ABSTRACT The start-up friction coefficient is a parameter which is very useful for engineers who design hydrodynamic bearings

Bouyer J

Fillon M

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Downloaded FProceedings of the STLE/ASME International Joint Tribology Conference 
IJTC2008 
October 20-22, 2008, Miami, Florida, USA 
  
IJTC2008-71099 
AN EXPERIMENTAL INVESTIGATION ON FRICTION COEFFICIENT IN PLAIN 
JOURNAL BEARINGS DURING START-UP 
 
Bouyer J. and Fillon M. 
Laboratory of Solid Mechanics,  
University of Poitiers, UMR CNRS 6610, 
SP2MI, Bd Marie et Pierre Curie, BP 30179,  
86962 Futuroscope Chasseneuil Cedex, FRANCE.  
ABSTRACT 
The start-up friction coefficient is a parameter which is 
very useful for engineers who design hydrodynamic bearings. 
It is most often issued from an approximation. Several studies 
can be found in the literature on this topic but most of them are 
concerned with air bearings or are only numerical. Some 
studies are more general and deal with the transient thermal 
behaviour in journal bearings as well as in thrust bearings. 
Other studies deal with the friction coefficient during running, 
at a fixed rotational speed. The aim of this study is to provide 
experimental measurements of the bush torque during start-up 
of plain journal bearings, varying the specific pressure. Thus, 
the friction coefficient at start-up (deduced from 
measurements) is obtained for four different bearings varying 
feeding conditions, radial clearance and length. 
INTRODUCTION 
The start-up friction coefficient in hydrodynamic bearings 
has been the subject of several published studies, but most of 
them are dedicated to air bearings or are only numerical. We 
will present here a brief review of papers related to this topic. 
In 1989, Dufrane et al. [1] analyzed seizure of journal 
bearings due to the insufficient lubrication by a simple 
numerical model, relating the seizure time to the bearing 
operating parameters. They concluded that, in high-speed 
bearings, the seizures could occur within seconds of operation. 
Sun et al. [2] presented in 1994 a paper on the starting 
characteristics of an unlubricated journal bearing. They showed 
numerically that the motion of the journal in an unlubricated 
bearing consists of various combinations of rolling and sliding. 
Harnoy [3] presented in 1995 a model to evaluate friction 
during start-up in hydrodynamic journal bearing. He showed 
that it is possible to reduce the wear of the bush by increasing 
start-up acceleration and that a flexible support could reduce 
the maximum friction force and energy losses. Pistner [4] 
studied in 1996 several examples of industrial bearing pads that rom: https://proceedings.asmedigitalcollection.asme.org on 06/29/2019 Terms of Usehave wiped during a cold start-up. He demonstrated that the 
face temperatures of the thrust shoes increase rapidly during 
start-up, causing a catastrophic deflection of the pads which 
induced the rupture of the oil film. He concluded that this 
problem could be avoided by preheating the bearing pads 
before start-up. Ettles et al. [5] studied in 2003 the effects of 
start-up and shut-down on hydro-generator thrust bearings. 
Based on the observations made by Pistner [4], they developed 
a two dimensional model of transient thermoelastic effects in 
bearing assemblies. They proposed several procedures using 
jacking oil lifts as to avoid seizure during the start-up and shut-
down of thrust bearings. In 2005, Wang et al. [6] studied both 
the effects of start-up time and start-up load on the temperature 
distribution in the oil film. They showed that the fluid film 
forms quickly at the beginning of the start-up and also that the 
film temperature is lower in a rapid start-up than in a slower 
start-up. Lu et al. [7] presented in 2006 a study on the lift-off 
speed in journal bearings. They analyzed the characteristics of 
the Stribeck curve varying load, oil temperature and oil type. 
They continued in 2007 [8] with an experimental investigation 
of grease lubricated journal bearings. They discussed the 
advantages of grease instead of oil as a lubricant for heavily 
loaded bearings operating at relatively low speeds. Ünlü et al. 
[9] conducted the same year a study which deals with the 
determination of the friction coefficient in journal bearings. 
They developed a new test rig and a method to measure friction 
coefficient of journal bearing, but they were not interested in 
the friction at start-up. 
Most of these works are numerical while the published 
experimental data focuses mainly on the consequences of 
seizures caused by high friction during start-up or on friction in 
steady-state operating conditions.  
Thus, the aim of this study is to provide experimental 
measurements of the bush torque during start-up. The 
experiments presented here have been made under transient 
regime, varying static load and for four different bearings. 1 Copyright © 2007 by ASME 8
: http://www.asme.org/about-asme/terms-of-use
DownEXPERIMENTAL RIG 
The experimental apparatus being well described in a 
previous work [10], only some details on the test bearing will 
be given here. 
The equipment of the bearings was reduced in order to not 
disturb torque measurement. Only few thermocouples were 
inserted at the film/bush interface. The torque measurement 
device is composed of a shaft which is attached on each side of 
the bush supporting assembly and allows the frictional torque 
to be transmitted to the torque transducer, placed in the front of 
the test bearing. A view of the measurement apparatus is given 
in Fig. 1.  
 
Figure 1. Torque measurement device. 
 
Table 1 gives the geometrical parameters of the different 
bearings. The bearings were all driven with the same stainless 
steel shaft. The operating conditions are given in Tab. 2 just 
below. The test where performed the same way for all the 
bearings: a 10s acceleration from 0 to nominal speed followed 
by a 50s maintain at nominal speed and stop. 
 
Bearing #1 #2 #3 #4 
Grooves 2 1 1 1 
Length(mm) 0.08 0.08 0.06 0.04 
Bush diameter(mm) 99.96 100.015 99.96 99.96 
Radial clearance(µm) 90 117.5 90 90 
Table 1. Geometrical parameters. 
 
Parameters Units Value 
Nominal speed Rpm 1000 
Applied load kN 1 – 8 
Supply pressure MPa 0.1 
Supply temperature ºC 39 
Lubricant grade - ISO VG 32 
Table 2. Operating conditions. 
RESULTS 
In order to ensure a good repeatability of the 
measurements, the tests presented here were performed after 
the thermal equilibrium of the test rig was achieved. Each test 
was repeated at least six times. Moreover, according to the 
results presented in a previous work [11], the acceleration time 
and the feeding temperature, which have no influence on the 
maximum torque measurement, are fixed and respectively 
equal to 10s and 39°C for all the tests.  
Test 
Bearing Torque 
transducer 
loaded From: https://proceedings.asmedigitalcollection.asme.org on 06/29/2019 Terms of UseThe uncertainty on specific pressure is constant, whatever 
the value considered. Hence, being ±0.0125 MPa, it ranges 
from ±10% for the low specific pressure to only ±1% for the 
higher value. The uncertainty on torque measurement varies 
from ±0.7 N.m for the low specific pressures to ±1.8 N.m for 
the higher ones.  
 
 
Figure 2. Torque during start-up under 3 kN static load. 
 
The measurements presented in fig. 2 are obtained on 
bearing #1 for a static load of 3 kN, i.e. a 0.375 MPa specific 
pressure. The evolution of torque during start-up can be 
separated in three zones. The first zone (from 0 to vertical 
green dash doted line) corresponds to a peak which is due to 
direct contact between the shaft and the housing. The torque 
evolution is from 0 to 23 N.m followed by a fall towards 0.22 
N.m which corresponds to the introduction of oil in the contact. 
The second zone (between vertical dashed lines) shows the 
increase in torque due to the acceleration of the shaft. Once the 
nominal speed is attained (third zone), the torque remains 
nearly constant and equal to 1.3 N.m. 
 
 
Figure 3. Maximum torque vs static load 2 Copyright © 2007 by ASME 8
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DownloFigure 3 shows the results obtained for the four bearings at 
several values of static load. Whatever the bearing considered 
the maximum torque at start-up increases linearly with the 
increase in static load. One can note that the slope increases for 
the shorter bearings, and decreases for the bearing with higher 
radial clearance.  
The evolution of the friction coefficient as a function of 
specific pressure is presented in Fig. 4. The friction coefficient 
for bearing #1 increases with the specific pressure towards a 
value around 0.14, and remains almost constant with increasing 
specific pressure from 0.4 to 1 MPa. For bearing #2, which has 
the higher radial clearance, there is a slight decrease of friction 
coefficient around the same value of 0.14. The bearing #3 
shows higher friction coefficient but quite constant, around 
0.16, whatever the specific pressure considered. The shortest 
bearing friction coefficient slightly increases with specific 
pressure from 0.13 to 0.16 but the mean value remains around 
0.14.  
 
 
Figure 4. Friction coefficient vs specific pressure 
 
We can then conclude that the friction coefficient at start-
up doesn’t depend on the specific pressure applied to the 
bearing. Its value remains in a domain which varies around 
0.15 ± 0.01. This value is very close to that which can be found 
in the literature for the friction coefficient between steel and 
bronze. Indeed, the value of the friction coefficient for 
bronze/steel contact is 0.15-0.18 for dry contact and 0.1-0.13 
for lubricated contact. At start-up, there is a direct contact 
between the shaft and the bearing which can be neither dry 
with a high friction coefficient nor partially lubricated with a 
lower friction coefficient.  
CONCLUSION 
 The measurement of the torque at start-up is not an easy 
task. Nevertheless, the tests showed that the torque increases 
linearly with the specific pressure and that the friction 
 
 
aded From: https://proceedings.asmedigitalcollection.asme.org on 06/29/2019 Terms of Usecoefficient remains almost constant whatever the specific 
pressure considered. 
In order to understand exactly what occurs in the first 
revolution of the shaft, future work will be conducted to 
determine the mechanisms of the transition between contact 
and hydrodynamic lubrication.  
ACKNOWLEDGMENTS 
The authors would like to thanks Mr. R. Branlé, technician 
at the laboratory for his help on this work. 
REFERENCES 
[1] Dufrane, K.F. and Kannel, J.W., 1989, "Thermally Induced 
Seizures of Journal Bearings", J. Tribology, 111(2), pp. 
288-292. 
[2] Sun, D.C. and Jing Xu, 1994, "A study of the starting 
characteristics of an unlubricated journal bearing", Proc. 
ASME/STLE Tribology Conference, Maui, Hawaii, USA. 
[3] Harnoy, A., 1995, "Model-Based Investigation of Friction 
During Start-Up of Hydrodynamic Journal Bearings", J. 
Tribol., 117(4), pp. 667-673. 
[4] Pistner, C.A., 1996, "Some Effects of Start-Up Transient 
Loads on Shoe Bearings for Large Hydraulic 
Pump/Turbines", Trib. Trans., 39(1), pp. 93-98. 
[5] Ettles, C.M., Seyler, J. and Bottenschein, M., 2003, "Some 
Effects of Start-Up and Shut-Down on Thrust Bearing 
Assemblies in Hydro-Generators", J. Tribol., 125(4), pp. 
824-832. 
[6] Wang, X., Zhang, Z. and Pan, J., 2005, "A Transient 
Thermoelastohydrodynamic Analysis of a Tilting-Pad 
Thrust Bearing During Start-Up", Proc. WTC2005,  
Washington, D.C., USA. 
[7] Lu, X. and Khonsari, M., 2006, "On the lift-off speed in 
journal bearings", Trib. & Lub. Technol., 3, pp. 40-46. 
[8] Lu, X. and Khonsari, M., 2007, "An experimental 
investigation of grease-lubricated journal bearings", J. 
Tribol., 129(1), pp. 84-90. 
[9] Ünlü, B.S., and Atik, E., 2007, "Determination of friction 
coefficient in journal bearings", Mat. & Design, 28, pp. 
973-977. 
[10] Brito, F.P., Bouyer J., Fillon M. and Miranda A.S., 2007, 
"Experimental Investigation of the Influence of Supply 
Temperature and Supply Pressure on the Performance of a 
Two Axial Groove Hydrodynamic Journal Bearing", J. 
Tribol., 129(1), pp. 98-105. 
[11] Bouyer, J. and Fillon, M., 2007, "Behaviour of a 
hydrodynamic journal bearing : torque measurement 
during start-up", Proc. ROTRIB’07, Bucharest, Romania. 
 
3 Copyright © 2007 by ASME 8
: http://www.asme.org/about-asme/terms-of-use


IJTC2008-71099 AN EXPERIMENTAL INVESTIGATION ON FRICTION COEFFICIENT IN PLAIN JOURNAL BEARINGS DURING START-UP

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IJTC2008-71099 AN EXPERIMENTAL INVESTIGATION ON FRICTION COEFFICIENT IN PLAIN JOURNAL BEARINGS DURING START-UP

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