1. Rolling bearing code

The rolling bearing code is a product symbol that uses letters and numbers to represent the bearing structure, size, tolerance grade, technical performance and other characteristics. The national standard GB/T272-93 stipulates that the bearing code consists of three parts:

Prefix code Basic code Postfix code

The basic code is the basis of the bearing code. Both the prefix code and the postfix code are supplementary to the bearing code. They are only used when there are special requirements for the bearing structure, shape, material, tolerance grade, technical requirements, etc. In general, they can be partially or completely omitted.

The basic code indicates the basic type, structure and size of the bearing. It consists of the bearing type code, size series code and inner diameter code.

Type code Size series code Inner diameter code

(1) Bearing type code Use numbers or letters to represent different types of bearings. For details, see the database.

(2) Size series code It consists of two digits. The first digit represents the width series (radial bearings) or height series (thrust bearings), and the second digit represents the diameter series. The size series indicates that bearings with the same inner diameter can have different outer diameters, and the same outer diameter can have different widths (or heights), thereby meeting various different load-bearing requirements.

(3) Inner diameter code Indicates the size of the nominal inner diameter of the bearing, expressed in numbers.

Example 1: Bearing 2 32 24

2-Type code, spherical roller bearing; 32-Size series code; 24-Inner diameter code, d=120mm;

Example 2: Bearing 6208-2Z/P6

6-Type code, deep groove ball bearing; 2-Size series code; 08-Inner diameter code, d=40mm; 2Z-Bearing with dust covers on both ends; P6-Tolerance grade meets the standard requirements of grade 6.

2. Type, structure and model selection

1) Machinery has various working conditions. In order to meet these specific use requirements, different types of bearings are needed to ensure actual needs. According to the shape of the rolling element, rolling bearings can be roughly divided into ball bearings and roller bearings; according to the main direction of the load, they can be divided into radial bearings and thrust bearings. Table 1 is a comparison of the general characteristics of ball bearings and roller bearings. Table 1 Comparison of general characteristics of ball bearings and roller bearings According to the national standard GB/T272-93, the commonly used types of rolling bearings can be found in the database.

2) Selection of rolling bearing types There are many types of rolling bearings. When selecting, the following factors can be considered to make a selection.

1. The size, direction and nature of the load: ball bearings are suitable for light loads, and roller bearings are suitable for heavy loads and impact loads. When the rolling bearing is subjected to pure axial load, thrust bearings are generally selected; when the rolling bearing is subjected to pure radial load, deep groove ball bearings or short cylindrical roller bearings are generally selected; when the rolling bearing is subjected to pure radial load and small axial load, deep groove ball bearings, angular contact ball bearings, tapered roller bearings and self-aligning balls or self-aligning roller bearings can be selected; when the axial load is large, angular contact ball bearings and tapered roller bearings with large contact angles can be selected, or radial bearings and thrust bearings can be combined together, which is particularly suitable for extremely high axial loads or special requirements for greater axial rigidity. 2. Allowable speed: There are great differences due to different types of bearings. Generally speaking, bearings with low friction and low heat generation are suitable for high speeds. When designing, it is necessary to strive for the rolling bearing to work under conditions below its limit speed. 3. Rigidity: When the bearing is under load, elastic deformation will occur at the contact between the bearing ring and the rolling element. The deformation is proportional to the load, and the ratio determines the size of the bearing rigidity. Generally, the rigidity of the bearing can be improved by preloading the bearing; in addition, in the bearing support design, considering the combination and arrangement of the bearings can also improve the bearing support rigidity.

4. Self-aligning performance and installation error: After the bearing is installed in the working position, it is often installed and positioned poorly due to manufacturing errors. At this time, the bearing is often subjected to excessive loads due to shaft deflection and thermal expansion, causing early damage. Automatic self-aligning bearings can overcome the defects caused by installation errors by themselves, so they are suitable for such purposes.

5. Installation and disassembly: Tapered roller bearings, needle roller bearings and tapered roller bearings, etc., belong to the type of bearings with separable inner and outer rings (so-called separable bearings), which are easy to install and disassemble.

3. Rolling bearing damage

When disassembling and inspecting the rolling bearing, the failure and cause of the bearing damage can be judged according to the damage of the bearing.

1. Metal peeling on the raceway surface The rolling elements and the inner and outer ring raceways of the bearing are subjected to periodic pulsating loads, thereby generating periodic contact stress. When the number of stress cycles reaches a certain value, fatigue spalling occurs on the rolling element or the working surface of the inner and outer ring raceways. If the load on the bearing is too large, this fatigue will be aggravated. In addition, improper bearing installation and shaft bending will also cause raceway spalling. Fatigue spalling of the bearing raceway will reduce the running accuracy of the shaft and cause vibration and noise in the mechanism.

2. Bearing burns The raceways and rolling elements of the burned bearings have tempering color. The causes of burns are generally insufficient lubrication, the quality of the lubricating oil does not meet the requirements or deterioration, and the bearing is assembled too tight.

3. Plastic deformation The uneven pits on the contact surface between the raceway and the roller of the bearing indicate that the bearing has plastic deformation. The reason is that the local stress on the working surface of the bearing exceeds the yield limit of the material under the action of a large static load or impact load. This situation generally occurs in bearings rotating at low speeds.

4. Bearing race cracks The causes of cracks in the bearing raceway may be too tight bearing fit, loose outer or inner ring of the bearing, deformation of the bearing containment, poor surface processing of the installed bearing, etc.

5. The reason for the cage to break is insufficient lubrication, rolling element crushing, skewed raceway, etc.

6. The metal of the cage adheres to the rolling element. The possible reason is that the rolling element is stuck in the cage or insufficient lubrication.

7. Severe wear of the raceway may be caused by foreign matter falling into the raceway, insufficient lubricating oil or inappropriate lubricating oil grade. The main factors affecting rolling bearings are: load conditions, lubrication conditions, assembly conditions, environmental conditions, and material or manufacturing accuracy.

IV. Inspection and troubleshooting of bearings during operation

 

The inspection items during operation include the rolling sound, vibration, temperature, lubrication status of the bearing, etc. The specific conditions are as follows:

1. Rolling sound of the bearing

Use a sound meter to check the size and sound quality of the rolling sound of the running bearing. Even if the bearing has slight damage such as peeling, it will emit abnormal and irregular sounds, which can be distinguished by a sound meter.

2. Vibration of the bearing

Bearing vibration is very sensitive to bearing damage. For example, peeling, indentation, rust, cracks, wear, etc. will be reflected in the bearing vibration measurement. Therefore, by using a special bearing vibration measuring device (frequency analyzer, etc.), the size of the vibration can be measured, and the specific situation of the abnormality can be inferred through frequency analysis. The measured values ​​vary depending on the use conditions of the bearing or the installation position of the sensor, so it is necessary to analyze and compare the measured values ​​of each machine in advance to determine the judgment standard.

3. Bearing temperature

The bearing temperature can generally be inferred from the temperature outside the bearing chamber. If the bearing outer ring temperature can be directly measured using the oil hole, it is more appropriate. Usually, the bearing temperature rises slowly as the operation starts and reaches a stable state after 1-2 hours. The normal temperature of the bearing varies depending on the heat capacity, heat dissipation, speed and load of the machine. If the lubrication and installation are appropriate, the bearing temperature will rise sharply and abnormally high temperature will occur. At this time, the operation must be stopped and necessary preventive measures must be taken.

4. Lubrication

4.1 The role of bearing lubrication

Lubrication has an important impact on the fatigue life, friction, wear, temperature rise, vibration, etc. of rolling bearings. Without normal lubrication, the bearing cannot work. Analysis of the causes of bearing damage shows that about 40% of bearing damage is related to poor lubrication. Therefore, good lubrication of bearings is an effective measure to reduce bearing friction and wear. In addition, bearing lubrication also has many functions such as heat dissipation, rust prevention, sealing, and impact mitigation. The role of bearing lubrication can be briefly described as follows:

a. A layer of oil film is formed between the two rolling surfaces or sliding surfaces in contact with each other to separate the two surfaces and reduce the friction and wear of the contact surfaces.

b. When oil lubrication is used, especially when circulating oil lubrication, oil mist lubrication and oil spray lubrication are used, the lubricating oil can take away most of the friction heat inside the bearing and play an effective heat dissipation role.

c. When grease lubrication is used, it can prevent foreign matter such as dust from entering the bearing and play a sealing role.

d. Lubricants have the function of preventing metal corrosion.

e. Extend the fatigue life of the bearing.

4.2 Comparison of grease lubrication and oil lubrication

The lubrication methods of bearings are roughly divided into grease lubrication and oil lubrication. In order to give full play to the function of the bearing, it is important to use the lubrication method according to the use conditions and purpose of use.

4.3 Grease lubrication: Grease is a lubricant composed of base oil, thickener and additives. When choosing, you should choose a grease that is very suitable for the use conditions of the bearing. Due to different trademarks, 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. Too much grease increases the temperature.

4.4 Selection of grease: 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. The actual working temperature of the bearing should be 10-20℃ lower than the dropping point. The use temperature of synthetic grease should be 20-30℃ lower than the dropping point. When selecting grease according to the bearing load, grease with small needle penetration should be selected for heavy loads. In addition to small needle penetration, it must also have high oil film strength and extreme pressure performance when working under high pressure. When selecting grease according to environmental conditions, calcium-based grease is not easily soluble in water and is suitable for dry and less water-rich environments.

4.5 Oil lubrication: Under high speed and high temperature conditions, oil lubrication can be used when grease lubrication is no longer suitable. A large amount of heat can be taken away through the circulation of lubricating oil. Viscosity is an important characteristic of lubricating oil. The size of the viscosity directly affects the fluidity of the lubricating oil and the thickness of the oil film formed between the friction surfaces. The viscosity of the lubricating oil at the working temperature of the bearing is generally 12-15cst. ​​The higher the speed, the lower the viscosity should be, and the heavier the load, the higher the viscosity should be. Commonly used lubricants include mechanical oil, high-speed mechanical oil, turbine oil, compressor oil, transformer oil, cylinder oil, etc.

Oil lubrication methods include:

a. Oil bath lubrication: Oil bath lubrication is the most common lubrication method, suitable for lubrication of low and medium speed bearings. A 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 lower than the center of the lowest rolling element.

b. Dripping oil lubrication: Dripping oil lubrication is suitable for bearing parts that require quantitative supply of lubricating oil. The amount of oil dripping is generally one drop every 3-8 seconds. Excessive oil will cause the bearing temperature to increase.

c. Circulating oil lubrication: Use an oil pump to transport the filtered oil to the bearing parts. The lubricating oil after passing through the bearing is filtered and cooled before use. Since the circulating oil can take away a certain amount of heat and cool the bearing, this method is suitable for bearing parts with higher speeds.

d. 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 the bearing and prevent impurities from invading. This method is suitable for lubrication of high-speed and high-temperature bearing parts.

e. Spray lubrication: Use an oil pump to spray high-pressure oil into the bearing through a nozzle, and the oil injected into the bearing flows into the oil tank through the other end of the bearing. When the bearing rotates at high speed, the rolling element and the cage also rotate at a relatively high speed to form an airflow around the air. It is difficult to deliver the lubricating oil to the bearing using general lubrication methods. At this time, the lubricating oil must be sprayed into the bearing using a high-pressure spray method. The nozzle should be placed between the inner ring and the center of the cage.

5. Selection of rolling bearing oil

The spindle bearing lubricating oil using rolling bearings is generally considered comprehensively according to the product of the bearing inner diameter d and the spindle speed n, that is, the speed factor dn, the bearing operating temperature T, and the load P borne by the bearing.

The following is a selection of recommended lubricating oil grades for rolling bearings. The types of lubricating oil can be FC or FD bearing oil, HL hydraulic oil, TSA turbine oil, DAD compressor oil, gasoline engine oil or cylinder oil, etc., depending on the product type, working conditions and viscosity grade.

1. The bearing working temperature is -30-0℃, the applicable oil viscosity (40℃) for normal load (3MPa) is 18-32mm2/s, and the viscosity grade is 22, 32; the applicable oil viscosity (40℃) for heavy load or impact load (3MPa-20MPa) is 18-50mm2/s, and the viscosity grade is 32, 46.

2. Bearing operating temperature 0-60℃, speed factor (dn value) below 15000mm.r/min, applicable oil viscosity (40℃) 32-70mm2/s for normal load, viscosity grade 32, 46, 68, applicable oil viscosity (40℃) 70-192mm2/s for heavy load or impact load (3MPa-20MPa), viscosity grade 68, 100, 150; speed factor (dn value) 15000-75000mm.r/min, applicable oil viscosity (40℃) 32-50mm2/s for normal load, viscosity grade 32, 46, applicable oil viscosity (40℃) 42-90mm2/s for heavy load or impact load (3MPa-20MPa), viscosity Grades 46, 68, 100; speed factor (dn value) 75000-150000mm.r/min, applicable oil viscosity (40℃) 15-32mm2/s for normal load, viscosity grades 15 and 32, applicable oil viscosity (40℃) 32-42mm2/s for heavy load or impact load (3MPa-20MPa), viscosity grade 32; speed factor (dn value) 150000-300000mm.r/min, applicable oil viscosity (40℃) 9-12mm2/s for normal load, viscosity grades 5, 7, 10, applicable oil viscosity (40℃) 18-32mm2/s for heavy load or impact load (3MPa-20MPa), viscosity grade 15.

3. Bearing operating temperature 60-100℃, speed factor (dn value) below 15000mm.r/min, suitable oil viscosity for normal load 40℃ 110-192mm2/s (100℃ 9.3-12.5mm2/s), viscosity grade 150, suitable oil viscosity (40℃) 195-240mm2/s (100℃ 15-24mm2/s), viscosity grade 150 for heavy load or impact load (3MPa-20MPa); speed factor (dn value) 15000-75000mm.r/min, suitable oil viscosity for normal load 40℃ 70-120mm2/s (100℃ 5.6-9.3mm 2/s), viscosity grade 68, 100, heavy load or impact load (3MPa-20MPa) applicable oil viscosity (40℃) 110-192mm2/s, viscosity grade 150; speed factor (dn value) 75000-150000mm.r/min, ordinary load applicable oil viscosity 40℃ 50-90mm2/s (100℃ 5.6-9.3mm2/s), viscosity grade 46, 68, 100, heavy load or impact load (3MPa-20MPa) applicable oil viscosity (40℃) 70-120mm2/s, viscosity grade 100

VI. Main factors affecting bearing vibration

1. The vibration system composed of stiffness, flexibility, mass, damping, etc. determined by the bearing structure design parameters will produce vibrations corresponding to its mechanics under operating conditions

2. Vibration caused by deviations in manufacturing and processing. The smaller the deviation, the smaller the vibration. The roundness, waviness and surface roughness of the steel ball surface are the factors that have the greatest impact on vibration among all bearing parts, followed by the shape error and surface roughness of the inner and outer ring grooves, and then the bearing cleanliness and lubricant quality.

3. Operating conditions such as installation conditions, speed, axial load direction, value, etc.