Decision-Support System for Bearing Lubrication

Gerard Schram, SKF
Tags: bearing lubrication

Selecting the correct lubricant for a particular bearing is a crucial step for rolling element bearing longevity. Knowledge of greases and oils in association with bearing operation is essential. This knowledge has been encapsulated into an Internet-enabled computer program called LubeSelect (www.aptitudexchange.com). The program allows users to choose from a range of grease and oil types dependent on the particular operating conditions of the bearing application.

Appropriate lubrication is crucial in bearing applications. SKF has developed deep knowledge of this topic through fundamental understanding of lubrication, empirical test results and application experience. Lube Select, a knowledge-based system, contains the company’s understanding of bearing lubrication. The system ranks greases and oil types based on their relevance to the application conditions.

Companies need to differentiate themselves with high-quality products, and by exploiting their unique knowledge. Knowledge management introduces ways to optimize the creation and flow of knowledge through the company, for example, the use of expert systems for decision-support. Knowledge can be represented in best practice examples (generalized rules of physical relations with empirical parameters). Different knowledge-based solutions are available depending on the nature and maturity of the knowledge. In line with the dot.com revolution, expert systems can be accessed on the Web.

Lubricant selection has traditionally been based on oil viscosity only. The goal of adequate lubricant viscosity was to build up a thin film of lubricant to prevent direct metallic contact between the 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 (online) catalogs. The ratio of the viscosity of a particular (base) oil at the operating temperature to the required viscosity needed in the bearing application is called the viscosity ratio (denoted by a kappa). A rule of thumb is that values between 1.0 and 2.5 are desired, but this depends on various other bearing and lubricant factors as well, as discussed below.

A rudimentary expert system, based on viscosity ratio calculations, was developed for grease selection. That system served as a base for recommendations that further differentiated between greases by adding selection criteria other than the viscosity ratio.

A recently developed database, LuBase, contains product information on a wide range of greases. The system allows for simple 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.

Although the viscosity ratio is still a useful parameter, the complexity of grease lubricant performance with special additives, solids, thickeners, base-oil types and other factors is not fully represented. Instead, one must consider all performance characteristics of the grease lubricants light of the bearing application.

Searching on consistency, thickener type, etc. is a step in the right direction. However, the most important translation from application conditions to desired grease properties is missing. LuBase and other systems operate by allowing users to search through a grease database. LubeSelect, on the other hand, provides lubricant suggestions based on application conditions.

Working Principles
LubeSelect can be consulted in two ways:

  1. Via application conditions. The conditions describe the bearing environment and are mapped to the performance characteristics of individual greases or oil types. The best-rated lubricants are ranked first.
  2. Via application profiles. For a range of typical bearing applications, grease advice is formulated based on proven-practice examples.


Figure 1. Lubricant Selection via Application Conditions.
Enables Following Conditions to be Specified:
Vertical Shaft, Outer Ring Rotation; and
Special Requirements such as High Rust
Protection, Low Noise, Biodegradability, etc.

Selection via Application Conditions
The first step in selecting a lubricant is to get basic information about the bearing application. The user supplies about 30 application parameters. For particular requirements, the user can differentiate importance. Requirements are high rust protection, low noise, biodegradability, etc.

If application conditions are outside the standard range or special conditions, a wider range of greases is considered. These preferred greases are differentiated by their individual performance on bearing-specific criteria. Assuming a range of criteria, specific performance tests are applied (such as a test-rig for noise level measurements, or one of the many lubricant standard tests). After considering the application conditions, the grease’s total score is determined as a fuzzy number between 0 and 1 (no fit = 0, full fit = 1).

The relevant fuzzy logic calculations are selected from fuzzy decision theory - a computational intelligence methodology. In essence, this means that:

  • The scoring on different performance criteria is (weighed) averaged,
  • The user selects importance factors to weigh the criteria, and
  • A score of zero on one criterion cannot be compensated by other criteria.

After assessment, the grease lubricants are ranked according to their individual scores. The final scores are expressed in percentages. The grease’s rating, including its scoring is the key outcome of selection advice (Figure 2). A range of non-SKF grease lubricants is included as well. This range can be extended by consulting the SKF Engineering and Research Center about the criteria.


Figure 2. LubeSelect Suggests a Range of
Alternative Lubricants. Apart From the Scoring, Viscosity
Ratio and Grease Life Values are Also Given. Individual
Information and Reporting Functions are Provided
by Clicking on One of the Grease Designations.

The system is further extended by various rules for exception handling and ranking. The recommended grease(s) are accompanied by calculated grease life (or relubrication interval), viscosity ratio, 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 (Figure 3). The additional information is provided for the oil types as well. With the information, the user can select a particular oil from any supplier.

 

 


Figure 3. LubeSelect Suggests a Range of Alternative Oil
Types. Depending on the Viscosity Ratio at Operating
Temperature, the Required Oil Viscosity is Provided.
Additional Input is Given Concerning the Viscosity
Ratio at Peak Temperature and the Start-up Torque
at Minimum Temperature.

Selection via Application Profiles
Another approach in selecting a lubricant is to consider proven-practice application profiles. In cooperation with SKF business segments, lubrication experts, application engineers and about 50 application profiles (for example, pulp and paper applications) are included.

The introduction of a knowledge-based system such as LubeSelect is an effective way to apply key lubrication knowledge to daily decisions. Interactive advisory systems in related knowledge areas are in development (http://www.aptitudexchange.com).


Figure 4. Application Profile Report for
Suction Rolls in a Paper Mill

Editor’s Note
LubeSelect does not factor some product qualities that could impact a lubrication selection decision. This includes factors such as grease oxidation stability, worked stability, wash resistance, lubricating properties of the thickener, etc.

While LubeSelect is a significant accomplishment and is likely a useful tool for the correct product selection, there is still room for the input from the supplier to assist in selecting a best-fit application.


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