This gets the shaft up on the oil film and decouples the end bell from the shaft, so that a realistic natural frequency can be determined. A simple way to determine a mode shape is to take a transfer function at equally spaced points along the structure, then plot the normalized amplitudes of the imaginary components above the location of the test points on a scaled plot. If velocity data are used, plot the normalised values of the real part of the transfer function. DC Motors - The spectrum of the current to a DC motor can be used to find problems with SCRs or firing circuits. It is much simpler to look at the current spectrum than to try to see the problem with vibration.
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.