Determination of rpm from vibration spectral plots based on percentage RPM

A vibration analyzer for use in determining a rotational speed. The vibration analyzer includes an input for sensing vibration data, a memory for storing the vibrational data, and a processor. The processor produces a spectral plot of the vibrational data, locates peaks in the spectral plot, receives an input rotational speed, and scans the spectral plot in predetermined rotational speed increments to provide a candidate rotational speeds. For each candidate rotational speed, a number of associated harmonics is identified, closest peaks in the spectral plot to the candidate rotational speed and its harmonics are located, gaps between the closest peaks and the candidate rotational speed and its harmonics are measured and summed, and a sum of the gaps is recorded. The candidate rotational speed that is associated with a minimum sum is selected as the nominal rotational speed.