Vibration spectrum analysis with vibration analyzer

          


Vibration is an important characteristic of the return mechanism. Using data collector, such as vibration analyzer to collect vibration information of mechanical equipment running state, and then through vibration spectrum analysis, we can quickly and accurately diagnose the causes of faults, such as rotor imbalance, shaft bending, bearing damage and looseness, shafting misalignment, so as to achieve early fault detection, diagnosis quickly and timely, conclusion fixed-point quantitative, mechanism Clearly understand the purpose. Mechanical vibration analyzer is the basic instrument for condition monitoring and predictive maintenance in the factory, and it is also a powerful tool for equipment reliability management and TPM. It is easy to operate, suitable for equipment maintenance and spot check personnel, and also suitable for production operators. It is used to measure, record and track the machine status, find abnormalities, and diagnose and trend monitor common machine vibration faults.

 

Most common faults of rotating equipment:


It mainly includes the following faults:

Resonance ● unbalance ● misalignment ● shaft bending

● mechanical looseness ● motor problem ● sliding bearing problem

● rolling bearing problem ● gear problem ● belt problem

● fan problem ● pump problem ● compressor problem ● turbine problem

 

Resonance is a common problem in rotating machinery. The resonance of rotating parts such as shafts is usually called critical speed. Resonance exists in all parts of a structure, even in pipes, cement floors, etc. it is important to avoid the machine running at the frequency that causes resonance. A simple way to identify resonance is to compare the vibration values of the same bearing in three directions: horizontal, vertical and axial. If the vibration in one direction is more than three times of that in other directions, the machine may have resonance in that direction.



 

The way to solve the resonance problem is to change the rotating speed of the machine under possible conditions, and the common solution is to change the mass or rigidity of the machine structure.  



 

When the center of gravity of the rotating parts is inconsistent with the center of rotation, that is, when the mass is eccentric, imbalance occurs. The unbalanced rotor produces centrifugal force, which damages the bearing and reduces its service life. A displacement of the center of gravity of just a few millimeters can cause very large forces. The unbalance causes obvious frequency rotation vibration.



 

Misalignment refers to the misalignment of two coupled axes. If the state centerline is parallel, it is called parallel misalignment. If the axis centerline intersects at one point, it is called angular misalignment. In reality, the misalignment is a combination of two types.



 

The vibration caused by shaft bending is similar to misalignment. Shaft bending may be caused by uneven heating of rotor caused by motor rotor cage bar fault. If the bending occurs in the center of the shaft, the dominant vibration is 1 x rpm, and if the bending occurs in the proximity, coupling, the dominant vibration frequency will be 2 x rpm.  



 

There are two kinds of mechanical looseness, rotating and non rotating. Rotating looseness means that there is too much space between the rotating and fixed parts of the machine; non rotating looseness means that the gap between the two fixed parts is too large. Both of them produce excessive 1x RPM harmonic vibration in three measuring directions.



 

The motor has the same faults as other rotating machines, but some faults are unique to the motor. Such as rotor thermal bending, air gap eccentricity, rotor looseness, eccentric rotor, coil looseness, rotor cage bar failure, etc.

 

The problem with plain bearings is that they often produce vibration peaks at frequencies less than 1 x rpm, known as subsynchronous peaks. Sometimes there are even harmonics of these peaks, indicating very worn bearings.  

Oil film whirl: oil film whirl is the oil film that excites the frequency vibration between 0.38 x rpm and 0.48 x rpm. The vibration is caused by the abnormally large margin and small radial load. It exerts a pressure on the oil film and drives the shaft to move along the bearing. Whirling of the oil film can cause the oil to not lubricate the shaft. Changing the viscosity and pressure of the oil and the related load will also affect the oil whirl.   



 

The vibration caused by bearings is called bearing tone. All rolling bearings produce a certain degree of tone. The more serious the bearing is worn, the higher the degree of bearing tone.  

The vibration produced by each fault bearing is not an accurate 1 x RPM harmonic frequency, that is, the asynchronous vibration components. In addition to these components, the fault of the bearing produces broadband noise.   



 

? in a flawless gearbox, the relative dominant tone appears at the meshing frequency, which is the number of teeth multiplied by the speed (RPM frequency). When the gearbox is used for a period of time, the meshing composition of the gear decreases because the edge of the tooth is slightly rounded. However, the continuous wear will increase the meshing vibration level again. This vibration level is also affected by the alignment of the gear shaft.



 

Worn or loose belts produce the vibration of belt frequency and its harmonic frequency. In a system with two pulleys, the second harmonic frequency usually dominates. The basic belt passing frequency FBF is calculated as follows:

FBF  = π (D/L) RPM

FBF = basic belt passing frequency

D = pulley diameter

L = belt length RPM = speed of pulley D (Hz)

The basic belt frequency is always less than 1x rpm.

Eccentric pulley produces high radial 1x component vibration, especially in the direction parallel to the belt (radial refers to the direction from the sensor to the pulley Center).

The axial 1xrpm vibration and the axial harmonic frequency FBF of the basic fluctuation frequency of the belt are produced by the poor belt wheel pair.

If the tension of the belt is not correct, the belt will produce vibration of natural frequency, which is taken in a large range.

 

Fans are usually produced on blades