According to the working conditions of the grease parts for automobile, the basic requirements for grease: proper consistency, good heat resistance, water resistance, wear resistance, rust resistance, corrosion resistance and colloidal stability, etc.
Consistency refers to the degree to which plastic materials such as grease resist deformation when subjected to force. Consistency is a characteristic of plasticity and a general concept reflecting the deformation and flow resistance of grease.
The evaluation index of grease consistency is cone penetration, which is generally measured by cone penetration meter. The larger the cone penetration value, the weaker the structural force of the grease system, that is, the smaller the consistency, the easier it is to deform and flow.
Suitable consistency must be considered in selecting grease. The American Grease Association (NLGI) divides the grease into 9 grades according to the working taper of the grease at 25℃, which has been widely used internationally, and the specific classification method is shown in Table 2-22.
Table 2-22 NLGI Consistency classification and tapering range units :0.1 mm
2. Low Temperature Performance
cars used in cold areas, require grease can still maintain good lubrication performance. The low temperature performance of grease is evaluated by similar viscosity and low temperature torque.
Similar viscosity refers to the ratio of shear force to shear rate when grease flows at a certain temperature and shear rate, which generally decreases with the increase of temperature. Because the similar viscosity of grease is based on the two fixed conditions of temperature and shear rate, the two conditions should be indicated for the similar viscosity.
Low temperature torque indicates the degree to which grease can block the rotation of low speed ball bearings when used at low temperature. Generally, the minimum temperature of the bearing during one week of rotation within 1 min is measured with a torque of 9.8 N·cm, which is the minimum operating temperature of the grease.
3. high temperature performance
Temperature has a great influence on the fluidity of grease, the temperature increases, the grease becomes soft, so that the grease adhesion performance is reduced and easy to lose. In addition, it is easy to increase the evaporation loss, oxidation deterioration and oil separation of grease at higher temperature. Lubricating grease with good high temperature can maintain its adhesion at higher service temperature, and its deterioration failure process is also slow.
The main indexes to evaluate the high temperature of grease are drop point, evaporation and leakage.
The drop point of the grease is its minimum temperature at specified test conditions, and the grease reaches a certain fluidity. It mainly depends on the type and content of thickener. The higher the drop point, the better the heat resistance of the grease.
Evaporation of greases is the percentage of mass loss of greases due to evaporation under specified test conditions. the evaporation of the grease mainly depends on the type of base oil used, the fraction composition and molecular weight. the evaporation amount can qualitatively represent the upper limit use temperature of the grease.
Another index to evaluate the high temperature performance of grease is to measure the leakage of grease bearing. The greater the leakage, the worse the high temperature working performance of grease.
4. water resistance
The water resistance of the grease indicates the resistance of the grease to structural and consistency changes after water. After the grease absorbs water, it will dissolve the thickener and reduce the drop point, causing corrosion, thus reducing the protective effect.
Grease water resistance evaluation index is the amount of water loss, generally according to the SH/T 0109-2004《 grease water resistance determination method.
5. corrosion resistance
The corrosion resistance of grease refers to the performance of grease to prevent corrosion and corrosion of parts, which is generally evaluated by corrosion resistance test, corrosion test, determination of free alkali and free organic acid.
6. mechanical stability
Mechanical stability refers to the ability of grease to resist consistency changes under mechanical working conditions. It mainly depends on the strength of the thickener fiber itself, the attraction of the indirect contact of the fiber and the amount of thickener. The grease with poor mechanical stability is easy to dilute or even lose when used, which affects the life of the grease.
7. colloidal stability
Colloidal stability refers to the ability of grease to maintain colloidal structure against temperature and pressure, that is, the stability of base oil combined with thickener.
Grease is a colloidal dispersion system, the stability of colloidal structure is often affected by temperature, pressure and varying degrees of damage, so that the base oil fixed in the fiber space skeleton is separated out, serious will make grease deterioration. Therefore, it is necessary to have proper requirements for the oil separation of grease, and the amount of oil separation is an important index to evaluate the colloidal stability of grease.
8. oxidation stability
The oxidation of grease produces corrosive substances, destroys the structure of grease, causes metal corrosion and shortens the service life of grease.
The oxidation stability of grease is its ability to resist oxidation in storage and use and keep its properties from changing permanently, mainly depends on the oxidation resistance of base oil.
9. extreme pressure abrasion resistance
The grease film formed by the grease coated on the surface of the metal in contact with each other can withstand loads from the axial and radial directions. The characteristic of the grease film to bear the load is called the extreme pressure of the grease.
Abrasion resistance refers to the ability of grease to prevent wear and tear of metal by holding a grease film between the surfaces of moving parts. The thickener of grease itself is oily agent and has better anti-wear property.