Page 6 - 810 Trianing Book Extract
P. 6
B Vibration analysis principles on rotating machines
Other machine components on the shaft
The rotating motor shaft turns the pump shaft through
the coupling. Every time an impeller vane passes the
sensor on the top, it creates a small vibration. If the
pump has 10 impeller vanes, the sensor sees vibration
from a vane 10 times for every shaft revolution.
(The pump impeller is going the same speed as the
pump shaft.) A sine wave shows 10 complete cycles
(see igure 11).
The larger mass of the motor and pump shaft will
cause more vibration amplitude (magnitude) than the
impeller. The sine wave for the pump impeller vibration
will be smaller in amplitude than the shaft vibration,
Figure 10
but more cycles (10) for one complete shaft rotation. In
igure 11 see how the vibrations of the impeller vanes
on the shaft = the cycles of the sine wave.
10 vibrations per shaft revolution = 10 cycles per shaft revolution
(Rotating impeller vanes) (Sine wave)
Figure 11
Complex waveforms
If we superimpose the two sine waves (shaft and pump impeller) it might look like igure 12.
1 revolution of shaft
10 impeller vibrations per 1 shaft rotation
Figure 12
28 Section 1: Basics of proactive maintenance and vibration testing
