Abstract: Based on the simulated experimental study on the load distribution of four-row composite bearings, the detection method of bearing radial load distribution is proposed, which provides reliable experimental means and test data for the research of improving the life of work roll bearings in Baosteel 2050 finishing mill. The experimental results have important reference value for the research of bearing failure mechanism.

Key words: Experimental research test technology Bearing life roll bearing

In recent years, with the rapid development of automobile, home appliance and other industries, users have higher and higher requirements for steel quality, and the shape control means of rolling mill is also stronger and stronger. For example, the widely used HC, CVC, PC and other technologies have a strong ability to control the shape of the plate. However, due to the application of the above profile control technology, the structure of the rolling mill is more complicated, the stress state of the work roll bearing and the bearing seat deteriorates, and the bearing melting accidents often occur. The 1700 continuous rolling mill introduced by WisCO from SMS Company of Germany has melted more than 100 sets of bearings between 1981 and 1984 [1]. In the aluminum foil rolling mill introduced by DAVY in 1990s, bearing melting occurred many times when the rolling load and rolling speed were lower than the designed capacity. Baosteel 2050 hot tandem mill finishing mill F4 ~ F7 frame using imported SKF bearings, its life is far from the design level, 1996 ~ 1997 SKF bearings abnormal damage more than 30 sets, resulting in serious economic losses.

In view of the above situation, many scholars have carried out theoretical and experimental research on how to improve the life of roll bearings, and taken corresponding measures, such as improving machining accuracy, improving lubrication conditions, increasing bearing clearance and so on. However, the problem has not been fundamentally solved [2], and the bearing melting phenomenon is still very serious. To this end, the Steel Rolling Institute of our university and Baosteel Hot Rolling Plant cooperated, aiming at F4 frame of the 2050 finishing mill, to study the failure mechanism of work roll bearings from both theoretical and experimental research, and made a breakthrough. The theoretical analysis of bearing failure causes is referred to reference [6]. This paper mainly describes the experimental methods and results.

1. Roller bearing structure and failure form

Baosteel 2050 hot tandem rolling mill is a CVC four-high mill. Four cone roller bearings are used on the drive side of the work roll, and the combined bearings produced by SKF company of Germany are used on the operating side. Figure 1 shows the burn-out of the bearings. Field statistics show that the bearing melting accident mainly occurs on the combined bearing on the operating side of the rolling mill, and the failure form is mostly local spalling of the bearing roller and the outer ring. This phenomenon shows that the bearing near the roll side is subjected to large local contact stress in the rolling mill, and local fatigue spalling occurs under the repeated action of the stress, resulting in the bearing burn. Because of this, it is the focus of this experiment to study the load distribution of composite bearing and find out the cause of uneven load distribution.

2. Experimental scheme design

Baosteel Hot rolling plant is a large-scale continuous production enterprise, the experiment must be carried out under production conditions, and there must be no factors affecting production in the experiment. Moreover, the work roller bearing works under the condition of high speed and heavy load, and the impact of a large amount of cooling water brings many difficulties to the experiment. In order to ensure the smooth progress of field measurement, it is necessary to conduct laboratory simulation test first to lay the foundation for field measurement.

2.1 Bearing radial load distribution measurement scheme

To accurately determine the radial load distribution of bearings, the most effective way is to install the sensor in the bearing seat, so that it can directly feel the radial force of each column of bearings, but due to the bearing structure and sealing requirements, this test program is quite difficult, and no relevant reports have been seen. Through in-depth theoretical analysis and repeated experiments, a kind of embedded sensor is designed and installed in the work roller bearing seat. In order to ensure the normal operation of the bearing, the design, installation of the sensor and the design of the bearing seat must meet the following conditions:

① The embedded sensor must have high sensitivity, good sealing, strong anti-interference ability and long service life;

②The sensor should be installed in the position where the radial load of the bearing is maximum, and can accurately react the change of the load;

③ Because the sensor is located in the elastic system, the stiffness of the sensor should have a certain proportional relationship with the overall stiffness of the bearing seat to ensure the output sensitivity of the sensor;

④ Because the experiment is carried out under production conditions, the bearing seat installed with the sensor should meet the strength and stiffness requirements of the rolling mill production. Therefore, it is necessary to design the experimental machine according to the similarity theory, and carry out repeated experiments to investigate the reliability of the experimental scheme.

In order to meet the above conditions, the bearing seat was first analyzed by the boundary element method [3], which provided a theoretical basis for the design of the experimental scheme.

2.2 Design of simulation experiment prototype

In order to make the simulation experiment more close to the reality, the similarity theory is proposed

The simulation experiment prototype of the work roll bearing seat component of the rolling mill in 2050 is calculated.

The geometric size and material selection of the model follow the following rules [4].

The material of the bearing seat of the rolling mill is ZG270-550, which is also selected in the model. The maximum bending force of the rolling mill is 1000kN, and the bending force of the simulated prototype is 40kN. During the experiment, the bending force is simulated by loading the jack to the bearing seat ear. The bearing seat is fixed on the test bench, and a mandrel is installed in the bearing seat. The mandrel is rotated by the motor to simulate the roll movement, and the working condition of the model is similar to that of the real one as far as possible. However, due to the complex structure of the 2050 hot tandem mill and the harsh working environment, it is difficult to achieve the exact same load conditions. In spite of this, the results of the simulation experiment have important reference value for the study of the feasibility of the test scheme and the common problems affecting the bearing life.

2.3 Test system selection

Because of the complex structure of the tested object, the sensor used must be specially designed. The resistance strain gauge sensor composed of strain gauge and elastic element can be designed according to the structure and stress state of the measured object. The measured signal has no parameters, so the test system composed of resistance strain gauge and recorder is selected. The system is reliable, strong anti-interference and suitable for field measurement. In order to improve the reliability of the test results, two groups of strain flowers are affixed to the corresponding position of the bearing seat surface as a supplement to the sensor test results. The installation position of the sensor and the composition of the test system are shown in Figure 2.

3. Simulation experiment process and result analysis

The purpose of static simulation experiment is to investigate the response of embedded sensor and strain flower to bearing radial load. The conditions of the static experiment are as follows: the simulated prototype does not rotate, and the jack supports the boss on both sides of the bearing seat to simulate the effect of the bending roll force. The load is indicated by the force measuring head located on the jack.

3.1 Response of embedded sensor to roll bending force

FIG. 3 (a) and (b) show the calibration curve of the embedded sensor after it is loaded into the bearing seat. FIG. 3 (a) shows the response of radial force sensors No. 1-4 to roll bending force (see FIG. 2 for the positions of the four sensors). The application point of roll bending force is located on the center line of bearing seat, the horizontal coordinate represents the change of roll bending force, and the vertical coordinate is the output value of each sensor. It can be seen from the figure that the output of each sensor has good linearity and high sensitivity when the bending force changes.

FIG. 3 (b) shows the response of the radial force sensor to the offset load of the combined bearing. The offset load is caused by artificially moving the loading device so that it deviates from the bearing center line. The horizontal coordinate indicates the position of the jack, the 0 point corresponds to the center line of the combination bearing, the positive value indicates the shift to the roll side, and the negative value indicates the shift to the operating side. When the jack moves 40mm to the direction of the roller body, the signal of No. 1 and No. 2 sensors increases significantly, and conversely, the signal of No. 3 and No. 4 sensors increases significantly. It shows that the embedded sensor is sensitive to the change of off-load and can fully meet the test requirements of bearing radial load distribution.

3.2 Response of strain flower to roll bending force

Under the action of roll bending force, the outer surface of bearing seat has a great change of stress. In this experiment, 45° strain flower was used, with strain flower No. 1 located on the roll side and strain flower No. 2 located on the operating side. The stress value of the test point is determined by equation (1) [5].

With the change of the position of roll bending force, the strain and stress measured by the two groups of strain flowers are shown in Table 1.

The middle position in the table corresponds to the 0 position in Figure 3 (b), and the deviation value corresponds to Figure 3 (b). It can be seen from the data in the table that the output of strain flower also changes greatly with the change of the application point of roll bending force. When the load moves to the roll side, the maximum principal stress of strain flower No. 1 increases obviously, and vice versa, the maximum principal stress of strain flower No. 2 increases, indicating that the bearing off-load can cause the change of stress distribution on the bearing seat surface. By comparing FIG. 3 (b) and Table 1, it can be found that when the load application point changes, the change law of the sensor and strain flower is consistent. However, according to the data results, the output of the two groups of strain flowers lacks regularity due to the asymmetric axial structure of the bearing seat, coupled with installation, constraints and other factors. When the load position changes, the maximum principal stress of the strain flower No. 1 changes significantly, while the maximum shear stress of the strain flower No. 2 changes significantly, which lacks intuitive comparison. Because the embedded sensor is relatively independent, it can intuitively reflect the load change of each column of bearings, and is a simple and reliable test method.

3.3 Dynamic simulation of bearing radial load distribution

In order to verify the feasibility of the test scheme in the working state of the rolling mill, it is necessary to carry out dynamic experiments on the experimental device. The speed of the experimental device is 0 ~ 19 r/min, and the roll movement is simulated by the mandrel rotation. At the same time that the mandatum rotates, the jack loads the bearing seat to simulate the effect of roll bending force. The average value of the measured data under various states is shown in Table 2.

As can be seen from Table 2, the dynamic test results are consistent with the static test results. However, in the dynamic case, although the applied roll bending force is smaller than that in the static test, the output of the sensor is relatively large, indicating that the radial force of the bearing is not only related to the roll bending force, but also related to the machining accuracy of parts, assembly accuracy, vibration of the mechanism and other factors. Therefore, the static experiment can only be used as the qualitative analysis of the feasibility study of the scheme, and can not replace the dynamic process as the final result.

4. Field measurement results

Using the above experimental scheme, the research group carried out three large-scale field measurements on the load distribution of the operating side bearing of the lower work roll of F4 stand of Baosteel 2050 hot tandem mill in August 1999 and May 2000, and obtained a series of important data. The transformation according to the test results has obtained obvious results. It is fully affirmed by Baosteel leaders and field technicians, and provides successful experience for improving bearing life of large strip rolling mill. Due to space limitation, only one set of data is listed to illustrate the experimental results. For more detailed information, refer to the bearing load test report of Baosteel 2050 hot tandem rolling mill.

FIG. 4 (a) and (b) respectively show the radial load distribution of bearings before and after the transformation of F4 bearing seat by using the patented technology "roll bearing seat self-defense device". The experimental conditions of both are shown in Table 3.

In the figure, four curves respectively represent the stress situation of four sensors in a rolling pass, with No. 1 and No. 2 located on the side of the roll body and No. 3 and No. 4 located on the operating side. Before the transformation, at the biting stage, the force of No. 1 and No. 2 sensors is about 5 times that of No. 3 and No. 4 sensors, indicating that there is serious biased load in bearings. After the modification, the force of No. 1 and No. 2 sensors is significantly reduced, and the force of 4 sensors tends to be uniform, indicating that the roller bearing seat self-defense device has a remarkable self-defense effect and effectively reduces the bearing off-load. After nearly 1 year of operation test, the bearing life is significantly improved.

The self defense device has reliable life and good operation. Therefore, it was decided on the spot that the F1-F7 stands of the finishing mill group of hot continuous rolling were all equipped with self-defense devices and the bearing seats were reformed. It can be seen from the curve variation trend that when the roll bending force is applied, the partial load borne by the bearing under no-load is much larger than that under rolling. This conclusion can be confirmed by the roll system stress analysis, which will not be detailed here.

5. Conclusions

The experimental method of load distribution of work roller in F4 stand of Baosteel 2050 hot tandem mill is studied and proved by practical application.

(1) The embedded sensor can be used to directly measure the radial load distribution of bearings, and eliminate the mutual interference between the columns of bearings, providing a new radial force testing method for bearings.

(2) The experimental results show that the change of the bearing radial load can also be detected by mounting on the outer surface of the bearing seat, but the relationship between the bearing seat and the force is not linear, which will bring a lot of trouble to the data processing.

(3) Compared with static and dynamic test results, there is a big difference between the two, indicating that the bearing radial load is not only related to load, but also related to a variety of factors such as bearing installation and motion state, so the static test results cannot be used to replace the dynamic process.

(4) The bearing seat and sensor designed according to the similarity theory have reasonable structure and reliable operation, and fully meet the requirements of simulation experiments.

(5) The field measured results show that the design of the bearing radial load sensor is reasonable, the experimental scheme is correct, and the roll bearing seat self-defense device has obvious self-defense effect, which can effectively alleviate the bearing off-load and improve the bearing life.

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