Mismatched, worn or stretched belts generate vibrations at the belt frequency and its harmonics. In a system with two sheaves the second harmonic is usually dominating. The The fundamental belt pass frequency is calculated as following:
FBF = πfD/L
FBF = Fundamental belt pass frequency
D = Diameter of the sheave
L = The length of the belt
f = The rotational speed (Hz) of the sheave D
The fundamental frequency is always lower than 1X.
Eccentric sheaves generate high radial 1X components, especially in the direction parallel with the belts. (Radial means the direction from the transducer to the centre of the sheave.)
Bad alignment of a sheave generates axial vibrations at 1xRPM and axial harmonics of the fundamental wave frequency, FBF, of the belt.
If the belt tension is not correct standing wave vibrations appear in the belt. The frequency can vary within a large range.
|Source of vibration||Exciting frequency||Dominating direction||Amplitude||Spectral characteristic||Comments|
|Worn or streched belts||Multiples on belt frequency (B). 2B usually strongest||Radial in line with the belts||May be irregular pulsing if 2B is close to the speed of one of the sheaves.||B is always less than 1X|
|Eccentric and/or unbalanced sheavesr||1 X axle||Radial||Constant||Often mistaken for unbalance.|
|Bad alignment of belt or sheave||1 X driving||Axial||Constant||Verify with stroboscope|
|Belt resonance||Varying||Radial||May be irregular||Belt resonance related to speed|
|Bad belt tension||May produce belt resonance. May increase wear of bearing.|