If a bearing is to live up to its design specification, it must have the proper lubricant for its application. Selecting the correct lubricant can only be done with a fundamental, and thorough, understanding of grease lubrication and formulations, from empirical test results and application experience.
Engineers with SKF have considerable experience and knowledge of lubricants and the ways they perform in bearing applications. Now, this knowledge has been captured in a computer program called SKF LubeSelect, so customers worldwide can seek advice on their specific applications.
SKF is one of the world’s largest developers and suppliers of bearings systems. The SKF LubeSelect program can be accessed at www.aptitudexchange.com.
Evolution of lubricant selection
Traditionally, choosing a lubricant has been based on the ratio between the desired and the actual base-oil viscosity, the goal being to build up a thin film of lubricant to prevent direct metallic contact between rolling elements, raceways, and cages. Depending on the various bearing operating conditions, the required oil viscosity can be determined with bearing manufacturers’ recommendations with the help of (on-line) catalogues. The ratio between the viscosity of a particular (base) oil at the operating temperature, and the required viscosity needed in the bearing application, is called the viscosity ratio (denoted by k, the Greek letter ‘kappa,’).
In the late 1980s, a rudimentary expert system based on viscosity ratio calculations was developed for grease selection. The system served as a base for developing recommendations that further differentiated between greases by adding selection criteria other than viscosity ratio.
More recently, a database — called SKF LuBase, also available at www.aptitudexchange.com — has been developed, containing product information on a wide range of greases. This system allows simple searches — e.g., searches based on viscosity or consistency. Other bearing suppliers have introduced similar databases and a search facility based on consistency, thickener type, base-oil type and viscosity ratio.
The viscosity ratio is still a useful parameter, especially for oil lubrication. But because of the complexity of modern grease lubricants, with their special additives, solids, thickeners, base-oil types and other factors, the viscosity ratio does not completely represent the performance of a grease lubricant today. Instead, all the performance characteristics of the grease lubricant must be considered carefully in light of the bearing application.
Searching by consistency, thickener type, and other values, is a step in the right direction. But, the most important translation from application conditions to the desired grease properties was missing. SKF LuBase and other systems operate by allowing users to search through a grease database. In contrast, SKF LubeSelect provides lubricant suggestions based on application conditions can be consulted two ways.
1. Application conditions. The conditions describe the bearing environment as completely as possible, and are mapped onto the performance characteristics of individual greases or oil types. Statistically suitable lubricants are ranked for reference.
2. Application profiles. For a range of typical bearing applications, grease advice is formulated based on proven-practice examples.
Selection via application conditions
The first step in selecting a grease is to get basic information about the bearing application. The program works with input from about 30 application parameters, supplied by the customer. For particular requirements, the customer can differentiate according to their importance. For example, high rust protection, low noise, and biodegradability. Given the conditions, standard greases are checked by generalized rules and, when applicable, recommended first.
If the application conditions are outside the standard range or special conditions apply, a wider range of SKF-tested greases are considered. These preferred greases are differentiated according to their individual performance on bearing-specific criteria. Assuming a range of criteria, specific performance tests are applied. After considering the application conditions, the total score of a grease is determined as a “fuzzy” number between 0 and 1 (no fit = 0, full fit = 1) out of the individual scorings of the required criteria.
The relevant fuzzy logic calculations are selected from fuzzy decision theory, a computational intelligence methodology. Essentially, this means that:
After assessment, the greases are ranked according to their individual scores. The final scores are expressed in percentages by multiplying by 100%. A rank of the greases, including their scoring, is the key outcome of the grease selection advice. A range of non-SKF grease lubricants is included as well. This range can be extended; consult the SKF Engineering and Research Center about the criteria.
The system is further extended by various rules for exception handling and ranking, for example, for specific greased-for-life bearing variants. The recommended greases are accompanied by calculated grease life (or re-lubrication interval), viscosity ratio, warnings on high start-up (viscous) friction, grease quantity, and other grease information.
The oil (type) lubrication advice is mainly based on temperature and viscosity ratio. The result is a list of possible oil types, and the desired viscosities. The additional information is provided for the oil types as well. With this information, the user can select a particular oil from any supplier.
Selection via application profiles
A completely different approach of selecting a grease is to consider proven-practice application profiles. In cooperation with SKF business segments, lubrication experts and application engineers, about 50 application profiles (according to industry) with recommended greases are offered. The profiles can be selected from straightforward standard descriptions.