When lubricating grease on bearings and other friction parts, its internal friction replaces the solid friction between the friction surfaces of parts. Therefore, the viscosity of grease has a great influence on the power consumption of machinery using grease. If the grease used has a high viscosity, it is obvious that there will be more friction loss.Basic Precautions for Grease Storage
Basic Precautions for Grease Storage
The property that the viscosity of lubricating grease changes with the shear speed makes it have special adaptability when it is used in machinery with constant speed change. When the speed is high, the viscosity of the lubricant is required to be low. At this time, the damage of the grease structure is intensified, the fiber is oriented, and the viscosity becomes low.
When the speed is slow, the viscosity of the lubricant is required to be large, while the viscosity of the grease is relatively large when the shear speed is low. The change of grease viscosity with shear speed basically meets the requirements of mechanical speed change on lubricant viscosity bearing.Basic Precautions for Grease Storage
The viscosity of grease at very low shear speed is closely related to the starting of lubricated friction parts. Since the viscosity of the grease is high when the shear speed is small, if the viscosity of the grease is too high, the starting resistance will increase. Especially at low temperature, the viscosity of grease increases, which will affect the low-temperature starting and even cause difficulties.
In fact, when the machine is started, the force required to overcome the flow resistance of grease at small shear speed is much greater than that required to overcome the strength limit. For example, the shear stress limit of a lithium grease at a temperature of 40 ℃ is no more than 686.5pa (7gf / cm2), while its flow resistance is 2452pa (25gf / cm2) at 2.5s-1 at the same temperature.
It can be seen that the viscosity of the grease at low temperature and low shear speed has a great influence on the low temperature starting performance of the grease. For lubricating grease used in low temperature or wide temperature range, its low temperature viscosity needs to be specified.Basic Precautions for Grease Storage
1、 Sliding temperature and oil film holding capacity
The grease covers the metal surface and prevents the external air and water from contacting with the metal, thus protecting the metal surface. If the covering layer causes the grease to no longer adhere to the metal surface due to the change of ambient temperature, the grease has no protective effect.
The slip temperature indicates the high temperature limit of grease adhesion to vertical metal surfaces. The slip temperature of grease depends on the drop point, surface capacity, phase state and viscosity temperature coefficient of grease, as well as the coating thickness on the metal surface.Basic Precautions for Grease Storage
The surface of ordinary mild steel will rust within 10min if it contacts with water and air at room temperature. Applying protective grease can delay the rusting time. The thicker the oil film, the slower the transmission of oxygen and water vapor. The higher the density of the oil film, the lower the permeability.
At a fixed temperature, the weight of the uniform oil film per unit area is measured, which is called the oil film holding capacity. The industry standard SH / t0334-92 is a method for measuring the retention capacity of lubricating grease film.Basic Precautions for Grease Storage
2、 Rust resistance
Due to its good adhesion characteristics, lubricating grease can maintain enough lipid film on the metal surface to prevent water vapor, air, acidic and corrosive gas or liquid from eroding the metal surface through the lipid film. Therefore, lubricating grease can protect the metal surface from erosion more than lubricating oil. However, the lipid film formed by grease on the metal surface is permeable. Generally speaking, the thicker the lipid membrane, the slower the air and agricultural gas pass. Similarly, the greater the density of the lipid membrane, the lower the permeability.Basic Precautions for Grease Storage
There are several test methods for testing the antirust performance of lubricating grease:Basic Precautions for Grease Storage
1. The industry standard SH / t0333-92 is a method for measuring the protective performance of lubricating grease, which is suitable for measuring the ability of lubricating grease to prevent metal corrosion when the humidity increases.Basic Precautions for Grease Storage
2. GB / t2361-92 is a damp heat test method for antirust grease, which is suitable for measuring the antirust performance of antirust grease on metal under high temperature and high humidity conditions.Basic Precautions for Grease Storage
3. GB / t5018-85 is a test method for the corrosion resistance of lubricating grease, which is suitable for measuring the corrosion resistance of tapered roller bearings coated with lubricating grease in wet state. This method is formulated with reference to ASTM d1742 test method for corrosion resistance of lubricating grease.Basic Precautions for Grease Storage
3、 Corrosion resistance
The thickener of lubricating grease and base oil itself do not corrode metals. There are many reasons for the corrosion of lubricating grease, mainly due to the oxidation of acidic substances. Generally speaking, too much free organic acid and alkali will cause corrosion.
Corrosion test is to detect whether the grease has corrosive effect on metal. There are several measurement methods and different test conditions, but they all observe the discoloration or spots on the metal sheet at a certain temperature and test time to judge the corrosivity of the lubricating grease. Basic Precautions for Grease Storage
The determination methods include the inter industry standard SH / t033l-92 grease corrosion test method, SH / t0080-91 rust proof grease corrosion test method and GB / t7326-87 grease steel sheet corrosion test method. GB / t7326-87 is widely used, which can be divided into method a and method B. The general selection condition is 100 ℃ for 24h. Take out the copper strip after the test.
After washing, method a is to compare the test copper strip with the copper strip corrosion standard color plate to determine the corrosion level. Method B is to check whether the test copper is discolored. This method can only be used to determine the corrosivity of lubricating grease to copper strip. Method a is equivalent to ASTM d4048 grease steel sheet corrosion method, while method B is equivalent to jisk2220-1984.5.5 copper sheet corrosion test method.Basic Precautions for Grease Storage
4、 Water resistance
The water resistance of grease indicates the water absorption of grease under atmospheric humidity. The lubricating grease is required not to have the ability to absorb water during storage and use. After the grease absorbs water, it will dissolve the withering agent and change the structure, reduce the dropping point and cause corrosion, so as to reduce the protective effect.
Some greases, such as some compound calcium base greases, harden due to absorbing moisture in the atmosphere, gradually lose the lubricating capacity of the grease and affect the actual use.Basic Precautions for Grease Storage
The water resistance of lubricating grease mainly depends on the water resistance and emulsifying property of thickener. Hydrocarbon based Thickener (such as ground wax) has good water resistance and is not easy to absorb water and emulsify. Sodium soap conditioning agent has poor water resistance. It is easy to absorb water and will be dissolved by water.Basic Precautions for Grease Storage
The industry standard SH / t0109-92 is a method for measuring the water spray resistance of lubricating grease.
In addition, the extended working cone penetration method in GB / t269-91 is used. Add 10% water into the working device. After 100000 times of operation, the water resistance of lubricating grease is measured by the change of cone penetration value. The United States also has ASTMD4049 grease water spray test method to evaluate the adhesion of grease to metal surface when water spray grease samples.Basic Precautions for Grease Storage