Belt Drive Problems

Worn, Loose or Mismatched Belts

Typical Spectrum

 worn

Belt frequencies are below the RPM of either the motor or the driven machine. When they are worn, loose or mismatched, they normally cause 3 to 4 multiples of belt frequency. Often 2x belt frequency is the dominant peak. Amplitudes are normally unsteady, sometimes pulsing with either driver or driven RPM. On timing belt drives, wear or pulley misalignment is indicated by high amplitudes at the timing belt frequency.


Belt / Sheave Misalignment

Typical Spectrum

alignment.gif

Misalignment of sheaves produces high vibration at 1x RPM predominantly in the Axial direction. The ratio of amplitudes of driver to driven RPM depends on where the data is taken as well as on relative mass and frame stiffness. Often with sheave misalignment, the highest axial vibration will be at the fan RPM.


Eccentric Sheaves

Typical Spectrum

eccentric.gif

Eccentric and/or unbalanced sheaves cause high vibration at 1x RPM of this sheave. The amplitude is normally highest in line with the belts, and should show up on both driver and driven bearings. It is sometimes possible to balance eccentric sheaves by attaching washers to taperlock bolts. However, even if balanced, the eccentricity will still induce vibration and reversible fatigue stresses in the belt.


Belt Resonance

Typical Spectrum

resonance.gif

Belt Resonance can cause high amplitudes if the belt natural frequency should happen to approach or coincide with either the motor or the driven machine RPM. Belt natural frequency can be altered by either changing the belt tension or the belt length. Can be detected by tensioning and the releasing belt while measuring response on sheaves or bearings.


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