Rolling bearing

First, the rolling bearing code

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

Prefix code basic code post code

The basic code is the basis of the bearing code. The pre-code and post-code are all supplements to the bearing code. They are only used when there are special requirements for the bearing structure, shape, material, tolerance class, technical requirements, etc. The general situation can be partially or completely omitted.

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

Type code size series code inner diameter code

(1) Bearing type code Use different numbers or letters to indicate different types of bearings. See the database for details.

(2) Size series code consists of two digits. The first digit represents the width series (radial bearing) or the height series (thrust bearing), and the last digit represents the diameter series. The dimension series means that bearings of the same inner diameter can have different outer diameters, while the same outer diameters have different widths (or heights), thereby being used to meet the bearing capacity of various requirements.

(3) Inner diameter code Indicates the nominal inner diameter of the bearing, which is represented by a number.

Example 1: Bearing 2 32 24

2-type code, spherical roller bearing; 32-size series code; 24--inner 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-bearings with dust cover on both ends; P6-tolerance class conforms to the standard 6 class.

Second, the type structure and model selection

1) The machine has various working conditions. In order to meet these specific use requirements, different types of bearings are needed to ensure the actual needs. According to the shape of the rolling element, the rolling bearing can be roughly divided into a ball bearing and a roller bearing; according to the main direction of the load, it can be divided into a radial bearing and a thrust bearing. Table 1 compares the general characteristics of ball bearings and roller bearings. Table 1 Comparison of general characteristics of ball bearing and roller bearing 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 type There are many types of rolling bearing. When selecting, consider the following factors to select.

1. Size, direction and nature of the load: Ball bearings are suitable for withstanding light loads and roller bearings are suitable for heavy and impact loads. When the rolling bearing is subjected to pure axial load, thrust bearing is generally used; when the rolling bearing is subjected to pure radial load, deep groove ball bearing or short cylindrical roller bearing is generally used; when the rolling bearing is subjected to pure radial load, there is still little For axial load, deep groove ball bearings, angular contact ball bearings, tapered roller bearings and self-aligning balls or spherical roller bearings are available. When the axial load is large, angular contact with large contact angle can be selected. Ball bearings and tapered roller bearings, or the combination of radial and thrust bearings, are particularly suitable for very high axial loads or especially for large axial stiffness.

2. Allowable speed: There are large differences depending on the type of bearing. In general, bearings with low friction and low heat generation are suitable for high speeds. The design is designed to work with rolling bearings below their limit speed.

3. Rigidity: When the bearing is under load, the bearing ring and the rolling element will be elastically deformed at the contact point. The deformation amount is proportional to the load, and the ratio determines the rigidity of the bearing. Generally, the rigidity of the bearing can be improved by pre-tightening of the bearing; in addition, in the design of the bearing support, the bearing rigidity can be improved by considering the combination and arrangement of the bearings.

4. Self-aligning performance and installation error: After the bearing is loaded into the working position, the installation and positioning are often poor due to manufacturing errors. At this time, the bearing is often subjected to excessive load due to deflection and thermal expansion of the shaft, causing early damage. Self-aligning bearings can overcome the defects caused by installation errors and are suitable for such applications.

5. Installation and disassembly: Tapered roller bearings, needle roller bearings and tapered roller bearings, etc., belong to the type of bearings that can be separated inside and outside the ring (so-called separate bearings), easy to install and disassemble.

Third, the damage of the rolling bearing

When the rolling bearing is disassembled and inspected, the bearing failure and the cause of damage can be judged according to the damage of the bearing.

1. Raceway surface Metal peeling bearing rolling elements and inner and outer ring raceways are subjected to periodic pulsating loads, resulting in cyclically varying contact stresses. When the number of stress cycles reaches a certain value, fatigue peeling occurs on the rolling element or the inner and outer ring raceways. If the bearing load is too large, this fatigue will be exacerbated. In addition, the bearing is not installed correctly, the shaft is bent, and the raceway is peeled off. Fatigue spalling of the bearing races reduces the accuracy of the shaft and causes vibration and noise in the mechanism.

2. Bearing burn burned bearings have a tempering color on the raceway and rolling elements. The causes of burns are generally insufficient lubrication, the quality of the lubricant does not meet the requirements or deterioration, and the bearing assembly is too tight.

3. Uneven pits appear on the contact surface between the raceway and the roller of the plastic deformation bearing, indicating that the bearing is plastically deformed. The reason for this is that under a large static load or impact load, the local stress of the working surface exceeds the yield limit of the material, which generally occurs on bearings rotating at low speed.

4. Bearing ring crack The bearing ring is cracked because the bearing is too tight, the outer ring or inner ring of the bearing is loose, the bearing part of the bearing is deformed, and the surface of the bearing is poorly processed.

5. The cage is broken due to insufficient lubrication, broken rolling elements, and skewed races.

6. The reason why the metal of the cage adheres to the rolling elements is that the rolling elements are caught in the cage or insufficiently lubricated.

7. Serious wear of the raceway may be due to foreign matter falling into the race, insufficient lubricant or improper lubricant grade. The main factors affecting rolling bearings are: load, lubrication, assembly, environmental conditions and material or manufacturing accuracy.

Fourth, bearing operation inspection and fault handling

The inspection items in operation include the rolling sound, vibration, temperature, and lubrication status of the bearing. The details are as follows:

Rolling sound of the bearing

The sounder is used to check the size and sound quality of the rolling sound of the running bearing. Even if the bearing is slightly peeled off, abnormal sounds and irregular sounds are emitted, which can be distinguished by the sounder.

2. Bearing vibration

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

3. Bearing temperature

The temperature of the bearing is generally estimated by the temperature outside the bearing. If the oil hole can directly measure the temperature of the bearing outer ring, it is more suitable. Usually, the temperature of the bearing rises slowly as the operation starts, and reaches a steady state after 1-2 hours. The normal temperature of the bearing varies with the heat capacity, heat dissipation, speed and load of the machine. If the lubrication and installation parts are suitable, the bearing temperature will rise sharply and abnormal temperature will occur. At this time, it is necessary to stop the operation and take necessary precautions.

4. Lubricate

4.1 The role of bearing lubrication

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

a. Forming an oil film between the two rolling surfaces or sliding surfaces that contact each other to separate the two surfaces, reducing friction and wear on the contact surface.

b. When using oil lubrication, especially when using circulating oil lubrication, oil mist lubrication and fuel injection lubrication, the lubricating oil can take away most of the friction heat inside the bearing and effectively dissipate heat.

c. When grease lubrication is used, foreign matter such as dust can be prevented from entering the bearing and sealing.

d. Lubricants have the effect of preventing metal corrosion.

e. Extend the fatigue life of the bearing.

4.2 Comparison of grease lubrication and oil lubrication

The lubrication method of the bearing is 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 use a lubrication method depending on the conditions of use and the purpose of use.

4.3 Grease lubrication: Grease is a lubricant composed of base oil, thickener and additives. When selecting, you should choose a grease that is very suitable for the conditions of use of the bearing. Due to the different trademarks, there will be great differences in performance, so you must pay attention when choosing. Commonly used greases for bearings include calcium-based greases, sodium-based greases, calcium-sodium-based greases, lithium-based greases, aluminum-based greases, and molybdenum disulfide greases. The amount of grease filled in the bearing is preferably 1/2-1/3 of the internal space of the bearing. It should be reduced to 1/3 at high speed. Excessive grease causes the temperature to rise.

4.4 Selection of grease: When selecting grease according to working temperature, the main indicators should be dropping point, oxidation stability and low temperature performance. The dropping point can generally be used to evaluate high temperature performance. The actual working temperature of the bearing should be lower than the dropping point of 10-20 °C. . Synthetic grease should be used at a temperature below the drop point of 20-30 °C. When selecting the grease according to the bearing load, the grease with a small penetration should be selected for the heavy load. When working under high pressure, in addition to the small penetration, it also has higher 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 drying and low moisture environments.

4.5 Oil lubrication: Under high-speed and high-temperature conditions, oil lubrication can be used when grease lubrication is not suitable. A large amount of heat can be taken away by the circulation of the lubricating oil. Viscosity is an important characteristic of lubricating oil. 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 12-15 cst. The higher the speed, the lower the viscosity, and the heavier the load, the higher the viscosity. Commonly used lubricating oils include mechanical oil, high-speed mechanical oil, turbine oil, compressor oil, transformer oil, cylinder oil, and the like.

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. Part of the bearing is immersed in the oil groove. The lubricating oil is taken up by the rotating bearing parts and then flows back to the oil tank. The surface should be slightly below the center of the lowest rolling element.

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

c. Circulating oil lubrication: The oil pump is used to transport the filtered oil to the bearing components, and the lubricating oil after passing through the bearings is filtered and cooled for use. Since the circulating oil can take a certain amount of heat to cool the bearing, this method is suitable for bearing parts with higher rotational speed.

d. Spray lubrication: The dry compressed air is mixed with the lubricating oil through a sprayer to form an oil mist. In the jet bearing, the air flow can effectively cool the bearing and prevent the intrusion of impurities. This method is suitable for the lubrication of high speed, high temperature bearing components.

e. Injection lubrication: The high-pressure oil is injected into the bearing through the nozzle by the oil pump, 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 form a gas flow at the relatively high rotation speed of the surrounding air. It is difficult to send the lubricating oil to the bearing by the general lubrication method. At this time, the lubricating oil must be sprayed by the high-pressure injection method. In the bearing, the position of the nozzle should be placed between the inner ring and the center of the cage.

Five, the choice of rolling bearing oil

The spindle bearing lubricating oil using the rolling bearing is generally considered in consideration of the product of the inner diameter d of the bearing and the spindle rotational speed n, that is, the speed factor dn, the bearing operating temperature T, and the bearing P subjected to the bearing.

The selection of the recommended lubricant grade for rolling bearings is given below. Lubricating oils can be selected from FC or FD bearing oil, HL hydraulic oil, TSA turbine oil, DAD compressor oil, gasoline engine oil or cylinder oil, depending on product type, working condition and viscosity grade.

1. Bearing working temperature -30-0 °C, normal load (3MPa) Applicable oil viscosity (40 °C) 18-32mm2 / s, viscosity grade 22, 32; heavy load or impact load (3MPa-20MPa) applicable oil viscosity (40 °C) 18-50mm2/s, viscosity grade 32, 46.

2. Bearing working temperature 0-60 ° C, speed factor (dn value) 15000mm.r / min or less, normal load applicable oil viscosity (40 ° C) 32-70mm2 / s, viscosity grade 32, 46, 68, heavy load or impact Load (3MPa-20MPa) Applicable oil viscosity (40°C) 70-192mm2/s, viscosity grade 68, 100, 150; speed factor (dn value) 15000-75000mm.r/min, common load applicable oil viscosity (40°C) 32-50mm2/s, viscosity grade 32, 46, heavy load or impact load (3MPa-20MPa) applicable oil viscosity (40 °C) 42-90mm2 / s, viscosity grade 46, 68, 100; speed factor (dn value) 75000 -150000mm.r/min, general load applicable oil viscosity (40 °C) 15-32mm2 / s, viscosity grade 15, 32, heavy load or impact load (3MPa-20MPa) applicable oil viscosity (40 ° C) 32-42mm2 / s , viscosity grade 32; speed factor (dn value) 150000-300000mm.r/min, common load applicable oil viscosity (40 ° C) 9-12mm2 / s, viscosity grade 5, 7, 10, heavy load or impact load (3MPa- 20MPa) Applicable oil viscosity (40 ° C) 18-32mm2 / s, viscosity grade 15.

Six major factors affecting bearing vibration

1. The vibration system consisting of stiffness, compliance, mass, damping, etc., determined by the bearing structural design parameters, will produce vibrations that are compatible with its mechanics under operating conditions.

2. The vibration caused by the deviation in the manufacturing process, the smaller the deviation, the smaller the vibration. The roundness, waviness and surface roughness of the steel ball surface are the most influential factors on the vibration of each bearing component, followed by the shape error and surface roughness of the inner and outer ring channels, again the bearing cleanliness and lubricant Quality and so on.

3. Operating conditions such as installation conditions, rotational speed, direction of load applied to the axial direction, magnitude, etc.