Mechanical looseness can be of very different nature: not enough tightening between different mechanical elements (loosening of bolts, fracture of fixations, etc.), increase of tolerances produced by wear and tear (excessive gaps in rolling element bearings, sleeve bearings, gears, etc.), etc. Looseness manifests in the vibration measurements provided that there is a source of excitation due to a problem other than looseness, such as unbalance, misalignment, etc., so that small external excitation forces can result in high vibration amplitudes, in the presence of looseness.

Looseness is characterized by the presence in the frequency spectrum of multiple harmonics of the shaft rotating speed. The vibration levels experienced by the machine are often important. Depending on the evolution of the looseness, harmonic peaks of the rotating frequency (1x, 2x, 3x, etc.), half harmonics (1,5x, 2,5x, 3,5x, etc.) and even subharmonics of that same frequency (0,5x) can appear in the spectra. On the other hand, the time waveform will be characterized by being erratic and without any pattern of repetition.

We can make a general classification of looseness, distinguishing between looseness of rotating elements (rolling element bearings, sleeve bearings, couplings, rotors, etc.) and structural looseness (benches, casters, anchor bolts, etc.). Each one of these has its own characteristics that are discussed below.