Factors Affecting the Vibration of Rolling Bearing

2016/02/23  Bmschem

Factors affecting the vibration of rolling bearing:

1 Eccentric Vibration

      At present, vibrating screen uses vibration exciters which are mostly eccentric shaft and box-type exciters. The eccentric shaft vibration exciter has convenient installation and adjustment, while its cost is higher and the eccentric distance can not be adjusted. Box-type vibration exciter, adopted fan-shaped eccentric block whose relative position is adjustable, can achieve adjustable vibrating force so as to attain adjustable amplitude.

     When vibration exciter works, the centrifugal force generated by eccentric mass bends the eccentric shaft and leads to the relative deflection of bearing inner and outer rings. Rotational frequency and harmonic frequency are generated due to the eccentricity and cause vibration. So inertia force and inertia couple, generated in the operation, will cause dynamical reaction and vibration of the bearing, destroy the stable working state of bearing and other parts, generate high-frequency vibration. According to the vibration spectrum analysis of vibrating screen, strong vibration of screen box elastomer will be caused when the vibration frequency of vibration exciter is same to a certain natural frequency of screen box’s elastic vibration.

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

2 Geometric Accuracy of Bearing

     Large exciting force of vibrating screen makes bearing bear a large radial force, which produces strong vibration. The higher the bearing precision is, the smaller the vibration will be. Rollaway nest, especially the waviness on the surface of rolling element, has a great influence on bearing vibration. The clearance and relative motion between rolling element and holder, inside and outside rolling surface will cause bearing vibration. This is due to the high spin frequency of rolling element, the contact between working surface and inside and outside rollway, the 4:3:1 vibration generated by rolling element, ferrule and holder. So we should improve the surface working accuracy of rolling element to reduce bearing vibration firstly. Waviness is a kind of form error between roundness and circular degree and is produced in the grinding process of rolling element. To control its production is difficult, so we should seek other ways to control the vibration of rolling element.

3 Bearing Diametral Clearance

    Radial internal clearance no matter too big or too small will cause bigger vibration of bearing system. Too small radial clearance causes high frequency vibration and too big leads to low frequency vibration. The test results of 3E3626 vibrating screen with large size bearing show that due to overlarge radial clearance, the radical natural frequency of bearing elastic system is reduced, easy to produce resonance and larger low frequency vibration. This is due to the large acceleration generated at the impact point of rolling element and ferrule. In the early stage of shock, high frequency compression waves, which are independent of mechanical quality and object shape, are produced and introduced into the inside of metal. In the later stage of shock, mechanical force will produce mechanical vibration whose frequency is lower than shock compression wave. Therefore, overlarge radial internal clearance will intensify the vibration of bearing on transmission bands.

     Test analysis showes that overlarge bearing radial clearance brings bearing system with large impact vibration. Due to large radial force, too small radial clearance leads to rapid rising of friction temperature and burn of high temperature bearing. In addition, the holder also has a greater radial run-out with the increasement of radial clearance, which results in big vibration.

4 Coordination

   The coordination between the outer ring and bearing hole affects vibration transmission. Tight fit deforms rollaway nest, increases form error and results in the increasing of vibration. Loose fit can make oil films in clearance generate damping effect. There is a big difference in material properties between outer rings and bearing holes. When rubber damping rings are placed between them, vibration transmission can be restrained.

5 Friction and Lubrication

    Bearing is the main vibration source which is difficult to control by vibrating screen. Vibrating screen relys on exciting force to maintain work so bearing bears a large radial force. In the working process of vibrating screen, large exciting force will cause the elastic vibration of bearing system. If the lubrication is bad, greater friction will be produced and bearing temperature will rise too high. The thermal expansion is too large, which makes radial clearance significantly reduced and friction increased, then the temperature further increases. Therefore, a lot of vibrating screens adopt large bearing clearance. Overlarge radial clearance reduces radial natural frequency of bearing system. At the same time, it increases radical run-out possibility of rolling elements after disturbance and the energy of impacting ferrule during the run-out. It thereby increases the vibration value of high-frequency parts and produces strong high-frequency vibrations.