General Problem: Belt is worn out or is loose on the pulley.
How to detect it in a vibration spectrum: We would find peaks at “belt frequency” (or better known as “fundamental belt pass frequency”) and its harmonics. The highest amplitude peak in the series will often be the twice the belt rate frequency.
The fundamental forcing frequency for such a problem is known as the “belt rate” or “fundamental belt pass frequency”. It is the rate at which a point on the belt passes a fixed reference point. It is always less that the speed of either pulleys (driving and driven).
Calculation of Belt Pass frequency as follows:
Driven RPM =Driving RPM x Driving sheave diameter/Driven sheave diameter
Belt freq = Pi x Sheave RPM x Sheave diameter/Belt length = PixDxN/BL
Where Pi = 3.1416
Spectrum: Look for the belt rate peak (sub-synchronous) and harmonics. Sometimes the belt rate peak may be cut off by the high pass filter, but the harmonics will be present. Remember we are looking for the 2 times belt rate frequency to confirm the problem.
Time waveform: If the belts are simply worn then the time waveform will not be the best analysis tool. If a belt has a distinct point of damage then there will be an event in the waveform once per belt revolution. This provides a useful distinction to discern the exact nature of the problem.
Strobe: A strobe is a very useful tool. If you use the strobe to freeze the movement of the belts then you can inspect them without stopping the machine. You can also detect slip on multi-belt systems.