Q: What are the main properties of bearing grease? What are their characteristics?

A: Bearing grease has the following main properties:


1) Thixotropy


The most basic characteristic of grease is thixotropy. When an external force is applied, the flow of the grease gradually softens, showing a decrease in viscosity, but once at rest, the consistency increases again after a period of time (very short). This characteristic is called thixotropy. This characteristic of the grease determines that it can be lubricated where it is not suitable for lubricating oil and thus shows its excellent performance.


2) Viscosity


The apparent or similar viscosity of a grease is usually expressed. The temperature and shear rate must be indicated when specifying the viscosity of the grease. Similar viscosity index can be used to control its low temperature fluidity and pumping performance.


3) Strength limit


The strength limit of grease is the minimum shear stress required to guide the sample to start flowing. The strength limit of grease is a function of temperature. The higher the temperature is, the smaller the strength limit of grease will be. The lower the temperature is, the greater the strength limit of grease will be.


4) Low temperature fluidity


One of the important indexes to measure the cryogenic performance of grease is cryogenic torque, that is, the degree to which the grease blocks the rotation of low-speed flowing bearing at low temperature (below -20°C). The cryogenic torque of grease is expressed by the average value of the starting torque and the torque after 60MM rotation.


5) Drop point


The minimum temperature at which the grease reaches a certain fluidity under specified conditions is called the drop point. The drop point of the grease can help to identify the type of grease and roughly estimate the maximum service temperature of the grease. Generally speaking, for soap base grease, its service temperature should be lower than 20~30°C drop point, the higher the drop point, the better the heat resistance.


6) Evaporation


The evaporability (degree) of the grease indicates the degree of volatilization of the grease when the grease is used for a long time under the condition of high temperature. The less the evaporability, the better. The evaporability of the grease depends mainly on the properties of the lubricating oil and the fraction composition.


7) Colloidal stability


The colloid stability of the grease refers to the stable colloid structure of the grease at a certain temperature and pressure to prevent lubricating oil from separating out of the grease, that is, the ability of the grease to resist oil separation. The amount of oil precipitated from grease is usually converted into mass fraction. The colloid stability of grease reflects the grease in long-term storage and practical application of time oil trend, if the colloid stability of grease is poor, under the action of heat, pressure, centrifugal force and other easy to occur serious oil, leading to a rapid decline in life, and make the grease thickened and dried, lost lubrication.


8) Oxidation stability


Oxidation stability refers to the ability of grease to resist the action of heat and oxygen during long-term storage or long-term use at high temperature, so as to keep its properties from permanent changes. As a result of oxidation, free alkali content is often reduced or free organic acid content is increased, drop point drops, appearance color becomes dark, strange odor appears, consistency, strength limit, similar viscosity drops, corrosive products are generated and damage the structure of the grease material, resulting in soap oil separation. Therefore, in the long-term storage of grease, it should be stored in a dry and ventilated environment to prevent sunlight exposure, and it should regularly check the change of free alkali or free organic acid, corrosive and other items to ensure its quality and performance.