.⁷

Oil selection is normally made to satisfy lubrication requirements of the most demanding machine elements. While ISO viscosity grades up to 1,000 and higher can be used, many mist systems employ a mineral gear oil in the ISO VG68 to VG460 viscosity range (68 to 460 cSt at 40°C) with anticorrosion, antiwear, and extreme pressure properties.

Past problems with wax and additive separation from mist oils are now avoided by using naphthenic base stocks, and by not using oil additives that might be deposited upon encountering water contamination.

Mist oils also contain special additives to improve atomization, promote condensation on rotating surfaces, and to reduce fogging and stray mist in the surroundings. These mist formulations typically provide 20 to 30 percent more usable oil delivery than normal mineral oils. Automotive engine oils should not be used because their mistability varies widely.

With their lower lubrication needs, ball bearings in electric motors and related pumps commonly employ a bearing oil of 100 to 150 cSt (at 40°C) for summer use and 32 cSt for colder months. Minimum cubic feet/minute of mist to be fed to each bearing is specified by U.S. Motors as D (inches shaft diameter) x R (rows of balls)/20 using mist of approximately 0.4 to 0.65 cubic inches of oil/hour/standard cubic feet per minute (scfm) of air flow.

Limitations 
While mist systems have dramatically reduced maintenance and operating problems, establishing flow rates in a system has proved troublesome and the following details commonly need attention:

• The air stream in feed lines must be kept laminar, below approximately 24 feet/second, because turbulence causes oil particles to impact the pipe wall and be removed from the air stream before reaching delivery points. At abnormally low velocities, on the other hand, oil droplets may also settle out prematurely.
• Performance is sensitive to temperature. Even when not required by viscosity considerations, heaters are often employed to stabilize the oil/air ratio under widely varying ambient temperatures. When used, air heaters are usually accompanied by oil reservoir heaters.
• Spray mist involves environmental hazards. Vent lines are needed at lubrication points for collecting stray mist which has not been classified. OSHA requirements state that in an eight-hour period, a person can be exposed to no more than five milligrams of oil per cubic meter of air.

References

1. Ehlert, D. “Oil Mist Lubrication in the Hydrocarbon Processing Industry.” Machinery Lubrication, May-June 2004.
2. Bloch, H. and Shamim, A. Oil Mist Lubrication: Practical Applications. Fairmont Press, Inc., Lilburn, Georgia. 1998.
3. Schrama, R. “Oil Mist vs. Air-Oil for Consumable Lubrication Systems.” Lubrication Engineering, Vol. 49, p. 9-17. 1993.
4. Reiber, S. “Oil Mist Lubrication.” CRC Tribology Data Handbook. E. Booser (editor), CRC Press LLC. 1997.
5. Alemite Corporation. Oil Mist Application Manual. Charlotte, North Carolina. 2004.
6. Towne, C. “Practical Experience with Oil Mist Lubrication.” Lubrication Engineering, Vol. 39, p. 496-502. 1983.
7. Khonsari, M. and Booser, E. Applied Tribology – Bearing Design and Lubrication. Wiley & Sons, New York. 2001.