The Tribological Influences on the Failure Modes of
Automobile Wheel Bearing
RPI MANE-6960
Friction, Wear, and Lubrication of Material
December, 12 2012
Timothy J.Pond Jr
Table of Contents
Table of Contents………………………………………………………………………………….2
List of Figures…………………………..…………………………………………………………2
List of Tables………………………...……………………………………………………………2
Introduction……………………………………………………………………….……………….3
Discussion…………………………………………………………………………………………3
COMSOL Calculation………………………………………………………………….…………7
Failure Modes…………………………………………………………………..…………………9
Conclusion……………………………………………………………………….………………11
References………………………………………………………………………………………..12
Table of Figures
Figure 1: Thrust Bearing Reference (6)………………………………………………………...…3
Figure 2: Typical Roller Bearing Assembly for Vehicle Use……………………………………..4
Figure 3: Stirbeck-Hersey Curve………………………………………………………………….5
Figure 4: Wear Rate as a Function of Relative Load………………………………………….…..6
Figure 5: Lubrication-Life Correction Factor as a Function of Lambda………………………….7
Figure 6: Pressure of Lubrication………………………………………………………………....7
Figure 7: Velocity Profile……………………………………………………………………...….8
Figure 8: Strength of Material……………………………………………………………………..8
Figure 9: Pressure Area that was Compared………………………………………………………9
Figure 10: Lubrication Comparisons…………………………………………………………..….9
Table of Tables
Table 1: Friction Coefficients………………………………………………………………….….4
Table 2: Lubricant Material Properties…………………………………………………………....9
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Introduction:
Friction is when two surfaces resist movement between one another. Friction can be created by sliding
or by rolling. Friction creates heat, resistive load, and wear reduction of material. Rolling friction is
significantly less than sliding friction in the same loading scenario. It is practical to use rolling friction
over sliding friction when given the opportunity.
Friction is created in almost any application of work. Bearings are used to reduce friction in load
application. Many things in today’s society use bearings (cars, heavy machinery, tools, and computers).
Bearing use the principle of rolling friction. Rolling friction has been discovered to be significantly less
than sliding friction. Without this discovery, work on large or heavy objects would take weeks/years
instead of hours/days and advancement of our society would have taken much longer than it has.
Discussion:
Rolling friction has been proven to be less than sliding friction. Ever since the advent of the wheel, it has
been the preferred method of transmitting work onto an object. The use of rolling contact principles in
industry is due to the lower power loss vs. sliding friction. One of the most familiar uses of bearings
today are those used in automobiles. Everyone uses or is familiar with the basic concept of the
automobile. Wheel bearings are what are responsible for moving the automobile effectively. Most
automobile use thrust bearings in the wheel assemblies. Figure 1 depicts a Thrust roller bearing. Figure
2 depicts the trust roller bearing in a wheel a hub assembly. The thrust bearing uses rolling friction to
eliminate some of the friction associative with movement. The internal rollers of the bearing assembly
follow a track assembly.
Figure 1: Thrust Bearing Reference (5)
Prior to this design, bearing design was much simpler and contained no internal rollers. This in turn
relied heavily upon the performance of lubrication. Without the benefit of rollers and an internal track
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assembly side loads were difficult for a bearing assembly to resist. Much lubrication was needed to
account for rubbing of the internal assemblies and the heat generated as a result.
Figure 2: Typical Roller Bearing Assembly for Vehicle Use
Lubrication is needed for most if not all rolling contact cases. If, in the hypothetical simple case, rolling
was to take place in a dry state without deformation or sliding, and no wear occurs, lubrication would be
unnecessary. The potential advantage of lubricated rolling contacts over sliding contact in respect of
friction is depicted in Table 1 of typical ranges for friction coefficients, from Reference 2.
Table 1: Friction Coefficients
The main reasons for lubrication rolling contact bearings can be summarized below:
Prevent metal-metal contact between races and rolling elements at points of sliding
To eliminate any harmful effects of surface irregularities that cannot be completely removed
even by the most careful polishing
To support the sliding contact between the cage and the rolling elements and /or shoulder
To carry away the heat developed in the bearing
To protect the highly polished surfaces from corrosion
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Thus the main functions of a lubricant can be classified as lubrication heat transfer and
protection.
A number of lubrication behaviors can be identified based on the type of lubricant involved and its film
thickness. A way of depicting some of these behaviors is by using the Stribeck Curve. Stribeck
performed comprehensive experiments on journal bearings around 1900. In these experiments he
measured the coefficient of friction as a function of load, speed, and temperature. He did however have
difficulty condensing this information into an easily usable form. Some years later Hersey performed
similar experiments and devised a plotting format based on a dimensionless parameter. The curve he
created was come to be known as the Stirbeck-Hersey curve. This curve is depicted in Figure 3 below
from Reference (7). The Stirbeck-Hersey curve uses the coefficient of friction as a function of the
viscosity of the liquid (z), Velocity (N) and load (P) all together as a determined parameter (ZN/P).
Figure 3: Stirbeck-Hersey Curve
The thickness of a lubrication application can vary widely within a bearing assembly. Surface
interactions start to take place when film thickness of the lubricant becomes progressively thinner. This
regime is referred to as the mixed lubrication regime. In this regime asperity interactions and fluid film
effects are combined as friction increase. At low values of a ZN/P parameter then one enters the realm
of boundary lubrication. This regime is characterized by the following:
Highly complex, involving metallurgy, physical and chemical absorption, corrosion, catalysis, and
reaction kinetics.
Formation of protective surface films to minimize wear surface damage
Formation of these films is governed by the chemistry of the film-forming agent, as well as the
surface of the solid and other environmental l factors
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The effectiveness of these films in minimizing wear is determined by their physical properties,
which include shear strength, thickness, surface adhesion, film cohesion, melting point or
decomposition temperature and solubility.
Besides the Stribeck Hersey curve (Figure 3), an idealized plot of wear rate as a function of relative load
can also delineate the various lubrication regimes and some wear transitions. These relationships are
depicted in Figure 4 from Reference (7). Minimum film thickness formula from Reference (7) can be
summarized in Equation (1).
Equation (1)
For Pure Cylindrical roller contact K=infinity and the term in the parentheses in the above equation
equals 1.
Figure 4: Wear Rate as a Function of Relative Load
The measure of effectiveness of a lubricant film can be determined by Lambda or Equation (2)
Equation (2)
Surface roughness can be calculated using the Root Mean Squared (rms) in Equation 3. Sigma (surface
roughness) is the rms surface finishes of the contacting bodies are used to determine the composite
surface roughness
Equation (3)
The lambda ration can be used as an indicator of rolling element performance and life. Figure 5 depicts
this relationship. For lambda<1, surface smearing or deformation, accompanied by wear, will occur on
the rolling surface. For 1<lambda<3, some surface glazing can occur with eventual roller failure caused
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by classical subsurface originated rolling-element fatigue. Figure 6 shows the effect of lambda on rolling
element fatigue life determined by Reference (7). At lambda>3, minimal wear can be expected with a
long life. Failure will eventually be by classical subsurface-originated rolling element fatigue.
Figure 5: Lubrication-Life Correction Factor as a Function of Lambda
COMSOL Calculation:
Figure 6 depicts a COMSOL simulation of a simple thrust bearing profile. As the inside ring remains
stationary the outside ring rotates at a speed of 5 m/s. The velocity profile of this analysis is depicted in
Figure 7. This best depicts how most thrust bearings work. Figure 8 provides a closer look at the
interaction between the inner and outer surfaces of the bearing.
Figure 6: Pressure of Lubrication
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Figure 7: Velocity Profile
Figure 8 depicts the pressure distribution of the thrust bearing assembly and how the peak pressures
appear just to the right of the internal bearings closest point to the outer bearing. This is consistent
with what was calculated by hand. This is also true of all other bearing pinch points within the assembly.
The gap between bearing surfaces has been set to .002 m in this analysis. Equation 2 can be used to
determine film thickness for a given bearing assembly. 0.002 m was used in this analysis to depict the
pressure and velocity profile while still being able to see the minor roller geometry. Actual film
thickness calculated for the dimensions of this assembly are much smaller and would require a much
smaller scale that requires more time to run than available computing power for this project.
Figure 8: Strength of Material
Several different oils were evaluated in the COMSOL analysis. This was done to determine the affects of
oil material properties on a given geometry. The material properties for the various oils used for this
comparison are included in Table 2 below.
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Table 2: Lubricant Material Properties
The results for the pressure distribution for a given section of the thrust bearing assembly are depicted
in Figure 10. The Section of the thrust bearing analyzed is highlighted in green in Figure 9.
Figure 9: Pressure Area that was Compared
Pressure (Pa)
3000
2000
Oil 2
Oil 4
1000
Oil 3
0
0.00E+00 2.00E-02 4.00E-02 6.00E-02 8.00E-02
-1000
Oil 1
Length
Figure 10: Lubrication Comparisons
Failure Modes:
Several failure modes have been identified below as a majority of the problems associated with bad
wheel bearings. Wheel bearings can be damaged for any number of reasons. Those failures can
manifest into serious problems to other components in an automobile and lead to minor expense to
repair or loss of the vehicle and intern its operator. Exceeding the load capacity of a bearing assembly
can damage the housing unit or the rollers themselves and cause either one to yield to stresses that the
system was not designed to handle. Foreign particles introduced into the assembly (sand, metal
shavings) can accelerate the wear between the two internal loaded surfaces of the bearing assembly.
Lack of lubrication can also hasten the failure of a bearing assembly. Without lubrication friction,
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corrosion and wear are all accelerated under a given load compared to a lubricated system. Any one of
the above identified scenarios can lead to vehicle symptoms described below from Reference 6.
Wheel Noise
As wheel bearings fail, an increase in noise from the wheel can sometimes be heard. This is
often a grating or grinding noise that will progressively get louder as the vehicle is driven faster.
Wheel Loosening
Although excessive wheel noise is the most common symptom of bad wheel bearings, many
times a vehicle with bad wheel bearings will not have any noise. One way to determine if there
is a problem with a suspected wheel bearing is to pull on the wheel in question and pulling on it.
Wheel Rotation Restriction
One failure method of weal bearings is corrosion. As the wheel bearings corrode they will lose
the smooth machined exterior of the rollers. This will in turn prevent the tire from rotating
smoothly. IF one wheel assembly contains a bad bearing, then it can cause the vehicle to pull to
one side or another.
Abnormal Tire Wear
A bad wheel bearing can cause the tire to rotate lose than it was designed. As the tire rotates in
an irregular pattern, the wear pattern can also be irregular. This will also cause the vehicle to
pull to one side more frequently. Abnormal tire wear can also shorten the life span of a tire,
requiring more frequent tire changes if not diagnosed in time.
Front Suspension Looseness
The symptoms caused by bad wheel bearings are often the same as symptoms caused by bad
front suspensions on a vehicle. A silent wheel bearing problem can sometimes only be properly
diagnosed after all irregularities in front end suspensions have been eliminated. Bad wheel
bearings can also cause accelerated wear of suspension systems.
All of the above identified failure modes can lead to increased coast of vehicle repair. If the underlying
root cause of the failures can be identified and prevented, then automobiles will become more reliable.
As basic understanding for material interactions in friction, lubrication, and wear applications increases,
more steps can be taken to prevent the above identified failures.
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Conclusion:
As the density of the lubrication increases so does the maximum pressure seen by the assembly. When
designing a system and selecting a given lubrication, this must be considered. Reaction loads of a
bearing and length of surface life must also be considered. One of the primary functions of lubrication is
to facilitate movement, given it be sliding or rolling friction. Rolling friction has been found to be more
effective at transferring work onto an object. The primary bearings used for the wheels on automobiles
are thrust bearings. Thrust bearings have been analyzed in this paper and are the ideal wheel bearing
for this instance because they limit any side loads that may exist as a result of a bearing assembly
wobbling or loosely following a track assembly.
The other functions of lubrication, such as to protect against wear, corrosion, surface irregularities, and
possible contaminants must be assessed against what has been evaluated already inn this paper. Only
then can the service life of a bearing assembly be maximized. One of the larger influences on service life
was found to be the surface condition of bearing surfaces or Lambda. The smoother the surface, the
better the bearing. This can be compensated for by lubrication performance but this is not as reliable as
the bearing dimensions. As machining techniques advance so do the performance of bearing used
through industry. The concepts covered in this paper however will need to be considered for any
bearing assembly regardless of how advanced a design.
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References:
1.)
http://permanent.access.gpo.gov/LPS112295/LPS112295_19880004998_1988004998.pdf
Mechanisms of Rolling Contact Spalling Final Report (Vanderbilt Univ.)
A.M Kumar, S.M. Kulkarni, V. Bhargara, G.T. Hahn, and C.A. Rubin, May 30, 1987
2.)
http://www.knovel.com/web/portal/basic_search/display?_EXT_KNOVEL_DISPLAY_bookid=4642
Rolling Contacts, Stolarski, T.A.; Tobe S. © 2000 John Wiley and Sons
3.)
http://www.gassavers.org/f5/wheel-bearings-rolling-resistance-question-6868.html
Gas savers website
4.)
http://www.fra.dot.gov/downloads/Research/ord0006iii.pdf Website for Rolling friction
5.)
http://en.wikipedia.org/wiki/Tapered_roller_bearing
6.)
http://www.ehow.com/list_5939456_signs-bad-wheel-bearings.html Symptoms of bad bearings
7.)
Lubricant Effects on Bearing Life, Erwin V. Zaretsky, Lewis Research Center Cleveland Ohio, NASA
Website for Taper roller bearings
Technical Memorandum 88875
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