Output (High Speed) Shaft
To reduce the vibration of the output shaft, a modified tilting-pad bearing was selected. The axial width of the bearing was
increased from 0.6-in to 0.9-in, while maintaining the same
radial clearance and preload (Table 1 and Figure 15).
Figure 15. New wider tilt pad (top) and original (bottom) for
lower bearing on high speed output shaft.
This was an alternate bearing design,
provided by the OEM. This reduces the bearing load and increases the effective damping
provided by the bearing because it reduces the
stiffness for approximately the same damping. This lowers the predicted critical speed
slightly, but also reduces the amplification
factor as well. The vibration at the probes
is reduced (Figure 16), which compares the
predicted maximum amplitude at the shaft
probes with both the modified and original
bearing.
Bearing Type
L/D
Cd
m
Original
5 pad, Load-Between-Pad
0.35
0.006
0.4
Table 1. Original and modified bearings.
Modified
5 pad, Load-Between-Pad
0.6
0.006
0.4
Bode and polar plots for the pump with
the original and modified bearings are shown
in Figures 17 to 20. As can be seen, the synchronous amplitude has been reduced from
0.6-mils to 0.15-mils, which is very close to
the predicted change shown in Figure 16.
Results
These modifications were made to the three
pumps in May through August of 2002.
Since the modifications, there has not been
a pump failure. After the bearing modifications, the synchronous amplitude of the
vibration dropped from 0.6-mils to approximately 0.15-mils.
The total cost of these modifications to
all three pumps was less than $50,000. The
increase in reliability of these pumps has eliminated the need for a $500,000 pump replacement project that was planned to install new
pumps to increase the unit reliability.
Figure 16. Predicted unbalanced response at shaft probes with modified and original lower bearing.
Conclusions
The reliability of these pumps was greatly
increased by the combination of several
minor modifications that reduced the synchronous vibration, increased the buffer fluid
