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BEARINGS FEATURE


GREASE IS THE WORD... when thinking about rolling bearings


Dr Steve Lacey, engineering manager at Schaeffler UK, looks into the importance of selecting the most suitable grease for rolling bearings


S


electing the correct lubricant is critical to ensure the functional reliability


and optimum operating life of a rolling bearing. Failure statistics show that a significant proportion of premature rolling bearing failures are directly or indirectly related to the lubricant used, with the main causes being unsuitable lubricants (20%), aged lubricants (20%) and insufficient lubrication (15%). When selecting the right grease a


number of application-related factors need to be considered including the type of bearing, operating speed, temperature and load.


CHARACTERISTICS The characteristics of a grease fundamentally depend on three properties. Base oil type & viscosity: The


viscosity of the base oil – often mineral oils or synthetic oils – is responsible for the formation of the lubricant film. It is important that synthetic oils are differentiated according to their type (polyalphaolefin, polyglycol, ester, etc.), as these possess very different characteristics. Thickeners: Typical thickeners used


include metal soaps or metal complex soaps; and organic or polymer types such as polycarbamide are becoming increasingly important. Additives: All greases contain additives.


A distinction is made between additives that have an effect on the oil itself (oxidation inhibitors, viscosity index improvers, etc.) and those that have an effect on the bearing or the metal surface (e.g. anti-wear additives, corrosion inhibitors, friction value modifiers). Greases are classified in terms of their principal components: thickener and


base oil. They are produced in various consistencies, which are defined as NLGI grades. These are determined by the ‘worked penetration’ of the grease according to ISO 2137. The higher the NLGI grade, the harder the grease. Preferred greases for rolling bearings are those with NLGI grades of 1, 2 or 3.


SELECTING THE RIGHT GREASE Bearing type: A distinction needs to be made between point contact (ball bearings) and line contact (needle roller bearings and cylindrical roller bearings). In ball bearings, each overrolling


motion at the rolling contact places strain on only a relatively small volume of grease. In addition, the rolling kinematics of ball bearings exhibit only relatively small proportions of sliding motion. The specific mechanical strain placed on greases in bearings with point contact is therefore significantly less than in bearings with line contact. Typically, greases with a base oil viscosity ISO VG 68 to 100 are used. In rolling bearings with line contact,


higher requirements are placed on the grease. Not only is a larger grease quantity at the contact subjected to strain, but sliding and rib friction is also to be expected. This prevents the formation of a lubricant film and would therefore lead to wear. As a countermeasure, greases should be selected that exhibit a higher base oil viscosity (ISO VG 150 to 460 or higher). Anti-wear additives may also be required and consistency is normally NLGI 2. Speed: The speed parameter of the


bearing should always be a good match for the speed parameter of the grease. This depends on the type and proportion


/ DESIGNSOLUTIONS


of the thickener, the base oil type and the proportion of base oil. The speed parameter of a grease is not a material parameter but depends on the bearing type and the required minimum running time. As a general guide, for rolling bearings rotating at high speeds or with a low requisite starting torque, greases with a high speed parameter should be selected. For rolling bearings rotating at low speeds, grease with a low speed parameter is recommended. Temperature: The temperature range of the grease must correspond to the range of possible operating temperatures in the rolling bearing. The operating temperature range is dependent on the type and proportion of thickener, the type and proportion of base oil, the production quality and production process. Load: For a load ratio C/P <10 or P/C


> 0.1, greases are recommended that have higher base oil viscosity and anti- wear additives. These additives form a reaction layer on the metal surface that provides protection against wear. These greases are also recommended for bearings with an increased proportion of sliding motion (including slow running) or line contact, as well as under combined radial and axial loads. There are, however, other factors that


also need to be considered, including: water and moisture; oscillations, shock and vibration; seals; mounting position; plus legal and environmental regulations (e.g. food-grade greases).


Schaeffler T: 0121 313 5870 www.schaeffler.co.uk Enter 226


DESIGN SOLUTIONS | MAY 2015 27


Failure statistics show that a significant proportion of premature rolling bearing failures are directly or indirectly related to the lubricant used, with the main causes being unsuitable lubricants (20%), aged lubricants (20%) and insufficient lubrication (15%)


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