Vibration is a secondary effect reflecting the problem which is actually of electrical origin. DC Motors - The current spectrum from a DC motor can also be used to find tuning problems with DC drives. Improperly tuned drives will produce frequencies at the oscillation rate of the instability. These frequencies can also appear in the vibration spectra and are very difficult to analyse since they do not have a mechanical origin. Gear boxes which have common prime factors between the teeth of intermeshing gears can produce subharmonics of the toothmesh frequency.
Side bands are often the result of modulation by a defective component in a gear box.
However, beware of making hasty conclusions when analysing a planetary gear box, where modulation naturally occurs due to a continually varying transmission path caused by rotation of the planetary gears. When making torsional measurements, pulse trains need to be placed on anti-nodes and strain gauges installed on nodes. .