Diagonoses gearbox-in-field

gear Reducer

1. Diagnosis of Gearbox Faults
in the Field
A. El-Shafei
RITEC
Cairo, Egypt

2. Synopsis
•
C
ase Studies of Gearbox Diagnosis
•
T
echniques used
–
S
pectral Analysis
–
T
ime Waveform Analysis
–
E
nvelope Analysis
–
P
hase Analysis
–
ODS Analysis

3. Synopsis
•
W
ide range of cases
–
M
isaligned Gearbox
–
C
asing Distortion
–
C
racked Tooth
–
T
ooth Resonance
–
T
ooth Loading
•
C
ases from Cement, Steel and
Petrochemical Industries

4. Gearbox Analysis
•
Difficult
–
S
ignificant activity in gearbox spectrum •
P
articular attention in monitoring
–
H
eavy gearbox casings
•
P
reclude use of seismic sensors
•
W
hen used can mislead the analyst
–
V
ariety of faults occur in a gearbox •
I
nstallation Faults
•
W
ear
•
O
verload

5. Gearbox Analysis
•
R
equires considerable knowledge in both
gearbox fundamentals and vibration analysis tools

6. VIBRATION TECHNIQUES FOR
GEARBOX ANALYSIS AND
DIAGNOSIS
•
S
pectral Analysis: Main Tool
–
V
ibration at a known frequency
–
V
ibration at an unknown frequency
–
S
ingle frequency vibration •
U
sually high 1X or high GM
•
M
ost difficult to analyze
•
N
eed more tools to study

7. VIBRATION TECHNIQUES FOR
GEARBOX ANALYSIS AND
DIAGNOSIS
•
GM Vibration
–
High GM frequency in the spectrum does not in itself indicate a problem in the gearbox.
–
Significant change of the GM, or th
e appearance of multiples of GM
or sidebands, indicate a gear meshing problem, probably wear.
–
Many problems have same general symptoms. Narrow band analysis used to distinguish between various meshing problems.
–
Backlash results in a family of
sidebands (characteristic of
frequency modulation) around the gear mesh frequency.
–
An eccentric gear would result in sidebands to the GM at the frequency of the shaft speed of the eccentric gear.
–
Excessive wear would, in general,
result in the presence of both
multiples of the GM frequency as well as sidebands.
–
Common factor problems result in subharmonic
m
ultiples of the
GM frequency, and can be high even for a new gearbox. However, this gearbox will have accelerated wear.

8. VIBRATION TECHNIQUES FOR
GEARBOX ANALYSIS AND
DIAGNOSIS
•
T
ime waveform
–
D
irectional forces
•
P
hase
•
ODS Analysis
–
A
ctual visualization of machine or structure
•
E
nvelope Analysis
–
a
ccentuation of the moderate frequency vibration that
would be otherwise obscured in a normal spectrum
•
R
esonance Testing
–
N
atural Frequencies and Critical Speeds

9. TOOTH RESONANCE
PROBLEM: BUMP TEST
•
C
ement Industry
•
C
ement Mill Girth Gear
–
H
uge gear surrounding the mill body
–
15 rpm
•
T
he girth gear set changed
–
D
ifferent manufacturer
–
H
igh vibration 14 mm/s rms
a
t pinion bearings
–
P
itting on the Girth Gear

10. Figure 1 Pitting of the Girth Gear

11. •
I
nspection of measured spectrum at pinion
bearings
–
G
ear Mesh frequency 67 Hz
–
B
eating with another component at 65 Hz
–
S
ource of the 65 Hz component unknown
•
B
eating Confirmed by time waveform
•
U
nknown high frequency activity around 132 Hz
•
P
hase measurements at the pinion bearings
tracking the gear mesh frequency indicated abnormal twisting at the bearings
•
P
robably caused by the beating phenomenon

12. Figure 2
Spectrum Showing beating with Gear Mesh
Frequency

13. Figure 3 Beating in Time Waveform

14. Bump Test
•
D
iagnosis
–
B
eating between the unknown frequency and the
gearmesh
f
requency causing the high vibration and
pitting of the gear teeth
•
B
ump test to investigate source of unknown
frequency
–
P
inion Bearings, skid, mill body, pinion and gear teeth
•
T
he bump test on girth gear showed resonant
frequencies of 62 Hz and 129 Hz

15. Figure 4

Bump test showin
g resonant frequency of
62 Hz

16. •
T
he resonant frequency of 62 Hz of the girth gear teeth was deemed
to be the beating frequency with
the gear mesh frequency, thus
amplifying the vibration and causing the gear pitting.
•
T
he resonant frequency at 129 Hz
als
o
appears to explain the
vibration activity at
132 Hz.
•
T
he difference of 3 Hz between t
he sought after frequenc
ies and the
measured tooth resonance frequenc
ies, can be explained by
–
t
he resonance test was conducted at stand still
–
a
t no load.
•
It is conceivable that the teet
h natural frequencies at load and
speed, for such a huge gear, s
hould have some considerable
stiffening both due to loading and due
to the centrifugal force,
even
though it is operating at a low speed.

17. •
T
hus the final diagnosis was that the gear-pitting
problem is caused by resonan
ces in the gear teeth at the
gear meshing frequency.
•
T
his causes a profound beating between the two
frequencies, resulting in multiple impacts of the teeth during their meshing cycles, causing the pitting in the gear teeth.
•
B
ased on the results of this analysis, the owner decided
to replace the new gear set with the old worn gear set from the original equipment manufacturer (OEM), since they do not exhibit these teeth resonance problems, pending the arrival of a new gear set from the OEM.

18. GEARBOX VIBRATION DUE TO TOOTH LOADING: ENVELOPE
ANALYSIS
•
C
ement Industry
•
C
ompactly designed gearboxes driving
cement mills
•
3900 KW motors drive the gearboxes
•
G
earbox mounted on fluid film bearings
•
G
earbox classes 4 MW class, 8 MW
class,..etc.
•
4
MW gearboxes: critically loaded

19. •
C
ondition monitoring program
•
S
everal spectra monitored at all gearbox
bearings
•
O
ne monitored gear mesh frequency at 348 Hz
–
M
onitored monthly along with its multiples in a
1000 Hz velocity spectrum
•
G
M frequency remained constant at less than 3
mm/s rms
f
or nearly 3 years
•
O
nly seemingly benign sidebands started to
appear of amplitude 0.2 mm/s (indicating some wear).

20. Figure 5 Normal Spectrum on
the Gearbox showing GM frequency

21. •
O
n a routine inspection: excessive wear
pattern was visually identified.
•
H
owever, no indication of excessive wear
was evident neither in the monitored spectra nor time waveforms.
•
T
o improve condition monitoring: measure
envelope spectra.
•
R
outinely collected for machines on rolling
element bearings not fluid film bearings.

22. Fig. 6
Envelope Spectra showing huge sidebands to the GM frequency

23. •
E
nvelope spectra indicated the presence of the
GM and its multiples, plus huge sidebands that are of the same order of magnitude as the GM.
•
C
learly abnormal.
•
M
achines on rolling element bearings would fail
well before the sidebands reach the value of the fault frequency, even in an envelope spectrum.
•
G
ear set was replaced with a new set

24. Figure 7 Envelope Spectra after repl
acing the gear set sh
owing no sidebands

25. •
S
ince then, envelope spectra are routinely
measured on these gearboxes: more sensitive to gear condition.
•
H
ypothesis:
–
E
nvelope spectrum detects stress waves
–
H
eavily loaded gearboxes: heavy tooth
loading produces stress waves
•
A
ct as carrier to GM, GM modulated by 1X
–
E
nvelope spectrum detects these modulation
effect (sidebands): gives early warning.

26. MISALIGNMENT PROBLEM:
TIME WAVEFORM ANALYSIS
•
C
hiller System
•
U
sed for air conditioning in a major Saudi
building
•
M
ounted on fluid film bearings
•
G
earbox had a single stage, GM of 4590
Hz, with motor speed at 30 Hz and compressor speed at 68.5 Hz
•
P
roximity probe monitoring system

27. •
O
ne gearbox consistently operated at its
alarm level
•
M
anufacturer suggested increasing the
alarm level
•
O
wner decided to investigate the root
cause of this problem
•
S
pectra and time waveforms were
measured on casing and from panel

28. Figure 8
Cas
ing High frequency Spec
tra

29. •
Low amplitude casing spectra
•
C
asing time waveform had no significant
pattern
•
P
anel displacement spectra indicated
multiples of running speed at low amplitude
•
K
ey: panel displacement time waveform

30. Figure 9

Panel Dis
p
lacement Time Wave
form Indic
a
ting Misalignment Problem

31. •
S
haft travels about 5 µm in the negative direction and
about 25 µm in the positive direction.
•
M
oreover, the time waveform is spiked in the positive
direction.
•
G
ear is providing directional forces pushing the shaft to
travel into the upper part of the bearing.
•
T
his is a clear pattern of misalignment in gearboxes [5].
•
M
isalignment was corrected at the next shut down.
•
T
he machine has since then operated well below the
alarm level.
•
T
he key to diagnosing this problem was from the
analysis of the time waveform shape.

32. CRACKED TOOTH
PROBLEM: CREST FACTOR
•
S
teel Industry
•
B
aseline data for a brand new state-of-the-art steel plant
in Egypt
•
C
omplete vibration data collected: generally within ISO
limits
•
A
n intermediate rolling stand, driven by a 7 MW DC
motor, output shaft speed of 75 rpm (1.25 Hz), driven by a double reduction gearbox with GMs of 46.25 Hz and 30 Hz.
•
S
pectral measurement on the gearbox casing indicated
very low amplitude levels at less than 0.3 mm/s.
•
W
ell within ISO standards.

33. Figure 10
Spectrum of Gearbox
Vibration

34. •
T
ime waveform for the same point, which is on
the high speed shaft motor side, indicated pulsations at the running speed of 1.25 Hz.
•
T
his is quite worrisome for such a brand new
gearbox under warranty
.
•
M
anufacturer (based on ISO standards) initially
rejected the idea that there is a problem with this brand new gearbox
•
W
hen presented with crest factor data, they
changed their mind.

35. Figure 11
Puls
es in Time Waveform

36. Table 1 Crest Factor Data for the Points on the Gearbox
First Stage
Point
Point No.
Direction
Units
rms
V
alue
Peak
Value
Crest F
a
ctor
(peak/rms)
HS
1
3
Axial
m
m/sec
0
.245
1.36
HS
1
3
Horizontal
mm/sec
0.309
2
.9
9.4
HS
1
3
Vertical
mm/sec
0.14
0.6
4
.3
LS 1
5
Axial
m
m/sec
0
.166
1.64
9.9
LS 1
5
Horizontal
mm/sec
0.368
1
.7
4.6
LS 1
5
Vert
ica
l
mm
/sec
0.341
1
.32
3
.9
5.6

37. •
H
aving crest factors in the range of
2 to 4
is not uncommon for many sharp non- harmonic signals.
•
U
sually, crest factors above
5
would
indicate a problem
–
C
rest factor approached
10
on some points
for this gearbox.
•
T
his is a sure sign for a problem.

38. •
T
he reason for the pulsations is yet to be
determined.
•
P
ulses at 1x in gearboxes are usually associated
with gear tooth cracks.
•
A
s the cracked tooth enters a loading cycle once
per revolution, it produces a sharp peak, while the vibration energy, represented by the rms value, does not change much.
•
T
he crest factor is thus a valuable tool for
diagnosing tooth crack problems

39. VIBRATION OF GEARBOX
SUPPORTING STRUCTURE:
ODS ANALYSIS
•
O
il and Gas Industry
•
V
ertical air cooling fan supported on a horizontal steel
frame with a horizontal motor and a bevel gearbox.
•
O
ne of 16 units supported on an elevated steel frame,
used to cool air for 3 Gas Turbines 9 MW each.
•
T
he units were experiencing frequent shear failures of
the shaft connecting the motor and the gearbox at the input shaft of the gearbox.
•
T
he manufacturer suggested the use of a larger
diameter shaft.
•
H
owever, this resulted in accelerated wear in the
gearbox.

40. MNDE

Br
g
.

MDE

Br
g
.

G
B
B
r
g.
1
G
B
B
r
g.
2
G
B
B
r
g.
3
G
B
B
r
g.
4
F B
r
g
.

Ba
s
e
P
l
a
t
e

Figure 12 Schematic layout of
the machine and measurement points
definition

41. •
M
ain exciting frequency is at 6x (blade passing
frequency).
•
A
bump test was conducted, but was not
conclusive, because of transmitted vibration
•
A
n ODS was conducted at 6x, the exciting
frequency
•
M
ain deflection occurs in the middle of the
frame, right where the repeated failure occurs, both in the horizontal and vertical directions.

42. Figure 13
Base Plate ODS-Horizontal Direction

43. Figure 14 Base Plate ODS-Vertical Direction

44. •
T
he reason of the failures is quite clear: the
flexibility of the supporting structure at the gearbox input shaft position results in shaft failure.
•
I
ncreasing the shaft diameter only transmitted
the problem to the gearbox, resulting in the accelerated wear.
•
T
he root cause of the problem is the flexibility of
the supporting structure.
This supporting
structure should be stiffened.

45. Conclusions
•
V
ariety of gearbox vibration problems from various
industries.
•
T
he problems represent a set of perhaps unusual cases,
where a variety of vibration analysis tools had to be used.
•
T
echniques used
–
S
pectr
a
l Analysis
–
T
ime Waveform Analysis
–
E
nvelope Analys
is
–
P
hase Analy
s
is
–
O
DS Analysis
•
A
ll these tools have to be used for effective gearbox
analysis and diagnosis.