Design reasonable parameters such as load, amplitude and vibration frequency, and select appropriate bearing type and accuracy grade

Abstract: through the analysis of the mechanism and causes of the vibration of the rolling bearing of the vibrating screen, we can ensure the normal operation of the vibrating screen by designing reasonable parameters such as load, amplitude and vibration frequency, selecting appropriate bearing type and accuracy grade, reasonably determining the coordination between the bearing and relevant parts, improving the rigidity of the bearing seat, and adopting necessary pre tightening measures and good lubrication.

Key words: rolling bearing; Vibration sieve; Bearing accuracy; Coordination quality; Vibration

The working conditions of the rolling bearing of the vibrating screen are very bad. It not only has to bear a large impact load in the adjusted state, but also completes circular, elliptical or linear motion when rotating around its central axis, and bears high radial acceleration and centrifugal force. Most vibrating screen drive shafts also require a certain shaft deflection, resulting in forced vibration and impact vibration of the bearing. Although the vibration of the bearing itself generally does not affect the amplitude and vibration frequency of the vibrating screen (the amplitude of the vibrating screen is generally 10 times or even more than ten times higher than the amplitude of the bearing), the small amplitude vibration of the frequency component of the bearing will cause resonance of adjacent parts, thus affecting the screening effect of the vibrating screen; At the same time, the vibration of the bearing and the resonance of the relevant parts will cause the vibrating screen to appear / vibrate 0 (the vibrating screen vibrates up and down with a large amplitude), which will greatly reduce the service life of the bearing, screen and other parts. For the vibrating screen, if the bearing itself is poorly manufactured or improperly installed and used, the bearing vibration may also become one of the main factors for the failure of the vibrating screen. Therefore, it is of positive significance to study the vibration mechanism and control method of the bearing for the normal screening of the vibrating screen and to improve the service life of the bearing, screen and other parts.

1 Mechanism of rolling bearing vibration

The main reason for the vibration of rolling bearing is that the rolling bearing deforms under the action of load when the rolling bearing rotates.

1.1 Vibration of bearing ring

The vibration of rolling bearing is basically caused by the vibration of bearing ring, which is related to the structure, rigidity, manufacturing accuracy, installation conditions and service conditions of the bearing. If there are large radial ripples on the raceway of the bearing inner ring, the bearing outer ring will generate radial vibration when the inner ring rotates. The ripple of the inner ring of the bearing will not only cause the radial vibration of the outer ring, but also cause the axial vibration of the outer ring. When the bearing is subjected to axial load, the axial relative position of the inner ring and the outer ring of the bearing will change. Therefore, when the inner ring rotates, the outer ring of the bearing will also generate axial vibration. When the vibrating screen is working, for the bearing rotating at high speed, the vibration frequency caused by the ripple may be equal to the rotation frequency of the exciter shaft, that is, resonance will occur.

1.2 Vibration of cage

The vibration of the cage is mainly caused by the collision between the cage and the vibrating rolling body when the bearing rotates. When the vibrating screen works, various alternating loads such as impact load will cause violent vibration of the cage, which is generally considered to be self-excited vibration of the cage caused by sliding friction between the rolling body and the cage.

Cage vibration is mainly generated in ball bearings and tapered roller bearings, which is easy to occur under the conditions of too large radial clearance, poor lubrication performance, and fluctuation of torque transmitted on the shaft of vibrating screen exciter.

1.3 Rolling body vibration

In the ball bearing and roller bearing of large-scale vibrating screen, when the bearing mainly bears radial load and rotates at low speed, the rolling body will fall down and vibrate. It is found in the test that when the bearing is under radial load and the grease performance is poor, the vibration sound of "click" and "click" will be heard at the initial stage of operation, which is mainly due to the vibration sound generated by the sudden acceleration of the rolling body and the collision with the cage after leaving the load area. This impact sound is unavoidable but will disappear after a period of operation.

When the bearing operates under the radial impact load of the vibrating screen, there are only a few rolling elements inside the bearing bearing bearing the load. Due to the elastic contact with the ferrule / spring 0 support, the rolling elements will generate periodic vibration through the radial load action line (and the rotating shaft center will move vertically or horizontally). Especially when the vibration frequency of the vibrating screen is low, this kind of vibration is more obvious.

1.4 Friction vibration

Friction vibration mainly refers to the abnormal vibration of rolling bearings. Generally speaking, friction vibration is easy to occur on large ball bearings, especially when grease lubrication is used, especially when the lubrication performance of grease is poor. According to the test and observation, the friction vibration of rolling bearing is often directly related to the properties of lubricating oil. When the lubricating oil performance is not good or clean, friction vibration is easy to occur, and it is also related to the processing quality of the outer raceway of the rolling bearing. If a special precise processing method is adopted for this, the friction vibration can be better controlled.

The vibration of rolling bearing is often the result of the above vibration. The test shows that the surface waviness of bearing parts is the main factor that causes the bearing vibration under the normal condition of bearing operation.

2 Factors affecting bearing vibration

2.1 eccentric vibration

At present, most of the exciters used in the vibrating screen are eccentric shaft exciters and box type exciters. The eccentric shaft exciter is easy to install and adjust, but the cost is high, and the eccentric distance cannot be adjusted; The box type exciter adopts the sector eccentric block with adjustable relative position, which can realize the adjustment of the excitation force and achieve the purpose of adjustable amplitude.

When the exciter works, the centrifugal force generated by the eccentric mass causes the eccentric shaft to bend, resulting in the relative deflection of the inner and outer rings of the bearing. Due to the eccentricity, the vibration caused by the rotation frequency and its harmonic frequencies will be generated. Therefore, the inertia force and inertia couple generated in the operation will cause the dynamic reaction force and vibration of the bearing, damage the stable working state of the bearing and other parts, and generate high-frequency vibration. The vibration spectrum analysis of the vibrating screen shows that when the vibration frequency of the exciter bearing is the same as a certain natural frequency of the elastic vibration of the screen box, the strong vibration of the elastic body of the screen box will be caused.

The vibration system composed of bearing and eccentric system can be regarded as a single degree of freedom system. Both the driving shaft and the driven shaft of the bearing have a certain resonance frequency. If a certain excitation frequency is close to the resonance frequency, resonance will occur. In addition, due to the centrifugal inertia force in the eccentricity, bending vibration will occur. If the rotational speed is close to the critical value, bending resonance will occur.

2.2 Bearing geometric accuracy

The large exciting force of the vibrating screen causes the bearing to bear a large radial force, thus generating strong vibration. The higher the bearing accuracy, the smaller the vibration. The waviness of the raceway, especially the surface of the rolling element, has the greatest impact on the vibration of the bearing. The clearance and relative movement between the rolling element and the cage and the rolling surfaces of the inner and outer rings will cause the vibration of the bearing. This is because the rolling element has a high spin frequency and the working surface contacts the inner and outer raceways at the same time. The vibration generated by the rolling element, the ring and the cage is approximately 4:3:1. Therefore, to reduce the bearing vibration, the surface machining accuracy of rolling elements should be improved first. Since waviness is a form error between roughness and roundness, which is produced by rolling elements during grinding, it is difficult to control it. Therefore, other methods should be sought to control the vibration of rolling elements.

2.3 Radial clearance of bearing

Too large or too small radial clearance will cause large vibration of bearing system. Too small radial clearance will cause high-frequency vibration, and too large radial clearance will cause low-frequency vibration. The test results of the large bearing 3e3626 of the vibrating screen show that the radial natural frequency of the elastic system of the bearing is reduced due to the large radial clearance, which is easy to generate resonance and generate large low-frequency vibration. This is because a large acceleration will be generated at the impact point of the rolling element and the ferrule. At the initial stage of impact, high-frequency compression waves independent of mechanical mass and object shape are generated and transmitted into the metal; At the later stage of impact, mechanical vibration with lower frequency than shock compression wave will be generated by mechanical force. Therefore, the large radial clearance increases the vibration of the bearing in the passband.

The test and analysis show that if the radial clearance of the bearing is too large, the bearing system will have large impact vibration. If the radial clearance is too small, the friction temperature will rise quickly due to the large radial force, and the bearing will be burned at high temperature. In addition, the cage will also produce large radial runout with the increase of radial clearance, and then generate large vibration.

2.4 Cooperation

The fit between the outer ring and the bearing hole will affect the transmission of vibration. A tighter fit will force the raceway to deform, increase the shape error, and cause an increase in vibration. Looser fit can make the oil film in the gap produce damping effect. The material properties of the outer ring and the (bearing seat) bearing hole are quite different, especially the rubber damping ring placed between them can suppress the transmission of vibration.

2.5 Friction and lubrication

The bearing is the main vibration source of the vibrating screen which is difficult to control. Since the vibrating screen is maintained by a large excitation force, the radial force on the bearing is large. During the working process of the vibrating screen, the large excitation force will cause the elastic vibration of the bearing system. If the lubrication is poor, there will be large friction, which will cause the bearing temperature rise to be too high and the thermal expansion to be too large, which will significantly reduce the radial clearance, thus intensifying the friction and further improving the temperature rise. For this reason, many vibrating screens generally use large clearance bearings, such as 3e3626 and 3g3624. However, due to the excessive radial clearance, the radial natural frequency of the bearing system is reduced, and the possibility of radial runout of the rolling element after being disturbed and the energy of impacting the ring during the runout are also increased, thus increasing the vibration value of the high-frequency part and generating strong high-frequency vibration.

3 Control method of rolling bearing vibration of vibrating screen

3.1 Reasonable selection of bearings

Rolling bearings are purchased parts for most vibrating screen manufacturers and users. Therefore, how to reasonably select bearing types and geometric dimensions in the design process of vibrating screen is an important factor to reduce bearing vibration. In order to reduce the vibration of bearings, bearings with good rigidity and high vibration resistance shall be generally selected. Fig. 1 shows the comparison of radial stiffness of several rolling bearings. It can be seen from the figure that the radial rigidity of needle roller bearing is the highest and that of ball bearing is the worst. However, needle roller bearings and roller bearings are sensitive to the geometric accuracy and assembly quality of parts, while deep groove ball bearings are on the contrary, with poor rigidity and insensitive to assembly quality. Therefore, it is ideal to select deep groove ball bearings from the perspective of reducing vibration requirements. However, in support with high rigidity requirements, such as large vibrating screen, tapered roller bearing shall be adopted; Self aligning roller bearing can be selected for vibrating screen with large load, severe impact and swing load. Split combined cage made of high-strength brass is selected in the test to improve the limit of cage on the freedom of rolling elements and enhance the stability of rolling elements during rotation. In the test, the polyurethane cushion damping ring is designed to be installed on the outer ring of the bearing, and the effect is ideal.

A) Centripetal ball bearing; B) Double row cylindrical roller bearing; C) Tapered roller bearing; D) Needle roller bearing

Fig. 1 radial stiffness of rolling bearing

3.2 Reasonably determine the coordination of bearing and relevant parts

The fit of the outer ring in the bearing hole will directly affect the vibration of the bearing. If the fit is too tight, it will not only directly improve the transmission of vibration, but also force the raceway to deform, affect the shape error of the raceway, reduce the radial clearance, and cause the vibration to increase. Therefore, the fit between the outer ring and the bearing hole should be appropriately loose. Proper matching can make the oil film at the contact between the bearing and the bearing hole damp the vibration of the outer ring. However, the loose fit of the outer ring will also cause vibration. The roughness and tolerance of the journal matched with the bearing inner ring shall strictly meet the requirements of the selected bearing accuracy grade. If the rolling bearing is installed too tightly on the journal that does not meet the requirements, even if the bearing accuracy is high, the error of the journal will inevitably be transmitted to the inner ring raceway, and large vibration will also occur during the operation. Therefore, in order to eliminate this kind of vibration, in addition to controlling the manufacturing accuracy of the bearing itself, special attention should be paid to the control of the interference during the processing and assembly of the journal during the use and maintenance of the vibrating screen.

Damping treatment is applied between the inner and outer rings of the bearing. The rolling elements of the bearing can be made into hollow rolling elements or damping materials can be added inside the hollow rolling elements to reduce the vibration of the bearing.

3.3 Improve the rigidity of bearing pedestal and necessary pre tightening measures

The stiffness of the bearing pedestal has a great influence on the bearing vibration. Increasing the stiffness of the bearing pedestal can reduce the bearing vibration. Fig. 2 shows the relationship between bearing seat wall thickness T and bearing vibration obtained by using S092 bearing vibration measuring instrument. It can be seen from the curve in the figure that under the same rotating speed, the peak value of bearing vibration decreases with the increase of wall thickness. Therefore, by changing the axial thickness and its shape, the stiffness of the bearing seat can be increased, thereby reducing the vibration of the bearing. In addition, the bearing preload can change the radial clearance of the bearing and increase the contact rigidity. Therefore, preloading of bearing is also an effective measure to reduce bearing vibration.

Fig. 2 Effect of bearing pedestal wall thickness on Vibration

3.4 Good lubrication

Good lubrication also plays a great role in reducing vibration. It can improve the friction performance of rolling elements, raceways and cages, but it is necessary to control the injection amount of grease or lubricating oil and adopt appropriate lubrication methods. Too much lubricating oil will make the bearing agitate at high speed and induce contact resonance. In the test, the lubrication structure of the eccentric exciter of the vibrating screen was improved. The method of circulating ring oil lubrication was adopted to reduce the vibration of the bearing and greatly improve the service life of the eccentric exciter bearing.

4 Concluding remarks

Based on the analysis of the vibration causes and the discussion of the vibration mechanism of the rolling bearing of the vibrating screen, the control methods of the vibration of the rolling bearing of the vibrating screen are summarized through the experimental research, and the following conclusions are reached:

(1) according to the design parameters and specific working conditions of the vibrating screen, the bearing shall be reasonably selected from the aspects of bearing type, accuracy, clearance and cage form.

(2) during the use and maintenance of the vibrating screen, special attention shall be paid to the machining accuracy of the shaft diameter matched with the bearing inner ring and the control of the interference during assembly.

(3) the peak value of bearing vibration decreases with the increase of the wall thickness of the bearing seat, so changing its axial thickness and shape to improve its rigidity can effectively reduce the bearing vibration.

(4) rubber or polyurethane cushion is set between the bearing outer ring and the bearing seat as the damping ring, and the effect is ideal.

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