Long reading but here is support for the oil from the grease is what really lubricates Your bearings.
1.1.4 Lubricating Film with Grease
With lubricating greases, bearing lubrication is mainly effected by the base oil, small quantities of which are separated
by the thickener over time. The principles of the EHD theory also apply to grease lubrication. For calculating the viscosity ratio = /1 the operating viscosity of the base oil is applied. Especially with low values the thickener and the
additives increase the lubricating effect.
If a grease is known to be appropriate for the application in hand – e.g. the FAG Arcanol rolling bearing greases (see
page 57) – and if good cleanliness and sufficient relubrication are ensured the same K2 values can be assumed as for
suitably doped oils. If such conditions are not given, a factor from the lower curve of zone II should be selected for determining the a23II value, to be on the safe side. This applies especially if the specified lubrication interval is not observed.
The selection of the right grease is particularly important for bearings with a high sliding motion rate and for large and
heavily stressed bearings. In heavily loaded bearings the lubricating effect of the thickener and the right doping are of
Only a very small amount of the grease participates actively in the lubricating process. Grease of the usual consistency
is for the most part expelled from the bearing and settles at the bearing sides or escapes from the bearing via the seals.
The grease quantity remaining on the running areas and clinging to the bearing insides and outsides continuously separates the small amount of oil required to lubricate the functional surfaces. Under moderate loads the grease quantity remaining between the rolling contact areas is sufficient for lubrication over an extended period of time.
The oil separation rate depends on the grease type, the base oil viscosity, the size of the oil separating surface, the grease temperature and the mechanical stressing of the grease. The effect of the grease thickener becomes
apparent when the film thickness is measured as a function of operating time. On start-up of the bearing a film
thickness, depending on the type of thickener, develops in the contact areas which is clearly greater than that of the
Grease alteration and grease displacement quickly cause the film thickness to be reduced, fig. 13. In spite of a possibly reduced film thickness a sufficient lubricating effect is maintained throughout the lubrication interval. The thickener and the additives in the grease decisively enhance the lubricating effect so that no life reduction has to be expected.
For long lubrication intervals, the grease should separate just as much oil as needed for bearing lubrication. In this way, oil separation over a long period is ensured. Greases with a base oil of very high viscosity have a smaller oil separation rate. In this case, adequate lubrication is only possible by packing the bearing and housing with grease to capacity or short relubrication intervals.
The lubricating effect of the thickener becomes particularly evident in the operation of rolling bearings in the mixed friction range.
13: Ratio of the grease film thickness to the base oil film thickness as a function of operating time Grease film thickness
Base oil thickness
0 10 20 30 40 50 120
Lubricant in Rolling Bearings
Functions of the Lubricant in Rolling Bearings