Practical Solutions to Machinery and Maintenance Vibration Problems
Chapter 13, Additional Vibration Sources
Section 1, Vibration Due to Pulleys/Sheaves and Drive Belts
Sometimes the length and tension on a drive belt results in a resonance within the belt whereby it acts like a resonant string. The resonance mode shape usually can be seen with a strobe after putting a chalk mark along the edge of the belt and watching it while vibrating. The belt will appear to vibrate in a mode shape typical for a specific resonance (most often, the 1st resonance mode, but for very long belts, the 2nd resonance mode occasionally occurs). This is not to be confused with vibration due to an unbalanced belt.
Almost all variations of shaft-to-shaft pulley misalignments result in higher than usual axial vibration. Sometimes the primary frequency is at 2 x rpm, but more often it is at 1 x rpm. Multiple V belts with unequal tensions also cause a high axial vibration. Sometimes the rigidity of the support system prevents it from being measured or seen, but pulley/sheave misalignment very often causes excessive thrust bearing wear. Replacing with belts of equal tension will usually cure the problem. Multiple belts molded in one piece are supposed to prevent this problem but do not if the sheaves are misaligned.
Drive and driven pulleys with multiple or single belts should be in the same line of action or in the same plane. If not in the same plane through cocking of one sheave relative to the other, or if parallel but not in alignment, then excessive vibration results. Proper alignment in the same plane cures the problem. Make sure that the grooves are aligned rather than basing alignment on the sides of the pulley.
Although relatively rare, a poorly made belt can within itself be out of balance. This results in excessive vibration at the low frequency of 1 x rpm of the belt (not of the rotor).
Pulley/sheave eccentricity results in a vibration frequency of 1 x rpm of the eccentric pulley/sheave. However, it can be distinguished from an unbalanced rotor. For example, an unbalanced sheave will show the usual characteristic of relatively equal amplitudes in both the horizontal and vertical directions. However, an eccentric sheave will show its largest amplitude in the direction of the line connecting the drive and driven sheaves.
For an unbalanced sheave, readings taken at the same bearing in directions 90° to each other will show an approximate 90° phase relationship. However, for an eccentric sheave, phases measured at the same bearing in directions 90° to each other will show a phase relationship of either approximately 180° or 0°. This is similar to what is often seen due to coupling/shaft misalignment.
This textbook contains only part of the information in our Practical Solutions seminar.