Basic knowledge of rolling bearing

1 Type and code of rolling bearing

1. Type and structure

Mechanical equipment has a variety of different conditions, in order to meet these specific requirements, different types of bearings are needed to ensure the actual needs. According to the shape of rolling element, rolling bearing can be divided into ball bearing and roller bearing; According to the main load bearing direction, it can be divided into radial bearing and thrust bearing. The following table compares the general characteristics of ball bearings and roller bearings.

Comparison of general characteristics between ball bearing and roller bearing

2. Rolling bearing model code

2.1 basic code

The basic code is used to indicate the bearing's inner diameter, diameter series, width series and type, generally up to five digits, which are described as follows:

(1) The bearing inner diameter is represented by the first and H digits from the right of the basic code.

For the bearing with the usual inner diameter d = 20-480mm, the internal diameter of the bearing is generally a multiple of 5, and these two digits indicate the quotient of the bearing inner diameter dimension divided by 5.

For example: 04 means d = 20mm; 12 means d = 60mm, etc.

For bearings with an inner diameter of 10 mm, 12mm, 15mm and 17mm, the inner diameter codes are 00, 01, 02 and 03 in sequence.

For bearings with inner diameter less than 10 mm and greater than 500mm, the internal diameter indication method is specified otherwise, please refer to gb/t272.

(2) The diameter series of bearings (i.e. the change series of bearings with the same structure and same inner diameter in terms of outer diameter and width) are represented by the third digit from the right of the basic code. 

For example: for centripetal bearing and centripetal thrust bearing, 0,1 represents a very light series; 2 for light series; 3 represents the medium series; 4 for heavy series. Except for the ultra light series with 1, the thrust bearing is consistent with the centripetal bearing.

(3) The width series of bearings (i.e. the series of changes in bearing width with the same structure, inner diameter and diameter series) are represented by the fourth digit from the right of the basic code.

When the comparison of width series 13-4 in Figure 13-4 is 0 series (normal series), the width series code o can not be marked in most bearings, but for the aligning roller bearing and tapered roller bearing, the width series code 0 should be marked.

Diameter series code and width series code are collectively referred to as dimension series codes.

(4) The bearing type code is represented by the fifth digit from the right of the basic code (for cylindrical roller bearing and needle roller bearing, etc. type code is letter).

2.2 post code

The post code of bearing is to use letters and numbers to express the structure, tolerance and special requirements of materials of the bearing. There are many contents of post code. Here are some common codes.

(1) The internal structure code is the different internal structure of the same type of bearing, which is represented by letters following the basic code.

For example, the contact ball bearings with contact angles of 15 °, 25 ° and 40 ° respectively use C, AC and B to indicate the difference of internal structure.

(2) The tolerance grades of bearings are divided into two, four, five, six, six and zero, and they are 6 grades, which are from high to low in sequence, and their codes are /pz, /p4 '/ps, /p6, /p6x and /po. In tolerance class, 6x is only applicable to tapered roller bearings; Grade 0 is a general level, which is not marked in the bearing code.

(3) The common bearing radial clearance series is divided into 1 group, 2 group, 0 group, 3 group, 4 group and 5 group, which is composed of 6 groups, and the radial clearance is from small to large in sequence. Group 0 clearance is a common type of clearance, which is not marked in bearing code, and other clearance groups are represented by /ci, /cz, /c3, /c4 and / CS respectively in bearing code.

2.3 pre code

The bearing pre code is used to represent the bearing parts, and is represented by letters. If l is used to represent the separable collar of the separable bearing; K represents the rolling body and cage assembly of the bearing, etc.

There are many types of rolling bearings in practical application, and the corresponding bearing codes are also complex. The code introduced above is the most basic and common part of bearing code. If you are familiar with this part of the code, you can identify and select the common bearings. For detailed code method of rolling bearing, please refer to gbt272.

2、 Inspection and troubleshooting of bearing in operation

The inspection items in operation include rolling sound, vibration, temperature and lubrication status of bearings, and the specific conditions are as follows:

1. Rolling sound of bearing

The rolling sound of the bearing in operation and the sound quality are checked by sound detector. Even if the bearing has slight peeling and other damage, abnormal and irregular sound will be produced, and it can be distinguished by sound detector.

2. Vibration of bearings

Bearing vibration is sensitive to bearing damage, such as peeling, indentation, corrosion, crack, wear, etc. will be reflected in bearing vibration measurement. Therefore, the vibration can be measured by using special bearing vibration measuring device (frequency analyzer, etc.), and the abnormal specific conditions can not be inferred by frequency division. The measured values are different due to the bearing conditions or the sensor installation position, so it is necessary to analyze and compare the measured values of each machine in advance and determine the judgment criteria.

3. Temperature of bearing

The temperature of the bearing, generally, can be inferred from the temperature outside the bearing chamber. If the oil hole can directly measure the temperature of the outer ring of the bearing, it is more suitable. Generally, the temperature of the bearing rises slowly with the operation, and reaches a stable state after 1-2 hours. The normal temperature of the bearing is different due to the heat capacity, heat dissipation, speed and load of the machine. If the lubrication and installation are suitable, the bearing temperature will rise rapidly and abnormal high temperature will occur. At this time, the bearing must stop running and take necessary preventive measures. According to a large number of test data, table 4-1 lists the average temperature of outer ring when bearing is working in various machines for reference. Due to the influence of lubrication, speed, load and environment, the value of the table only indicates the approximate temperature range. The thermal sensor can monitor the working temperature of the bearing at any time, and automatically alarm or stop to prevent the shaft accident when the temperature exceeds the specified value.

3. Lubrication of bearings

1. Grease lubrication

Grease is a lubricant composed of base oil, thickener and additive. When selecting, the grease which is suitable for bearing service conditions should be selected. Due to the different trademark, there will be a great difference in performance. Therefore, attention must be paid to when selecting. The common lubricating grease of bearings includes calcium based grease, sodium grease, calcium sodium grease, lithium grease, aluminum based grease and molybdenum disulfide grease. The number of grease filled in the bearing is suitable for filling 1 / 2-1 / 3 of the bearing internal space. It should be reduced to 1/3 at high speed. Excessive grease increases the temperature.

2. Selection of grease

When selecting grease according to the working temperature, the main indexes shall be drop point, oxidation stability and low temperature performance. Drop point can be used to evaluate high temperature performance generally. The actual working temperature of bearing shall be lower than 10-20 ℃ of drop point. The service temperature of synthetic grease shall be lower than 20-30 ℃ of drop point. When selecting grease according to bearing load, grease with small penetration should be selected for heavy load. In addition to the small penetration, the oil film strength and extreme pressure performance are also higher when working at high pressure. When choosing grease according to environmental conditions, calcium based grease is not easy to dissolve in water, and is suitable for drying and less water environment.

3. Oil lubrication 

Under the condition of high speed and high temperature, oil lubrication can be used when grease lubrication is not suitable. Through the circulation of lubricating oil, a large amount of heat can be taken away.

Viscosity is an important characteristic of lubricating oil. The viscosity directly affects the fluidity of lubricating oil and the thickness of oil film formed between friction surfaces. The viscosity of lubricating oil is 12-15cst under bearing operating temperature. The higher the speed, the lower the viscosity, the heavier the load, the higher the viscosity. The common lubricating oil includes mechanical oil, high-speed mechanical oil, turbine oil, compressor oil, transformer oil, cylinder oil, etc.

4. The oil lubrication method includes:

(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 groove. The lubricating oil is taken by the rotating bearing parts, and then the oil surface of the oil tank shall be slightly lower than the center of the lowest rolling body.

(2) Oil drop lubrication

Oil drop lubrication is suitable for bearing parts that need to supply lubricating oil quantitatively. Oil drop is usually one drop every 3-8 seconds. Excessive oil will cause the bearing temperature to increase.

(3) Circulating oil lubrication

The filtered oil is sent to the bearing parts by oil pump, and then used after filtering and cooling through the lubricating oil after bearing. Because the circulating oil can take away certain heat and cool the bearing, this method is suitable for bearing parts with high speed.

(4) spray lubrication

The dry compressed air is mixed with the lubricating oil through the sprayer to form an oil mist. In the jet 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.

(5) Spray lubrication

High pressure oil is injected into the bearing through the nozzle by 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 body and cage also form air flow at a relatively high rotating speed. It is difficult to send lubricating oil to the bearing by general lubrication method. At this time, the lubricating oil must be sprayed into the bearing by high pressure injection, and the position of the nozzle should be placed in the bearing