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Oil Mist Lubrication

History Of Oil Mist Lubrication

The Oil Mist principle was developed by a bearing manufacturer in Europe during the 1930s. The problem that nurtured this development was the inability to satisfactorily lubricate high-speed spindle bearings on grinders and similar equipment. The speed of these bearing was too high for grease lubrication, and liquid oil generated too much heat through fluid friction, necessitating an expensive recirculating system. Continuous thin-film lubrication with Oil Mist provided a solution. The purging and slight cooling effects of the carrier air gave additional benefits. The Oil Mist generator resulted later from this development and used a small amount of air to produce a dense concentration of small oil particles. About 97% of these particles could be transmitted to the bearings without condensing in the piping, regardless of the distance of the bearings from the Oil Mist generator itself.

In 1958, air heaters were developed because it was discovered that, by heating the air used to generate Oil Mist, oils of just about any viscosity could be atomized. Many applications, subject to extremes in ambient temperature, use air heaters to ensure a constant oil/air ratio regardless of the oil viscosity. Today Oil Mist is still used to lubricate high speed spindles in grinders. Included in the increasing range of Oil Mist applications are systems applied to all types of other machine tools, web and sheet processing equipment, belt and chain conveyors rolling mills, vibrators, crushers, centrifuges, kilns, pulverizers, ball mills, dryers and liquid processing pumps.

Mist Generator Systems

Oil Mist is a centralized system in which the energy of compressed gas, usually air taken from the plant supply, is used to atomize oil. Oil is then conveyed by the air in a low pressure distribution system to multiple points of lubricant application.

The compressed air is passed through a venturi. Oil, siphoned from a reservoir by the air flow, is atomized into a fine spray. Baffles downstream from the venturi nozzle causing the larger oil particles to coalesce and return to the reservoir. The remaining air-oil-mixture is Oil Mist. Oil Mist contains oil particles (droplets) averaging about 1-1/2 microns in diameter (.00006 inches), which can be conveyed through distribution piping (mist manifold), at velocities up to 24 feet per second, to application fittings (mist fittings which meter oil to bearing housings). Air-borne oil particles are then "wetted out" by impinging upon bearing surfaces rotating at sufficient speed to cause adherence and the formation of larger drop sizes. Because there are no moving parts in the basis Oil Mist generator system, and because the system pressure is very low (manifold pressures from 5 to 40 inches of water), it is a reliable lubrication method. Also, the system can be interlocked with machine operation or an alarm system to ensure proper functioning. Note that, even when malfunctions occur, most bearings will operate for hours on an existing film of lubricant.

Principle Of Oil Mist Generation

Advantages Of Oil Mist

This system is ideal in that a metered amount of oil (approaching optimum) can be provided to the bearing for proper lubricity. Lubricant friction itself is virtually eliminated. Keep in mind that grease is used only as a "carrier" for those oils within it which provide lubricity. Other than that, it is an impediment to rotation, causes frictional heat, can carry and recycle abrasive particles to the bearing, and doesn’t guarantee good dispersion (i.e., coating of all surfaces of the bearing at all times to prevent corrosion).

The oil mist provides a flushing action with fresh, clean lubricant and acts in a slight manner as a bearing coolant. The carrier air used to distribute the oil provides additional benefits in that it maintains bearing housings under slight positive pressure and the "outward" air flow prevents the entrance of contaminants. Where mist systems run continuously - and when motors are inoperative - problems with condensation reaching bearing surfaces are eliminated.

Maintenance "human factor" problems are reduced: Over-greasing, lack of replenishment of the proper grease and the right amount at the right time.

Our Position: Product Modifications

It has been determined that our horizontal ball bearing motors are compatible with oil mist systems, keeping in mind the following necessary features and modifications:

  1. "Regreasable" motor construction.
  2. Open bearings with bearing caps.
  3. LUBRIFLUSH or straight-through lube system.
  4. Grease inlet in upper bracket quadrant (standard).
  5. Grease (condensate) drain near 6 o’clock position. This maintains a "dry sump" system and minimized accumulation of churning of oil.
  6. Drain should not be located directly in air flow from fan (copper or galvanized steel tube extensions should be supplied where this is the case).
  7. Grease inlet hole threads must accommodate standard mist fittings (1/8 and ¼ inch present standard generally suitable but check particular mist generator supplier for specific size and thread). Vents should be about twice the area of inlets.
  8. Winding treatments used for CORRO-DUTY® and VPI are suitable. EXAR 150 leads must be used. Silicon rubber should be avoided due to potential swelling. (Very small particles of mist will enter the motor regardless of bearing caps, seals, etc.).
  9. Motors should be supplied with grease intact, for protection during shipping and storage. Before connecting to a mist generation system, vents (grease drain hole) must be opened. Then the grease inlet channel should be cleared with clean, dry, low pressure compressed air so that mist will flow into the bearing cavity.

These features are reflected in the following chart:

NEMA Frame - CORRO-DUTY® Motors

(Except Hazardous Duty)

Frame Cast Iron
Bearing
Cap
Bearings Mist Inlet Mist Outlet Insulation
& Leads
140 Construction not suitable
180 Add

Bearing

Caps

Modify

To Open

Bearings

Check
Mist
Supplier
For Hole
Size &
Thread
SE & PE
Bracket
Grease Drain
to be at

6 0'Clock

Standard
Winding
Treatment

Use EXAR 150
Leads or an
Equivalent Impervious
to Oil

210
250
280
320
360
400
440

*Add Vent Extension.

At the present time we will supply modified TEFC motors which are compatible with Oil Mist systems, but will not provide system components or assume total system responsibility.

As a motor supplier, our position is that the bearing must "see" a proper flow, mixture and quality of mist. There should be no pre-condensation of the mist waxing of the lubricant (clogged nozzles) etc.

General guidelines are as follows:

Flow (CFM) should follow the relationship -

CFM = DR/20

D = Shaft Diameter (inches)
R = Rows of balls
20 = Constant for heavy service (use 40 for moderate service)

Note, however, that a conservative standard of 0.4 CFM has been used successfully for most NEMA frames.

Mixture should be:

Oil / air ratio = .4 to .65 cubic inches oil / hour / CFM air

Lubricant:

Use a quality bearing oil formulated for oil mist lubrication, 600 SSU viscosity at 100ºF for summer months or 150 SSU at 100ºF for colder months.

Note that the system should be designed such that bearing cavity pressure will be approximately 5 inches of water minimum. This will prevent contaminants from entering this cavity.

Also note that bearing rolling elements must have a velocity of 200 linear feet per minute minimum when mist fitting is used in order for oil particles to "wet out" as they impinge upon ball surfaces.

Liability:

Since we have no control over lubrication other than the initial amount of grease and since this grease is eventually purged by the mist, we should provide a conditional statement to our Warranty similar to the following:

"The mist system must provide a minimum of CFM flow to each bearing per the relationship of 0.05 D (shaft diameter inches) x R (rows of balls). Mixture of mist should be approximately 0.4 to 0.65 cubic inches of oil / hour / CRM air. A quality bearing oil must be used and have 600 SSU viscosity (100ºF) in summer or 150 SSU viscosity (100ºF) in colder months. The mist generate must be run continuously when the motor is inoperative. Also, the system should have adequate alarms to flag a malfunction and effect shutdown of the motor."

This statement is at present a general guideline only and may change as we gain experience and/or propose motors for specific applications.

Vertical Motors:

Currently vertical motors are not approved for oil mist lubrication. Further lab testing and/or prototyping in the field may be required. In the interim, however, refer any inquires of significant volume to our headquarters office in St. Louis.