The displacement, velocity and acceleration characteristics of vibration are measured to determine the severity of the vibration and these are often referred to as the ‘amplitude’ of the vibration.
In terms of the operation of the machine, the vibration amplitude is the first indicator to indicate how good or bad the condition of the machine may be. Generally, greater vibration amplitudes correspond to higher levels of machinery defects.
Motion below 10 Hz (600 cpm) produces very little vibration in terms of acceleration, moderate vibration in terms of velocity and relatively large vibrations in terms of displacement (see Figure 2.11). Hence, displacement is used in this range.
In the high frequency range, acceleration values yield more significant values than velocity or displacement. Hence, for frequencies over 1000 Hz (60 kcpm) or 1500 Hz (90 kcpm), the preferred measurement unit for vibration is acceleration.
It is generally accepted that between 10 Hz (600 cpm) and 1000 Hz (60 kcpm) velocity gives a good indication of the severity of vibration, and above 1000 Hz (60 kcpm), acceleration is the only good indicator.
Since the majority of general rotating machinery (and their defects) operate in the 10–1000 Hz range, velocity is commonly used for vibration measurement and analysis.
