Operating temperatures, the rate at which lubrication must be supplied, and the design of the equipment may make the use of oil impractical or impossible. For instance, wheel bearings in automobiles would be very difficult to lubricate with oil without sophisticated recirculating systems, and even then only extremely high-viscosity oils could withstand the loads. The solution is the use of semi-solid oils, or greases. Greases are used at points where oil will not provide proper lubrication.
The engineers who design a particular component must take into account which grease will provide the necessary lubrication to provide long life over a wide temperature range. Machinery manufacturers provide either pressure or cup fittings for the application of the grease.
General purpose lubricating greases are usually based on lithium, calcium or sodium soaps and include some EP greases and lithium complex grease. Specialty greases [high-specification greases] are formulated to meet rigorous specifications and based on complex soaps combined with highly refined or synthetic base oils and additives. While mineral oil based grease is the most common. Others are: water based lubricants, industrial grease, high temperature grease, dry film lubricants, conveyor lubricants, bearing grease, molybdenum lubricant, synthetic grease, dry lubricant bearing grease, and molybdenum grease - to name a few. Requirements for oxidation 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 Grade 1, soft greases, are used for high speeds and low pressures (light loads); the Grade 2, medium greases, are used for medium speeds and medium pressures (medium loads); the Grade 3, hard greases, are used for slow speeds and high pressures (heavy loads).
Specific lubrication instructions are always listed for specific applications, 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.
In the past, the diversity of specifications for automotive service greases, as established by the original equipment manufacturer (OEM), made it difficult for the marketer of lubricating greases to make available all the many specified products. Finally, the Society of Automotive Engineers (SAE), the American Society for Testing and Materials (ASTM) and the National Lubricating Greases Institute (NLGI), forming a tripartite group. The three worked to establish ASTM D 4950, the current standard for automotive service greases. As a result, OEM's, suppliers and consumers have a common language to use when describing lubricating greases for automotive service.
ASTM D 4950 includes specifications for two category groups: chassis lubricants (letter "L" prefix) and wheel bearing lubricants (letter "G" prefix). Performance classifications within these groups result in two letter designations for chassis greases (LA and LB) and three for wheel bearing greases (GA, GB, and GC) The automotive industry is in general agreement that the highest performance category, to date, in each group (LB and GC) is suitable for service relubrication.
Following the development of these categories, the Institute implemented its NLGI service classification as a means of identifying to the consumer the two highest performance levels: GC and LB; the combination, GC-LB, represents a multi-purpose category. In the future, NLGI will revise the system concurrently with any changes made to ASTM D 4950.
To facilitate easy, accurate identification to all users, NLGI has made available to the industry an identifying symbol, i.e., the NLGI Certification Mark, to be displayed on grease packaging. Those OEM owner's manuals that illustrate this mark will advise users to use only those greases carrying this mark on the label.
Ball and Roller Bearing Grease: Ball and roller bearing grease is for general use in equipment designated to operate at temperatures up to 300°F. For temperature applications above 300°F, high-temperature, electric motor, ball and roller bearing grease must be used. Categories GA, GB and GC greases are used in these applications.
Extreme Pressure Grease: Extreme pressure grease has antirust properties and is suitable for lubrication of semienclosed gears, or any sliding or rolling metal surfaces where the load may be high and where the equipment may be exposed to salt spray or moisture. It is intended for use in temperature ranges within 0° to 140°F.
Chassis Grease: Commonly used for suspension system components such as ball joints and spring shackles. Categories LA and LB greases are used in these applications.
Grease Cup Lubrication
Dirt in lube oil will generally settle out, but dirt in grease will remain mixed within the grease and will become abrasive. For this reason, you must be particularly careful to prevent contamination, especially where grease cups are used. Before you open the container, carefully remove all dirt from the exterior. Do NOT allow any dirt to enter either the opening or the grease cups. You should frequently empty, clean, and refill the cups with fresh grease.
Pressure fittings provide an easy means for lubricating numerous low-speed, lightly loaded, or widely separated bearings. They are not, however, good for use on electric generators and motors, as pressure fittings used on these units may force grease out of the bearing and onto the windings. Pressure fittings are similar to those on an automobile where grease guns are used for lubrication. You must use only one type of grease to fill a grease gun. You should mark the grease gun to identify the type of grease it contains so that you will not use the wrong lubricant. Before using the grease gun, you should clean the pressure fittings and gun tip. Apply pressure to the fitting until grease comes out around the edges of the bearing. In bearings fitted with felt or other seals, you must be careful to avoid breaking the seals by over-pressure. If you use excessive pressure while you are lubricating the needle type of roller bearings, you may unseat the needles. Grades most common in use in engine rooms are ball and roller bearing grease and extreme pressure grease.