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Avoid Bearing Failures by Selecting the Appropriate Grease

Premature bearing failure can be prevented by choosing the most appropriate grease for a rolling bearing. Grease selection is also vital in achieving high reliability and optimal operating life of a rolling bearing. Statistics present that a large portion of premature rolling bearing failures are directly or indirectly related to lubricants used. Reports show that main causes of failures include unsuitable lubricants (20%), aged lubricants (20%), and insufficient lubricants (15%).

Lubricating oils like mineral and synthetic oils are sometimes suggested for use with rolling bearings undergoing extreme operating conditions like high temperatures or radiation. However, most bearing manufacturers recommend usage of greases.

Grease characteristics essentially depend on three properties: base oil type & viscosity, thickeners, and additives. The principal components by which greases are classified deal with thickeners and base oils.

Base oil viscosity is responsible for lubricant film formation. Commonly used base oils can either be mineral or synthetic oils. When picking synthetic oils, it is also important to differentiate these according to type (polyalphaolefin, polyglycol, ester, fluoro oil, etc) as these have very distinct characteristics.

Standard thickeners used include metal soaps or metal complex soaps. Also becoming increasingly important are organic or polymer thickeners like polycarbamide.

Additives are found in all greases. There are additives that affect oil (oxidation inhibitors, viscosity index improvers, detergents, etc.) and additives that affect bearings or metal surfaces (anti-wear additives, corrosion inhibitors, friction value modifiers).

Greases are formulated in multiple consistencies or NLGI grades, determined by ‘worked penetration’ according to ISO 2137. Higher NGLI grade means harder grease. Preferred greases for rolling bearings have NGLI grades of 1, 2, or 3.

Selecting the right grease for a rolling bearing involves a number of application-related factors that should be considered including bearing type, operating speed, temperature, and load. Factors such as mounting position, sealing, shock and vibration, and legal/environmental regulations should be noted as well.

Bearing Type involves differentiating between point contact (ball bearings) and line contact (needle roller bearings and cylindrical roller bearings). In ball bearings, mechanical strains placed on greases in bearings with point of contact is greatly less than in bearings with line contact. Usually, these bearings use greases with base oil viscosity ISO VG 68 to 100. For rolling bearings with line contact, higher requirements are placed on the grease. Greases selected for this bearing type must show a higher base oil viscosity ISO VG 150 to 460 or higher. Rolling bearings may also require anti-wear additives with consistency usually that of NLGI 2.

Bearing speed parameter should always be a suitable match for the grease’s speed parameter. This factor relies on the type and proportion of the thickener, the base oil type, as well as the proportion of base oil. A grease’s speed parameter is based on bearing type and required minimum operating time. As a general rule, for rolling bearings rotating at high speeds or with a low requisite starting torque, greases with high speed parameter are advised. Greases with a low speed parameter must be selected for rolling bearings rotating at low speeds.

Temperature range of greases must match the range of possible operating temperatures in the rolling bearing. The operating temperature range depends on the type and proportion of thickener, type and proportion of base oil, production quality, and the production process. Also dependent on the production quality and process is the stability of the grease at high temperature. For reliable lubrication and an acceptable grease operating life, it is recommended that greases should be chosen according to bearing temperature that usually occurs in the standard operating range. Some other factors to also consider include the upper operating temperature of grease, the dropping point, and the lower operating temperature.

For a load ratio C/P < 10 or P/C > 0.1, recommended greases must have higher base oil viscosity and anti-wear additives. The additives create a reaction layer on the metal surface, providing wear protection. These greases are suitable for bearings showing a greater proportion of sliding motion (including slow running) or line contact, and under combined radial and axial loads.

Having a good understanding of the factors listed above leads to proper grease selection for rolling bearings. Usage of appropriate greases avoids potential bearing failures that in turn results to increased machinery and facility productivity. At the Industrial Lubricant Store, you can choose from a range of food grade and non-food grade lubricants and greases to ensure optimal performance of your equipment. Browse our product offerings by visiting www.theindustriallubricantstore.com/products today!

About the Author
Randy Renick
Randy Renick has a Bachelor's degree from LSU. He is an STLE Certified Lubrication Specialist and has a 29 year work history in Industrial Lubrication. He is currently a Lubricant Consultant at The Industrial Lubricant Store.