The vibration spectrum can provide clear indication of
certain fault conditions, but when you see a large peak at the running speed
(1X) what will your diagnosis be? What if you also see peaks at 2X and 3X? Now,
if you are monitoring a large fan with a history of the building up on the fan
blades, then you may reasonably conclude that the high 1X peak indicates
unbalance. But in the more general case, how do you distinguish between
unbalance, bent shaft, looseness, resonance, eccentricity, misalignment, cocked
bearing, and other fault conditions? This is where phase analysis is your
friend.
Once upon a time phase analysis was difficult to perform
because most people owned single channel vibration analyzers. But with a two-channel
analyzer, and two vibration sensors, it is very easy to perform phase analysis.
By simply placing one sensor vertically on the bearing and one sensor
horizontally you can determine if unbalance exists. By comparing the vibration
from one end of the machine to the other (in the same axis) you can confirm the
unbalance diagnosis and assess whether it requires single-plane balance or two-plane.
Comparing phase axially across a coupling, and radially across the coupling can
help you diagnose and confirm misalignment.
I could go on and on, but phase analysis the best tool for
distinguishing between all of the listed fault conditions and more.
This tip is provided by Jason Tranter, Managing Director of Mobius Institute.
For more condition monitoring tips, continue to visit the IMVAC – International Machine Vibration Analysis Conference – blog every week. To learn more about IMVAC and the event nearest to you, visit vibrationconference.com.
