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Wowk-TP.pdf
An Investigation into Experimental Wear
Coefficient Determinations for PTFE Based, SelfLubricated Bearing Materials in Sliding Contact
Final Project for MANE 6960
Christopher Wowk
Friction, Wear, and Lubrication of Materials
Dr. Ernesto Gutierrez-Miravete
December 8, 2013
Contents
1.0
INTRODUCTION
1
2.0
THEORY AND METHODOLOGY
1
2.1.
Adhesive Wear Model
3
2.2.
Summary of Investigated Experiments
3
3.0
RESULTS AND DISCUSSION
5
3.1.
Comparison of Wear Coefficient Values
5
3.2.
Effects of Misalignment/Edge Loading on Bearing Wear
9
3.3.
Conclusions
4.0
REFERENCE LIST
11
11
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1.0 Introduction
Use of self-lubricated bearings in industry is increasing in applications where external lubrication is
undesirable due to: maintenance costs of continually manually applying lubrication, cost and complexity
of automatic lubrication systems, and environmental impacts of release of the lubricant into the
environment. Unlike the metallic lubricated bearings they are meant to replace, self-lubricated bearings
do not have a long proven service history. In order to determine wear rates and other bearing
performance measures, testing is required. Full scale testing of bearings is typically cost and time
prohibitive for large scale, long life bearing applications. Because of this, small scale laboratory testing
and full scale accelerated wear testing is required to predict the wear and service life of self-lubricated
bearings. The US Army Corps of Engineers Construction Engineering Research Laboratory (CERL) has
investigated the use and has started implementation of self-lubricated bearings at its hydropower and
navigation facilities [1]. Bearing applications at these facilities include guide rollers, lock gate guide pins,
tainter valve strut arm components, and pintle bearings. These components can be characterized as high
load – low speed applications.
Due to limited application history of self-lubricated bearings in high load – low speed applications, CERL
underwent a test program to characterize the wear properties of multiple commercially available selflubricated bearings. All of the self-lubricated bearings tested were blends of polytetrafluoroethylene
(PTFE) and filler materials. In general, pure PTFE is a poor bearing material due to its limited
mechanical properties and its tendency to cold flow under load [2]. The wear and mechanical properties
of PTFE based composites are increased due to the addition of filler materials, such as glass and carbon,
or solid lubricants, such as Molybdenum Disulfide or graphite. Due to the large amount of literature
available on the wear of PTFE composites, the wear results of the PTFE based self-lubricated bearing
materials tested by CERL [1] will be compared against a similar test program performed by
Gawarkiewicz [3] and laboratory tests of similar PTFE based composites performed by Khedkar [4].
Although the large scale and laboratory scale materials tested are not identical, it is the intent of this
investigation to characterize and compare PTFE based bearing materials as a whole, not individually.
2.0 Theory and Methodology
Wear occurs at the surface of two or more components in contact and is important to understand for
machinery requiring precise alignment. The phenomenon of wear is heavily influenced by the contact
between the inherent roughness present on all engineering surfaces. Contact between high points on the
surfaces, known as asperities, leads to removal of material from the softer material in the contact pair [5].
This removal of material can lead to misalignment in machinery, unacceptable damage to the surface of
the components, and generation of debris that can affect the operation of not only the components in
contact, but components in their vicinity as well.
Archard [5] suggests that there are ten different classifications of wear mechanisms. Due to the
complexity and microscopic nature of wear, multiple wear mechanisms can affect a component. The
wear mechanism(s) occurring at any time is dependent on the type of type of loading, operating
environment, and properties of the materials in the contact pair. The different types of wear mechanisms
are tabulated in Table 1.
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Table 1 - Classifications of Wear Mechanisms (adopted from [5])
Adhesion and Transfer
Materials weld at sliding asperity tips, is transferred to the harder member,
possibly grows in subsequent encounters and is eventually removed by fracture,
fatigue, or corrosion
Corrosion Film Wear
A film formed by reaction with the environment or the lubricant is removed by
sliding
Cutting
Plastic Deformation
Surface Fracture
A sharp particle or asperity cuts a chip
The surface is worked plastically. Cracks form, grown, and coalesce forming
wear particles
If nominal stress exceeds fracture stress of a brittle material, particles can be
formed by fracture
Surface Reactions
One material dissolves or diffuses into another
Tearing
Elastic material can be torn by a sharp indenter
Melting
High generated temperatures can cause wear by melting
Electrochemical
The difference in potential on the surface due to a moving fluid can cause a
material to go into solution
Fatigue
The surface is worked elastically. Micro-cracks form, grow and coalesce
forming wear particles
It is generally recognized that, for metallic components in constant sliding contact under low load with
polymer based composites, such as self-lubricated bearings, the total wear of the softer material is
dependent on the load applied to the bearing, the sliding speed, the duration of sliding, and the
characteristics of the materials in contact. For a constant load and speed, the wear rate of the bearing will
be relatively high in the early stages, but will drop to a lower, fairly constant value after a brief “runningin” period [6]. During the “running-in” period, worn particles of the PTFE composite transfer to the
rubbing surface of the metallic component. This transfer of particles changes the surface characteristics
of the metallic component, and the wear rate decreases and becomes fairly constant. During this period,
known as mild wear, the total volume of worn material increases with time/sliding distance. Figure 1,
taken from [6], shows typical wear behavior for PTFE composites.
Figure 1 - Typical Wear of PTFE Composites (from [6])
This paper will focus on the steady-state, mild wear region of the curve shown in Figure 1. During this
period of wear, adhesive wear is assumed to be the wear mechanism responsible for removal of bearing
material.
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2.1. Adhesive Wear Model
The adhesive wear model proposed by Archard [5], sometimes known as Archard’s Law, is used in this
paper to compare the wear coefficients for different commercially available, PTFE based, self-lubricated
bearing materials that were calculated based on the published results of two different test programs.
Published wear coefficients for other generic PTFE based composites based on small scale laboratory
testing were also used for comparison.
Archard’s law states that, for sliding contact, the worn volume per unit sliding distance, w, can be
calculated as:
Eq (1)
Where Vw is the volume of worn material, L is the length the materials have slid in contact, Fn is the force
pressing the two bodies into contact, H is the hardness of the softer material in the friction couple, and
is the dimensionless wear coefficient. If the total sliding distance is the result of sliding at a constant
velocity, the equation above can be used to determine the worn depth per unit time. This is typically a
reported value from testing.
Eq (2)
Where is the worn depth per unit time,
is the wear coefficient, P is the nominal contact pressure, and
V is the nominal sliding velocity. is a dimensionless value, however, since the hardness of the PTFE
bearing materials are not documented for all of the materials used in the testing and is assumed to be
similar between them, this paper will account for the term /H simply as the dimensional wear
coefficient, K, which has the units mm3 N-1 m-1. The wear coefficient, K, can be calculated as:
Eq (3)
This value can be used to compare the experimental results, as it is independent of the test’s speed and
load.
2.2. Summary of Investigated Experiments
As previously stated, test results from two rather large scale bearing tests and one laboratory scale test
were used to compare wear coefficients of different PTFE bearing materials. The details of each of the
experiments are detailed in Table 2.
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Table 2 - Summary of Investigated Experiments
Reference
CERL [1]
Gawarkiewicz [3]
Khedkar [4]
Testing Activity
CERL
Gdansk University,
Poland
LSU
Number of Materials
Tested
11
4
7
Counter Surface/
Surface Finish
Stainless Steel
0.4 μm
Stainless Steel
0.8-1.2 μm
Stainless Steel
0.4 μm
Load Type
Oscillating
(Rotating)
Oscillating
(Planar Sliding)
Magnitude
3300 psi (23 MPa)
29 MPa
5N
Bearing Form
Journal
Flat
Flat
Bearing Size
5 inch (127 mm)
Not Specified
62.5 mm Disk
Duration
120 hrs
Various
1 km
Run in Period
Excluded from
Results?
Yes
No
No
Reported Results
d/t
mils/100 test hrs
K, wear coefficient
mm3 N-1 m-1
K, wear coefficient
mm3 N-1 m-1
Unidirectional (Pin
on Disk)
The experiments investigated in this paper use different forms of motion to create sliding contact between
the bearing material and counter surface. This paper explores whether the method of testing sliding
contact between impacts the resulting wear coefficient. In testing performed by CERL, a shaft was placed
through a bearing made of the test material. A test rig applied a force to the bearing housing, which held
the bearing material being tested and created contact between the bearing and shaft. A hydraulic cylinder
was used to rotate the shaft in four and fifteen degree swings at a constant velocity. This test was meant
to be representative of the motion bearings see in service in hydropower applications. Testing performed
by Gawarkiewicz also used small oscillatory motion to create sliding contact between the bearing material
and counter surface, however, in their testing, flat specimens were used for planar sliding contact.
Khedkar’s testing used a commercial pin on disk tribometer to generate uni-directional sliding contact
between a test disk of PTFE material and a stainless steel ball. Schematics of the test setups are shown in
Figure 2.
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Figure 2 - (a) CERL Test Set Up (b) Gawarkiewicz Test Set Up (c) Schematic of a
Typical Pin on Disk Tribometer used by Khedkar
The CERL testing also included wear measurements of the tested bearing materials with simulated
misalignment due to a double tapered shaft. The results of that testing are discussed later in this paper.
3.0 Results and Discussion
The wear coefficients of the materials tested in the previously mentioned testing are compared against
each other and typical values for PTFE composites published by Neale [6] in Section 3.1. Section 3.2
discusses CERL’s wear test results for simulated bearing misalignment. The actual results are compared
against predicted values based on ABAQUS modeling of the reduced contact area.
3.1. Comparison of Wear Coefficient Values
Wear coefficients for materials tested by Gawarkiewicz and Khedkar were included in the published
results of their experiments. The results of CERL testing only published the depth of wear per 100 test
hours for each of the bearings tested. The wear coefficient had to be calculated from the published wear
depth and the test rig design pressure and velocity using Equation 3. The calculated and published wear
coefficients from the experiments investigated are shown in Table 3.
It can be seen that the wear coefficients calculated from the CERL testing are orders of magnitude lower
than the wear coefficients published by the other two experiments. There are multiple reasons that
explain the difference in wear coefficient values. Potential reasons will be discussed for the
Gawarkiewicz and Khedkar results separately.
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Table 3 - Calculated and Published Wear Coefficients
Reference
CERL
Material
Gawarkiewicz
Khedkar
K
K
K
Calculated/P
Calculated/P
Calculated/P
3 -1 -1 ublished
3 -1 -1 ublished
ublished
(mm N m )
(mm N m )
(mm N m )
3
-1
-1
Delrin AF100
2.0E-07
Calculated
Not Tested
-
Not Tested
-
Devatex I
6.0E-09
Calculated
1.8E-07
Published
Not Tested
-
Lubron TF
2.4E-08
Calculated
Not Tested
-
Not Tested
-
Fiberglide
5.7E-08
Calculated
Not Tested
-
Not Tested
-
ORKOT TXM
7.0E-08
Calculated
3.8E-07
Published
Not Tested
-
KARON V
2.7E-08
Calculated
1.8E-07
Published
Not Tested
-
Tenmat T12
5.9E-06
Calculated
Not Tested
-
Not Tested
-
Tenmat T814
1.2E-07
Calculated
7.2E-06
Published
Not Tested
-
Thordon TRAXL SXL
1.5E-07
Calculated
Not Tested
-
Not Tested
-
Thordon TRAXL HPSXL
4.6E-09
Calculated
Not Tested
-
Not Tested
-
Devatex II
5.2E-08
Calculated
Not Tested
-
Not Tested
-
Pure PTFE
Not Tested
-
Not Tested
-
9.4E-04
Published
75% PTFE 18% C 7%
graphite
Not Tested
-
Not Tested
-
8.5E-05
Published
85% PTFE 15% Glass
Fibers
Not Tested
-
Not Tested
-
7.0E-04
Published
75% PTFE 25% Glass
Fibers
Not Tested
-
Not Tested
-
3.0E-04
Published
75% PTFE 20% Glass 5%
MoS2
Not Tested
-
Not Tested
-
1.0E-04
Published
97.5% PTFE 2.5% PPDT
Fibers
Not Tested
-
Not Tested
-
4.0E-04
Published
90% PTFE 10% PPDT
Fibers
Not Tested
-
Not Tested
-
1.2E-04
Published
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Materials tested in the Gawarkiewicz experiments were identical to some of the materials tested by
CERL. One would expect similar wear coefficients to be determined by both experiments. Table 4
shows a direct comparison between the identical materials tested by both CERL and Gawarkiewicz. It
can be seen that the coefficients published by Gawarkiewicz are anywhere from 4 to 60 times greater than
the calculated wear coefficients from the CERL data. The Gawarkiewicz data suggests that the materials
experience more wear for a given load and speed. One possible reason for the difference in values is the
initial roughness of the counter surface. Although the counter surface was stainless steel in both
experiments, the shafts used by CERL were finished to 0.4 μm, whereas the disks used by Gawarkiewicz
were slightly rougher, 0.8-1.2 μm. It is well established that a rougher counter surface will lead to
increased wear for PTFE based, and most other bearing materials. Another possible reason Gawarkiewicz
values are higher is because the experiment did not account for the higher rate of wear that occurs during
the “running in” period. CERL testing allowed for 48 hours of occasional oscillation with the test load on
the bearing prior to the start of the experiment. This period, which CERL used to account for creep and
set of the bearing, also allowed PTFE to transfer to the counter surface and transition from the “running in
period” to the mild wear region prior to any wear measurements. This suggests that the wear coefficient
calculated from the CERL experimental data is more representative of the actual wear coefficient in the
region of mild wear. A graphic depiction of the two wear coefficients is shown in Figure 3. It can be
seen that if “run in” is allowed prior to testing, the measured wear coefficient should be much lower than
if “run in” wear is included in the calculation of the wear coefficient. It is important to note however, that
for machinery designers, the wear that occurs during bearing run in is important to account for, as it can
result in larger than planned for clearances and eccentricity in the equipment. Wear coefficients that are
calculated as were by Gawarkiewicz will give higher predictions for wear in a shorter time, which may be
beneficial, such that the designer can conservatively design the machinery accordingly.
Table 4 - CERL vs. Gawarkiewicz Wear Coefficient Results
K
(mm3 N-1 m-1)
Material
CERL
Gawarkiewicz
% Difference
Devatex I
6.0E-09
1.8E-07
2894%
ORKOT TXM
7.0E-08
3.8E-07
437%
KARON V
2.7E-08
1.8E-07
579%
Tenmat T814
1.2E-07
7.2E-06
5995%
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Figure 3 - Wear Coefficient Determination with and without Accounting for “Run In” Period
Differences between the Khedkar data and the other two experiments can be mainly explained by the
differences in materials tested. Khedkar tested generic PTFE composites; whereas CERL and
Gawarkiewicz tested proprietary blends of PTFE based self-lubricating bearing material. It is expected
that the proprietary blends offer enhanced wear performance over less expensive generic PTFE
compounds. Additionally, as with Gawarkiewicz, Khedkar did not account for the increased wear during
the “run in” period. Extended testing was performed by Khedkar on the three compounds that had the
lowest wear coefficients in the one kilometer portion of testing. These compounds were tested to a
distance of five kilometers of sliding contact. The wear coefficients determined for the longer duration
tests were much lower than those determined for identical materials for the shorter duration test. For
additional information on this testing, see Khedkar. This supports the previous rationale that explains the
differences between the CERL and Gawarkiewicz results and reinforces the applicability of the wear
model that assumes a period of high rate of wear during bearing “run in,” followed by a period of a lower
rate of wear when the bearing material has transferred to the counter surface. Khedkar also documented
the presence of wear particles in the wear track of the samples during unidirectional testing. The presence
of wear particles suggests that three body abrasive in addition to adhesive wear may have increased led to
the increased wear measured during the test.
Neale published order of magnitude ranges for typical wear coefficients for dry rubbing bearing materials.
The order of magnitude wear coefficients published by Neale are shown in Figure 4. CERL wear
coefficients were generally lower than what is typical for filled PTFE compounds and PTFE with glass
weave and thermoset resin (typical general composition of the commercial bearing materials).
Gawarkiewicz wear coefficients were consistent with Neale values for filled PTFE compounds and PTFE
with glass weave and thermoset resin. Khedkar values were greater than the Neale values for pure PTFE
and PTFE with glass weave and thermoset resin. Although the values from the experiments differ from
those published by Neale, it must be noted that none of the wear coefficients are drastically out of the
range suggested by Neale, and that, as previously discussed in this paper, different values for the wear
coefficient can be determined experimentally depending on whether or not “run in” wear is accounted for.
There is no data on the method used to determine Neale data, so that cannot be commented on.
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Figure 4 - Order of Magnitude Wear Coefficients for Typical Dry Rubbing
Bearing Materials (from [6])
3.2. Effects of Misalignment/Edge Loading on Bearing Wear
The experiments performed by CERL also investigated the effects of misalignment on the wear of the
PTFE bearing materials tested. Misalignment between the shaft and the bearing was simulated by using a
double tapered shaft, allowing contact between the shaft and bearing only at the edges of the bearing.
Due to this reduction in contact area, it is expected that the amount of wear measured at the edge of the
bearing will increase due to increased contact pressure.
ABAQUS CAE was used to investigate the differences between the contact area of aligned and
misaligned shafts. Figure 5 shows plots of the contact area for an aligned and misaligned shaft. It can be
seen that contact occurs over much of the projected area of the shaft for an aligned shaft. The contact
area decreases to approximately one half of that of an aligned shaft for the misaligned shaft.
Figure 5 - (a) ABAQUS Model Used to Investigate Bearing to Shaft Contact (b)
Aligned Shaft Contact Pressure Results (c) Misaligned Shaft Contact Pressure
Results
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It is expected that with the reduction of contact area, the contact pressure at the edges of the misaligned
bearing will increase, and as such, so will the total amount of wear. From the ABAQUS results, it was
determined that, for the simulated misalignment tested by CERL, the contact area decreases to
approximately half of that the aligned bearing. This would correlate to twice the pressure at the contact
surface of the misaligned bearing compared to an aligned bearing, and assuming that the wear coefficient
calculated previously is correct, twice the total amount of wear. The predicted total wear due to
misalignment and the actual wear are shown in Table 5.
Table 5 - Predicted vs. Actual Total Wear of Misaligned Bearings
Material
K
3
-1
-1
(mm N m )
Total Wear During
Aligned Testing
(μm)
Predicted Total Wear
due to Edge Load
(μm)
Actual Total Wear
due to Edge Load
(μm)
Delrin AF100
2.0E-07
16.82
33.65
6.58
Devatex I
6.0E-09
0.52
1.04
3.44
Lubron TF
2.4E-08
2.04
4.08
Not Tested
Fiberglide
5.7E-08
4.91
9.81
1.55
ORKOT TXM
7.0E-08
6.07
12.13
18.04
KARON V
2.7E-08
2.29
4.57
3.08
Tenmat T12
5.9E-06
504.81
1009.62
Not Tested
Tenmat T814
1.2E-07
10.18
20.36
9.72
Thordon TRAXL SXL
1.5E-07
12.98
25.97
6.00
Thordon TRAXL HPSXL
4.6E-09
0.40
0.79
12.56
Devatex II
5.2E-08
4.45
8.90
5.76
It can be seen from the results above that the reduction in contact area does not solely account for the
difference in wear between an aligned and misaligned bearing. In some cases, the predicted wear is less
than the actual wear for the misaligned case. This suggests that for those cases, the contact area
prediction from the ABAQUS model was greater than the actual contact area due to misalignment. For
other cases, the predicted wear was less than the actual wear, suggesting that the ABAQUS model
underestimated the contact area. It is important to note that the ABAQUS model used a general material
model for PTFE to model the bearing. Differences in the elastic modulus between the actual bearing
materials and the ABAQUS model may account for some of the error seen in the predicted results.
Additionally, for the bearing materials highlighted in Table 5, the total wear in the misaligned case was
less than that of an aligned bearing. Additional inspection of the worn bearings is required, rather than
speculation, to better understand the limited wear misalignment, as it does not agree with the wear model
suggested by this paper.
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3.3. Conclusions
This paper investigated the wear coefficients of self-lubricated bearing materials as they were tested by
CERL, Gawarkiewicz, and Khedkar. All of the materials investigated were PTFE based, however CERL
and Gawarkiewicz tested proprietary blends from commercial vendors, whereas Khedkar tested generic
PTFE composites. The following motions were used to create sliding contact between a stainless steel
counter surface and the PTFE based bearing material: oscillatory rotary contact (CERL), oscillatory
planar sliding (Gawarkiewicz), and unidirectional sliding (Khedkar).
It was shown that different test methods yielded different wear coefficients for the subject materials.
Khedkar and Gawarkiewicz did not account for the “run in” period, where bearing material is transferred
to the counter surface at a high rate, which resulted in higher wear coefficients. CERL allowed for
bearing “run in” prior to wear measurements, which resulted in lower wear coefficients. Although they
did not exactly agree, wear coefficients determined by the experiments were similar to those published by
Neale.
The effects of misalignment on the wear of bearings were also explored. An ABAQUS model was
generated using typical values of PTFE to determine the reduction in contact area due to misalignment.
Predicted values of total wear based on the reduced contact area were compared to actual test results from
CERL. The predicted values did not generally agree with the actual values, indicating that further
investigation and possibly more refined material models are required to accurately predict the effect of
misalignment on self-lubricated bearings.
4.0 Reference List
[1]
[2]
[3]
[4]
[5]
[6]
CERL Technical Report 99/104, “Greaseless bushings for hydropower applications: program,
testing, and results,” 12/1999.
Lancaster, J.K. “Dry bearings: a survey of materials and factors affecting performance,” Tribology
(December 1973) p 219-251.
Gawarkiewicz, R. and Wasilczuk, M. “Wear measurements of self-lubricating bearing materials in
small oscillatory movement,” Wear 263 (2007) p 458-462.
Khedkar, J. Negulescu, I. and Meletis, E. “Sliding wear behavior of PTFE composites,” Wear 252
(2002) p 361-369.
Archard, J.F. “Wear theory and mechanisms,” Wear Control Handbook, American Society of
Mechanical Engineers, New York, 1980, p 39-79.
Neale, M.J. “Plain bearing materials,” Tribology Handbook, Elsevier, 1995, p A5.1-A5.5.
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