Use of Bearing vibrometer

2018-09-21 15:37:47

In factory, there are countless rotating equipments, large and small, daily inspection equipment or whenever the equipment is overhauled and installed, it is necessary to measure the vibration amplitude of the bearing to assess whether the equipment meets the operating conditions. The vibrometer is one of the more frequently used tools in power plants, so the correct use of the vibrometer is our basic skill. I have always been ignorant of the use of the vibrometer. For this reason, I have consulted the relevant information on the Internet. I will share it with you.

First, the vibration meter we use

The vibration meter used by the operator, model HG-2508, is a pocket-sized instrument that measures vibration and temperature, with an accelerometer and temperature sensor. Usually we only use its vibration measurement function, and the vibration is divided into three parameters: displacement, velocity and acceleration. There are two parameter setting switches on the top of the vibrometer. The upper left corner is used to select high frequency (HI), low frequency (LO) or temperature, and the upper right corner is used to select displacement, velocity or acceleration. Before using the vibrometer to measure vibration, first check whether the parameters are set correctly, and then check whether the probe is connected properly.

According to the needs of the measurement, before the measurement, respectively, the two wave switches on the top of the instrument are toggled to make the instrument in the measurement state of acceleration, speed or displacement, and then press the measurement button to measure.

When measuring acceleration, place the switch in the acceleration gear so that the display indicates that the unit arrow points to “m/s2” and simultaneously dials the frequency gear according to the actual need to point the frequency indication arrow to “high frequency (HI)” or “low frequency (LO) file. .

When measuring speed, place the switch in the speed range and indicate that the unit arrow points to “mm/s”.

When measuring the displacement, place the switch in the displacement position, indicating that the unit arrow points to "mm".

Note: When measuring speed or displacement, the frequency has no effect, but the switch should be set to either high frequency or low frequency.

Second, measuring amplitude, vibration speed or vibration acceleration?

The amplitude is the representation, and the velocity and acceleration are the extent of the rotor's exciting force. Vibration displacement, generally used for vibration evaluation of low-speed machinery; vibration speed, generally used for vibration evaluation of medium-speed rotating machinery; vibration acceleration, generally used for vibration evaluation of high-speed rotating machinery. The practical vibration velocity of the project is the effective value of the velocity, which is the energy of the vibration; the acceleration is the peak value used to characterize the impact force in the vibration.

Devices with the same amplitude may have different vibration states, so the vibration velocity is introduced. Displacement, velocity, and acceleration are all metrics for vibration measurements. In terms of concept, the measurement of displacement can directly reflect the stress conditions on bearings, fixing bolts and other fixtures. For example, by analyzing the displacement of the sliding bearing on the turbine, the position and friction of the shaft inside the bearing can be known. Speed reflects the fatigue stress experienced by bearings and other related structures, which is an important cause of failure of rotating equipment. The acceleration reflects the combined effects of various forces inside the device.

For simple harmonic vibration, all three are sinusoidal curves with phase differences of 90 degrees and 180 degrees, respectively. The speed peak is 2πf times the displacement peak, and the acceleration peak is 2πf times the speed peak. Of course, pay attention to the peak-to-peak value generally used for displacement, the effective value for speed, and the peak for acceleration. Also note that the displacement measured in the field is the relative vibration of the shaft and the bearing. The velocity and acceleration are the absolute vibration of the bearing. Assuming that the speed of a vibration is constant, it is 5mm/s. You can calculate for yourself that if it is a low-frequency vibration, its displacement will be large, but the acceleration is small. The high-frequency vibration displacement is extremely small and the acceleration is large. Therefore, the displacement is generally used in the low frequency region, the velocity is used in the intermediate frequency, and the acceleration is used in the high frequency region, but the range of use also overlaps. The displacement value reflects the vibration range of the device in space, so its peak-to-peak value is taken. The effective value of the velocity is proportional to the energy of the vibration, and its magnitude represents the magnitude of the vibration energy. Acceleration is proportional to force, generally with its peak value, which indicates the largest impact force in vibration. Equipment with large impact force is more susceptible to fatigue damage. There is no standard of acceleration.

Third, the practical application

In field applications, displacement is the best measurement method for low speed equipment (generally referred to as speed less than 1000 RPM). The equipment speed of the power plant is mostly between 600~4200r/min, and the corresponding vibration frequency is 10~70Hz, which belongs to low frequency vibration. Therefore, more vibration displacement values are selected. For those devices with small acceleration and large displacement, a compromise method is generally adopted, that is, speed measurement is adopted. For high-speed or high-frequency equipment, sometimes the displacement is small and the speed is moderate, but the acceleration measurement is very important for equipment with high acceleration. When the vibration velocity value and the displacement value are both large, the high-frequency acceleration and the low-frequency acceleration value can be separately measured, and then the two values are compared: when the high-frequency value is lower than the low-frequency, the vibration is mainly caused by the low frequency, and the speed should be Standard value judgment. At the same time, the causes of low frequency faults may be considered, such as unbalance, misalignment, shaft bending, looseness of the base, etc.; when the high frequency value is greater than 4 or 5 times of the low frequency value, the vibration is mainly caused by the high frequency. Consideration may be given to causes of high frequency failures such as wear of rolling bearings, tooth surface friction, impact or broken teeth.

It is also necessary to understand the working principle and application choices of the sensor. It is mentioned that the displacement measured by the eddy current sensor and the application of the acceleration sensor by the displacement of the two integrated outputs are completely different. The eddy current sensor measures the relative motion between the bearing and the shaft. The acceleration sensor measures the vibration at the top of the bearing and then converts it into displacement. If the entire bearing vibrates very much, the relative motion of the shaft and the bearing is small, the eddy current sensor cannot reflect such a state, and the acceleration sensor can. Both sensors measure two different phenomena. With this in mind, you can see why many experienced engineers combine eddy current sensors with accelerometers to observe both the vibration of the bearing relative to the ground and the vibration of the shaft relative to the bearing. In this way, a more complete machine state can be obtained.