High Speed Gearbox Failure Root Cause Analysis
20,000 HP Rated 1800 RPM Input-8000 RPM Output
By Abdulrahman Alkhowaiter June-2022
From: Khowaiter, Abdulrahman O
Sent: Wednesday, June 11, 2008 4:28 PM
To: Plant Engineer
Cc: Plant & Central Engineering
Subject: Compressor 20,000 HP Gearbox Failure Model SG-710/5
Background:
Last week, the-KG-322 HP-Compressor 20,000 HP rated reduction gearbox initiated a
compression train shutdown on high gearbox casing vibration. Previous to this incident, the
gearbox had been operating for four years since year 2004, and has never been opened for
maintenance since vendor commissioning. However, the plant applies intensive online condition
monitoring to these gearboxes, including fully instrumented Radial/Thrust/Seismic vibration
protection, Radial/Thrust bearing temperature imbedded sensors, and monthly lube oil quality
monitoring. The facility has a total of 12 similar Special-Purpose gearboxes, all motor driven at
constant speed, with three identical size HP units, and so far, this is the first recorded failure
Page 1 of 10
High Speed Gearbox Failure Root Cause Analysis
20,000 HP Rated 1800 RPM Input-8000 RPM Output
By Abdulrahman Alkhowaiter June-2022
incident on any of the 12 gearboxes. The gearboxes were packaged by the compressor
manufacturer for their compressor skids and all gearboxes were manufactured in Europe. From
a process parameter point of view, this gearbox has not been exposed to any unusual operating
load conditions for the past four years, such as: over-speed (constant speed motor driven), excess
driver torque motor torque data was given to Gearbox OEM during design in 2002, or excess
driven machine torque, such as seizing of compressor in its casing.
After the shutdown, the gearbox inspection hatch was opened, and interior damage to the bull
gear was noted. The remaining drive train such as compressor and motor driver were all found
in good condition. The gearbox was removed from field and sent to the central shops division.
The facility personnel have made a very good preventative maintenance step by inspecting the
gearbox and discovering the internal faults, before major damage has occurred to this critical
gearbox.
Page 2 of 10
High Speed Gearbox Failure Root Cause Analysis
20,000 HP Rated 1800 RPM Input-8000 RPM Output
By Abdulrahman Alkhowaiter June-2022
Failure Analysis:
The Gosp-1 HP compressor gearbox at central shops was opened yesterday for analysis of failure.
As can be seen from the photos, the Bull gear has moved axially from its design position by a
distance of approximately 12 cm. This led to rubbing of the bull gear with the oil spray supports
and radial bearing housing, and this produced vibration which caused a seismic vibration
shutdown on Hi-Hi vibration. Examination of the gearbox upper and lower halves revealed that
the gearbox is a steel fabrication of heavy duty construction, with high stiffness, and advanced
design. All welds were of excellent quality, so the gearbox casing or footing were not contributing
to this incident. All pictures above are the as-found condition, no cleaning has been applied. Both
bull and pinion radial–thrust bearings and shaft journals were found to be in as-new condition.
The following are critical points for the analysis:
1. All gearbox bearings were found in excellent condition, with no signs of internal rubbing,
lack of lubrication, poor quality lubrication, or misalignment of wear pattern. There were no
signs of varnishing on any of the radial-thrust bearings, which is also an indicator that they
have not been exposed to excessive temperatures. If the gearbox was subjected to some
unusual torque loads, a typical finding would be overheating and brown varnish stains on the
HS radial bearings, however, no evidence was found. The fact that all radial bearings Babbitt
surface were found with excellent surface condition is further proof that they were not
exposed to vibration beyond the manufacturer design limits during the four years of
operation. All gearbox historical vibration-temperature data is available with the facility. The
shaft journals and thrust collars were found in excellent condition.
2. The gearbox low speed and high speed shaft coupling hubs were found to be in proper place,
with no movement out of position, looseness, or mechanical damage. No significant
indication of over-torque was found, and the same applies to the HS-LS flexible coupling
spools. The coupling bolts are in good condition.
3. The bull gear rotor design is a two piece fabricated construction; forged bull gear, with center
bored, to allow shrink-fitting to a large diameter, high stiffness low speed shaft. No keys are
used for torque transmission, so by design, all shaft torque to bull gear is transmitted by the
frictional shrink interference fit only. The low speed shaft diameter is high, giving a large
cylindrical surface area to achieve a sufficient shrink fit. However, the actual shrink fit used
on this gearbox during factory assembly was at first unknown until the gear was disassembled
in the shop. There is always a possible difference between design shrink fit, and the actual
shrink fit at assembly due to the possibility of human error. OEM stated in a recent letter that
the design shrink interference fit of the shaft to the wheel bore is 0.379mm minimum, and
maximum interference fit of 0.44 mm diametral. The LS Shaft Diameter (nominal) is 360.44
Page 3 of 10
High Speed Gearbox Failure Root Cause Analysis
20,000 HP Rated 1800 RPM Input-8000 RPM Output
By Abdulrahman Alkhowaiter June-2022
mm. By changing to imperial units, the OEM design shrink fit is 0.015 to 0.0175 inch on the
14.2 inch OD shaft, which means that the design interference fit range is from 0.001 to 0.0012
inch, per inch diameter. The actual as found interference once disassembled, was only 0.008
inches diametral, far below OEM standards.
4. The bull gear teeth in general, except for the last 2cm flank that were exposed to edge
rubbing, are in good condition. The pinion teeth are not severely damaged, but there is a
tooth face wear pattern most likely resulting from the short period when the primary failure
mode of bull gear movement occurred, and the continuous rubbing of bull gear caused a
wobbling and a modulating teeth pattern since they have operated for hours at this condition,
under load. However, please note that the pinion teeth unusual surface pattern is a
secondary damage, and not primary.
5. At initial impressions, it was suspected that some type of over torque situation led to the bull
gear rotating on the LS shaft, and overpowering the shrink fit. However, the pictorial evidence
attached above from the opened gearbox, shows that the bull gear did not rotate on the shaft
for the first 12 cm of axial displacement. This means that an over torque scenario never
occurred as a primary failure mode, or the shaft underneath the bull gear would have been
heavily scored and damaged from the resistance to over torque. All evidence points to loss
of bull gear to LS shaft shrink fit during normal operation, and gradual axial slipping toward
the opposite bearing, until rubbing contact occurred and heavy vibration resulted. This
slipping is assisted by the normal axial force acting on single helical gears. We have never
opened this gearbox before, and no overspeed incidents are possible due to constant speed
driver, and no evidence of external over torque to gearbox was found, then the only possible
root causes of failure are: Insufficient shrink fit by design, or insufficient shrink fit during
assembly, or relaxation of initial shrink fit due to bull gear forging heat treatment process
errors. There is no physical evidence of severe overheating of the gearbox during operation
which would lead to any bull gear unusual thermal growth and looseness. The only sign of
overheating in the pictures above are the secondary damage after the bull gear sidewall
rubbed other parts during operation.
Conclusions:
From the evidence observed, the root cause of gearbox failure is attributed to loss of bull gear
shrink fit as a result of insufficient design interference fit or factory assembly errors such as
incorrect bore/shaft machining dimensions. In general, we consider the present gearbox design
shrink fit limits of only 0.001 to 0.0012 inch/inch diameter, to have borderline safety factor to
overcome the effects of differential thermal expansion between shaft and gear, combined with
Page 4 of 10
High Speed Gearbox Failure Root Cause Analysis
20,000 HP Rated 1800 RPM Input-8000 RPM Output
By Abdulrahman Alkhowaiter June-2022
the reduction in shrink fit due to expansion of bore under centrifugal stresses at normal operating
speeds. However, since the shaft to gear holding power is in conflict with total tensile stress in
gearbox which arises from centrifugal forces and tooth stress, it is possible that OEM limited
shrink fit to 0.0012 inch/inch diameter to reduce overall stresses on gear.
The primary root cause is the as found manufacturing defect of having only 0.008-inch
interference total which led to a primary failure mode of axial displacement of the bull gear (not
LS shaft). Overall, the gearbox is of high quality, heavy duty construction, as is evident by its
condition now after the failure. All major mechanical parts such as bearings were so heavily
designed that they show no wear or tear under the failure conditions. Therefore, we consider
this error something that can be rectified and permanently solved. We have the capability to
perform an engineering analysis of the Bull Gear operating stresses and conditions, and applying
a shrink fit that will resist all possible normal operating stresses, without overstressing the yield
strength of the bull gear forging material. However, we recommend at this stage, to leave all
parts in the as-found condition, allow OEM to complete their investigation, and propose
improved shrink fitting design parameters to this gearbox LS rotor.
Recommendations:
1. Install a new gear set into this gearbox, as these gearboxes are normally purchased with spare
gear sets. Insure that shrink fit is measured at shops and increased to new dimensional
settings to be agreed upon with OEM and myself.
2. Inspect all remaining 11 gearboxes, using the inspection hatch, for any visible signs of bull
gear displacement, within 1-month maximum from receipt of this failure report. Remove any
gearbox found with bull gear displacement, to Central Shops, without delay.
3. Inspect and calibrate all gearbox seismic transducers, as these transducers are responsible
for protection from similar failures. This can be done on-line, by isolating the transducer, and
applying a shaker to test output to monitors. This should be done within two weeks’ time
period.
4. Issue written letter to both plants operation personnel, warning that no compression train
shall be restarted if the train was shut down by the gearbox seismic or displacement
transducers.
5. Arrange scheduled gearbox overhauls at Central Shops for all remaining 11 gearboxes, to
measure and upgrade the bull gear shrink fits.
Page 5 of 10
High Speed Gearbox Failure Root Cause Analysis
20,000 HP Rated 1800 RPM Input-8000 RPM Output
By Abdulrahman Alkhowaiter June-2022
Page 6 of 10
High Speed Gearbox Failure Root Cause Analysis
20,000 HP Rated 1800 RPM Input-8000 RPM Output
By Abdulrahman Alkhowaiter June-2022
Page 7 of 10
High Speed Gearbox Failure Root Cause Analysis
20,000 HP Rated 1800 RPM Input-8000 RPM Output
By Abdulrahman Alkhowaiter June-2022
Page 8 of 10
High Speed Gearbox Failure Root Cause Analysis
20,000 HP Rated 1800 RPM Input-8000 RPM Output
By Abdulrahman Alkhowaiter June-2022
Page 9 of 10
High Speed Gearbox Failure Root Cause Analysis
20,000 HP Rated 1800 RPM Input-8000 RPM Output
By Abdulrahman Alkhowaiter June-2022
Page 10 of 10