A number of factors determine the effective-ness of oil film lubrication. They include load, temperature, viscosity, flow rate, speed, align-ment, condition of the bearing surfaces, running clearances, and purity of the lubricant. Many of these factors, of course, are interrelated and interdependent.

A lubricant must stick to the bearing surfaces and support the load at operating speeds. More adhesiveness is required to make a lubricant stick to bearing surfaces at high speeds than at low speeds. At low speeds, greater cohesive-ness is required to keep the lubricant from being squeezed out from between the bearing surfaces.

Large clearances between bearing surfaces require the use of a lubricant with a high viscosity and cohesiveness that will provide an adequate lubricating oil film. The larger the clearances, the greater resistance the lubricant must have so it will not be pounded out. (If the lubricant is pounded out, the lubricating oil film will be destroyed.) High unit loading of a bearing will also require the use of a lubricant with a high viscosity. A lubricant that is subjected to high loading must be sufficiently cohesive to hold together and maintain the oil film.

GREASES

Operating temperatures, the rate at which lubrication must be supplied, and the design of the equipment may make the use of oil impractical. Greases are used at points where oil will not provide proper lubrication. Machinery manufacturers provide either pressure or cup fittings for the application of the grease. The maintenance requirement card (MRC) and the equipment guide list (EGL) will provide all the necessary information for the proper lubrication of a component: the type of grease required, the type of grease fitting on the component, the specific location of the fitting, and any additional information you may need to perform the lubrica-tion procedures in a safe, efficient manner. You MUST follow specific lubrication instructions, since some greases are for general use and others are for special purposes. You will better under-stand maintenance problems involving lubrication if you are familiar with the principal factors of the composition and classification of greases.

Some lubricating greases are simple mixtures of soaps and lubricating oils. Others are more unusual, such as silicones and dibasic acids, which are exotic liquids. These may be thickened with metals or inert materials so that enough lubrication is provided. Requirements for oxida-tion inhibition, corrosion prevention, and extreme pressure performance are met by the addition of special substances (additives).

Lubricating greases are supplied in three grades: soft, medium, and hard. The soft greases are used for high speeds and low pressures (light loads); the medium greases are used for medium speeds and medium pressures (medium loads); the hard greases are used for slow speeds and high pressures (heavy loads).