Page 114 - Rotating Machinery Pratical Solutions to Unbalance and Misalignment

Rotating Machinery: Practical Solutions

not the case. This type of misalignment will manifest itself into the
same failures of bearings as shaft misalignment.
Figure 7-8 shows why using a straightedge to align pulleys or
sheaves can lead to errors. Note that the two pulleys have a dif-
ferent face thickness. Aligning these pulleys with a straightedge
causes the sheave centers to be offset. Be sure to measure the
thickness of any pulleys that will be aligned using this method. It
is far better to use the pulleys’ center-lines as a reference.

Figure 7-8. Errors in Using Straightedge to Align Pulleys

Regardless how V-belt and chain drives are aligned, the
amount of axial vibration is a sure sign of the preciseness of the
alignment. In general, when the axial vibration exceeds 1/2 the
highest vertical or horizontal vibration, misalignment or a bent
shaft should be suspected.
To determine if the cause of vibration is related to the belt
itself, the rpm of the belt needs to be determined. The following
formula determines the belt rpm.

rpm = 3.14159 × Pulley Diameter/Belt Length (7.1)

If the vibrations are at belt rpm or a multiple of belt rpm, the
belt(s) should be suspect. Belt defects can be in the form of miss-
ing material, uneven widths, lumps on the belt, cracks, hard spots,
soft spots, and others. Regardless, these will show up at belt rpm.
Multi-belt applications may exhibit frequencies that do not coin-
cide with belt rpm, since several belts could have defects at differ-
ent locations.
Belt tension is very important in assuring the long life of the
belts and equipment. Although belts that are too loose may not be