Lubrication.Life adjustment factors

        Friction and wear are reduced by separating rollers and races with a lubricant film to minimize metal to metal contact. The major factors in selecting a lubricant are speed, lubricant base oil viscosity and temperature.

Building a lubricant film

        As speed and viscosity increase, thickness of lubricant film increases. As temperature increases, lubricant film thickness decreases. The lubricant film should be sufficient to cover the average peaks on the bearing surface by a ratio of at least 1.25. Ranges from 3 to 7.5 are described as elasto-hydrodynamic lubrication (abbreviated EHD or EHL). Adequate lubrication is defined as a ratio of 1.25 to 3.0. As the ratio falls below 1.25, some metal to metal contact will occur with a corresponding loss of L10 life. An a23 factor is used to adjust the L10 life for lubrication. L10 life is calculated based on adequate lubrication, then multiplied by the a23 factor, see a23 Va/Vr chart in the Life adjustment factors secton.
 

Grease lubrication

        Grease lubrication is easier than oil to retain in the bearing offering lower lubricant losses and improved sealing. Grease also offers better protection against corrosion to the roller surfaces. A grease typically consists of three components; a thickener (sometimes called a soap), a base oil and additives. The oil in the grease has an ISO-VG rating. In most cases, this is the key to selecting the grease. At speeds in excess of 200,000mm dn, greases with synthetic base oils are recommended. Please consult our technical department for proper grease selection.
        The National Lubricating Grease Institute (NLGI) has designated consistency grades for greases based upon the amount of thickener in the grease. The standard recommended grease for Cooper is a No.2 or No.3 consistency grade with an EP additive. The exception to this is a central pumped system where No.1 is used for it “pumpability”.
        A lithium complex thickener is used for normal applications operating at temperatures between 32°F and 180°F. When water resistance is required, an aluminum complex thickener can be used. Aluminum complex greases are not compatible with some types of grease therefore, the bearing must be solvent cleaned of other greases prior to adding an aluminum complex based grease.
        The initial pack of grease depends on speeds expressed as dn, where dn=shaft dia. x rpm. The initial pack should be used to coat the rolling surfaces of the bearing during installation. Initial amounts are shown in the Full pack grease volumes page.

Oil lubrication

        Oil lubrication can be broken down into three major categories; recirculating oil systems, constant level and oil mist.
        Recirculating oil systems use a pump to provide a continuous flow of oil to the bearing which is then recaptured, cooled, filtered and recirculated.
        A constant level oiler is the simplest method for delivery of oil lubrication to a bearing. The oiler maintains a constant level in the bottom of the bearing. Ideal conditions for oiler use would be bearing temperature less than 140°F (60°C), load through center, with low to moderate speeds.
        An oil mist system uses compressed air to atomize oil and spray it into the bearing. Conveying oil with filtered air maintains a positive pressure in the cartridge which is an effective method for keeping out contaminant. Oil mist systems are especially effective for high speeds

 Grease for initial lubrication

        The initial amount of grease is determined by adjusting the full pack volume for the operating speed and temperature. If the operating temperature is above 180°F(80°C), the bearing should be packed with 25% of the full pack amount regardless of speed. For temperatures below 180°F(80°C), the following chart should be used.
Speed in this case is expressed as dn (shaft diameter x rpm). The proper initial grease pack percentage for various dn ranges is shown below.
      

 Selection of base oil viscosity grade (ISO-VG)

        To determine the proper viscosity grade, the Cooper part number, operating temperature and speed (rpm) must be known.
Select the appropriate bearing geometry factor from the table below for the given shaft size and Series.
Multiply the geometry factor by the bearing rpm to obtain the velocity factor. Starting with the VG 150 graph on the next page (Temperature range charts), draw a vertical line from the calculated velocity factor and a horizontal line from the bearing operating temperature.
        If the lines intersect inside the shaded area, a grease or oil containing a base oil viscosity grade (ISO-VG) of 150 should be suitable for use.
        If the intersection of the lines is outside the shaded area on the VG 150 graph, follow the same procedures as above to determine if a VG 220 or VG 460 would be suitable.
For conditions not covered by the chart and graphs, please contact our technical department. 

(Origin: http://www.cooperbearings.com/tech05.htm)

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