Diagnosis
of Rolling Element Bearing Faults using Envelope
Analysis
Arul Muthukumarasamy, Suri Ganeriwala
SpectraQuest Inc., 8201 Hermitage Road, Richmond, VA 23228
Tel: (804)261-3300 www.spectraquest.com
July, 2009
Published: July
2009
Abstract
This technical
note focuses on the diagnosis of the localized faults in rolling
element bearings using the envelope
detection techniques. These bearing faults usually result in strong
harmonics of the fault frequencies along with sidebands in the
spectrum of the vibration signals. Advanced envelope techniques
are developed to identify and analyze the fault frequencies, in
order to interpret the type of faults from the results. The envelope
signal provides more diagnostic information about the fault frequencies
and modulation. Experiments and data acquisition were performed
using SpectraQuest’s Bearing Prognostics Simulator and examples
involving outer and inner race faults are presented.
Keywords: Envelope analysis, Rolling element bearings, Inner and
outer race faults
Full Text (PDF)
Introduction
The rolling
element bearings play a prominent role in today’s machinery.
They are widely used in motors, gas turbines, pumps and many
other machines. Unlike the long-life fluid
bearings, rolling element bearing often encounters metal-to-metal
contacts which result in a consequent wear. A series of force impacts
are produced when the rollers or balls rolling over a local fault
in the bearing that eventually wears out the bearings. Hence, an
early detection of any deterioration in the bearing is necessary
to avoid major machine failures. So, usually the vibrations signals
from the bearings are analyzed and any variations in them due to
localized faults are detected to judge the state of the rolling
element bearings.
The typical faults in rolling element bearings include corrosion
in outer race, inner race and rolling elements, fatigue pitting
and cage damage. An impulsive shock is produced exciting the high
frequency resonances of the structure of the bearing system, when
a fault on the rolling element strikes either the inner or outer
race or rolling elements strike a fault on outer or inner race.
In case of large machines and large faults, the resonances are
excited often lower in frequency than 30 kHz. This impulse incurred
while shaft rotating will create a modulating phenomenon. The periodicity
and amplitude of the impulses depend on the shaft speed, location
of the defect, geometry of the bearing and type of bearing load.
The bearing fault frequencies can be categorized as BPFO (ball
passing frequency outer race), BPFI (ball passing frequency inner
race), BTF (ball fault frequency), and FTF (fundamental train frequency),
whose calculations are shown in the appendix [1].
Figure 1 shows the acceleration signals and the envelope signals
(by amplitude demodulation) produced by faulty components of a
rolling element bearing. Inner race faults are typically modulated
by the shaft speed that is the rate at which they pass through
the load zone as illustrated in Figure 1. Likewise, the rolling
element faults are modulated by the cage speed or fundamental train
frequency (FTF). Outer race faults are not modulated as they tend
to occur usually in the load zone. Also, the ball fault strikes
the outer or inner race which results in two shocks per basic period
as defined by the ball spin frequency (BSF).
Fig 1: Acceleration and envelope signals from localized faults
in rolling element bearings (Adapted from [1])
The faults in a defective rolling bearing signal are difficult
to diagnose through a simple FFT spectrum analysis as they are
cyclo-stationary (periodically time-varying statistics), have very
little energy and overwhelmed by noise and other vibrations. Also,
the resonances enhance higher frequencies and any jitter removes
the high frequencies in raw signal but showing them in the envelope.
Hence several signal envelope demodulation techniques have been
introduced to analyze these bearing signals.
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