Determination of Clearance of Crossed Roller Bearings in Low Temperature Environment

Abstract: The influence of the clearance of the crossed roller bearing in the low temperature environment is studied. Through the calculation of the clearance adjustment and the analysis and demonstration of the low temperature test test of the bearing, the law of the clearance change of the rolling bearing in the low temperature environment is summarized. and its calculation method.

The space environment simulator is a device for simulating the cold, hot, dark and vacuum environment of space for experimental testing of equipment working in space. Rolling bearings were used in the design and manufacture of a rotating test stand placed in a simulator. The multi-dimensional rotating test bench is developed for the first time in China. Therefore, the test and analysis of the change of rolling bearing clearance at low temperature can be used for reference in the design and development of similar products.

Crossed roller bearings, because they provide the performance of double-row bearings in a single-row space, have a high ability to resist overturning moments, so they are used in the design of space simulators that require small footprint and yaw rotation. Take the crossed roller bearing as an example for related discussion. In order to analyze the change of the clearance of the crossed roller bearing in the low temperature environment, we carried out the theoretical calculation of the bearing clearance in the low temperature environment, and on the basis of the theoretical data, simulated the low temperature working environment for testing and certification.

1. Analysis of the influence of low temperature environment on bearing work

1.1 Bearing working conditions Take CRB25025 crossed roller bearing as an example.

Working environment: The working environment of the rotating test bench is vacuum and low temperature (the vacuum degree is below 1×10-3Pa, the temperature is -50℃), and the rotation angular velocity is 5°/s (shaft diameter Φ250mm). The material of hollow shaft and bearing seat is 0Cr18Ni9, and the material of bearing is bearing steel 9Cr18. The structure diagram of the bearing part is shown in Figure 1.

The outer ring of the crossed roller bearing is of the upper and lower separation type. When assembling at room temperature, the dimension B2 is controlled by the end cover, that is, the distance e between the upper and lower outer rings is changed to adjust the size of the bearing clearance.

When the equipment works in the working environment, the temperature change of bearing-related assemblies is mainly affected by two aspects:

(1) The temperature of the working environment is lower than the normal temperature, and the temperature of the bearing and its matching shaft, bearing seat, etc. drops;

(2) When the rolling bearing is in operation, the friction between the rolling element and the raceway generates heat, and the temperature rises. Moreover, due to different heat dissipation conditions, a temperature difference will also be formed between the inner and outer rings.

In this example, the effect of frictional heat generation on the bearing is negligible due to the slow linear velocity of the bearing rotation. When choosing this working condition test, the mutual interference of the two factors can be avoided, and only the influence of the low temperature at point (1) on the bearing clearance can be considered. The temperature difference caused by frictional heat generation is discussed in other literatures.

1.2 Influence of temperature changes on bearings

Due to the different materials of the housing and the bearing, the shrinkage rate is also different. According to literature [1~2], at 30~-50℃, the average linear expansion coefficient of bearing seat and shaft material 0Cr18Ni9 is (15.85×10-6)/℃, and the average linear expansion coefficient of bearing material 9Cr18 is (9.50×10-6)/℃, relatively small.

When the temperature drops, the coefficient of linear expansion is relatively large, and the amount of shrinkage will also be larger. The relative fit of the width dimensions B1, B2, and the radius dimensions r1 and R2 of the inner and outer rings of the bearing has changed. If the original fit gap is 0mm, the following situations will occur after cooling:

(1) In the B2 dimension direction, the space formed by the bearing seat and the end cover is smaller than the width of the bearing outer ring, so that the gap e between the upper and lower outer rings of the bearing becomes smaller, and the bearing clearance decreases;

(2) In the dimension direction of B1, the space formed by the shaft and the end cover is smaller than the width of the inner ring of the bearing, so that the inner ring of the bearing is deformed and the bearing clearance is reduced;

(3) In the R2 dimension direction, the inner diameter of the bearing seat is smaller than the outer diameter of the outer ring of the bearing, so that the outer ring of the bearing is deformed and the bearing clearance is reduced;

(4) In the dimension direction of r1, the outer diameter of the shaft is smaller than the inner diameter of the inner ring of the bearing, which increases the matching clearance between the inner ring of the bearing and the shaft, but does not affect the clearance of the bearing.

Therefore, when the working temperature drops, the changes in the first three aspects will affect the bearing clearance.

2. Calculation of the effect of temperature change on bearing clearance

2.1 The influence of the radial fit change of the bearing and the bearing seat on the clearance

(1) Interference calculation after temperature change. As described in Section 1.2, the bearing and the bearing seat are reduced in radial fitting clearance due to temperature reduction and different linear expansion coefficients. When an interference fit occurs, the outer ring is deformed and radial displacement occurs. According to the literature [1], the relationship between the elongation of metal materials and temperature, where L1 is the length of the object at temperature T1; L2 is the length of the object at temperature T2; α is the average linear expansion coefficient of temperature T1 ~ T2. Since the shrinkage rate in the circumferential direction is proportional to the radius, when the temperature of each related part drops by 75°C (25～-50°C), the r seat is the inner diameter of the bearing seat at -50°C, mm; the r seat is the bearing seat when the temperature is 30°C ΔT is the absolute value of temperature change (case value is 75°C); αjun 1 is the average linear expansion coefficient of the bearing seat at 25～-50°C, 0Cr18Ni9 is (15.85×10-6)/ °C;

R2′ is the outer diameter of the outer ring at -50°C of the bearing, mm;

R2 is the outer diameter of the outer ring when the bearing is at 30°C (the case value is 155mm);

αjun 2 is the average linear expansion coefficient of the bearing at 25～-50℃, and 9Cr18 is (9.50×10-6)/℃;

The reduction in the clearance between the outer ring of the bearing and the bearing seat △R diameter=r’seat-R2′, and the above values ​​are substituted to obtain △R diameter=0.0775mm.

According to the working conditions of the bearing, the cooperation between the bearing and the bearing seat is determined to be Φ310H7/h5. Considering the influence of factors such as roundness, according to conventional experience, the fitting clearance after assembly at room temperature is calculated as 0mm. Then, the unilateral interference △Ry between the bearing and the bearing housing after the temperature change is 0.0775mm.

(2) Calculate the radial and radial displacement of the outer ring raceway rib of the bearing caused by the interference. According to the literature [3], when the bearing seat is a solid steel shell, the radial shrinkage △Re of the outer ring raceway rib of the bearing

△Ry is the unilateral effective interference between the outer ring and the bearing seat, mm;

r2 is the radius of the raceway rib of the outer ring of the bearing (the value of the middle diameter of the rib in the case is 143.7mm);

R2 is the nominal radius of the bearing outer ring and the bearing seat hole (the case value is 155mm);

Then △Re=0.072mm can be obtained.

As shown in Figure 2, the influence value △U1 on the bearing clearance is △U1=△Re·sin45°=0.051mm.

If the matching gap between the bearing and the bearing seat is increased to make up for it, it will affect the assembly accuracy of the bearing at room temperature. Therefore, the adjustment gap e of the outer ring is increased to compensate for the effect of temperature changes. 2.2 The influence of the change of the axial fit between the outer ring and the end cover on the clearance

As shown in Figure 3, in the axial direction, when the shrinkage rate of the outer ring of the bearing is smaller than that of the bearing seat, the e value will be reduced, and the clearance will be directly reduced. Amount of decrease in e value

B2 is the axial dimension of the outer ring of the bearing (the actual value of the case is 25mm);

After calculation, the △e value can be obtained as 0.0125mm.

Then, the axial fit of the bearing outer ring and the end cover changes, and the influence value on the clearance is △U2=△esin45°=0.009mm.

Since the temperature change directly affects the e value, it is necessary to increase the outer ring adjustment gap e to compensate.

2.3 The influence of the change of the axial fit between the inner ring and the end cover on the clearance

As described in Section 1.2, in the axial direction, when the shrinkage of the inner ring of the bearing is smaller than that of the shaft, the interference δ=△e1=0.0125mm is generated.

The axial compression of the inner ring of the bearing will cause deformation. Since the bearing axial clearance has little effect on the bearing assembly accuracy, it is sufficient to increase the axial clearance when the bearing inner ring is installed to compensate.

2.4 Compensation value of clearance adjustment gap e

According to the previous sections 2.1, 2.2 and 2.3, after the temperature changes, the influence value on the clearance is △U=△U1+△U2. Since the adjustment surface of the raceway and the end cover is 45° as shown in Figure 4, the theoretical compensation value of the clearance adjustment gap e (including the clearance of the upper and lower raceways) △e=2△U/sin45°=0.17mm.

3. test

After the theoretical calculation, the theoretical data is verified by experiments.

Install the bearing on the test device (at this time, the room temperature is about 25°C). When the outer ring end cover is installed, the clearance of the bearing is controlled by adjusting the clearance of the bearing axial gland. The clearance is controlled so that the bearing can be rotated flexibly without jamming, so that the clearance of the bearing is about 0.01-0.015mm. The method of simulating the working temperature is to use liquid nitrogen to cool the bearing detection device as a whole. Place the bearing detection device in a low temperature environment of liquid nitrogen. As the bearing temperature decreases, after reaching -20°C, manually rotate the bearing for every 10°C drop until it reaches -60°C. And take measures to ensure that the overall cooling of the bearing is uniform.

It can be seen from the test data that the temperature change value and the bearing clearance change are basically in line with the theoretical calculation.

Considering the influence of the compression of the bearing axial clearance adjustment washer and the pre-tightening of the gland during installation, the clearance adjustment washer is selected to be 0.20mm to increase the e value. The operating conditions for the test are the same as in Section 3.1. The test results are shown in Table 2.

It can be seen from the test results that due to the influence of factors such as the fact that the material properties in the assembly and calculation data cannot be completely consistent, the variation of the clearance in the test is about 20% larger than that in the calculation.

Taking into account the working environment temperature change allowance and other unforeseen factors, the thickness of the gasket during the actual assembly of the bearing is increased to 0.3mm to ensure that the clearance value after work is within the design range of about 0.09mm. The equipment works well during commissioning and subsequent work.

4. Conclusion

It can be seen from the above calculations that the factors affecting the clearance adjustment of the crossed roller bearing are related to the linear expansion coefficient and fitting properties of the shaft, bearing seat and bearing material, and have no particularity. Therefore, this calculation method can be applied in common rolling bearings, and the following rules can be summarized.

Since the linear expansion coefficient of bearing steel is generally small, when the bearing is installed at room temperature and works at low temperature, the influence of frictional heat between the raceway and the roller is excluded. When the temperatures of the bearing, bearing seat, shaft, etc. are equal, the clearance will still be reduced. small; and the lower the temperature, the greater the clearance reduction;

When the overall temperature changes, the effect on the clearance is greatest in the direction (radial) of the relative larger dimension, especially in the case of higher housing rigidity and thinner bearing walls.

Due to the fact that the material properties and other data are different from the actual, and the assembly accuracy and other factors are not considered, the theoretical calculation value and the actual test value have a certain difference, and the actual test change value will be larger.