Effect of grease viscosity on performance
The viscosity of lubricating oil decreases as the temperature increases, so the same lubricating oil has different viscosities due to different temperatures. This behavior is called “viscosity-temperature characteristic”. The viscosity-temperature characteristics of grease are more complex than that of lubricating oil, because the viscosity-temperature characteristics of this structural system also change with the change of shear force. The viscosity of grease at a certain temperature is a variable that changes with changes in shear speed. This viscosity is called similar viscosity, and the international unit of measurement is Pascal seconds (Pa·s). The similar viscosity of grease does not obey Newton’s laws of liquid flow and decreases as its shear speed increases. Due to the relative motion between the grease layers, the structural skeleton is destroyed. Therefore, the higher the shear speed, the more severe the structural skeleton is damaged, and the greater the reduction in the similar viscosity of the grease. When the shear rate continues to increase, the similar viscosity of the grease will no longer change after it approaches the viscosity of the base oil, but maintains Newtonian liquid properties. The change pattern of similar viscosity and shear speed of grease is called viscosity-speed characteristic. The more bitter the viscosity changes with shear speed, the greater the energy loss. Generally, the low-temperature service limit of the grease can be determined based on the allowable value of similar viscosity of the grease at low temperatures. The similar viscosity of grease also decreases as the temperature rises, but slowly, only a few hundredths or a few thousandths of that of the base oil. Because the resistance to grease flow is partly determined by the strength of the skeleton structure, which is less affected by temperature, the viscosity-temperature characteristics of grease are better than those of lubricating oil. Generally speaking, the change in viscosity of grease within the operating temperature range is much smaller than that of base oil. The viscosity-temperature properties of grease are determined by the characteristics and dosage of the thickener used, as well as the phase properties of the soap oil system, and have less to do with the viscosity of the base oil. The viscosity of grease changes with shear speed, making it particularly adaptable when used on machinery with frequently changing speeds. When the speed is high, the viscosity of the lubricant is required to be low. At this time, the structural damage of the grease is intensified, the fibers are oriented, and the viscosity becomes low. When the rotational speed is slow, the viscosity of the lubricant is required to be relatively high, while the viscosity of the grease is relatively high when the shear speed is low. The change in grease viscosity with shear speed basically meets the requirements for lubricant viscosity due to changes in mechanical speed. The viscosity of the grease at low shear speeds is closely related to the starting of the lubricated friction parts. Since the viscosity of grease is high at low shear speeds, if the viscosity of the grease is too high at this time, the starting resistance will increase. Especially at low temperatures, the viscosity of grease increases, which will affect low-temperature starting and even cause difficulties. In fact, when the machine is started, the force required to overcome the flow resistance of the grease at small shear speeds is much greater than the force required to overcome the strength limit. For example, the shear stress limit of a certain lithium grease at 40°C is not greater than 686.5Pa (7gf/cm2), and its flow resistance at the same temperature is 2452Pa (25gf/cm2) at 2.5s-1. It can be seen that , the viscosity of the grease at low temperature and low shear speed has a great influence on the low temperature starting performance of the grease. For greases used at low temperatures or within a wide temperature range, the low temperature viscosity needs to be specified.
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