The greater the viscosity of the grease, the better to judge whether the grease is good or bad?
April 09, 2022
Recently received a customer inquiry, is the viscosity of the grease better? In fact, the viscosity of grease at a certain temperature is a variable that changes with the change of shear rate. This viscosity is called similar viscosity. It is not enough to judge the quality of the grease from the higher the viscosity, the better.
First of all, let's take a look at what is the similar viscosity of grease? Similar viscosity is the viscosity measured at a certain temperature and a certain shear rate. It reflects the variable that the viscosity of the grease changes with the shear rate at a certain temperature, the unit is: Pa·S, and the similar viscosity of the grease decreases with the increase of the shear rate. When the shear stress exceeds its strength limit, the grease will flow, and internal friction will also occur when the grease flows. Similar viscosity characterizes the internal friction characteristics of the grease. The viscosity of lubricating grease is not exactly the same as that of ordinary liquids. The viscosity of ordinary liquids is a constant at a certain temperature and does not change with the shear velocity between the liquid layers. Ordinary liquids move according to Newton's law of fluid flow. The flow of grease does not obey Newton's laws of fluid motion. Its viscosity during flow is not a constant at a certain temperature, but a variable that changes with the shear rate between lipid layers. When the shear speed is small, its viscosity is large; when the shear speed increases, its viscosity becomes small; when the shear speed is large, its viscosity is small to a certain extent and remains constant. Similar viscosity of grease is an important basic characteristic of grease, which affects the performance of grease in machinery. When grease lubricates bearings and other friction parts, its internal friction replaces the solid friction between the friction surfaces of the parts. Therefore, the viscosity of the grease has a great influence on the power consumption of the machine using the grease. If a grease of similar viscosity is used, obviously the friction loss is also larger. The nature of the viscosity of grease changing with shear speed makes it very adaptable when used on machinery with frequent changes in speed. When the speed is high, the viscosity of the grease is required to be low. At this time, the directional structure of the grease fiber is severely damaged, and the viscosity becomes low at this time. When the speed is slow, the viscosity of the grease is required to be higher, and the viscosity of the grease is also higher when the shear rate is low. The change of grease viscosity with shear speed basically meets the requirements of mechanical speed change for grease viscosity. The viscosity of the grease at very low shear speeds has a lot to do with the start-up of the lubricated friction parts. Since the viscosity of the grease is large when the shear speed is small, if the viscosity of the grease is too high at this time, the starting resistance will be increased. Especially at low temperature, the similar viscosity of the grease increases, which will affect the low temperature start-up, and even make it difficult to start. In fact, at mechanical start-up, the force required to overcome the flow resistance of the grease at small shear rates is much greater than the force required to overcome the strength limit. It can be seen that the similar viscosity of lubricating grease is not as large as possible, nor can it represent the quality of lubricating grease. Therefore, it is the best way to select lubricating grease by considering various conditions and working conditions.
