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The effect of viscosity of lubricating oil on its performance in grease filling equipment

Release time:

2023-06-27

The effect of viscosity of lubricating oil on its performance in grease filling equipment

In grease filling equipment, the viscosity of lubricating oil decreases with the increase of temperature, so the same type of lubricating oil has different viscosity due to different temperatures. This property is called the "viscosity temperature characteristic". The viscosity temperature characteristics of lubricating grease are more complex than those of lubricating oil, as the viscosity temperature characteristics of this structural system also change with changes in shear force.

The viscosity of lubricating grease at a certain temperature is a variable that varies with the variation of shear speed. This viscosity is called similar viscosity, and the Unit of measurement is pascal. second (Pa • s). The similar viscosity of lubricating grease does not follow Newton's law of liquid flow and decreases with the increase of its shear speed. Due to the relative motion between the layers of lubricating grease, the structural framework is destroyed. Therefore, the higher the shear speed, the heavier the structural framework is destroyed, and the greater the decrease in similar viscosity of lubricating grease. As the shear speed continues to increase, the similar viscosity of the lubricating grease approaches the viscosity of the base oil and no longer changes, while maintaining Newtonian liquid properties. The variation law between the similar viscosity and shear rate of lubricating grease is called the viscosity velocity characteristic. The more bitter the viscosity changes with shear rate, the greater the energy loss. The low temperature usage limit of lubricating grease can generally be determined based on the allowable value of similar viscosity of lubricating grease at low temperatures.

The similar viscosity of lubricating grease also decreases with the increase of temperature, but the force is slow, only a few hundred or a few thousand times that of the base oil. Because the resistance of lubricating grease during flow is partly determined by the strength of the skeleton structure, which is less affected by temperature, the viscosity temperature characteristics of lubricating grease are better than those of lubricating oil Generally speaking, the viscosity change of lubricating grease within the temperature range of use is much smaller than that of base oil. The viscosity temperature properties of lubricating 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 little relationship with the viscosity of the base oil.

The viscosity of lubricating grease at low shear rates is closely related to the starting of the lubricated friction components. Due to the high viscosity of lubricating grease at low shear rates, excessive viscosity of lubricating grease at this time can increase starting resistance. Especially at low temperatures, the increase in viscosity of lubricating grease can affect low-temperature starting and even cause difficulties. In fact, during mechanical start-up, the force required to overcome the flow resistance of lubricating grease at low shear speeds is much greater than the force required to overcome the strength limit. For example, the Shear stress limit of a lithium grease at 40 ℃ is not more than 686.5Pa (7gf/cm2), while its flow resistance at 2.5s-1 at the same temperature is 2452Pa (25gf/cm2). It can be seen that the viscosity of the grease at low temperature and low shear speed has a greater impact on the low-temperature starting performance of the grease. For lubricating grease used at low temperatures or over a wide temperature range, its low temperature viscosity needs to be specified.