When lubricating bearings and other friction components with grease, its internal friction replaces the solid friction between the friction surfaces of the components. Therefore, the viscosity of lubricating grease has a significant impact on the power consumption of machinery using lubricating grease. If the viscosity of the lubricating grease used is high, it is obvious that there will also be more friction loss.

The viscosity of lubricating grease varies with shear rate, making it particularly adaptable when used on machinery with frequent speed changes. When the speed is high, the viscosity of the lubricant is required to be low. At this time, the structural damage of the lubricating grease intensifies, and the fibers are oriented, resulting in a decrease in viscosity. When the speed is slow, the viscosity of the lubricant is required to be high, while the viscosity of the lubricating grease is relatively high when the shear speed is low. The viscosity of lubricating grease changes with shear rate, which basically meets the requirements of mechanical speed changes for lubricant viscosity.

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, if the viscosity of lubricating grease is too high at this time, it will increase the starting resistance. Especially at low temperatures, the increase in viscosity of lubricating grease can further 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, a certain lithium grease has a shear stress limit of no more than 686.5Pa (7GF/cm2) at a temperature of 40 ℃, while its flow resistance is 2452Pa (25GF/cm2) at the same temperature of 2.5s-1. Therefore, it can be seen that the viscosity of the grease at low temperature and low shear rate has a significant impact on its low-temperature starting performance. Lubricating grease used for low or wide temperature ranges needs to have its low temperature viscosity specified.

1、 Sliding temperature and oil film retention capacity

Lubricating grease covers the metal surface, hindering external air and water from coming into contact with the metal, thus protecting the metal surface. If the covering layer changes due to environmental temperature, causing the lubricating grease to no longer adhere to the metal surface and slide off, then the lubricating grease is equivalent to not providing protection. The sliding temperature represents the maximum temperature limit at which lubricating grease can adhere to a vertical metal surface. The sliding temperature of lubricating grease is determined by its droplet point, surface capacity, phase state, viscosity temperature coefficient, and coating thickness on metal surfaces.

The surface of ordinary soft steel will rust within 10 minutes when in contact with water and air at room temperature. Applying protective grease can delay the rusting process. The thicker the oil film, the slower the passage of oxygen and water vapor. The higher the density of the oil film, the lower its permeability. At a fixed temperature, measuring the weight of a uniform oil film per unit area is called oil film retention capacity. The industry standard SH/T0334-92 is a method for determining the retention capacity of lubricating grease films.

2、 Rust resistance

Lubricating grease, due to its excellent adhesion properties, can maintain a sufficient lipid film on the metal surface, isolating water vapor, air, acidic and corrosive gases or liquids from penetrating the lipid film to corrode the metal surface. Therefore, lubricating grease is more effective than lubricating oil in preventing corrosion on the metal surface. However, the grease film formed on the metal surface is permeable. Generally speaking, the thicker the lipid film, the slower the air and agricultural gas flow. Similarly, the higher the density of the lipid membrane, the lower its permeability.

There are several test methods for testing the rust resistance of lubricating grease:

1. 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 humidity increases.

2. The national standard GB/T2361-92 is a wet heat test method for rust proof oils and fats, which is suitable for measuring the rust proof performance of rust proof oils and fats on metals under high temperature and high humidity conditions.

3. The national standard 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 damp conditions. This method is developed in accordance with ASTMD1742 "Test Method for Corrosion Protection of Lubricating Greases".

3、 Corrosion resistance

The thickener of lubricating grease and the 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, excessive free organic acids and bases can cause corrosion. Corrosion test is to detect whether lubricating grease has a corrosive effect on metals. There are several methods for measuring, and the test conditions are also different, but all of them are to observe the discoloration or spot formation on the metal sheet at a certain temperature and test time to determine the corrosiveness of the lubricating grease. The measurement methods include the industry standard SH/T033l-92 lubricating grease corrosion test method, SH/T0080-91 anti rust grease corrosion test method, and GB/T7326-87 lubricating grease steel plate corrosion test method. Among them, GB/T7326-87 is the most widely used, divided into two types: method A and method B. The general selection conditions are 100 ℃ and 24 hours. After the experiment is completed, remove the copper sheet and wash it. Method A compares the test copper sheet with the copper corrosion standard color plate to determine the level of corrosion. The second method is to check whether the test copper sheet has discoloration. This method can only be used to determine the corrosiveness of lubricating grease on copper sheets. The A method is equivalent to using ASTMD4048 "Corrosion Test for Lubricating Steel Sheets", while the B method is equivalent to using JISK2220-1984.55 "Copper Strip Corrosion Test Method" for ordering.

4、 Water resistance

The water resistance of lubricating grease indicates its water absorption performance under atmospheric humidity conditions. Lubricating grease is required to have no ability to absorb moisture during storage and use. After lubricating grease absorbs water, it will dissolve the withering agent and change its structure, reduce the drop point, and cause corrosion, thereby reducing its protective effect. Some lubricating greases, such as some composite calcium based greases, become hard due to absorbing moisture from the atmosphere, gradually losing their lubricating ability and affecting their actual use.

The water resistance of lubricating grease mainly depends on the water resistance and emulsifying properties of thickeners, and hydrocarbon based thickeners (such as wax) have good water resistance, are not easy to absorb water, and are not easy to emulsify. Sodium soap modifiers have poor water resistance and are easy to absorb water while also being dissolved by water.

The industry standard SH/T0109-92 is a method for determining the water resistance of lubricating grease.

In addition, the extended working cone penetration measurement method in GB/T269-91 is also used. Add 10% water to the workpiece and measure the water resistance of the lubricating grease based on the change in cone penetration value after 100000 operations. There is also ASTMD4049 Test Method for Water spray Resistance of Lubricating Grease in the United States, which is used to evaluate the adhesion of lubricating grease to metal surface when spray grease samples with water.