Relationship and conversion between vibration meter units: mm, mm/s, mm/s&#178%!

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Relationship and conversion between vibration meter units: mm, mm/s, mm/s²! Vibration can generally be expressed in the following three units: mm, mm/s, and mm/s², which correspond to amplitude, vibration velocity, and vibration acceleration respectively. Amplitude is a representation, while velocity and acceleration indicate the magnitude of the rotor excitation force. Vibration displacement: To be understood as distance, measured in mm; it is generally used for evaluating the vibration of machinery operating at low speeds ； Vibration velocity: To be understood as velocity, with the unit of mm/s; it is generally used for evaluating the vibration of machinery operating at moderate speeds ； Vibration acceleration: To be understood as motion acceleration, with the unit of mm/s²; it is generally used for evaluating the vibration of high-speed rotating machinery. The practically useful vibration velocity is the effective value of the velocity, which represents the energy of the vibration. Acceleration uses the peak value to represent the magnitude of the impact force in vibration. Speed describes how fast or slow motion is ； Vibration speed refers to the rate of vibration, namely the amplitude that can be generated in one second. For devices with the same amplitude, their vibration states can differ; therefore, vibration velocity is introduced. Displacement, velocity, and acceleration are all measurement parameters for vibration. Conceptually, the measurement of displacement can directly reflect the stress conditions on the bearing fixing bolts and other fasteners. For example, by analyzing the displacement of the sliding bearings on a turbine, it is possible to determine the position of the shaft within those bearings as well as the level of friction ； Speed reflects the fatigue stress experienced by bearings and other related components, and this is precisely a major cause of failures in rotating equipment ； Acceleration reflects the combined effect of various forces within the device. In terms of expression, all three are sine curves with phase differences of 90 degrees and 180 degrees respectively. In practical applications, for low-speed devices (with rotational speeds below 1000 RPM), displacement is the best measurement method. For those devices with small acceleration but large displacement, an incremental approach is generally adopted, that is, velocity measurement. For high-speed or high-frequency devices, where the velocity may be moderate despite a small displacement, but the acceleration can still be very high, using acceleration measurement is a very important approach. It is also necessary to understand the working principle of sensors and the options for their application. For example, the displacement measured using an eddy current sensor differs significantly from the displacement obtained by integrating the output from an acceleration sensor twice. Eddy current sensors measure 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 severely and the relative movement between the shaft and the bearing is minimal, the eddy current sensor will not be able to detect such a condition, whereas the acceleration sensor can. The two sensors measure two different phenomena. With this understanding, you can see why many experienced engineers use vortex sensors and acceleration sensors together, so as to be able to observe the vibration of the bearing relative to the ground as well as the vibration of the shaft relative to the bearing; in this way, a more complete picture of the machine’s condition can be obtained. For vibration at a single frequency, the peak velocity is 2πf times the peak displacement, and the peak acceleration is again 2πf times the peak velocity. Of course, it should be noted that peak-to-peak values are generally used for displacement, the root mean square value for velocity, and the peak value for acceleration. It should also be noted that the displacement measured on-site represents the relative vibration of the shaft and the bearing shells, while the velocity and acceleration measure the absolute vibration of the bearing shells. Assume that the velocity of vibration is constant at 5 mm/s. One can calculate for themselves that in the case of low-frequency vibration, the displacement will be large, but the acceleration will be small ； The high-frequency vibration displacement is extremely small, while the acceleration is very large. So generally, displacement is used in the low-frequency range, velocity in the medium-frequency range, and acceleration in the high-frequency range. However, their areas of use also overlap. The displacement value reflects the range of vibration of the equipment in space; therefore, the peak-to-peak value is used. The power industry generally employs displacement as a criterion for evaluation. The effective value of the speed is proportional to the energy of vibration, and its magnitude represents the amount of vibration energy; in the power industry, the effective value of the speed is generally used as the standard. Acceleration is proportional to force; generally, its peak value is used, as it indicates the maximum impact force during vibration. Greater impact forces cause equipment to suffer from fatigue and damage more easily. At present, there is no standard for acceleration. Acceptable vibration standards for rotors at different speeds: For rotors with a rated speed of 750 r/min, the bearing vibration value shall not exceed 0.12 mm ； The vibration value of the bearing in a machine with a rated speed of 1000 r/min should not exceed 0.10 mm ； The vibration value of the bearing in a machine with a rated speed of 1500 r/min should not exceed 0.085 mm ； For rotating machinery with a rated speed of 1500 r/min or higher, the vibration level of the bearings shall not exceed 0.05 mm ； The shaft runout should not exceed 4 mm.