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Entry type: FAQ Entry ID: 60681058, Entry date: 05/22/2012
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Structure and designation of grease for rolling-contact bearings

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QUESTION:

 

 

 

 

 

What are the constituents of lubricating grease for rolling-contact bearings and what do the grease designations mean?


ANSWER / REMEDY / INFORMATION:

     
   

The grease acts as a stable lubricating film designed to prevent dry friction in the bearing. The lubricating film is mainly formed by the oil in the grease. Operational and ambient conditions (e.g. speed, temperature, mechanical forces, vibrations, contamination, water, chemicals etc.) can vary significantly and must be taken into account when the grease is selected.

     
   

Grease for rolling-contact bearings comprises base oil, thickening agent and additives.

     
    The properties of the base oil (mineral or synthetic oil) govern the constitution of the lubricating film in the bearing. The thickening agent "holds" the oil, determines the oil separation properties of the grease and often consists of a metal soap (e.g. lithium-based). Additives can be used, for example, to alter the load bearing capacity and corrosion protection properties. The viscosity of the oil and the proportion of thickening agent determine the consistency of the grease.
     
   

The consistency of the grease is classified according to its level of penetration by NLGI (= National Lubrication Grease Institute).

Penetration into the grease within 5 s, measured using a test object in standardized test equipment.

The lower the number of the NLGI class, the "softer" the grease. Grease is also classified according to application temperature.

 

 

 

 

 

The lower service temperature defines the temperature as of which
the grease can be pressed out through a standardized nozzle (i.e. describes only its flow properties but is irrelevant with respect to lubricating properties).

 

 

 

 

 

The upper service temperature corresponds to the highest test temperature applied during standardized testing at which the grease will achieve a minimum service life defined in the standard (e.g. a service life in excess of 100 h is required for a lubricating grease service life F50
with FE9 test run A/1500/6000 according to DIN 51821).

 

 

 
   

For a really long service life / life lubrication, the maximum bearing operating temperature should always be around 20 – 30 K below the upper grease service temperature specified by the grease manufacturer.

     
    Example of lubricating grease designation defined by DIN 51 825:

 

   
Lubricating grease to DIN 51825  -   K 3 N -20  Li

      K:         Lubricating grease with mineral oil as base oil

       3:         Walk penetration class 3

      N:         Upper service temperature min. +140 °C ( "K" would be +120 °C)

    -20:        Lower service temperature -20 °C

      Li:         With lithium soap as thickening agent

 

     
   

The grease types specified in the operating instructions may be mixed
with the grease applied as the initial lubricant at the factory, exceed the requirements stipulated by DIN 51825 and therefore permit the specified greasing intervals.

If other greases are used that possibly fulfill only the minimum requirements laid down in DIN 51825, the greasing intervals must be halved.

     
    The specified regreasing intervals apply for normal loads, operation at the speeds specified on the rating plate, precision-balanced running, virtually neutral ambient air, and use of high-quality rolling-contact bearing grease as stated in the operating instructions.
     
     

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