Did you know?
That a perfect .000” alignment can
sometimes be the wrong thing to do? Rarely, if ever, do
two machines in a machine train generate the same temperatures
during operation. These factors can cause perfectly aligned
shafts to be misaligned out of tolerance while operating.
Let’s take use the following motor driven pump as
an example of this fact:
To calculate the amount of expected thermal growth use
the following formula:
Thermal Growth = (T Running - T Non-Running) x h x k
T = Casing Temperature in the Plane Of The Feet
h = Height from Base plate to Shaft Centerline
k = Coefficient of Expansion
In this example the motor will experience a rise in the
front foot of ~2mils and 1.8mils in the rear foot. The pump
will experience a greater amount of growth due to the fact
that the fluid cavity will heat up a greater rate. This
calculate out to ~6mils in the back and ~5.5mils in the
Assuming that the shaft centerlines were a perfect .000”
prior to operation, they would be about .002” out
during operation. These is right at the edge of acceptable
on most machines, and remember, if we were just within the
tolerance before operation, we would now be almost .004”
out of alignment.
To correct this from happening, we must intentionally lower
each machine to the amount of thermal growth expected once
zero is achieved. Depending on the method of alignment used
these dial indicator readings can be determined without
going through the step of aligning to zero and then removing
the applicable shims.
Reliability and Maintenance Management Consulting
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