The Relationship Between the Vbration Units mm, mm/s, mm/s²
Generally, the vibration measurement of rotating machinery can be expressed in the following three units: mm, mm/s, mm/s², that is, vibration displacement (amplitude), vibration velocity (vibration velocity), vibration acceleration.
Vibration displacement (amplitude): It can be understood as a distance in mm, which is generally used for low-speed mechanical vibration assessment;
Vibration speed (vibration velocity): It can be understood as speed, the unit is mm/s, and it is generally used for vibration evaluation of medium-speed rotating machinery;
Vibration acceleration: understood as motion acceleration, in mm/s², it is generally used for vibration evaluation of high-speed rotating machinery. The vibration velocity of engineering practice is the effective value of the velocity, which characterizes the energy of the vibration.
The vibration velocity of engineering practice is the effective value of the velocity, which characterizes the energy of the vibration. Acceleration is used as a peak to characterize the magnitude of the impact force in vibration.
Speed describes the speed of movement, and the speed of vibration is the speed of vibration, the amplitude that can be generated in one second. Devices with the same amplitude may have different vibration states, so vibration velocity is introduced.
Displacement, velocity, and acceleration are all measurement parameters for vibration measurement. Conceptually, the displacement measurement is a direct reflection of the stress profile on the bearing fixing bolts and other fixing parts. For example, by analyzing the displacement of the steam turbine sliding bearing, we can know the position and friction of the rotating shaft in the bearing; The velocity reflects the fatigue stress of the bearing and other related structures, which is one of the important causes of rotating equipment failure; Acceleration, on the other hand, reflects the combined effect of various forces within the device.
The expression of the three is sinusoidal curve, with a phase difference of 90 degrees and 180 degrees respectively. In the field, displacement is the best measurement method for low-speed equipment (speed less than 1000r/min). For those devices with small acceleration and large displacement, a compromise method is generally adopted, that is, velocity measurement is adopted. For high-speed or high-frequency equipment, the acceleration can be very high, even though the displacement is small and the speed is moderate, so it is important to use acceleration measurements.
It is also important to understand how the sensor works and how to use it, for example, the displacement measured with an eddy current sensor is completely different from the displacement measured by the accelerometer through two integrated outputs. Eddy current sensors measure the relative motion between the bearing and the shaft; Accelerometers measure vibration at the top of the bearing, which is then converted into displacement. If the whole bearing vibrates very strongly, and the relative motion between the shaft and the bearing is very small, the eddy current sensor cannot reflect such a state, while the accelerometer can. Two sensors measure two different phenomena.
With this in mind, you can see why many experienced engineers use a combination of eddy current and accelerometer sensors to observe both the vibration of the bearing relative to the ground and the vibration of the shaft relative to the bearing, which provides a more complete condition of the equipment.
For a single frequency vibration, the peak velocity is 2πf times the displacement peak, and the acceleration peak is 2πf times the peak velocity. Of course, it is necessary to pay attention to the peak value used for displacement, the effective value for velocity, and the peak value for acceleration. It should also be noted that the displacement measured in the field is the relative vibration of the shaft and the pad, and the velocity and acceleration measure the absolute vibration of the pad. Assuming that the speed of a vibration is 5mm/s, you can calculate by yourself that if it is a low-frequency vibration, its displacement will be very large, but the acceleration will be very small; The displacement of high-frequency vibrations is extremely small, and the acceleration is very large. Therefore, displacement is generally used in the low frequency area, velocity is used in the medium frequency, and acceleration is used in the high frequency area.
However, there is also an overlap in the scope of use. The displacement value reflects the range of vibrations of the device in space, so its peak-to-peak value is taken. The RMS value of velocity is proportional to the energy of the vibration, and its magnitude represents the magnitude of the vibrational energy. Acceleration is proportional to the force, generally used as its peak, its size indicates the maximum impact force in vibration, and the equipment with large impact force is more likely to fatigue and damage.
