134 CHAPTER 7 INVESTIGATION OF BEARING FAILURE 7.1 INTRODUCTION The rolling element bearings play a crucial role for the proper functioning of the gearbox used in the WTG. The reliability of bearing in the special environments like corrosive, high temperature, high power, high speed and high vacuum zone is very important. Many problems are noticed in the WTG gearbox during operation even though scheduled maintenance at stipulated interval is carried out. Oil analysis, drive train alignment and tower torque are linked to bearing faults. Bearing failure in the wind industries cannot be tolerated because it leads to catastrophic losses in power production due to down time, cost of repairing, replacement of parts and so on. A survey related with research study reveals that the bearing defects and the bearing failure are the main sources of gearbox failure in the wind turbine generator. Gearboxes are being used to convert high speed and low torque to high torque and low speed in general. Gearbox in the WTG is used in the opposite way for converting low speed and high torque from turbine rotor into low torque and high speed to an electric generator through shrink disc and fluid coupling. This is accomplished by using large size gears and bearings than that are being used in a typical machine drives. Generally, gearbox is not the most likely component to fail in the wind industries rather bearings are the most common parts susceptible to fail in the rotating machinery including the wind turbine generator gearbox. In general, machines do not break down or fail without giving some sign of warning, which is indicated through an
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134
CHAPTER 7
INVESTIGATION OF BEARING FAILURE
7.1 INTRODUCTION
The rolling element bearings play a crucial role for the proper
functioning of the gearbox used in the WTG. The reliability of bearing in the
special environments like corrosive, high temperature, high power, high speed
and high vacuum zone is very important. Many problems are noticed in the
WTG gearbox during operation even though scheduled maintenance at
stipulated interval is carried out. Oil analysis, drive train alignment and tower
torque are linked to bearing faults. Bearing failure in the wind industries
cannot be tolerated because it leads to catastrophic losses in power production
due to down time, cost of repairing, replacement of parts and so on. A survey
related with research study reveals that the bearing defects and the bearing failure are the main sources of gearbox failure in the wind turbine generator.
Gearboxes are being used to convert high speed and low torque to
high torque and low speed in general. Gearbox in the WTG is used in the
opposite way for converting low speed and high torque from turbine rotor into
low torque and high speed to an electric generator through shrink disc and
fluid coupling. This is accomplished by using large size gears and bearings
than that are being used in a typical machine drives. Generally, gearbox is not
the most likely component to fail in the wind industries rather bearings are the
most common parts susceptible to fail in the rotating machinery including the
wind turbine generator gearbox. In general, machines do not break down or
fail without giving some sign of warning, which is indicated through an
135
increased vibration. It is possible to determine the nature of severity of the
defect thereby predicting the machine’s useful life or failure point by repeated
measurement and analysis of the vibration in the machine, physical inspection
of the gearbox through oil analysis and etc.
7.2 BEARING FAILURE HISTORY IN THE WTG AND ITSCONCERN
A study was carried out to investigate the bearing failure in an
intermediate stage non-drive end of the gearbox used in the WTG. Table 7.1
gives the frequency of bearing failure in the WTG model under investigation.
It is observed from Table 7.1 that the bearing fails within 2 years of usage
against the recommended minimum design life of 10 years. In the event of
such pre-mature failure, it demands immediate replacement of bearing for
getting uninterrupted power supply from the wind turbine. Figure 7.1 shows
the general arrangement of drive train components in the wind turbine
generator. The wind turbine under investigation consist of a speed increasing
gearbox comprising of one planetary stage and two helical stages (Figure 7.2)
to achieve the final speed ratio of 1: 74.917. The position and description of
the bearings at the helical stage of the gearbox is given in Table 7. 2.
Figure 7.1 Arrangements of drive train components in the wind turbine generator
Figure 7.2 Sectional view of the gearbox
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Table 7.2 Bearing details
Position Intermediate Stage (IMS) High Speed Stage (HSS)Drive end Non-drive end Drive end Non-drive end
Helicalgear stage
Cylindricalroller bearing
230 NJ EC / C3 D5
Cylindricalroller bearing INA LSL 19 2330 A 0993
72 / V11
Cylindricalroller bearingNU 328 ECJ/
C3
Cylindricalroller bearing
NU 228 ECJ/C3Four point
angular contact ball bearing (QJ328 N2MA/C3)
The overloading of the wind turbine was due to fluctuating wind
force, non-synchronizing of pitching, sudden braking, sudden and frequent
grid drops and all these have induced an undesirable force on the system.
These forces not only damage the bearings, seals and the couplings but also
spoil the gearbox or the generator eventually. Apart from this, the flanks of
both the drive and the non-drive end pinion have pressure marks due to excess
axial movement of the shaft, scuffing wear and pitting wear when wind loads
are high. Besides, any bearing failure in the gearbox while the wind turbine
generator is in operational mode requires huge man hours and machine
stoppage to set right the turbine. Moreover idle of the turbine for service
during high wind season leads to customer dissatisfaction due to loss of
revenue and loss of power generation. Further, bearing fault will harm smooth
functioning of the gear trains. As discussed in section 5.2, if pinion fails at an
intermediate stage then either replacement of that pinion or overhauling the
gearbox itself is very difficult considering the tower height and weight of the
gearbox. The replacement of the gearbox and the lubricant attribute to 38% of
the cost of the turbine parts. Replacing a gearbox in a 1.25mW turbine may
cost more than rupees one crore considering the cost of the gearbox, crane
rental, labour and revenue loss.
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7.3 PROBLEM PHASE
Filter choke alarm was noticed in the WTG controller on
16th August 2010. On thorough inspection of the 2-stage (50 µm and 10 µm)
filter element (Figure 7.3) enormous metal particles are found in the retainer
plate (Figure 7.4) located at the bottom of the filter. The wind turbine was
stopped immediately and thorough gearbox inspection was done to pin point
the failure location. The study concluded that the failed component was the
intermediate stage non-drive end bearing (Figures 7.5(a) and 7.5(b)). It is also
discovered that the metal particles removed from the bearing got mixed up
with the lubricant and got meshed between the intermediate pinion and the
low speed gear (Figure 7.6). It was observed from site feedback that most of
the wind turbines required significant repair and even complete overhauls
within five to seven years or even before the benchmark time as the wind
turbine downtime is attributed to the gearbox related issues.
The bearing failure occurs very often at the non-drive end of the
intermediate stage. A team comprising of operation, maintenance and product
development engineers has been trying to explore the possibility of designing
heavier bearing and employing new bearing manufacturing process for
trouble free operation of the wind turbine. This chapter is focused to predict
how and why the bearing fails in the wind turbine generator gearbox and to
arrive the remedial measure to minimize the occurrence of failure in future.
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Figure 7.3 Choked gear oil filter
Figure 7.4 Metal particles at the retainer plate
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Figure 7.5(a) Failure in intermediate non-drive end bearing
Figure 7.5(b) Failure in intermediate non-drive end bearing
141
Figure 7.6 Metal particles over the gear teeth
7.4 INVESTIGATION ON BEARING FAILURE
7.4.1 Examination of Drive Train Alignment and Oil Analysis
The high speed shaft of the gearbox is coupled with the fluid
coupling with the help of brake disc, an axial damper and the ‘H’ spacer. The
coupling in turn is connected with the generator by means of ‘D’ flange, a key
and a locking screw. The technical team inspected the damaged bearing
(Figures 7.5(a) and 7.5(b)) and predicted that the failures may be due to
over-load by wind force or misalignment between the generator and the
gearbox.
Preliminarily, the current position of the asynchronous generator
shaft relative to the gearbox shaft was examined through laser-optical
alignment technique (Figure 7.7) to conclude the mode of failure, as most of
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the bearing failures in rotating machinery are attributed to mis-alignment in
the drive train components. A vertical offset misalignment of 0.14mm and
vertical angular misalignment of 0.12 mm /100 mm were found while
inspecting the shaft alignment using easy laser measurement and alignment
system. The generator foot positions were 0.07 mm at the front side and
-0.16mm at the rear end. Similarly, a horizontal offset misalignment of
0.30mm and horizontal angular misalignment of 0.08 mm/100 mm were
noticed. The generator foot positions were 0.22 mm at the front side and
-0.39mm at the rear end. According to the maintenance manager, the
acceptable limit of the horizontal and the vertical offset misalignment is 0.10
mm and the angular misalignment is 0.08mm/100mm and this slight
misalignment will not cause any catastrophic effect on the bearing life and
also on the operation of the gearbox. So it was confirmed from the alignment
study that the failure of the intermediate non-drive end bearing is not because
of the mis-alignment in the system.
Figure 7.7 Drive train alignment in the WTG
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Further, the gearbox under consideration is lubricated by mineral
oil having a viscosity index of 460 for bearing and gear. The outcome of an
oil analysis of a recent bearing failure location that was conducted in March
2010 is presented in Table 7.3.
Table 7.3 Oil analysis report
Test Protocol Unit Result TAN ASTM D664-01 mg.KOH,/gm. 0.96 Colour ASTM D1500:2007 ASTM 7.5 Flash point (COC) ASTM D92-05a °C. 268Kinematic viscosity (at 40 Deg. C.) ASTM D445-2006 mm2/s. 456