The following images are designed to provide typical examples of the vibration spectra that will result from different problems a machine might experience. They are probability based, so field testing should always be performed regardless of how 'sure' you are of the diagnosis.
Remember that any diagnosis made from plot interpretation can be essentially be characterised as an assumption based on an estimate.
Cocked Bearings and Shafts Bent @ Bearing
Housing Distortion: Soft Foot, Piping Stress, etc.
Belt Problems: Wear, Resonance, etc.
Belt Problems: Pulley Alignment
Belt Problems: Eccentric Pulley / Bent Shaft
Sleeve Bearings (Looseness/Rubs)
Rolling Element Bearings
Hydraulic / Aerodynamic
Pump Cavitation / Flow Turbulence
AC Motor Problems: Broken / Cracked Rotor Bars
AC Motor Problems: Uneven Air Gap
AC Motor Problems: Loose Rotor Bars / Windings
DC Motor Problems: Drive Problems
DC Motor Problems: Speed Fluctuations
Gear Problems: Misalignment
Gear Problems: Wear, Eccentricity, Backlash