NASA Technical Reports Server (NTRS) 19950008234: Space Shuttle Main Engine turbopump bearing assessment program

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by  NASA Technical Reports Server (NTRS)

10th Annual
Technicai and Business
Exhibition and Symposium
-
May 10 11,1!#4
Von Braun Civic Center
Huntsville, Alabama
SPACE SHUTTLE MAIN ENGINE TURBO-
PUMP BEARING ASSESSMENT PROGRAM
Barbara Spiegel Breithaupt
Marshall Space Flight Center
Huntsville, Alabama 358 12
TABES Paper
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NO. 94 621
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TABES 94-621
SPACE SHUTTLE MAIN ENGINE
TURBOPUMP BEARING ASSESSMENT PROGRAM
Barbara Spiegel Breithaupt
Marshall Space Flight Center
MSFC, Alabama 35812
ABSTRACT end. For easy reference, the bearings are
numbered from one to four from the pump to
This paper documents the work done the turbine end Most wear is found on the
on the bearing assessment program over the p-imp end bearings, particularly bearing
past two and a half years. The objective of the number two.
program is to develop a nondestructive HPOTP bearings deteriorate quickly fop
evaluation system for the space shuttle main many reasons. The balls wear the fastest of all
engine (SSME) high pressure oxidizer the bearing components and, in the process,
turbopumps (HPOTP's) which would be used lose their preload. Liquid oxygen is their only
to detect anomalies in installed bearings without lubricant and a poor one at that. It oxidizes all
component disassembly. Databases of various exterior surfaces, thus promoting metal to metal
signatures are obtained by slowly turning the contact. Pump speeds in excess of 27,000
pump shafts before and after an engine firing. rimin and high temperatures in the contact
These signatures are then analyzed and stress areas are also significant conmbuting
compared to the original signatures to more factors.
accurately predict bearing wear. During flight, no internal
instrumentation is allowed inside the
turbopumps. Only externally mounted strain
I. INTRODUCTION gages and accelerometers monitor bearing
health. When an engine returns for post flight
The SSME was the first reusable large checkout, various methods are used to
liquid rocket engine in the world. With a determine the condition of the bearings. The
planned design life of 55 starts, it promised to pump is disassembled if any bearing wear
be the most cost effective and optimum engine indicators are found in the flight data.
for space travel. However, since its inceptici~, However, by the time external instrumentiition
it has had bearing problems which limit its indicates a problem, significant wear has
reusability. Bearings within the engine's four already occurred. If no indicators are found,
high speed turbopumps must withstand a ve;y inspection methods are used to better determine
harsh environment and operate at peak the actual degree of bearing wear.
performance to avoid catastrophic engine Several diagnostic methods are used on
failure. As a safety precaution, bearings within all of the turbopumps. It is standard checkout
one of the SSME turbopumps are replaced procedure to perform manual torque tests on
every three flights, which is time-consuming, them to verify that their shafts are fne to rotate.
costly, and frequently appears to have been Another inspection method is the microwiggle
unnecessary. test whereby the shaft is rotated and radial
Of the four turbopumps on an engine, clearance is measured instead of travel. This
the HPOTP by far has the most unpredictable test provides some HPOTP pump end bearing
and troublesome bearing wear problems. It information as well as turbine end; however, it
contahs two matched pairs of angular contact is still in the development stage.
bearings. Two of these are located near the Two inspection procedures are availablc
pump end and the other two near the turbine for the HPOTP turbine end bearings. The first
TABES 94-621
is called microshaft travel and involves a push SSME's without component disassembly. The
and pull test of the turbopump whereby actual plan is to develop a database of various types
shaft displacement is measured This of signatures obtained from slowly turning the
diagnostic nlethod reveals reliable dau about HPOTP shafts before and after an engine
the actual condition of the two bearings as a W g . T hese signatures would in nun be
pair. Second, the number three bearing can be analyzed and compared to the original
borescoped for evidence of visible damage to signatures to more accurately pndict bearing
the bearing components (i.e., spalling, pitting, wear.
etc.). We began by conducting literature
Unlike the turbine end bearings, the searches and researching past work done on
HPOTP pump end bearings currently have no similar projects. We investigated the'cumnt
reliable inspection method or diagnostic tool. methods used to determine bearing wear and
Consequently a conservative 2000-s limit is identified the arcas we were most intensred in
placed on these bearings to ensun mission pursuing. Then we laid the ground work by
safety. This amounts to a maximum of three establishing a test facility, acquiring a test
flights before pump disassembly. If this limit article, and forming a working group.
could be replaced by a reliable inspection To expedite testing, we obtained a spm
method, then the engine could possibly fly for HPOTP for our test facility. This would enable
a total of nine flights before it would have to be us to determine concept feasibility mart quickly
tom down. At an expense of $1.5 million per and concisely. From the beginning, we were
tear down, that would amount to a cost savings always cautious not to develop a technique that
of $3 million per engine. would work well in a laboratory setup but fail
Recognizing the critical need for such a miserably in real life application. Every
diagnostic tool, Mahall Space Flight Center concept we consider must be capable of
(MSFC) engineers have developed the bearing operating in a test stand environment.
assessment program. This rvsearch and The first pump we received was
development program has been in existence fc>r HPOTP unit 23 15, which we hiid briefly
almost 3 years and in that tics we have before it was replaced with IiPOTP unit 08 10.
studied several different methods used to Most all of our testing has ken done with this
determine bearing wear. We have also pump. It is a phase 11 development pump with
developed a prototype unit called the automated several configuration modifications. The pump
torque sensor (ATS) and are in the process of end bearings have accumulated approximately
qualifying it for service. 6,000 s of run time while the turbine end
In this report we describe the pr~gran bearings have 10,000 s. Even though the
and methodology used to develop such a bearings have much time on them, we still
.
nondestructive evaluation (NDE) system In consider the pump to have good bearings. A
sections 111, IV, and V, the testing prog.,.itms microwiggle test was perfomed and no
involving acoustic emission, vibration, and appreciable wear was found on any of the
torque arc discussed and their results are bearings. This only verified the pndictions
presented. The prototype diagnostic tool and made from the internal isolator strain gage hot
its use are explained in sections VI and VII. In fire data which showed low amplitude cage
section VIlI, an overall surnnlary is given and frequency responses which can be comlated to
are:
our recommendations presented The uneven ball wear in the HPOTP pump end
report concludes with a discussion of our bearings.
future program plans. The next logical step after preparing a
test facility was to assemble a bearing
11. PROGRAM PLAN assessment team. We composed this team of
individuals from three different arcas of
The Support Equipment Branch, EP44, expertise. The following disciplines wen
of MSFC's Propulsion Laboratory has been represented: mechanical, electrical, and data
working on an innovative program to develop a analysis.
bearing wear detection system for the SSME Once a foundation was in place, our
HPOTP's. This NDE system would be used to first task was to develop a system to dry-spin
detect anomalies in bearings on assembled the turbopump. For this, we fmt used as
TABES 94-621
much off-the-shelf hardware as possible and In our own vibration testing, we
then eventually used the ATS itself to drive the attached surface-mounted transducers to the
pump. We designed and even fabricated the pump housing, slowly rotated the shaft, and
support and interface hardware. analyzed the output signal for beaxing signature
With the ability to turn the pump came characteristics. We knew that the data analysis
the question as to how fast we could rotate it would be challengink b u s ea t speeds of 0 to
. . dry without damaging the bearings or - -y other 8 r/min, fundamental SSME bearing fault
component. Rocketdyne, primary contrdctor frequencies an all less than 1 Hz.
for the SSME, has never given us a maximum To begin, we had Physical Acoustics
speed limitation but advised us to nun as Corporation and Briiel and Kjm Instruments
slowly as possible. To be conservative, we perform preliminary testing on HFOTP unit
decided to limit our speed to a maximum of 8 0810. Both companies' tests were
r/min. With a functional test article in place, inconclusive since the background noise was
we were now ready to begin a comprehensive indistinguishable from the pump noise. They
testing program. did, however, recommend continued testing
with seismic accelerometers.
111. ACOUSTIC EMISSION TESTING Since those preliminary tests, we have
conducted three rignificant test series. In our
One of the testing areas we have had first series, we utilized a spare keel latch motor,
little success with is acoustic emission (AE). which was developed for the Hubble Space
AE's are stress waves that arc produced when Telescope maintenance mission, to rotate the
materials are stressed. A structure must be pump. We designed the spool and infernal
sufficiently loaded in order to produce stress spline to mate with the pump. The motor only
waves. This method detects movement, not ran at a constant speed of 8 rlmin, which
existing geometric discontinuities. Since limited our testing, since throttle capability
MSFC government employees had limited would have allowed us to better distinguish
experience with acoustic emission testing, the pump rotational driven spectral components
, industry was surveyed. from stationary background and electronic
%
Two independent contractors performed noise. Thc test series consisted of four 15411
preliminary AE testing on HPOTP unit 08 10. runs at this constant speed. We used three
The results were inconclusive in one case and different types of accelerometers: four
in the other the NDE method was deemed not miniature compression driven piezoelectric
viable at such low speeds. Turning at 0 to 8 sensors with internal electronics, three seismic
r/min, the system did not geneiate enough spring-mass type units sensitive in frequency to
energy to mgger the sensors. Even though the dc and four shear-type piezoelectric sensors
rcsults were not promising, we still plan to driven by external charge amplifiers. These
continue testing in order to disprove or prove off-the-shelf transducers were mounted in three
feasibility. With the required authorization, we different tiers: the preburner pump flange, the
will increase the speed of the turbopump up to HPOTP housing weld 3 region, and the G3
50 r/min and test again. Perhaps this will flange.
generate enough energv to excite the sensors. Our test objectives wen to optimize the
selection of acquisition equipment and the
IV. VIBRATION TESTING mounting locations of the transducers. To
accomplish this, we classified the dynamic
The second testing area we explored noise floor data prior to the pump rotation and
involved conventional vibrational analysis. As then determined the external drive's influence
beari~gsb egin to wear, they demonstrate on the data. The dynamic signals from the dry-
marked increases in vibration levels. spin testing were evah ated and the external
Piezoelectric accelerometers are traditionally environment influenct s on the dynamic data
used to convert this vibratory motion into an were idell tified.
electrical signal. These transducers are widely It was obvious tiam our test results that
used in industry to determine bearing health there was not enough energy in the system.
and predict failure. The miniatun accelerometers had not
functioned properly. We found that we needed
TABES 94-621
to work on understanding the frequencies fiom The ATS was envkimed fmm the very
external sources. As for identifying bearing beginning of the bearing assessmnt pmgram.
defect frequencies, the overall results wen The prototype was planned as an engineering
inconclusive. unit for development purposes. Jts
We did learn that we needed the ability quinrnents gradually evolved over time. The
to vary the motor speed. TLs would make it finished product is fully functional and reliable.
much easier to distinguish bearing related The ATS is 5attery powered, computer
spectral components from environmental and controlled, and easy to use. It was designed to
instrumentation noise. A slow throttling be portable and durable for operation on test
capability would also prove beneficial. Prior to stands. The complete ATS system is
the actual test, we should have performed composed mainly of two parts, a torque test
decoupled testing in order to isolate the feed head and a data acquisition system. The entin
thro~ghfr om the drive mechanism to the unit fits in two cases which have a combined
HPOTP data. Also we needed to record a shaft weight of less than 40 lb.
rotational speed data channel, i.e., key phascr The torque test head contains a 100 in-
information, to better enable us to locate lb torque transducer, a motor with a speed
bearing related frequencies. reducer, an optical encoder, various couplings,
a ball bearing, and a torque sensor motor
V. TORQUE TESTING control printed wiring assembly. All of these
components are encased in a container with a
In addition to the standard NDE nmovable lid to protect them from the
methods of AE and vibration, we also elements. The test head mates with an interface
-
investigated a new approach dynamic torque rod via a in square drive at the container's
analysis. Little if any work has ever been done open end and hard mounts to the pump by way
investigating SSME torque signatures for of a flange.
bearing signature content. Since torque checks The data acapisition system consists .
).
must be performed on all pumps, it seemed a 486SLC-25 notebook PC and a docking
logical to tap into this available resource. station. The laptop is configured with an 80-
Currently torque tests are routi~ely megabyte hard drive, 4 megabytes of RAM,a
performed on turbopumps during checkout to 3.5-in floppy disk drive, an ac-dc adaptor, and
determine breakaway and running torque. The an Ni-Cad rechargeable battery pack.
test consists of a techcician with a Snap-on Other important nardware included with
torque wrench hand turning the pump while a the system are the ATS battery pack and its
quality inspector reads the dial indicator ups& accompanying battery charger, an interface
down. Needless to say the test is rather crude, cable, and a B~yoneNt eil-Concelman (BNC)
and a more sophisticated test could possibly breakout box. This box serves as an interface
produce useful data. Since the test must be between the laptop and a data recorder. The
performed any way, it seemed reasonable to ATS has a BNC output of the unfiltered analog
have it automated so that the data could be torque signal for recoding to high-frequency
stored and analyzed for later use. Because the analog ape. From the Leginning, the bearing
unit is hard mounted to the pump, the tests are assessment team felt that the ATS should have
more accurate and consistent. No human error a BNC connector in order to record the analog
is involved, and no side loaL are induced into signal before the A/D conversion rocess. The
the pump. The aut() mated test is definitely analog data would be used to ve& the ATS's
more precise in m asuring static torque, since proper operation and expand our high
the accuracy of a ' ,and held torque wrench is frequency data processing options.
only plus or mint s 2 percent within the upper The accompanying software program
80 percent of tho scale. Moreover, the dynamic was written especially for the ATS and controls
content of the available torque signature could its entire operation. The p r o pi s menu
prove useful in determining bearing health. driven and very user-friendly. It offers two
levels of security with both an operator and a
supervisor password. All activity is rtcorded
VI. AUTOMATED TORQlJE SENSOR in a log that can only be ~cctssedb y the
_ - .
, r*l
TABES 94-621
supervisor. In order not to damage a pump by with good M n g sa nd verify that the ATS
accidentally cxceeded its maximum torque was rtcording torque data accurately. In order
limit, the value can be preset. The e n 6u nit to do this, the tern used state-of-the-art
will shutdown if this maximum torque is accelerometers and seismic transducers. The
exceeded. ATS drove the pump at a constant speed of 4
To run a tcst, the operator can either rimin. Thne different typcs of data were
select a previously loaded test set or select new recorded which included high frequency torque
parameters via pull down menus. Many data, key-phaser data, and acceleration data.
options an available and can be set in any The actual test ran for 10 min, with 9 min at
sequence. The speed at which the pump m s maximum speed and 30 s at the bcginnicg and
can be set anywhere from 0 to 4 rimin. The end at zero speed.
duration of the test can be estaSlished by either In the data reduction, two diffant
selecting the number of revolutions or the frequency regions were explored, those below
number of seconds. One to four data points 100 Hz and those below 20 kHz. In the low
can be recorded per degree of rotation. It is frequency analysis, the acquired torque
also possible to aixays start a test in a signatures were examined for fundamental
designated zero position. If this option is bearing def, *ts. In the high fnquency
selected, the unit will rotate to the start position analysis, bearing diagnostic information was
before the test begins. searched for using envelope detection
Once all the parameters have been techniques.
chosen, the operator must verify his selections In the time and waveforma nalysis, the
before the test can begin. During an actual test, results looked promising. As exp-d the
the torque, angular position, and rimin levels accelerometers registered very lou ,iiergy
are dyr lamically displayed. Upon completicln, levels. However, the geneid shape of the
the results can be viewed along with the torque signature was consistent wit!! those:
average values for the running and breakaway previously taken in test series orle and nvc
torqw. Data can be stored in ASCII on either This showed that the torque signature from the
the hard disk or on a diskette. pump was repeatable using totdly different
sensors and data acquisition systems.
VII. COMBINATION TESTING The frequency analysis also
demonsmted favorable resu1:s. In the 0 to
After the first test series, we were eager 100-Hz spectrum, no dish*, shable
to begin recordirIg torque and vibration dam cor,.ponents were found in the ,accelerometer
simu!taneously. The team configured an off- data. However, some discrete low frequency
the-shelf torque sensor, motor, and tachometer activity was noted in the torque data. The 0 to
intl~a workabie unit. We also designed and 20 Hz spectrum taken from the ATS was
fab:;cated the support and interface structure remarkably similar to that taken in a previcvs
ourselves to expedite testing. test series.
Before the test began, we performed a The high frequency torque data looked
manual torqut heck on the pump. Data from very prmising sitice the signature content was
the torque sensor, the tachometer, and the repeatable from test to tes; wi;h different
3 accelerometers was simultaneously recorded transducers. No obvious bearing defect
. ,
The same transducers were mount& in similar frequencies were found as we had expected
locations to the previous test series. During with good bearings. Howsver, some
our test, we gradually increased the pump's unidentified spectral components were noticed
speed to a maximum of 4 rimin during a 30- which provide merit for further study. Jr:
min time interval. We slowly ramped up to 4 conclusion, the team decided thilt a HPGTP
r/min, held this speed for 15 min, and then with severely worn bearings was ileeded as a
slowly ramped back down. The results of this test article to unequivocally prove concept
successful test will be mentioned later. feasibility.
When the ATS was delivered, we were
again enthusiastic to begin a third test series. VIII. CONCLUSION
In this test, we could achieve a twofold
objective. We wanted to haseline our pump
To date, the bearing asessment team breaks at a much higher torque level of 370 in-
has made important smdes. Of the three lb.
different NDE methods that were considered, Ourultimategoaltodttcrminetht
we have enjoyed the tnost sw%s wi;h. torque. relationship between torq~sig natures and
AE and vibration still need much mwt WG& bearing wear is still forcmost irl our minds.
for their verificaiiou. Ou accon;p:ishments art Since the likelihood of obtaining a turbopump
muly. We hat: 3asdincil a HPOTP which to to test with severely worn bearings is not
the best dour knowleSge has good bearings, favorable, the bearing and seal materials testa
and thus have exceLenr ~ikrencde." .m ;I, *Le will be used This unit is conveniently located
used for later comparisons. The very at MSFC and runs a set of W g su ntil they
deveiqr?-,nto f a prototype toque ricvke was arc worn. *Ino bjective will be to estaolish a
signifia .i. 5 itself. We ha% verit-m !ht the torque signature for the unit with new bearings
ATS call x x dR ocketdyne WCll P stah and thal acquire the torque signam at the end
toque data itczurately and consiscntly. of the bearing test when there is likely
Thus, it is nz recommm&tim ;hat the substantial bearing distress. Upon disassembly
ATS be incorporated into tht standani the bearings will be analyzed and the wear
nirbop~mpch eckout pnx*urcs 10 replace tk-. correlated to our data &servations. This
maaual torque c he& for bxh the Rccl:%iyne approach will give us fast results and embk us
and Ratt and Whitney SOTP's. 'ih em is no to familiarize ourselves with data and to
doubt that t!! automated method is better than develop data analysis criteria.
the cmtly used manual one. In addition, the As a paraliel effort we will test the ATS
data tzrlrcn can be stored and later retrieved for on an assembled SSME. However in order to
study and anal jsis. The data has tnc potential accomplish this, precision intaface hardware
of one day being used to detem21e t!e amount must be designed to mount the unit to an
of HPOTP pump end bearii~*~-,A . Additional installed HPOTP. A support bracket will hold
systems couid be developcu k ru te at Stennis the test head in place and guide the interface rod
Space Center, Kennedy Space Zrer, and to mate with the pump through the prebumer
Rwkeviy ne. pump inlet duc.. Since attachment involves a
,Missioli s2t.y has alwe;-; men a blind insul1atio;r through a duct, this guide is
primary goal of the space shuttle cjrr?.gram needed so me ATS will not induce any
This ATS would only enhance the SSMF's additional loads into the HPOTP. Once it is
performance and reliability. It would greatly demonstrated that we can take successful
improve bearing diagnostics ar tht- - provide measurements on both Rocketdyne and Ratt
'
another degree of confidence to ensure shuttle and Whimey HPOTP's, a fairly extensive
safety. The savings in time and money are database of flight and developmental HPOI'F's'
secondary but are still worth serious can be established. Then our formidable task
consideration during these times of severe of data analysis will begin.
budget cats. The ATS itself wiil be optimized by
making the unit lighter and shorter for ease of
XI. FUTURE PIAXS handling and installation. The software
package will be enhanced to accOIIlIllOdate
Although much has been accomplished, mon userdefined parameters and to enable the
much more work is left. The program is by no operator to manipulate the data immediately
means finished Our research with acoustic foll0wir.g the test. Finally, this system may
emission and vibration sensors will still one diylk hdapted f ai ndustrial use to
continue but with a new angle. The team is in perform quick and reliable bearing checks and
the process of designing an advanced ATS that thus provide another level of quality assurance.
wi;i be capable of turning t4t turbopump a
maximum of 50 rlrnin. Fater n -. ational speeds
will ixx is emgy levels in the system ard
hoy-ttd:y better excite the monitoring sensors.
Pisd vmctd ATS will also have arii n d
motor md torque capacity to handle the
dtmate ?rim snd Whiwey HPOTP which