Greases are preferred lubricants in rolling bearings and their performance strongly influences the bearing performance. More than 90% of the total number of rolling bearings are grease lubricated. The rest is lubricated with oil for better results (low friction). Lubricating greases are highly structured colloids, composed of base oil, thickener and performance enhancing additives. A good performing grease (oil) gives low friction, low wear rates, a high resistance to corrosion and/or a long life. In addition, greases also have sealing properties. Many of bearings are sealed for life. These bearings are filled with only 30 to 40% of its free volume with grease. This leads to a low level of friction (and therefore low heat development and therefore long grease life), which can only be obtained if the raceways are cleared with grease. After all, grease in the raceways leads to churning and therefore drag forces. An optimum filling and optimum grease provides the raceways with lubricant (mainly base oil) such that layers are created with a thickness in the order of magnitude of the EHL film thickness. These films are very thin and the required feed of lubricant from the un-swept area to maintain these layers can therefore be very low. A good performing bearing has therefore a completely filled un-swept area in which the rheology of the grease is such that the bulk of the grease stays there, while slowly releasing lubricant to the tracks.
This is not the initial situation after filling the bearing with grease. Much of the grease will be located in-between the rolling elements. During the early rotation of sealed for life bearings, the grease is therefore moved by the balls/rollers and rings of the bearing. This results in flow of the grease. Grease will be pushed forward but also sideways into the un-swept area. Part of it will flow back into the tracks. After all, due to the non-uniform initial distribution, some of this grease will push against the grease that is already there. This grease cannot be pushed any further because it will be obstructed by the seals and can therefore only flow back into the track. The complexity of the flow is further increased by the fact that the radial surface speeds of the rolling elements in the cage pockets have opposite signs creating a flow circulation in these areas. This non-uniform flow is called “churning”. Churning continues until most of the grease is pushed away from the swept area side-ways on the bearing shoulders and onto the seals/shields forming so-called reservoirs.
This side flow is called “channeling”. After all, the balls/rollers are ploughing themselves through the grease, ultimately forming a channel. Another term that is often used is “clearing” : the balls/rollers clear the raceways from (bulk) grease. Some of the grease will end up under the cage forming a reservoir there.
This churning of the grease, results in high shear leading to mechanical degradation of the grease . Bearings covered by shields showed more grease movement with longer churning periods due to the relatively small unswept area. The high drag forces also lead to an increased temperature. This combination of high temperature and large shear causes so-called “thermo-shear induced grease degradation”. The property that describes the resistance to this degradation is the “mechanical stability” of the grease. Flow of the grease is governed by (apparent) viscosity and the onset of flow by the yield stress. Both will change by this degradation. It is believed that the duration of the churning phase is governed the yield stress of the grease and its change during churning.
In general, the degree of grease degradation is influenced by the chemical composition of the grease, the temperature, shear rate and time of shearing. Hutton defines the effectiveness of clearing/channeling in three levels:
1.Good channeling/clearing: this results in a short churning phase, leading to only a short increase of the bearing temperature in the initial stage of bearing operation, followed by a more-or-less constant temperature where the height of the temperature is low, due to good lubrication.
2.Poor channeling/clearing: this results in a long churning period, leading to a higher bearing temperature for a longer period. This is caused by track replenishment from the grease located in the un-swept area of the bearing. This continuous flow/shear leads to more grease degradation.
3.Non channeling: the grease that is pushed into the un-swept area does not stay there, which causes a continuous replenishment of grease in the running tracks. This results in a continuous mechanical degradation and softening leading to grease leakage from the bearing.
The channeling properties are expected to be important during the churning phase. The change of grease properties during this phase will also have an effect on the quality of lubrication during the bleeding phase and therefore grease life.
Text; MSMechanic and Science Research portal