Page 139 - Rotating Machinery Pratical Solutions to Unbalance and Misalignment

Advanced Machine Alignment

that answers obtained from the graphical method and calculator
method are the same (when rounded off to the nearest mil). Gen-
erally, the calculator method will provide the exact answer, while
the human eye tends to round off the graph, or the pencil point or
straightedge slips slightly.
Regardless of the method employed, when the proper shims
are added and the machine properly torqued, the indicators
should zero in the 6 o’clock position. Often, due to crush, torque,
dirt and other discrepancies, more than one alignment attempt
will be required to obtain zeros.

THE GRAPHICAL METHOD

The first step in the graphical solution is selecting and laying
out the vertical and horizontal scales on the graph. Note that the
graph is a Cartesian coordinate system as studied earlier. Gener-
ally, the vertical scale can be set at one square equals one mil.
Unless the machine is severely misaligned this should be ad-
equate. The horizontal scale should be determined from the ma-
chine measurements. If thermal growth is to be considered, the
scale must be such that both the stationary and adjustable ma-
chines will fit on the graph.
For instance, if the distance from the rim to the stationary
machine’s outboard foot were 48 inches, and the distance from the
rim indicator stem to the outboard foot of the adjustable machine
were 56, then a scale should be determined so that 104 inches
would fit on the graph. Assuming that the graph paper is 8-1/2 by
11 and that the horizontal scale will be laid out on the 11-inch side,
a scale of one inch equals 10 inches could be used.
Starting at the left edge of the graph, a vertical line is drawn
near the edge to represent the outboard foot of the stationary
machine. A symbol representing a machine foot is drawn at the
top of the line, and labeled O.B. for outboard foot. A horizontal
line is drawn halfway up the graph, which represents the final
position of both machine shafts when they are aligned to each