The viscosity of a lubricant decreases as the temperature rises, so the same lubricant has a different viscosity depending on the temperature. This is called the “viscosity-temperature characteristic”. The viscosity-temperature characteristics of grease are more complex than those of lubricants because the viscosity-temperature characteristics of this structural system also change with shear force. The viscosity of a grease at a certain temperature is a variable that changes with the change in shear rate. This viscosity is called similar viscosity, the international unit of measurement for Pa (ska) . Seconds (Pa-s). Similar viscosity of grease does not obey the Newtonian law of liquid flow, with its shear speed increases and decreases. As the relative movement between the layers of grease, the structure of the skeleton is destroyed, so the higher the shear speed, the heavier the destruction of the structure of the skeleton, the grease similar viscosity reduction is greater. When the shear speed continues to increase, grease similar viscosity close to the viscosity of the base oil will no longer change, and maintain the Newtonian liquid properties. Grease similar viscosity and shear speed change law called viscosity – speed characteristics. Viscosity with shear speed change more obvious bitter, its energy loss is greater. Generally, according to the allowable value of similar viscosity of grease in low temperature to determine the low temperature use limit of grease. The similar viscosity of grease also decreases with the rise of temperature, but the more force slowly, only a few percent or a few thousandth of the base oil. Because the resistance of 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 lubricant. -Generally speaking, the change of viscosity of grease in the use temperature range is much smaller than that of base oil. The viscosity-temperature properties of grease is determined by the properties and amount of thickening agent used, as well as the phase properties of the soap-oil system, and the viscosity of the base oil is less relevant. The viscosity of grease with the shear speed change of nature, so that it in the speed often change the use of machinery with special adaptability. When the speed is high, the viscosity of the lubricant is required to be low, when the structure of the grease is damaged intensified, the fiber is oriented, and the viscosity happens to become low. When the speed is slow, the viscosity of the lubricant is required to be larger, and the viscosity of the grease is relatively large when the shear speed is low. The change of grease viscosity with shear speed is basically in line with the requirement of lubricant viscosity for mechanical speed change. The viscosity of grease at very small shear speed is highly related to the starting of the lubricated friction parts. Since the viscosity of grease is large when the shear speed is small, the viscosity of grease will increase the starting resistance. Especially in low temperature, the viscosity of the grease increases, more low temperature starting will be affected, and even cause difficulties. In fact, when the machinery starts, the force required to overcome the flow resistance of the grease in the shear speed is much larger than the force required to overcome the strength limit. For example, a lithium grease in a 40 ℃ shear stress limit is not more than 686.5 Pa (7gf/cm2), and it is at the same temperature in 2.5s-1 when the flow resistance of 2452 Pa (25gf/cm2), it can be seen that the viscosity of the grease in the low temperature low shear speed for the grease of low temperature starting performance. For low temperature or wide temperature range with grease need to specify its low temperature viscosity.  

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