A lumped node thermal model was developed representing the Space Shuttle Main Engine (SSME) liquid oxygen (LOX) turbopump turbine end bearings operating in a cryogenically cooled bearing tester. Bearing elements, shaft, carrier, housing, cryogen flow characteristics, friction heat, and fluid viscous energy are included in the model. Heat transfer characteristics for the regimes of forced convection boiling are modeled for liquid oxygen (LOX) and liquid nitrogen (LN2). Large temperature differences between the cryogenic fluid and baring contact surfaces require detailed nodal representation in these areas. Internal loads and friction heat are affected by temperature dependent operating clearances requiring iterations between bearing thermal and mechanical models. Analyses indicate a thermal-mechanical coupling resulting in reduced operating clearances, increased loading and heating which can contribute to premature bearing failure. Contact surfaces operate at temperatures above local saturation resulting in vapor rather than liquid in the contacts, precluding possible liquid film lubrication. Elevated temperatures can reduce lubrication, increase friction, and reduce surface hardness supporting a surface failure mode rather than subsurface fatigue

Cody, J. C.

New, L. S.

Tiller, B. K.

English

NASA Technical Reports Server

ADVANCED ROCKET ENGINE CRYOGENIC 
TURBOPUMP BEARING THEI?JUL MODEL 
Joe C. Cody, Linda New, aruce Tiller 
Aerospace Systems & Prod2~cts Division 
Spectra Research Systems, Huntsville Division 
Abstract 
A lumped node thermal model has been developed 
representing the Space Shuttle Main Engine (SSME) 
liquid oxygen (LOX) turbopump turbine end bearings 
operating in a cryogenically cooled bearing tester. 
Bearing elements, shaft, carrier, housing, cryogen 
flow characteristics, friction heat, and fluid 
viscous energy are included in the model. Heat 
transfer characteristics for the regimes of forced 
convection boiling are modeled for liquid oxygen 
(LOX) and liquid nitrogen (LN2). Large temperature 
differences between the cryogenic fluid and bearing 
contact surfaces require detailed nodal representa- 
tion in these areas. Internal loads and friction 
heat are affected by temperature dependent operating 
clearances requiring iterations between bearing 
thermal and mechanical models. Results compare 
favorably with limited data from the Marshall Space 
Flight Center (MSFC) Bearing Materials Tester (BMT). 
Analyses indicate a thermal-mechanical coupling 
resulting in reduced operating clearances, increased 
loading and heating which can contribute to premature 
bearing failure. Contact surfaces operate at 
temperatures above local saturation resulting in 
vapor rather than liquid in the contacts, precluding 
possible liquid film lubrication. Elevated tempera- 
tures can reduce lubrication, increase friction, and 
reduce surface hardness supporting a surface failure 
mode rather than subsurface fatigue. Future efforts 
include modeling the SSME liquid hydrogen turbopump 
bearings, model refinements, and further correlation 
of modeling techniques with BMT data. 
https://ntrs.nasa.gov/search.jsp?R=19850018568 2020-03-20T19:07:21+00:00Z
Introduction 
This thermal modeling development supports an 
overall MSFC Bearing Materials evaluation program 
designed to formulate and experimentally verify 
failure modes and life prediction models for high 
speed rolling bearings operating in cryogens. This 
research supports development and improvement of 
hardware such as the Space Shuttle Main Engine liquid 
oxygen (LOX) and liquid hydrogen (LH2) turbopumps 
which are highly efficient lightweight machines 
operating at high power levels and shaft speeds. The 
pump main shafts are supported by angular contact 
ball bearings operating in the cryogen being pumped. 
State of the art bearing codes are not adequate 
for thermal simulation of high speed rolling bearings 
operating in cryogens. Although sufficient for 
analysis of conventionally lubricated bearings, 
treatment of the various regimes of boiling, large 
temperature gradients, and significant variations in 
material properties is beyond the scope of these 
codes. The current thermal modeling effort is 
directed toward improved treatment of these important 
characteristics of cryogenic bearing systems, and to 
provide a better understanding of the thermal influ- 
ences on cryogenic bearing operating characteristics 
and life. Objectives are to develop and refine the 
bearing model, correlate results with BMT data, and 
provide a verified design tool for predicting the 
thermal characteristics of high speed rolling bear- 
ings operating in cryogens. 
Approach 
Model Description 
A state of the art thermal code "SINDA" was 
selected for modeling the cryogenic bearing system. 
This code has the capability for temperature and time 
varying boundary conditions, and temperature varying 
material properties. It also provides for modeling 
af fluid networks which is necessary to simulate the 
cryogenic fluid flow. The internal arrangement of 
t h e  BMT is shown i n  F i g u r e  1. S i n c e  t h e  r i g h t  and 
l e f t  b e a r i n g  p a i r s  are e s s e n t i a l l y  s i m i l a r ,  on ly  one 
b e a r i n g  p a i r  r e q u i r e s  modeling.  The cryogen,  a c t i n g  
a s  a  c o o l a n t ,  e n t e r s  a  manifold  a t  each end of ' t h e  
t e s t e r ,  f lows through each b e a r i n g  and e x i t s  v i a  a 
common f l o w  pa th .  Although t h e  t e s t e r  can be loaded 
i n  bo th  t h e  a x i a l  and r a d i a l  d i r e c t i o n s ,  t h e  c u r r e n t  
computer s i m u l a t i o n  t r e a t s  o n l y  t h e  a x i a l l y  loaded 
case .  S i n c e  t h i s  produces  a x i a l l y  symmetric l o a d i n g  
a  r a d i a l  s l i c e  of t h e  sys tem i s  a l l  t h a t  r e q u i r e s  
modeling. F i g u r e  2 ,  a schemat ic  of t h e  b e a r i n g  set ,  
i l l u s t r a t e s  a r e p r e s e n t a t i v e  r a d i a l  s e c t i o n  of t h e  
model. The b e a r i n g  is  a 57 mm b o r e  a n g u l a r  c o n t a c t  
b a l l  b e a r i n g  w i t h  t h i r t e e n  b a l l s .  
Due t o  t h e  l a r g e  t e m p e r a t u r e  d i f f e r e n c e s  i n  t h e  
b e a r i n g  c o n t a c t  s u r f a c e s  and c ryogen ic  f l u i d ,  l a r g e  
thermal  g r a d i e n t s  are p r e s e n t  i n  t h e  b a l l s  and r a c e s  
n e a r  t h e  h i g h l y  loaded  c o n t a c t s .  T h i s ,  i n  a d d i t i o n  
t o  t h e  extreme s e n s i t i v i t y  of t h e  f l u i d  h e a t  t r a n s f e r  
c o e f f i c i e n t  t o  s u r f a c e  t empera tu re ,  r e q u i r e d  many 
v e r y  s m a l l  nodes i n  t h e  c o n t a c t  a r e a s  of t h e  model. 
The n o d a l  r e p r e s e n t a t i o n  of t h e  i n n e r  r a c e ,  b a l l ,  and 
o u t e r  r a c e  f o r  t h e  57 mm LOX pump t u r b i n e  end b e a r i n g  
is shown i n  F i g u r e  3 .  There  are 642 nodes i n  t h e  
model. S u r f a c e  nodes are connected by a p p r o p r i a t e  
the rmal  r e s i s t o r s  t o  t h e  c r y o g e n i c  f l u i d ,  These 
r e s i s t o r s  are v a r i e d  a s  t h e  s u r f a c e  t empera tu re  of 
t h e  node changes t o  r e p r e s e n t  t h e  a p p r o p r i a t e  f l u i d  
h e a t  t r a n s f e r  regime. The b a l l  c o n t a c t s  t h e  i n n e r  
and o u t e r  r a c e s  i n  a s m a l l  e l l i p t i c a l  a r e a  t h a t  
v a r i e s  i n  s i z e  and l o c a t i o n  w i t h  speed and load .  The 
d e t a i l e d  noda l  r e p r e s e n t a t i o n  a l l o w s  f l e x i b i l i t y  t o  
account  f o r  t h e s e  v a r i a t i o n s  w i t h o u t  renoding t h e  
model. 
F l u i d  Heat T r a n s f e r  C o e f f i c i e n t s  
Wide v a r i a t i o n s  i n  corcpcnent s u r f a c e  tempera- 
t u r e s  c a r r i e s  t h e  f l u i d  h e a t  t r a n s f e r  mechanisms 
through t h e  range  of f o r c e d  c o n v e c t i o n  i n  l i q u i d ,  
h i g h  v e l o c i t y  f o r c e d  convec t ion  b o i l i n g ,  and f o r c e d  
convec t ion  i n  vapor .  S i n c e  t e s t  d a t a  a r e  a b s e n t  f o r  
h igh v e l o c i t y ,  h i g h  p r e s s u r e  f o r c e d  convec t ion  
2 boiling of cryogens, a superposition method was 
used to evaluate the appropriate flow regimes depend- 
ing on the surface superheat. 
Figure 4 provides a graphical illustration of 
the method for evaluating the heat transfer regimes 
as a function of surface superheat for LN2. As 
shown, when the forced convection heat flux equals 
the nucleate boiling flux incipient boiling is 
assumed to occur. As the surface superheat exceeds 
about 3.3"R the surface is assumed to be vapor 
blanketed and forced convection vapor is the mode of 
heat transfer. Since this indicates about an order 
of magnitude reduction in heat. transfer, the 
significance of the superheat value at which 
transition occurs becomes apparent. Heat fluxes for 
the different regimes were calculated from the 
following equations-the Dittus-Poelter equation for 
forced convection on all components except the balls, 
Katsnellson's equation for forced convection on the 
balls, the Borishanskiy-Minchenko correlation for 
nucleate boiling, axyl the Kutateladze correlation for 
the peak heat flux. This technique was tentatively 
verified with tester data as illustrated in Figure 5, 
where a comparison of predicted and measured bearing 
outer race temperatures is shown. The predicted 
values are provided as a function of surface 
superheat and indicate that a superheat range of 2 to 
6 degrees provides reasonable agreement with the 
measured BMT te5t data. A similar analysis was 
conducted for LOX . 
Neat Generated by Friction in the Bearing Contact 
Area 
Heat is generated in the raceway contacts due to 
ball spin and roll. Additional frictional heat, not 
yet included in the model, is generated by the ball 
contacting the cage, and the cage contacting the 
inner surface of the outer raceway. The cage effects 
are in the process of being incorporated into the 
model. 
The raceway f r i c y o n a l  h e a t  genera ted  i s  ob ta in -  
ed from t h e  SHABERTH b e a r i n g  code. T h i s  code a l s o  
p rov ides  b e a r i n g  o p e r a t i n g  c h a r a c t e r i s t i c s  such a s  
c o n t a c t  a n g l e s ,  o p e r a t i n g  c l e a r a n c e s ,  c o n t a c t  s t r e s s -  
e s ,  d e f l e c t i o n s ,  b a l l  and cage speeds  and b e a r i n g  
r e a c t i o n s .  Shown i n  F i g u r e  6 i s  an  example of t h e  
f r i c t i o n a l  h e a t  g e n e r a t e d  i n  t h e  i n n e r  r a c e  c o n t a c t s  
f o r  t h e  c o n d i t i o n s  no ted .  The two minimum p o i n t s  
i n d i c a t e  t h e  a r e a s  of b a l l  r o l l  and t h e  maximum v a l u e  
i s  caused by t h e  h i g h  r e l a t i v e  v e l o c i t i e s  due t o  b a l l  
s p i n .  S i m i l a r  i n f o r m a t i o n  may be genera ted  f o r  t h e  
o u t e r  r a c e .  These v a l u e s  a r e  used as i n p u t  t o  t h e  
b e a r i n g  the rmal  model. 
Heat Generated by Mechanical ly  Working t h e  F l u i d  
Cons iderab le  h e a t  is genera ted  i n  t h e  s h a f t  
b e a r i n g  system due t o  mechanical  work on t h e  c o o l a n t  
f l u i d .  Beat generated,, from f l u i d  s t i r r i n g  and 
f r i c t i o n  was e s t i m a t e d  f o r  each component. A BPTI: 
h e a t  b a l a n c e  provided a measure of t h e  t o t a l  h e a t  
i n t o  t h e  system. Est imated v a l u e s  were a d j u s t e d  t o  
match t h e  BMT h e a t  ba lance .  The e s t i m a t e d  v a l u e s  
were from 45% t o  80% t o o  low w i t h  t h e  l a r g e r  devia-  
t i o n  a t  t h e  lower s h a f t  speeds .  R e s u l t s  a r e  shown i n  
F i g u r e  7. Inc luded  are v a l u e s  of c o n t a c t  f r i c t i o n a l  
h e a t  f o r  t h e  l o a d i n g  c o n d i t i o n s  shown. T h i s  d a t a  
i l l u s t r a t e s  t h e  l a r g e  pe rcen tage  of h e a t  i n p u t  t o  t h e  
sys tem from f l u i d  work. 
Thermal Model R e s u l t s  
R e s u l t s  of t h e  b e a r i n g  the rmal  model show h i g h  
l o c a l  t empera tu res  and s t e e p  t empera tu re  g r a d i e n t s  i n  
t h e  b a l l l r a c e  c o n t a c t  a r e a s .  Examples are shown i n  
F i g u r e s  8, 9 and 10. F i g u r e  8 i l l u s t r a t e s  represen-  
t a t i v e  s u r f a c e  t empera tu res  and g r a d i e n t s  i n  t h e  
b a l l .  There  i s  a  wide v a r i a t i o n  i n  s u r f a c e  tempera- 
t u r e ,  and much of t h e  s u r f a c e  i s  above t h e  l o c a l  
s a t u r a t i o n  t empera tu re  (-241°F) a f  t h e  LN c o o l a n t ,  2 I n n e r  r a c e  t empera tu res  a r e  shown i n  F i g u r e  9. These 
s u r f a c e  t empera tu res  a r e  a l s o  above t h e  l o c a l  f l u i d  
s a t u r a t i o n  t empera tu re  a s  a r e  most of t h e  o u t e r  r a c e  
t r a c k  nodes shown i n  F i g u r e  10. These t empera tu res  
show that for the conditions analyzed the bearing 
contacts are operating in vapor rather than liquid. 
In addition, the maximum temperature shown in the 
ball track (461°F) is approaching temperatures that 
can affect surface hardness of the 440C steel. 
Thermal-Mechanical Interactions - 
The inner races of high speed rolling bearings 
reach a higher operating temperature than the outer 
races because of the increased ball spin at the inner 
race. This causes inner race growth relative to the 
outer race and has the effect of increasing loads and 
reducing internal operating clearances and contact 
angles. If bearing clearances and/or cooling capaci- 
ty is marginal, this effect can amplify causing 
increased loading and heating and possible premature 
bearing failure. Examples of this trend are shown in 
Figures 11 and 12. Figure 11 shows the reduction in 
internal clearance as the coolant flow is decreased, 
and Figure 12 shows the corresponding increase in 
component temperature. 
Comparison of Model and Test Results 
Shown in Figure 13 are comparisons of the 
measured and calculated outer race temperature and 
the coolant outlet temperature. These results show 
fair agreement between the measured temperatures and 
model predictions even though a slight difference in 
coolant flow rate exists because the thermal model 
predictions were conducted well before the actual 
test run and at that time the flow rates were not 
precisely known. BMT test data is limited at this 
time. As additional test data becomes available 
further verification and/or model refinements will be 
made. 
Conclusions 
Development of a bearing thermal model capable 
of accounting for two-phase cryogenic flow charac- 
teristics has revealed unexpected high temperatures 
in the loaded contacts. These contact areas operate 
at temperatures well above the local saturation 
temperature and are operating in a vapor rather than 
liquid environment. The lubrication benefits that 
might be expected from a liquid film separatixig the 
contacts does not occur. For some loading and 
coolant flow conditions, the model shows that there 
is a thermal-mechanical coupling that can cause loss 
of operating clearances, increased loading, and 
increased heat generation that can contribute to 
premature bearing failure. Elevated temperatures can 
reduce labrication, increase friction, and reduce 
surface hardness. These conditions support a surface 
failure mode rather than the subsurface fatigue 
experienced by conventional oil lubricated rolling 
bearings. 
Bearing modeling techniques are providing 
valuable tools for the assessment of new materials 
for high speed bearings operating in cryogens. 
However, due to the complexity of the system and lack 
of experimental data in many areas, especially high 
flow two-phase cryogenic heat transfer characteris- 
tics, additional data from the MSFC BMT is vital for 
verification of cryogenic bearing systems modeling 
methods. 
References 
1. SINDA Engineering Program Manual (Contract No. 
NAS9-10435) June 1971. 
2. Bearing Tester Data Compilation, Analysis, and 
Reporting and Bearing Math Modeling (Contract 
No. NAS8-34686), May 1984, Final Report. 
3. SHABERTH Computer Program Operation Hanual, 
Technical Report AFAPL-TR-76-90, October 1976. 
4. NASA Report No. 793, "Experiments on Drag of 
Revolving Disks, Cylinders, and Streamline Rods 
at High Speeds", by T. Theodorsen and A. Regier. 
FIGURE* 1 SHAFT, BEARINGS & CARRIER APPLIED LOADS 
flm 2 SWEfUTlC OF BEARIffi SET 
----  ' ----- 
I MIUSING 
I I 
I I 
I 
I ALL DltlENSIONS I N  IWCHES 
I 

Flm 5 Cuter b c r  Iwct r r p r r r t u n  As A Fwrt lon of 
L11 Eprhrt a t  Ibr Flus Condltlons 
(LOX Turbqprp Turblm End Wring - 57 r) 
L"z 
B q  42 a 
Axlal Lord 2540 l b  O u t n  L C I  
F l a  h t e  = 6.3 lb ls  - Iffi Set 
Tf" -305 OF 
Vapor Pmpcrtles Evaluated a t  TFl. 
I n n a  Race 
Locat ton 
of M e r  f463 m d  I 6 5  
-260 - 
Uf2 Test Oat, 
Perk Outer L c c  T a &  H e r s u r r m t  (T lwS 
. ------------- --------- ----- 
-270 . 
0 , s  10 I S  10 (Tw - TwT) a t  l*x Hut Flux. OF 
FIGURE 6 lmEI WE H u T  GEERATED AT COIRKT ELLICSE 
FIGURE 7 
HEAT GENERATED (KU) AT LOAD END OF THE WIT AS A FUNCTION OF 
SHAFT SPEED AND AXIAL LOAD (LN2 COOLANT) 
\ AXIN RVCTIDn ' 6MX) LO. RATE - 6.3 LBSlSEC PER BRG. SET 6: 200 COOLMT TEMPERATURE INTO BUG. SET -3OS.F (L IZ )  
FIGURE 9 INNER RACE TEHPERATURE DISTRIBUTION FOR LOX PUMP TURBINE END BEARING 
UIK REACTION = MWM 1.6. 
FLOW RATE . 6 . 3  LBS/SEC PER BRG.SET 
COOLANT TEMP. INTO BRG. SET . -305.F (LNZ) 
(TEttPS. I N  'F) 
I 
FIGURE 10 OUTER RACE TEMPERATURE DfSTRIBUTlON KR LOX P U R  TURBINE END BEARING 
FI@JR 11 OPERATING CLUIRAWCE VS COOLANT FLOH 
e 57 mn Angular Contact BGR 
s 30,000 RPIl 
o 6000 Lb Axlal  Load 
0 ,  I 1 
2 4 6 8 10 12 
COOLANT FLW RATE (LBISEC) PER BEARING SET 
0 57 nn BEAnltIG 
o 30.000 RR1 YUR SPEED 
o M)(M LB A X I M  REACTION 
I n L E r  cwum rap. -305 F (LMZ) 
0 f . 0.2 
- IWER RACE 
e f "  
COOLANT FLW RATE (LVYC) PER Wlffi SET 
OPERATING PARAMETERS 1 
SHAFT SPEED (RPM) I 
LN2 COOLANT FLOW RATE 
(LB/SEC PER ERG. P A I R )  
INLET COOLANT TEMPERATURE (OF 1 
COOLANT TEMPERATURE OUT 
OF LOADED BEARING (OF)  
OUTER RACE BEARING 
TEMPERATURE (OF)  I 
FIGURE 13 
COMPARISON OF MODEL PREDICTIONS AND TEST RESULTS 
BEARING MATERIALS TESTER DATA 
TEST TEST DATE # 2 0 1 N 0 4 0 9  4 - 2 6 - 8 4  TEST DATE 5 - 1 5 - 8 4  
I TEST X 2 0 1 N 0 5 0 1  I 
FIGURE 14 
COMPARISON OF MODEL PREDICTIONS AND TEST RESULTS 
BEARING MATERIALS 
BEARING THERMAL TESTER DATA 
MODEL RESULTS 
INLET COOLANT TEMPERATURE ( "F) 
COOLANT TEMPERATURE OUT OF 
LOADED BEARING (OF) 
OUTER RACE BEARING TEMPERATURE (" F) 


Advanced rocket engine cryogenic turbopump bearing thermal model

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