- Velocity: the speed of movement
- Acceleration: the force associated with movement
Velocity is the most useful measure of vibration because it gives us the same diagnosis across all frequencies. 0.5 ips at 1,000 rpm indicates the same vibration severity as 0.5 ips at 10,000 rpm.
Acceleration is the rate of change of velocity and is the measurement of the force being produced. Acceleration is typically expressed in units of mm/sec2 (millimeters per second squared) or gravitational forces (Gs: 1G = 32.17 ft/sec/sec). Acceleration is frequency reated, in that 1 g at 1,000 rpm is not the same vibration severity as 1 g at 10,000 rpm. Acceleration generally emphasizes high frequency peaks in a spectrum.
Amplitude can be expressed in terms of its peak value, or what is known as its Root-Mean-Square (RMS) value. The peak velocity amplitude of a vibrating machine is simply the maximum (peak) vibration obtained by the machine in a given time period. By contrast to the peak velocity amplitude, the RMS velocity amplitude of a vibrating machine tells us the vibration energy of the machine--the higher the vibration energy, the higher the root-mean-square velocity amplitude.