Analysis on the main causes of vibration failure of fan motor and some effective treatment methods

Fan vibration is a common phenomenon in operation. As long as it is within the vibration control range, it will not cause much impact. However, after the vibration of the fan exceeds the standard, it will cause damage to the bearing seat or motor bearing, loosening of the motor anchor bolts, damage to the fan casing, blades and air ducts, burning and heating of the motor, and other faults, which will reduce the working performance of the fan, and even make it impossible to work at all. Serious accidents may be caused by vibration, endangering personal health and working environment. The company has experienced friction between the impeller and the shell due to large fan vibration,

1、 Fan bearing box vibration

The common fault of the fan is the vibration of the bearing box, which can be preliminarily diagnosed through external detection. There are traces of faults caused by bearing box vibration, which is a process of vibration from small to large and occurring slowly. The company uses a vibration meter to regularly detect the vibration value of the bearing box of the fan, compare the vibration value, quickly make correct analysis and treatment, and carry out planned maintenance of the fan that may fail in advance, so as to ensure the safe and stable operation of the fan.

1. Vibration caused by unbalanced rotor mass

Most of the fan bearing box vibration occurred in the company is caused by the mass imbalance of the rotor system. The main reasons for the unbalance of rotor quality are: uneven wear or corrosion of impeller; There are uneven dust deposits or attachments on the impeller surface; The impeller is not dynamically balanced after repair welding; Loose parts or loose connections on the impeller.

The characteristics of vibration caused by rotor imbalance are shown by the data measured by vibration meter: (1) the vibration value is larger in radial direction and smaller in axial direction; (2) The vibration value increases with the increase of rotating speed.

For the vibration caused by rotor imbalance, we have developed a series of preventive measures. Because the induced draft fan used by the company is mainly to pump the asphalt flue gas generated in the baking furnace room out of the flue in time, the fan impeller is easy to be corroded, and the surface and other parts of the cavity are easy to stick ash, resulting in uneven ash deposition or attachments, resulting in fan rotor imbalance and fan vibration.

Therefore, the company requires the team members to measure the vibration of the fan every day and record the data. The technicians will make statistical tabulation and analysis of the measured data, and determine whether the fan impeller needs to be repaired, whether the dust and tar on the blade need to be cleaned, or whether dynamic balance correction is needed through the change of amplitude curve.

The curve change of fan bearing box vibration can be seen intuitively. When there is obvious change, we can take corresponding measures to deal with it, so as to avoid system shutdown caused by fan vibration fault. According to years of production practice, the impeller of the induced draft fan of the company is maintained and cleaned once a quarter, which ensures the smooth and reliable vibration of the fan, reduces the wear of the impeller, and protects the dynamic balance of the fan rotor.

2. Vibration caused by abnormal rolling bearing

(1) Poor bearing installation

When there are problems in the assembly of bearings, such as improper installation of bearings, brute force installation is used, resulting in bearing deformation; The installation is inclined, there is deviation or it is not installed in place, resulting in too small bearing clearance. The inner and outer rings are not in the same rotation center, resulting in misalignment.

Its vibration characteristics are: the axial vibration value is larger, and the radial vibration value is smaller; The vibration frequency is the same as the rotation frequency.

(2) Bearing surface damage

If the purchased bearings are of poor quality and the clearance is unreasonable; Fatigue damage caused by long-term overload operation and over life use; The lubrication is not in place, the lubrication method is incorrect, or the lubricating oil is selected incorrectly; Foreign matters enter and cause pollution, etc. It will cause damage to the bearing surface, wear and deformation of the ball, metal peeling off on the raceway surface, serious wear of the race raceway, fracture of the cage, reduce the operating accuracy of the shaft, and make the bearing seat vibrate.

The characteristics of vibration are as follows: the amplitude may be large in both radial and axial directions; The stability of vibration is poor, independent of load; The bearing housing is heated at the damaged bearing.

3. Vibration caused by insufficient stiffness of bearing housing foundation

The secondary grouting quality of the bearing pedestal foundation is unqualified, such as the movement of the foundation bedplate sizing block, the high foundation sizing block, the oil leakage of the bearing pedestal, the insufficient concrete strength, and the loosening of the anchor bolts, nuts, and gaskets will cause severe resonance. In serious cases, the anchor bolts will break, and the bolt holes of the bearing pedestal will break, resulting in the scrapping of the bearing pedestal.

Characteristics of this kind of vibration: the vibration value of the bearing box at the problematic anchor bolt is large, the amplitude is large in the radial direction, and the vibration frequency is generally an odd number of times the speed.

Handling of such faults: strengthen daily inspection and regularly tighten the bearing seat and motor anchor bolts and nuts.

4. Vibration caused by abnormal coupling

Because the induced draft fan and the motor shaft are not concentric, or there is no gap between the induced draft fan and the motor coupling, it will also cause severe vibration of the bearing seat and the motor. Its vibration characteristics are:

(1) The vibration is unstable. With the change of load, the amplitude is small at idle and large at full load. The larger the axis deviation is, the greater the vibration is;

(2) There is also obvious vibration on the output shaft side of the motor, which separates the induced draft fan from the motor coupling, and the motor operates alone, and the vibration disappears.

The method to deal with such vibration: re align the coupling center and adjust it to reduce the eccentricity of induced draft fan and motor shaft; Reasonably reserve the gap between induced draft fan and motor coupling.

2、 Vibration caused by critical speed of rotor

In the process of starting and accelerating the fan, when it reaches a certain speed, the fan will suddenly produce violent vibration. After passing this speed, the vibration will gradually decrease if it continues to accelerate. This phenomenon will also occur in the process of shutdown. The speed corresponding to the natural frequency of the fan is called the critical speed of the fan. The greater the mass of the shaft, the smaller the stiffness (slender shaft), the lower the critical speed, and vice versa. When the working speed of the fan is higher than the critical speed, it is prone to vibration.

Its vibration characteristics are: the relative vibration at the resonance of the object is large; The vibration frequency is the same as or close to the rotation frequency.

3、 Vibration of induced draft fan caused by vibration of air duct system

The vibration caused by the pressure pulsation and disturbance of the air flow in the air duct system mainly includes the vibration caused by the vortex pulsation of the bellows, the vibration caused by the local vortex of the air duct, the vibration caused by the insufficient stiffness of the fan casing and the air duct wall, and the rotation stall. Its vibration characteristics are: the pressure wave is often irregular, the amplitude increases with the increase of flow, the vibration is irregular, and the amplitude increases with the increase of load. The main measures to deal with this kind of vibration are fastening screws, strengthening volute and air duct or strengthening the base.