Effect of viscosity of Lubricating Grease on bearing performance

The viscosity of lubricating oil decreases with the increase of temperature, so the viscosity of the same kind of lubricating oil is different because of different temperature. This property is called "viscosity-temperature characteristic". The viscosity-temperature characteristic of grease is more complex than that of lubricating oil, because the viscosity-temperature characteristic of this structural system also changes with the change of shear force.

The viscosity of grease at a certain temperature is a variable that changes with the change of shear speed. This viscosity is called similar viscosity, and the international unit of measurement is Pa (ska). Seconds (Pa s). The similar viscosity of grease does not obey Newtonian liquid flow law and decreases with the increase of shear speed. Due to the relative motion between the grease layers, the structural skeleton is destroyed, so the higher the shear speed is, the more serious the structural skeleton damage is, and the greater the decrease of the similar viscosity of the grease is. When the shear speed continues to increase, the similar viscosity of the grease will no longer change when it is close to that of the base oil, but the Newtonian liquid properties will be maintained. The variation law of similar viscosity and shear velocity of grease is called viscosity-velocity characteristic. The more bitter the viscosity changes with the shear rate, the greater the energy loss. Generally, the low temperature service limit of the grease can be determined according to the allowable value of similar viscosity of the grease at low temperature.

The similar viscosity of grease also decreases with the increase of temperature, but it is only a few hundredths or one thousandth of that of base oil. Because the resistance of grease flow is partly determined by the strength of the skeleton structure, and the skeleton structure is less affected by temperature, the viscosity-temperature characteristic of grease is better than that of lubricating oil. Generally speaking, the change of viscosity of grease in the range of service temperature is much smaller than that of base oil. The viscosity-temperature property of grease depends on the property and amount of thickener used, as well as the phase properties of soap oil system, but has little relationship with the viscosity of base oil.

The viscosity of grease varies with shear speed, which makes it have special adaptability when used in machinery with frequently changing speed. When the speed is high, the viscosity of the lubricant is required to be low, when the structural damage of the grease is intensified, the fiber is oriented, and the viscosity just becomes lower. When the speed is slow, the viscosity of the lubricant is higher, while the viscosity of the grease is higher when the shear speed is low. The change of grease viscosity with shear speed basically meets the requirements of mechanical speed change on lubricant viscosity.

The viscosity of grease at low shear speed has a lot to do with the starting of lubricated friction parts. Due to the high viscosity of the grease at the shear speed, such as excessive viscosity of the grease will increase the starting resistance. Especially when the viscosity of grease increases at low temperature, the starting at low temperature will be affected and even difficult. In fact, when the machine starts, the force needed to overcome the flow resistance of grease at small shear speed is much greater than that needed to overcome the strength limit. For example, the shear stress limit of a lithium grease at-40 ℃ is not greater than 686.5Pa (7gf/cm2), but its flow resistance is 2452Pa (25gf/cm2) at 2.5s-1 at the same temperature, thus it can be seen that the viscosity of grease at low temperature and low shear speed has a great influence on the low temperature starting performance of grease. The low temperature viscosity of grease used at low temperature or wide temperature range needs to be specified.