Characteristics of Grease
author: Ann
2024-12-31
Thixotropy of grease
The thixotropy of grease refers to the property that when the grease is sheared, the viscosity decreases and softens, and when the shear force stops, the viscosity gradually recovers. When the grease is sheared, the contact parts between the individual soap fibers that constitute the continuous skeleton begin to slide and disengage, causing the system to flow from deformation. Under long-term or high shear, the soap fibers themselves will also be damaged and cut, so the viscosity decreases. After the shearing stops, the structural skeleton begins to recover. However, it takes a certain amount of time for the soap fibers to rearrange themselves, so the viscosity recovery is a slow process, and the re-formed skeleton is also different from the original one. For example, as the contact points of the soap fibers decrease, the strength of the structural skeleton is lower than before the damage, and the viscosity decreases. Conversely, as the number of soap fibers increases, the contact points increase, and the viscosity is greater than the original.
Rheological properties of grease
The relationship between shear rate and shear force of Newtonian and non-Newtonian fluids is the flow and deformation characteristics of grease when subjected to external forces, which are mainly manifested as follows:
(1) When grease is not subjected to external forces, it can maintain a certain shape like a solid, that is, it will not automatically flow when stationary.
(2) When subjected to a weak external force, it produces elastic deformation; after the external force is removed, it can return to its original position and shape, showing the elastic properties of a solid.
(3) When the external force applied is large enough, the grease deforms and flows, and it can no longer automatically return to its original position and shape. Therefore, the starting torque of grease on the mechanical moving parts is greater than that of liquid lubricants.
(4) During the flow of grease, as the shear stress increases, the soap fibers are oriented to varying degrees, which will cause the apparent viscosity (or similar viscosity) of the system to decrease. At this stage, the apparent viscosity of the grease decreases with the increase of shear rate.
(5) Under extremely high shear stress (high shear rate), the flow of grease is like that of Newtonian fluid, and the viscosity can remain constant and no longer change with the change of shear rate.
The significance of rheological properties and thixotropy of grease
The rheological properties and thixotropy of grease are of great significance to the use of grease. In the lubrication process of gears and bearings, the viscosity of grease decreases due to the relative sliding or rolling of the friction pair. Under the action of high shear force, the grease on the friction surface can form a fluid state, which is beneficial to the lubrication of mechanical parts. Once the operation stops, the viscosity of the grease returns to a certain level. For bearings, the grease can be kept inside the bearing without leakage; for gearboxes, the grease that has returned to a certain viscosity can play a sealing role to avoid leakage of the gearbox.
The thixotropy of grease refers to the property that when the grease is sheared, the viscosity decreases and softens, and when the shear force stops, the viscosity gradually recovers. When the grease is sheared, the contact parts between the individual soap fibers that constitute the continuous skeleton begin to slide and disengage, causing the system to flow from deformation. Under long-term or high shear, the soap fibers themselves will also be damaged and cut, so the viscosity decreases. After the shearing stops, the structural skeleton begins to recover. However, it takes a certain amount of time for the soap fibers to rearrange themselves, so the viscosity recovery is a slow process, and the re-formed skeleton is also different from the original one. For example, as the contact points of the soap fibers decrease, the strength of the structural skeleton is lower than before the damage, and the viscosity decreases. Conversely, as the number of soap fibers increases, the contact points increase, and the viscosity is greater than the original.
Rheological properties of grease
The relationship between shear rate and shear force of Newtonian and non-Newtonian fluids is the flow and deformation characteristics of grease when subjected to external forces, which are mainly manifested as follows:
(1) When grease is not subjected to external forces, it can maintain a certain shape like a solid, that is, it will not automatically flow when stationary.
(2) When subjected to a weak external force, it produces elastic deformation; after the external force is removed, it can return to its original position and shape, showing the elastic properties of a solid.
(3) When the external force applied is large enough, the grease deforms and flows, and it can no longer automatically return to its original position and shape. Therefore, the starting torque of grease on the mechanical moving parts is greater than that of liquid lubricants.
(4) During the flow of grease, as the shear stress increases, the soap fibers are oriented to varying degrees, which will cause the apparent viscosity (or similar viscosity) of the system to decrease. At this stage, the apparent viscosity of the grease decreases with the increase of shear rate.
(5) Under extremely high shear stress (high shear rate), the flow of grease is like that of Newtonian fluid, and the viscosity can remain constant and no longer change with the change of shear rate.
The significance of rheological properties and thixotropy of grease
The rheological properties and thixotropy of grease are of great significance to the use of grease. In the lubrication process of gears and bearings, the viscosity of grease decreases due to the relative sliding or rolling of the friction pair. Under the action of high shear force, the grease on the friction surface can form a fluid state, which is beneficial to the lubrication of mechanical parts. Once the operation stops, the viscosity of the grease returns to a certain level. For bearings, the grease can be kept inside the bearing without leakage; for gearboxes, the grease that has returned to a certain viscosity can play a sealing role to avoid leakage of the gearbox.
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