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【Air compressor oil】Super small knowledge of bearing lubrication

Single-phase air compressor

Lubricating oil is an indispensable lubricating material for bearings. It can protect bearings and reduce the hazards caused by bearings during operation.

Bearing lubrication methods are roughly divided into two types: grease lubrication and oil lubrication. In order to give full play to the function of the bearing, it is important to adopt the corresponding lubrication method according to the service conditions and purpose of the bearing.

1. Bearing grease

Grease is a lubricant composed of base oil, thickener and additives. When choosing, you should choose the lubricating grease that is very suitable for the service conditions of the bearing. Due to the different brands of lubricating grease, there will be great differences in performance, so you must pay attention when choosing.

Commonly used greases for bearings include calcium-based grease, sodium-based grease, calcium-sodium-based grease, lithium-based grease, aluminum-based grease, and molybdenum disulfide grease. The amount of grease filled in the bearing is suitable to fill 1/2-1/3 of the internal space of the bearing. It should be reduced to 1/3 at high speed. Excessive grease tends to increase the temperature of the bearing in operation.

2. Grease selection

When selecting grease according to the working temperature, the main indicators should be the dropping point, oxidation stability and low temperature performance. The dropping point can generally be used to evaluate the high temperature performance, and the actual working temperature of the bearing should be 10-20°C lower than the dropping point. The operating temperature of synthetic grease should be 20-30°C lower than the dropping point. When selecting grease according to the bearing load, grease with a small needle penetration should be selected for heavy loads. When working under high pressure, in addition to small penetration, it also needs to have high oil film strength and extreme pressure performance. When selecting grease according to environmental conditions, calcium-based grease is not easily soluble in water and is suitable for dry and less-moisture environments.

Single-phase air compressor

3. Bearing lubricating oil

Under the condition of high speed and high temperature, oil lubrication can be used when grease lubrication is no longer suitable. Through the circulation of lubricating oil, a lot of heat can be taken away. Viscosity is an important characteristic of lubricating oil. The size of viscosity directly affects the fluidity of lubricating oil and the thickness of the oil film formed between friction surfaces. The viscosity of lubricating oil at the working temperature of the bearing is generally 12-15cst. The higher the rotation speed, the lower the viscosity should be selected, and the heavier the load should be, the higher viscosity should be selected. Commonly used lubricating oils include mechanical oil, high-speed mechanical oil, steam turbine oil, compressor oil, transformer oil, cylinder oil, etc.

Single-phase air compressor

4. Lubricating oil method

(1) Oil bath lubrication: Oil bath lubrication is the most common lubrication method, suitable for the lubrication of low and medium speed bearings. Part of the bearing is immersed in the oil tank, and the lubricating oil is carried by the rotating bearing parts, and then flows back to the oil tank. The oil level should be slightly below the center of the lowest rolling element.

(2) Oil drop lubrication: Oil drop lubrication is suitable for bearing parts that require a quantitative supply of lubricating oil. The amount of oil drop is generally one drop every 3-8 seconds. Too much oil will cause the temperature of the bearing to increase.

(3) Circulating oil lubrication: The filtered oil is delivered to the bearing parts by an oil pump, and the lubricating oil after passing through the bearing is filtered and cooled before use. Because the circulating oil can take away a certain amount of heat to cool down the bearing, this method is suitable for bearing components with high speed.

(4) Spray lubrication: use dry compressed air to mix with lubricating oil through a sprayer to form oil mist, and spray it into the bearing. The airflow can effectively cool down the bearing and prevent the intrusion of impurities. This method is suitable for the lubrication of high-speed, high-temperature bearing components.

(5) Spray lubrication: use an oil pump to inject high-pressure oil into the bearing through the nozzle, and the oil injected into the bearing flows into the oil groove through the other end of the bearing. When the bearing rotates at high speed, the rolling elements and the cage also make the surrounding air flow at a relatively high rotation speed. It is difficult to send the lubricating oil to the bearing with the general lubrication method. At this time, the lubricating oil must be sprayed by high-pressure injection In the bearing, the location of the nozzle should be placed between the inner ring and the center of the cage.

Single-phase air compressor

Learned to choose lubricating oil, do you know when it is more appropriate to change the bearing grease?

Everyone may know that grease plays a very important role in the use of bearings. Then someone will ask, is it possible to use grease to lubricate the bearing indefinitely? When should the oil be changed and how much oil should be changed? In fact, these problems are complex issues of bearing maintenance technology.

To the question of whether grease can be used indefinitely, the answer in the affirmative is no. Because excessive use of grease is harmful to the bearing.

Grease has a good effect on bearings. It has good adhesion, wear resistance, temperature resistance, rust resistance and lubricity. It can improve high temperature oxidation resistance, delay aging, dissolve carbon deposits, and prevent metal wear debris. Accumulate with oil stains, improve mechanical wear resistance, pressure resistance and corrosion resistance.

However, if the filling amount of grease is more, the friction torque will be greater. With the same filling amount, the friction torque of sealed bearings is greater than that of open bearings. After the grease filling amount is equivalent to 60% of the internal space volume of the bearing, the friction torque will not increase significantly. This is because most of the grease in open bearings has been squeezed out, and the grease in sealed bearings has also been lost.

As the amount of grease filling increases, the temperature rise of the bearing increases linearly. With the same filling amount, the temperature rise of the sealed bearing is higher than that of the open bearing. It is generally believed that the grease filling amount of sealed rolling bearings should not exceed about 50% of the internal space at most.

Generally speaking, the lubrication schedule of bearings is based on time. Equipment suppliers often develop lubrication schedules based on operating hours. Also, equipment suppliers often include guidance on the amount of lubricant during their maintenance planning process. For the staff, it is very common to change the lubricating oil in a short period of time, and often too much lubricating grease is added.

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