Relationship and conversion between vibration meter units: mm, mm/s, mm/s²

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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: It can be regarded as the distance traveled; its unit is mm. It is generally used for evaluating vibrations in 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 vibration velocity that is practical in engineering 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, that is, 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 eclectic approach is generally adopted, namely velocity measurement. For high-speed or high-frequency devices—devices where, even though the displacement is minimal and the speed is moderate, the acceleration can still be very high—acceleration measurement is a crucial technique to employ. 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, which is then converted into displacement. If the entire bearing vibrates severely, but the relative motion between the shaft and the bearing is minimal, the eddy current sensor cannot detect such a condition; however, the acceleration sensor can. The two sensors measure two different phenomena. Understanding these points, you can see why many experienced engineers use vortex sensors in combination with acceleration sensors – this allows them to observe the vibration of the bearing relative to the ground, as well as the vibration of the shaft relative to the bearing; by doing so, a more complete picture of the machine’s condition can be obtained. Click to view -- Summary of chemical industry professional skills training courses in 2023. For a vibration at a single frequency, the peak velocity is 2πf times the peak displacement; the peak acceleration, in turn, is 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 shell, while the velocity and acceleration measurements refer to the absolute vibration of the bearing shell. 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 displacement due to high-frequency vibration is extremely small, while the acceleration is very large. Therefore, generally, displacement is used in the low-frequency range, velocity in the mid-frequency range, and acceleration in the high-frequency range. However, their areas of use also overlap. The displacement value indicates the range of spatial vibration of the device; thus, its peak-to-peak value is taken. In the power industry, displacement is generally used as a criterion for evaluation. The effective value of velocity is proportional to the energy of vibration; its magnitude represents the amount of vibrational energy. Currently, in almost all industries except for the power industry, the effective value of velocity is 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 should not exceed 0.05 mm ； The shaft runout should not exceed 4 mm.