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 has investigated the use of and has started implementation
of self-lubricated bearings at its hydropower and navigation facilities. 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, the
US Army Corps of Engineers underwent a test program to characterize the wear properties of multiple
commercially available self-lubricated bearings. All of the self-lubricated bearings tested were
composed of solid lubricants, like molybdenum disulfide and graphite, or blends of
polytetrafluoroethylene (PTFE) and filler materials. Due to the large amount of academic literature
available on the wear of PTFE composites, the wear results from the PTFE based self-lubricated bearing
materials tested by USACE will be compared against laboratory tests of similar PTFE based selflubricated bearing materials. Although the large scale and laboratory scale materials tested are not
identical, it is the intent of this investigation to characterize and compared PTFE based bearing materials
as a whole, not individually.
Theory and Methodology
Wear occurs at the surface of two or more components in contact. The pheonomenon of wear is heavily
influenced on 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. 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 the vicinity as well.
Archard 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 materials in the contact pair. The different types of wear mechanisms are tabulated in Table 1.
Table 1 - Classifications of Wear Mechanisms (adopted from (Archard))
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
Cutting
Plastic Deformation
Surface Fracture
Surface Reactions
Tearing
Melting
Electrochemical
Fatigue
A film formed by reaction with the environment or the lubricant is removed
by sliding
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
One material dissolves or diffuses into another
Elastic material can be torn by a sharp indenter
High generated temperatures can cause wear by melting
The difference in potential on the surface due to a moving fluid can cause a
material to go into solution
The surface is worked elastically. Microcracks form, grow and coalesce
forming wear particles
For the purposes of this report, only adhesion and transfer and fatigue wear will be focused on.
This paper will compare the wear results documented in the USACE test program against laboratory
experiments and classical wear models. A normalized wear constant, K, will be used to correlate wear
rates of PTFE based bearing materials tested by the USACE against unidirectional, pin on disk wear
testing performed at LSU. Additionally, the differences in wear performance due to shaft to bearing
misalignment explored during the USACE testing will be evaluated using ABAQUS CAE to determine
actual contact pressures due to the misalignment test setup. Finally, results of wet testing performed by
USACE will be compared to other laboratory tests of PTFE based materials in water to determine the
effect water on the wear rate of PTFE based composites.
Wear Model
The wear model used in this paper is an adhesive wear model and is a modification of Archard’s Law.
Per Reference (Archard) for sliding contact, the worn volume per unit sliding distance, w, can be
calculated as:
𝑤=
𝑉𝑤 𝐾𝐹𝑛
=
𝐿
𝐻
Where Vw is the volume of worn material, L is the length the material 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
K is the 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 equation will be used to
correlate the wear constants for different PTFE composites in this paper.
𝑑 𝐾
= 𝑃𝑉
𝑡 𝐻
Where d is the worn depth, t is the time in sliding contact, K is the wear coefficient, H is the hardness of
the softer material, P is the nominal contact pressure, and V is the nominal sliding velocity. Since the
hardness of the PTFE bearing material is not documented for all of the tested materials, this paper will
account for the term K/H simply as the wear coefficient, K. The final form of the wear model used in this
report is:
𝑑
= 𝐾𝑃𝑉
𝑡
A similar wear model is suggested for plain bearings in (Wear control handbook).
Summary of Experiments
USACE Wear Testing
The USACE tested 10 self-lubricated bearing materials. Of the 10 bearings tested, 8 of them contained
PTFE and filler materials. This experiment tested rotary journal bearings that were heavily side loaded
and underwent small oscillations at fairly low speed. Test details are below.
Shaft Details
5 in shaft/sleeve
Heat Treated 17-4 PH
Rc 28-32
16 microinches surface finish
Bushing Size
5.00 in nominal x 3.00 in
5200 lbf applied statically (3300 psi over projected area)
+-1000 psi variable load to simulate vibration
Test details
24 hour creep and initial wear test
120 hrs of testing at the following cycles
8 deg/sec oscillating between +-1 deg for 890 sec (3300 psi +-1000 psi)
6 deg/sec large oscillation between +-15 (one swing/10 secs) (3300 psi)
Sketch of the setup
The published results for dry wear, edge loaded wear, and wet wear are shown below.
Results
table
Results and Discussion
Large scale vs small scale test results
ABAQUS Contact pressure analysis
Wear of PTFE bearings in water
Reference List
[1]
CERL Technical Report 99/104, “Greaseless bushings for hydropower applications: program,
testing, and results,” 12/1999.
[2]
Lancaster, J.K. “Dry bearings: a survey of materials and factors affecting performance,” Tribology
(December 1973) p 219-251.
[3]
Gawarkiewicz, R. and Wasilczuk, M. “Wear measurements of self-lubricating bearing materials in
small oscillatory movement,” Wear 263 (2007) p 458-462.
[4]
Khedkar, J. Negulescu, I. and Meletis, E. “Sliding wear behavior of PTFE composites,” Wear 252
(2002) p 361-369.
[5]
Archard, J.F. “Wear theory and mechanisms,” Wear Control Handbook, American Society of
Mechanical Engineers, New York, 1980, p 39-79.
[6]
Neale, M.J. “Plain bearing materials,” Tribology Handbook, Elsevier, 1995, p A5.1-A5.5.