Page 150 - Rotating Machinery Pratical Solutions to Unbalance and Misalignment

Rotating Machinery: Practical Solutions

Vertical indicator readings:
Rim = –4 mils;
Face = 3 mils

What corrections should be made to assure proper alignment
in the vertical direction with the pump at normal operating con-
ditions?

Step 1. After a complete inspection for potential causes of unbal-
ance, and determining the motor would be the adjustable piece of
equipment, the equipment was checked for level and soft feet. The
pump was also checked for soft feet and all shims were removed,
cleaned and re-installed. After cleaning under all feet, the equip-
ment was re-torqued and the first set of readings was taken. Both
the graphical and the calculator methods were employed to assure
the correct movement was achieved.

Step 2. Both readings have their algebraic signs changed and the
rim reading is divided by two and plotted. A horizontal line is
now drawn to the face indicator line and the face reading marked
off from this point. The location of the face indicator reading is
also circled. A line is now drawn from the two circled points on
the indicator lines, through the lines representing the pump’s feet.
The distance to the cold alignment line is now measured.
In this example, the inboard foot shows a minus two (–2) and
the outboard foot shows a minus seven (–7). If no thermal growth
considerations were to be made, these readings would represent
the amount of shim material to be added to each foot.

Step 3. Next, the amount of thermal growth (shrinkage) needs to
be calculated. The pump case and support legs are constructed
from cast iron with a coefficient thermal expansion of .0063 mills
per inch per degree Fahrenheit. Using formula (8.2); Inboard: CL
= .0063 × 17 × (106 – 78) = 2.99 mils; Outboard: CL = .0063 × 22 ×
(138 – 78) = 8.3 mils.
These answers are rounded to the nearest .001 inch or 1 mil,