Lubricating Grease Shear Stability

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Lubricating grease is one of the most important and irreplaceable lubricating material. It provides the reliability and durability of service performance of the wide variety of modern technical devices. The production of this kind of lubricants is significantly less than total production volumes of lubricating oils. However, the number of friction points being lubricated by lubricating greases is well above those that are lubricated by oils. The number of friction points for which greases are used increases every year, i.e. the field of their application is essentially broadening. The increasing of the field of application of greases at stable production level is a result of significant improvement of the quality of these products, and development of multifunctional long-operating (“eternal”) greases produced with highly efficient base oils, new thickeners and different additives.

Mechanical properties of greases and their service performance are predetermined by the nature and structure of their structural matrix largely. Greases undergo different kinds of deformations while being filled into the friction points and, especially, during their operation. An application of force exceeding the fluidity limit during the deformation of greases leads to the destruction of structural matrix, and, as a result, the firmness of the grease decreases.

The structure of grease can be restored in the time of less intensive deformation or rest. Therefore, during the friction point operation the processes of destruction and restoration of grease structure combined with the change of their mechanical properties take turns continuously. While being applied, the grease can be destroyed to such an extent that it will not be able to withstand forces, which are trying to throw it away from the lubricated device. Normal operation of a friction point can be ensured only in the case of grease having enough resistance to a mechanical destruction, i.e. grease being mechanically stable. Consequently, the change of volumetric and mechanical properties of grease, for example, the change of breaking point, as a result of mechanical handling and the following its rest is called shear stability.

Shear stability of greases is regulated by a standard. According to this state standard, the firmness limit for rupture is determined for initial undestroyed grease, for destroyed grease, and for the grease that has rested for some specific time. The standard also sets the determination of destruction index and shear stability index.

In accordance with this standard, with the help of a method for shear stability assessment rheological state of grease in a friction point is assessed. Breaking point for rupture of initial grease is characteristic of its state in a bearing before its operation and also for that part of grease which is located out of friction area during the operation. Grease in a friction area, for example, bearing raceway, can be relatively characterized (depending on its deformation degree) by breaking point of a destroyed grease; and after the stop of bearing, depending on its length, it can be described by breaking point after rest.

Different grease depending on their composition (disperse medium, dispersed phase, presence of fillers, additives, etc.) and technology of production have significantly different dynamics for breaking points change.

Shear stability is an important service performance characteristic of greases that are used in the roller bearings and especially of those, used in all kinds of joints, flat bearings, sliding bearings, since in this case the whole volume of grease is involved in operation. Mechanically unstable grease, i.e. the one that is heavily destroyed and that cannot renew its initial properties after removal of deforming loads can flow out the friction point. A significant sealing of grease after its destruction and the following rest is also unwanted. Dry friction can appear in this case because of the grease not leaking to a friction area.

There are also other methods for shear stability assessment. According to methodologies of ASTM D 217 – 1150 and FTMS 313 shear stability assessment is conducted by determination of grease penetration after 100000 double hits (the mixing in penetrometer’s mixer. The other method is also based on the evaluation of grease penetration before and after its destruction in a device of Roll Schel Test (ASTM D 1831).

All these methods have some lacks in common, including methodological ones (for example, the time of experiment), and also practical ones (limits to application and different efficiency for different kinds of greases). Thus, the device and methodological recommendations for assessment of shear stability of lubricating greases is being developed. It should eliminate the lacks of methods mentioned above. This methodology will allow technicians to optimize the assessment of shear stability and conduct the determination of it for different kinds of lubricating greases with a goal of creating recommendations for improvement of greases production and service performance.

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