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Impact On Vibration
- Jan 12, 2019 -

Impact on vibration

In the past, simple mathematical subtraction was used to compensate for vibration levels in the case of low speed roller radial runout. If the amplitude of the vibration between the peaks is 1.6 mils, and the radial runout of the low speed rolls is also known, for example 0.45 mils, then (1.6-0.45) = 1.15 mils is considered true vibration.

In fact, this is not true, because the vibration and the low-speed roll radial runout are both waveforms, and can not be simply added or subtracted before filtering. Unfiltered vibrations contain the frequency components contained in all input signals. At operating speed, if a vibration signal is filtered by a specific frequency, for example, it can be expressed by amplitude and phase angle, it can be described as a vibration vector. As a vector, filtered vibrations at a given frequency (e.g., 1 or 2 times) can be compensated in a vector addition manner using filtered low speed rolls at the same frequency.

According to API 541, the filtered and compensated vibration displacement at the operating speed frequency should not exceed 80% of the unfiltered wave limit. In general, motor manufacturers do not use compensation, but in some cases it is useful. Compensation may also increase vibration depending on the angular position of the vector.

Factors affecting radial runout

The mechanical radial runout is the deviation between the measuring axis and the perfect cylindrical surface. It is primarily affected by manufacturing and assembly processes and changes in the motor over time during operation. Improper selection of cutting tools or machining parameters can result in higher surface finish. Mechanical damage such as scratches, nicks, and bends that occur on the bearing journal or detector track can affect the radial runout of the machine.

Since the measurement of the radial runout is made with reference to the bearing journal, if the trajectory of the detector is not concentric with the bearing journal, it can result in high maintenance, repair and overhaul costs. It is also affected by the following conditions:

The straight shaft is pressed into a curved rotor;

The curved shaft is pressed into a straight rotor;

Misalignment due to improper fixing between the motor frame and the bearing bushing;

The rotor is recessed or bent due to thermal instability within the rotor.

Electrical radial runout is a measure of the non-uniformity of the material of the shaft. When a non-contact eddy current detector is used to measure electrical radial runout, the interaction between the emitted magnetic field and the induced magnetic field is converted to distance. Any phenomenon that can change the magnetic interaction between the detector probe and the shaft can affect the radial runout. This includes non-uniformities in the texture of the material, non-uniformities in electromagnetic properties, or axes that have been magnetized. As a result of the forging or hot rolling process, the machining of the shaft can affect the metal properties of the material, thereby affecting the electrical radial runout.

According to the definition of the API, the low-speed roll radial runout of the motor and generator is a combination of electrical and mechanical combination, radial runout on a rotating shaft at a low speed of 200 to 300 rpm. Measurement. Since radial runouts can affect vibration readings and can cause measurement errors, it is important to understand the various influencing factors and how to eliminate them.

Monitoring the radial runout level during the manufacturing process helps to avoid equipment disassembly and return the rotor to the machine or grinder for rework. If the limit of the radial runout of the low speed roller is not met after the machine is assembled, it is costly for both the manufacturer and the customer.