What are lubricant-saturated polymer-matrixes and how do they work in application?

In essence, the term refers to a plastic structure with a multitude of very small interconnected cavities, made for example from PE. The advantage: a considerable amount of lubrication oil can be retained within the structure and is held there by the surface tension of the oil.

During manufacture, a polymer-lubricant mixture is injected into the free space of a bearing and upon completion of the hardening process, forms a solid structure. Therefore, small gaps form between the functional surfaces (rolling elements, raceways, cage) which enable the bearing to rotate freely. The bearing still has a cage, which is encased by the polymer-matrix, and acts as a stabilizing brace. During operation, the cage and the attached polymer-matrix rotate as one part, without any undue influence on the bearing’s operating behavior. 

Under the influence of centrifugal forces and temperatures, the polymer-matrix slowly releases the oil contained in the cavities. It is then deposited in optimal quantities directedly onto the surfaces which require lubrication. The release of oil increases with rising operating temperatures. At rest, part of the oil is reabsorbed into the polymer-matrix.   
Although lubrication oils are generally fully synthetic to allow for full exploitation of the lubrication system’s potential, different oils may be used depending on the manufacturer and application.
 

What are the main benefits of lubricant-saturated polymer-matrixes?

When compared to common grease lubrication, the polymer-matrix contains between two and four times as much oil. Therefore, when under difficult operating conditions and without the possibility for re-greasing, the bearing operating life can be increased considerably compared to standard, pre-greased bearings. 

As the lubricant attaches to and rotates with the bearing cage, constant displacement and working of the grease does not occur. Consequently, grease being pushed out of the bearing is minimized and bearing operating life is therefore increased. Additionally, contamination of the area surrounding the bearing with spilt grease – an undesirable side effect in certain industries, such as the electronics or food processing industry – is reduced.

The minimal loss of grease and the constant, focused provision of lubricant, even in adverse operating conditions, also improves the longevity of bearings in applications subject to high centrifugal forces and vibrations.

Furthermore, the lack of displacement and working of the grease reduces friction and rolling resistance, thereby increasing the operating efficiency of the application.
In the case of applications subject to very high humidity or with direct contact with liquid, the constant provision of lubricant from the polymer-matrix allows lubrication to be maintained under these very adverse operating conditions. The degrading emulsion of grease with water cannot occur and a washing away of the grease leading to a collapse of lubrication, and subsequent bearing failure, is effectively prevented. 

Finally, the polymer-matrix impedes the ingress of contamination into the bearing, as it typically fills the whole inside of the bearing.

From an engineering point of view however, the polymer-matrix itself should not be considered an effective seal - especially in applications with a high degree of contamination e.g. through dust or liquids. Depending on the application, additional seals in the bearing itself or immediately adjacent to the bearing should be used.

For which bearing types and applications can the lubricant-saturated polymer-matrix be used and what should be considered during the actual application?

Most premium bearing manufacturers can provide, upon specific order, the common radial bearing types with ball and roller rolling elements filled with a lubricant-saturated polymer-matrix. In general, bearing variants with a slim cage type (especially pressed steel cages) are preferred as they offer maximum free internal space to be filled with the lubricant.

Applications where this specific form of lubrication could be considered are those where:

• grease leakage / contamination with grease is not desirable or permissible (e.g. food processing, optical, electronics industry)

• a high level of humidity or direct fluid contact / pressure is to be expected, and seals against liquids cannot be used for reasons of friction and/or available space (e.g. food processing, packing industry, material handling)

• a significant increase in bearing operating life, combined with the bearing being maintenance-free is required (especially in cases with high centrifugal forces, when a bearing position is difficult to access, or where re-lubrication cannot be integrated into the application, e.g. material handling, textile machinery)  

• oscillating movement is prevalent and common lubricants are insufficient and/or lubrication methods are unsuitable to allow for a separation of the metallic surfaces

• a substantial decrease in friction compared with standard, pre-greased bearings is required (e.g. testing and measuring equipment)

Often a combination of several of the factors listed above may be a determining factor for the use of bearings with a lubricant-saturated polymer-matrix.

If the use of a bearing with a lubricant-saturated polymer-matrix is being considered, the following application factors should be observed:

• Technical characteristics of the lubricating oil used by the individual manufacturers (e.g. viscosity, approval for use in food processing) and suitability for the actual application requirements.
• Operating temperatures – depending on the manufacturer, the maximum permissible and long-term operating temperatures vary between 60°C and 100°C. If the bearings are to be mounted by heating, the mounting temperature should also be considered.   
• Operating environment which may require the use of additional seals in the bearing, bearing components made from stainless steel, etc.   
• Minimum operational load > 1% of the dynamic load rating
• Compatibility of the polymer matrix with chemical agents in the operating environment

*Always observe the individual manufacturer’s specifications!*

Which bearing manufacturers offer roller bearings with a lubricant-saturated polymer-matrixes?

Notable bearing manufacturers who can offer such products to specific order (under their own designations and with varying significant technical differences) are:

NSK – Molded Oil

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L11 (standard applications)

L12 (high speed applications)

NTN – Solid Grease

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LP03 (standard applications)

LP09 (food processing)

Schaeffler – Lubtect

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LT

SKF – Solid Oil

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W64

SNR – Lubsolid

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LSO (oscillating applications)

LSF (rotation, low temperatures)

LSHT (rotation, high temperatures)

LSV (vacuum)

LSA (food processing)

How can the use of bearings with a lubricant-saturated polymer-matrix be evaluated in practice?

Just based on the technical characteristics of this lubrication technology, it may be surprising that its use is not more common, with its application still lagging behind standard lubricants and lubrication methods. The reasons for this may be economic, as well as the limited, immediate availability of a broad range of products, which is important not only for initial widespread use but also for follow-up maintenance.

Header Image: iStock.com/Floaria Bicher