A Comparison of Bearing Material Wear Performance Tests: Pin-onDisc and Oscillating Bearing
by
Michael T. Peetros
An Engineering Project Submitted to the Graduate
Faculty of Rensselaer Polytechnic Institute
in Partial Fulfillment of the
Requirements for the degree of
MASTER OF ENGINEERING IN MECHANICAL ENGINEERING
Approved:
_________________________________________
Dr. Ernesto Gutierrez-Miravete, Project Adviser
Rensselaer Polytechnic Institute
Hartford, Connecticut
May, 2014
i
© Copyright 2014
by
Michael T. Peetros
All Rights Reserved
ii
CONTENTS
LIST OF TABLES ............................................................................................................ iv
LIST OF FIGURES ........................................................................................................... v
ACKNOWLEDGMENT ................................................................................................. vii
ABSTRACT ................................................................................................................... viii
1. Introduction and Background ...................................................................................... 1
1.1
The Use of Polymeric Materials in Bearing Applications .................................. 1
1.2
Bearing Material Wear Tests .............................................................................. 5
2. METHODOLOGY ...................................................................................................... 9
2.1
Selected Bearing Materials and Sample Configurations .................................... 9
2.2
Test Rigs ........................................................................................................... 13
2.3
Test Parameters ................................................................................................. 18
3. EXPERIMENTAL RESULTS .................................................................................. 20
3.1
Delrin® Acetal Results ..................................................................................... 21
3.2
UHMWPE Results ............................................................................................ 24
3.3
Molded Self-Lubricating Bearing Material Results ......................................... 27
4. CONCLUSIONS ....................................................................................................... 31
5. REFERENCES .......................................................................................................... 32
iii
LIST OF TABLES
Table 1 – Engineered Polymeric Bearing Materials Used for Testing .............................. 9
Table 2 – 440C Mating Surface Material Composition................................................... 12
Table 3 – Test Parameters................................................................................................ 19
Table 4 – Pin-on-Disk Test Matrix .................................................................................. 20
Table 5 – Oscillating Test Matrix .................................................................................... 20
Table 6 – Delrin® Acetal Sample Wear Results ............................................................. 23
Table 7 – UHMWPE Sample Wear Results .................................................................... 26
Table 8 – Molded Liner Sample Wear Results ................................................................ 29
iv
LIST OF FIGURES
Figure 1 – Example of Galling Damage Seen on Lubricated Bronze Bearings ................ 2
Figure 2 - Comparison of Wear Coefficient for Different Bearing Interfaces [1]............. 3
Figure 3 - Polymeric Bearing after same life cycle & operating conditions as Figure 1 .. 5
Figure 4 – Oscillating Bushing Wear Test Configuration [4] ........................................... 6
Figure 5 – General Pin-on-Disk Configuration ................................................................. 7
Figure 6 – Straight Journal Bushing Test Specimen Manufacturing Dimensions........... 10
Figure 7 – Test Bushings, Delrin® (L), UHMWPE (C), Molded Liner on CRES (R) .. 10
Figure 8 – Pin Test Specimen Manufacturing Dimensions ............................................. 11
Figure 9 –Test Pins, Molded Self-lube liner (L), Delrin® (C), UHMWPE (R) .............. 11
Figure 10 – Mating 440C Bushing Dimensions .............................................................. 12
Figure 11 – Finished 440C Mating Bushing.................................................................... 12
Figure 12 – Mating 440C Disk Dimensions .................................................................... 13
Figure 13 – Finished 440C Test Disk .............................................................................. 13
Figure 14 – Pin-on-Disk Test Rig CAD Rendering [5] ................................................... 14
Figure 15 – Pin-on-Disk Test Rig .................................................................................... 15
Figure 16 – Pin-on-Disk Test Rig Custom DASYLab UI during set up ......................... 16
Figure 17 – 10 Amp, 0-30 Volt DC Power Supply ......................................................... 16
Figure 18 – Oscillating Test Rig ...................................................................................... 17
Figure 19 – Oscillating Rig Operating During Testing ................................................... 18
Figure 20 – Delrin® POD Sample 3d3 Start and Finish Comparison – Pin Debris ........ 21
Figure 21 – Delrin® POD Pins and Disks After Testing – Variable Impact on Disks ... 21
Figure 22 – Delrin® Bushings and Mating 440C Sleeves – Burnished, Transfer Film .. 22
Figure 23 – Wear Indications in Load Zone (Marked with Yellow Paint) on Delrin®
Acetal ....................................................................................................................... 22
Figure 24 – UHMWPE POD Sample 8u3 Start and Finish Comparison – Pin Debris ... 24
Figure 25 – UHMWPE POD Pins and Disks After Testing – Variable Impact, Mostly
Transfer .................................................................................................................... 24
Figure 26 – UHMWPE Bushings and Mating 440C Sleeves – Minor Burnishing and
Transfer .................................................................................................................... 25
v
Figure 27 – UHMWPE Sample Showing Extrusion/Warping (L), Wear/Temperature
Indications (R) ......................................................................................................... 26
Figure 28 – Molded Liner POD Sample 8u3 Start and Finish Comparison – Pin Debris 27
Figure 29 – Molded Liner POD Pins and Disks After Testing – Variable Impact,
Transfer .................................................................................................................... 28
Figure 30 – Molded Liner Bushings and Mating 440C Sleeves – Minor Burnishing and
Transfer .................................................................................................................... 28
Figure 31 – Molded Liner Sample Showing Typical Wear Zone Burnishing; Mating
Sleeve Transfer ........................................................................................................ 29
vi
ACKNOWLEDGMENT
I would like to thank Professor Ernesto Gutierrez-Miravete for his patience and support
while I completed this project. I would also like to thank my colleagues at Kaman
Specialty Bearings (Kamatics Corporation), particularly Jim Collazo and the rest of the
Test and Development Laboratory for their assistance with the work described in the
following pages. Finally, I’d like to dedicate this work to my family and, most of all, my
wife, Annie. It is unlikely that I would be writing this if not for their love, support, and
continued encouragement.
vii
ABSTRACT
In the fields of tribology and engineering, the performance of bearing surfaces
within the machinery and mechanisms that humans have used throughout their more
recent industrial history is a critical issue.
Important bearing surfaces serve in
everything from a turbine in a power plant to a valve in an artificial heart. Many bearing
surfaces utilize metal-to-metal interfaces lubricated with some type of specially
formulated lubricant. When this type of system, or the maintenance that accompanies
one, is not practical or feasible, engineered bearing polymers have become important
anti-wear and friction reduction products. Their self-lubricating properties help reduce
friction and system wear without additional provisions.
Many industries rely on engineered bearing polymers to perform in existing
problem areas or in new challenging applications. Significant portions of the
development and the gains in market share that engineered polymers have made are due
in part to the tribological evaluations (friction and wear) to which they are subjected.
There are many different kinds of tribological evaluation tests, but two of the most
popular are the Pin-on-Disk (POD) and Oscillating (OSC) bearing tests.
Within
different circles, one test is often preferred over the other for either its apparent
similarity to the actual application, its availability, or its perceived level of conservatism
compared to the other method. The test that is decided upon is typically a result of
tradition or tribal knowledge within a group.
This project set out to determine if there was some discernable difference between
the POD test and the OSC test’s abilities to evaluate the wear performance of a bearing
polymer.
To do so, 5 samples each of three different bearing polymers (Delrin,
UHMWPE, and a molded liner system) were tested in the POD and OSC tests. Though
the measured and calculated specific wear rates in each test were small and limited to
one particular operating scenario, the findings pointed towards a consistently higher
level of conservatism in the POD test, warranting more potential testing with a wider
variety of bearing materials and operating parameters.
viii
1. Introduction and Background
The market for engineered polymeric bearing materials is important to the every
day function of transportation and industrial systems around the world. In every major
technological field, ranging from aerospace to marine and hydropower to human
prosthetics, the performance of bearing materials has become an increasingly studied
topic. Existing bearing applications are continually transitioned away from
environmentally unfriendly, poorly performing, and difficult to maintain technologies.
Though polymeric bearing materials have been widely used since the mid-20th century,
bearings manufactured from or containing engineered bearing plastics have become
more and more popular due to their highly customized, environmentally sensitive, and
advanced natures compared to traditional greased metal-to-metal bearings or traditional
rolling element (ball or needle) bearings.
New self-lubricating and reinforced bearing polymers are constantly being
engineered, investigated, and tested for better performance characteristics that may
improve the systems into which they are installed. Polymeric bearing materials are often
evaluated based on their coefficient of friction with a given mating surface, load carrying
capability, and their overall life expectancy for a given set of operating conditions. Most
tests that examine the overall life of a bearing material are typically observing, through
experimental means, the wear rate of the material. In many circles throughout the
bearing community, it is believed that the closer a test method is in nature to the real
world scenario for which the bearing material is being evaluated, the more accurate the
results are. In other instances, cost, speed, and simplicity are prioritized. This project
will compare the results of two standard tests: the pin-on-disc test and the oscillating
sleeve test.
1.1 The Use of Polymeric Materials in Bearing Applications
Polymeric bearing materials are used primarily for the very low coefficients of
friction that can be attained without the use of a supplemental form of lubrication, such
as the grease supplied to a bronze or copper-beryllium journal bushing or the lubricant
1
retained within a traditional high-speed ball or needle bearing. In the case of the bronze
alloy or copper-beryllium journal bushing, the base material is chosen for it’s low
hardness (relative to the harder mating surface material) and it’s resistance to galling and
fretting damage when operating dynamically against a dissimilar metal.
However,
without the presence of grease or other secondary lubricant, the bearing surface may still
become damaged under sufficiently high forces and oscillation speeds. The lubrication
must be continually or periodically applied through dedicated provisions (grease
grooves, lubricant paths, grease nipples) incorporated directly into the bearing, the
housing it is installed in, or the component that the bearing runs against. Even when the
bearing interface is properly lubricated, the lubricant may be extruded or squeezed out of
the load zone once a load is applied to the system. A thin film of lubricant may remain,
but the majority of the resultant bearing contact may be between the two metallic
components. Though the bronze or copper-beryllium has a lower hardness than the
mating material, the disparity between the two generally is not significant enough to
allow for continual operation without secondary lubrication. An example of the damage
than can occur in these instances is shown below in Figure 1.
Figure 1 – Example of Galling Damage Seen on Lubricated Bronze Bearings
2
When a bearing utilizing an engineered polymer is operated in a similar
environment, the disparity in hardness between the bearing material and the mating
material is normally much more significant, with the bearing plastic typically ranging in
hardness from 70-100 on the Rockwell M scale, while the typical stainless steel mating
component used in a bearing system may approach a hardness of 60 on the Rockwell C
scale. The bearing polymer’s significantly lower hardness relative to the mating surface
generally results in the bearing becoming the sacrificial element of the system in order to
protect the housing and the mating component. The bearing polymer also typically has
an inherently low coefficient of friction due to its molecular structure of long polymer
chains that have the ability to easily slip past one another when subjected to a shear
force. Figure 2, below, shows that wear rate for both similar and dissimilar metal-tometal interfaces is several orders of magnitude greater than non-metal against metal
interfaces as the amount of secondary lubrication decreases.
Figure 2 - Comparison of Wear Coefficient for Different Bearing Interfaces [1]
Bearing polymers also have the added benefit of containing or being made from
self-lubricating materials such as PTFE or graphite. The inclusion of these materials
3
helps to reduce the overall friction, which in turn results in lower wear rates. The
underlying process, particularly in the case of PTFE-based materials, that creates this
low coefficient of friction can be described as follows: as the bearing polymer surface
runs against the counter metallic surface, the PTFE crystalline chain structure undergoes
a process which creates fractured clumps of PTFE chains that are chemically reactive
with the mating surface. These chains adhere (through what is suspected to be a weak
van der Waals bond) to the mating surface and a transfer film is created. The bulk
portion of the PTFE interfaces with the transfer film, and this interaction allows the
crystalline structures of both surfaces to align and shear very easily. Essentially, the
transfer of the PTFE to the counter surface results in a PTFE-on-PTFE bearing surface,
causing the low coefficient of friction [2]. Under the appropriate conditions (speeds and
loads within the limits of the bearing polymer, mating surface conditions within industry
standard conditions of 40 HRC hardness or higher and 16 μin or better finish), the
bearing will only wear as quickly as this transfer film is worn out and replenished by
material (PTFE or otherwise) from the bulk bearing material. The wear rate of pure
PTFE has been shown to be relatively high, so it is often combined with other fillers that
reduce the total wear rate while still maintaining the low coefficient of friction generated
by the PTFE [3]. Figure 3, below, shows the condition of an engineered polymer bearing
that had operated in the same location and for the same duration as the metallic bearing
shown in Figure 1.
4
Figure 3 - Polymeric Bearing after same life cycle & operating conditions as Figure 1
1.2 Bearing Material Wear Tests
When evaluating the performance of a given bearing material, be it metallic,
metallic composite, plain polymer bulk, or polymer composite, there are several
different acceptable methods available. Each test can be used to determination one or
several of the critical bearing performance characteristics: wear rate or life, dynamic
load capacity, coefficient of friction, performance at temperature, and fluid
compatibility. Among the typical bearing tests used by the aerospace, automotive, and
industrial sectors are the oscillating bearing test, block-on-ring test, pin-on-disk, and
reciprocating line contact. For the purposes of this project, only the oscillating bearing
and pin-on-disk test will be evaluated.
The oscillating bearing test is a staple of the aerospace bearing standards,
currently governed by the SAE International (Formerly the Society of Automotive
Engineers). The method of dynamic loading and wear is similar to many reciprocating or
oscillating applications in these fields, so it is frequently used as direct analog to real
world operation. The desire to replicate the operating conditions of a given bearing
location thus often results in lengthy (weeks or months) test periods. The AS81934,
AS8943, and AS81820 journal bushing and spherical bearing specifications require that
5
the given bearing configuration pass several iterations of this test under varying
conditions prior to obtaining certification. In this test, a straight journal bushing or a
spherical bearing is installed into a CRES housing sized for a resultant light interference
or slip fit with the outer diameter of the bearing specimen. A hardened CRES shaft is
installed through the cylindrical bore of the bearing specimen and is simply supported on
either side of the test component [4].
Figure 4 – Oscillating Bushing Wear Test Configuration [4]
A load is applied to the bearing and it’s housing, in a direction normal to the axis
of the bearing bore, such that the pin is loaded in double shear. After statically preloading the bearing with the defined test load for a prescribed duration of time, the
bearing is oscillated a prescribed angle (typically 25°). One cycle consists of rotation
from 0° to +25°, +25° to 0°, 0° to -25°, and -25° back to 0°, for a total of 100° of travel.
The radial displacement of the bearing is measured through a dial indicator or electronic
sensor and recorded at prescribed intervals. After several thousand cycles (typically
25,000 to 100,000), the total change in radial displacement (taken to be the total wear
experienced by the bearing material) is compared to the allowable limit as defined by the
particular specification or design requirement [4]. Friction can be determined by
manually checking the loaded breakaway torque of the test bearing and defined intervals,
6
or through constant electronic monitoring of the system torque. The torque value, along
with the known radial (normal) load applied to the bearing can then be used to calculate
the coefficient of friction.
The pin-on-disk (POD) test is another common, simple test used to quickly
evaluate the tribological properties of materials and material combinations. As implied
by the name, a vertically oriented pin is run against a rotating disk with a specified
normal load applied along the vertical axis of the pin. The disk rotates in one direction
continuously until the pin, relative to the surface of the disk, travels a specified number
of rotations or linear distance. These values can be easily tracked using the
circumference of the pin’s diameter of travel (2 times the pin’s offset radius from the
disk center) to determine the distance traveled per revolution and by calculating the time
period required to complete one rotation at a given speed or through electronic
monitoring.
Figure 5 – General Pin-on-Disk Configuration
The end result of the test is a worn pin and/or disk; from which a total mass or
volume loss (worn material) can be calculated and converted into a specific wear rate
based on the cycles tested and relative distance traveled between the two samples. The
most direct method of determining the mass loss for each specimen is through the use of
an accurate digital scale after thoroughly cleaning the each test piece to remove any
potential wear debris or contamination. Once the change in mass has been determined,
the specific rate of wear can be determined using the following equation [3]:
7
K0 = Dm / L × F × r
Where:
[3]
K 0 is the specific wear rate
L is the sliding distance
F is the applied load
r is the density of the tested material
As noted earlier, traditional practice among aerospace, marine, and industrial
bearing designers is to test a component using an engineered polymer in an environment
most closely resembling the real-world operating environment. In most cases, the test
scenario that mimics the application scenario is the oscillating bearing test, either at
ambient temperature and under dry conditions, at elevated or reduced temperature, or
with the addition of various fluid, solid, and electromagnetic (sunlight, nuclear radiation)
contaminants that the bearing material is likely to be exposed to during it’s operational
life.
It is unclear as to whether or not that the continual rotation of the pin in the pinon-disk test is a more conservative test than the oscillating bearing test. Though the
actual distance should be equivalent when performing one type of test or the other, some
believe that because the pin-on-disc test travels over the entire circumference (at the
prescribed offset from the disk center), that the pin is exposed to a path of travel larger
relative to it’s contact area and must deposit a third body transfer film over a larger
surface, or more frequently. An oscillating test is sometimes perceived to operate over
the same confined surface area; therefore the transfer film only has to be maintained, not
continually replenished. Conversely, it could be said that because the oscillating bearing
test operates over the same ±25° arc length of the mating surface that the motion is more
focused, resulting in greater wear rates because the transfer film must be replenished
more frequently. It is the intent of the experiment described on the following pages to
determine if there is any legitimacy to this “tribal” (passed down from generation to
generation of workers within parts of the bearing industry) knowledge.
8
2. METHODOLOGY
2.1 Selected Bearing Materials and Sample Configurations
To evaluate the actual specific wear rates measured in an oscillating bearing test
and a pin-on-disk test and their level of conservatism relative to one another, a series of
engineered polymeric bearing materials were tested in both configurations. The three
engineered polymeric bearing materials selected were Delrin® Acetal, UHMWPE, and a
molded self-lubricating composite bearing liner.
Table 1 – Engineered Polymeric Bearing Materials Used for Testing
Engineered Polymeric Bearing Materials
Name
Delrin® Acetal
UHMWPE
Molded Self-Lubricating
Composite Bearing
Liner
Description
Polyoxymethylene (POM), also known as
acetal, is an engineering thermoplastic used in
precision parts requiring high stiffness, low
friction and excellent dimensional stability.
Ultra-high-molecular-weight polyethylene is a
subset of the thermoplastic polyethylene. It
has a very low coefficient of friction and is
self-lubricating; also is highly resistant to
abrasion.
This bearing liner conforms to the MIL-B8943
bearing specification. It is comprised of a
thermosetting epoxy resin, miscellaneous load
bearing fillers, and high amounts of PTFE.
This type of engineered bearing liner is widely
used across the aerospace and marine
industry.
Color
Density,
lbs./in3
Black
0.051
White
0.036
Brown
0.048
All three materials were chosen because of their inherent resistance to abrasion,
their self-lubricating properties, and their wide spread use as bearing materials across
many industries. All three materials were also selected because of their availability in
raw stock forms needed to manufacture both small diameter pins for the pin-on-disk test
and larger straight journal bushings for the oscillating bearing test.
The straight journal bushings were made from extruded round bar stock with a raw
outer diameter of 1.5 or 2 inches. Each component was single-point turned using
carbide cutting tools to the dimensions shown in Figure 6.
9
Figure 6 – Straight Journal Bushing Test Specimen Manufacturing Dimensions
For both the Delrin® and the UHMWPE, the manufactured straight journal
bushings were constructed entirely of the raw materials, with no metallic backing or
substrate. The molded self-lubricating bearing liner samples were constructed from 174PH CRES (corrosion resistant steel) in the H1150 condition, with a .010-.015 inch thick
layer of the bearing material applied to the 1.0010-inch nominal inner diameter. Due to
the properties of this type of bearing material, it is difficult to manufacture a solid sleeve
meeting the dimensional requirements shown in Figure 6 entirely of this product.
Figure 7 – Test Bushings, Delrin® (L), UHMWPE (C), Molded Liner on CRES (R)
10
The pin-on-disk (POD) test samples were manufactured to a nominal length of
0.625 inches and a nominal diameter of 0.1875 inches. Each sample was conventionally
machined using single point turning, per the schematic in Figure 8.
Figure 8 – Pin Test Specimen Manufacturing Dimensions
Figure 9 –Test Pins, Molded Self-lube liner (L), Delrin® (C), UHMWPE (R)
The desired mating components for both tests would be constructed from
stainless steel, have a high hardness, and have a surface finish that would promote a
reasonable amount of wear. The final material chosen was martensitic 440C CRES,
which is often used for mating pins and balls for bushings and spherical bearings in
aerospace applications. The original extruded bar stock material was heat treated to the
H900 condition and had a targeted finish of 32μin. The H900 condition provides a
resultant hardness of 55-62 on the Rockwell C scale, ensuring that the test pins or
bushings do not significantly scar the mating surface. The 32μin finish will ensure that
the mating surface asperities are not so large that the wear rate is highly accelerated or
that the bearing material fails catastrophically. It also provides a rough enough surface to
11
promote enough wear to liberate some of the bulk bearing material to create and
maintain transfer film. Just as the mating surface can have too poor of a finish and too
low a hardness to allow the bearing material to function properly, the mating surface
could theoretically be too hard and/or too smooth to allow certain self-lubricating
bearing materials to provide the wear life and low friction expected.
Table 2 – 440C Mating Surface Material Composition
Element
C
Cr
Ni
Mn
Si
S
% Comp.
.95-1.2
16-18
-
1.0
1.0
0.03
P
Mo
Others
0.04
0.75
Copper 2.5/4.5;
Cb and Ta 0.15/0.45
The mating bushings were machined from raw bar stock of at least 1.25 inches in
diameter, to the dimensions shown in Figure 9.
Figure 10 – Mating 440C Bushing Dimensions
Figure 11 – Finished 440C Mating Bushing
12
The mating disks were also machine from raw bar stock of at least 1.25 inches in
diameter, to the dimensions shown in Figure 10.
Figure 12 – Mating 440C Disk Dimensions
Figure 13 – Finished 440C Test Disk
2.2 Test Rigs
All testing performed for this project was conducted in the Test and Development
Laboratory at Kamatics Corporation in Bloomfield, CT. Matthew Lessard, an employee
of Kamatics Corporation, originally designed and built the pin-on-disk test rig in 2010
and 2011. The set-up was created following the operational criteria set forth in ASTM
G99. The test load is applied to the test pin and disk through a lever arm, which has
strain gauges attached to measure deflection in two axes, noting drag force and the
13
normal force. A proximity sensor measures the revolutions of the miter gears attached to
the test disk spindle. The test load is applied to the lever arm and pin by tightening a hex
nut that compresses a floating spring, exerting a reactionary load on the arm, which
pivots to force the pin against the disk [5].
The test pin is held within the machined aluminum pin holder (which itself is
pinned to the lever arm to transfer the load to the pin) using a threaded set-screw and the
test disk is retained using a hex socket head cap screw, designed to sit within the
countersunk through-hole seen in Figure 12.
Figure 14 – Pin-on-Disk Test Rig CAD Rendering [5]
14
Figure 15 – Pin-on-Disk Test Rig
All inputs (other than the test load, which is applied manually) and outputs are
controlled and monitored through the linked data acquisition system and the 10 amp, 030 V DC power supply (upgraded from the original 5 amp unit). Test iterations and data
recording are started through the customized DASYLab V11 user interface (UI), and the
applied load (and resultant pressure on the test pin) could be read from this same
terminal. Altering the coarse and fine voltage or current controls of the dc power supply
will adjust the rotational speed of the test disk. The resultant rotational speed and the
calculated surface speed seen by the test pin could be read directly from the custom
DASYLab UI. The software also determines the total linear distance traveled based on
the input test diameter (the pin offset from the test disk center) and the measured
rotational speed.
15
Figure 16 – Pin-on-Disk Test Rig Custom DASYLab UI during set up
Figure 17 – 10 Amp, 0-30 Volt DC Power Supply
The oscillating bearing test took place on one of the reciprocating, electric motor
driven test rigs that can be easily set up and broken down on one of the Kamatics T&D
Laboratory’s modular test tables. The test bushings were installed into a housing per the
recommended AS81934 test conditions (.0001 to .0011 inch interference fit). The
bearing assembly resembles the basic set up previously shown in Figure 4. Threading the
turnbuckle left or right, increasing or decreasing the amount of force being exerted on
the rig lever arm, adjusts the applied load. A spring scale indicates the load being applied
16
directly by the turnbuckle. This value is multiplied by a factor based on the lever arm
length to give the actual load being applied to the bearing assembly. The three-bar
linkage connecting the electric motor and the bearing assembly can be adjusted or
customized to produce the proper angle included angle of oscillation. The electric motor
can also be swapped out for other motors with varying speed ratings, or a gearing/pulley
reduction can be applied to further fine-tune the oscillation speed. A number of other
slightly different oscillating bearing set ups could have been utilized, but this
configuration was readily available at the time of testing.
Figure 18 – Oscillating Test Rig
17
Figure 19 – Oscillating Rig Operating During Testing
2.3 Test Parameters
To facilitate direct comparison between the pin-on-disk generated specific wear
rates and the oscillating test generated specific wear rates, the operating parameters were
first determined based on the limitations of the POD rig and then converted to matching
parameters on the oscillating rig. The Kamatics T&D Laboratory personnel previously
had made observations that bearing pressures (at the pin/disk interface) above 600 psi
and surface speeds of 30 to 50 fpm combined with certain polymeric bearing materials
may cause an overloading and subsequent burnout of the DC power supply. Noting this
observation, the bearing pressure was selected to be 1,260 psi, but with a surface speed
at the pin/disk interface of 8 fpm. One trial run was performed at this load and speed
and it was determined that the power supply would not be pushed beyond it’s
operational limits and the two parameters could be easily maintained by the system.
When a total distance traveled was considered for the POD tests, the limitations of
the oscillating rig were considered. In order to create a bearing surface speed in the
oscillating bushing test equal to the 8 sfpm selected for the POD test, it was calculated
that the oscillating test must run at 220 CPM if a ±25° operating angle (50° included
18
angle, 100° per cycle) were to be utilized. This speed was found to be within the
capabilities of the oscillating rig and its accompanying motor. Once the speed for both
tests was solidified, the distance traveled (POD) and number of equivalent cycles
(oscillating) could be selected. Since the inspiration for this test came primarily from the
aerospace bearing industry, a total number of cycles near the standard AS81820 and
AS81934 25,000 cycle mark was targeted. A final number of 29,670 oscillating cycles,
or approximately 2,158 linear feet was selected.
It was noted that since the POD test achieved a continuous surface speed and the
oscillating test technically did not maintain a truly constant speed due to the mechanics
of the three-bar linkage and the two rotational direction reversals present in each
oscillatory cycle. Therefore, if one wanted to achieve the same average speed in the
oscillating test as in the POD test, some alteration of the speed may be required. For the
purposes of this test, it was assumed that the mean surface velocity at the bearing
interface on the oscillating test would be equal to the desired 8 sfpm (varying between 0
sfpm and a peak of about 16 sfpm). This assumption was made because the effects of the
change in rotational direction on the actual speed experienced by the bearing surface is
seldom factored into any aerospace standard test or any accelerated lifecycle testing
based on a given bearing application. Maintaining the traditional test approach for both
the POD and oscillating test based on the author’s experience with said bearing
applications was desirable. The final conditions can be found in the table below.
Table 3 – Test Parameters
Applied Load
Resultant Bearing
Pressure
Oscillation Angle
Speed
Pin-onDisk
35 lbs.
Oscillating
630 lbs.
1,261 psi
N/A
8 fpm
19
±25°
220 cpm
3. EXPERIMENTAL RESULTS
The following tables outline the entire set of POD and oscillation test iterations
performed. Five samples of each of the three materials were tested in both bushing and
pin form, for a total of 15 POD tests and 15 oscillation tests. Prior to and after the testing
of both the pins and bushings, each test piece’s mass was recorded using a Mettler
Toledo JB1603-C/FACT digital scale, which is precise to 0.0001 gram.
Table 4 – Pin-on-Disk Test Matrix
Delrin® Acetal
UHMWPE
Molded
Bearing Liner
Pin #
1d1
2d2
3d3
4d4
5d5
6u1
7u2
8u3
9u4
10u5
11m1
12m2
13m3
14m4
15m5
Mating Disk
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Table 5 – Oscillating Test Matrix
Molded
Bearing Liner
UHMWPE
Delrin® Acetal
Bushing #
11
22
33
44
55
66
77
88
99
1010
1111
1212
1313
1414
1515
20
Mating Sleeve
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
3.1 Delrin® Acetal Results
Each of the Delrin® test pins showed visible signs of wear through varying levels of
polishing of the mating 440C test disks and through the accumulation of Delrin® wear
debris around the base of the pin. An example of such wear debris is shown in the
Figure below. Note that the image on the left was taken at the start of the test, while the
image on the right was captured at the completion of the test.
Figure 20 – Delrin® POD Sample 3d3 Start and Finish Comparison – Pin Debris
Figure 21 – Delrin® POD Pins and Disks After Testing – Variable Impact on Disks
21
Each of the Delrin® bushings also showed visible signs of wear, leaving a
discolored, burnished finish on the mating 440C sleeve in the area representing the load
zone. This likely represents a combination of transfer film material and polishing of the
sleeve material. The inner diameter of each bushing exhibited a polished surface in the
load zone with some small signs of wear scarring.
Figure 22 – Delrin® Bushings and Mating 440C Sleeves – Burnished, Transfer Film
Figure 23 – Wear Indications in Load Zone (Marked with Yellow Paint) on Delrin® Acetal
22
When the results were collected, the mass losses per sample and the specific
wear rates for each type of test were calculated for the Delrin® Acetal samples, and the
following table was generated.
Table 6 – Delrin® Acetal Sample Wear Results
POD
Delrin®
Acetal
Oscillating
Sample
Sample
Start
Mass, g
Sample
Finish
Mass, g
Specific Wear Rate
in3/lbs•ft
1d1
0.4018
0.4014
2.29E-10
2d2
0.4030
0.4017
7.44E-10
3d3
0.4023
0.3990
1.89E-09
4d4
0.4028
0.3861
9.56E-09
5d5
0.4013
0.3985
1.60E-09
1111
3.8105
3.8085
6.36E-11
1212
3.8267
3.8258
2.86E-11
1313
3.7690
3.7689
3.18E-12
1414
3.7501
3.7500
3.18E-12
1515
3.7698
3.7689
2.86E-11
Average
Specific
Wear,
in3/lbs•ft
2.80E-09
2.54E-11
It should be noted that the mass measurements of each sample were first
converted to pounds prior to calculating the specific wear rates in Imperial units. Based
on the data shown above, the pin-on-disk test samples were shown to have exhibited a
higher specific wear rate than the oscillating bushing test samples, by about 2 orders of
magnitude. However, samples 1d1 and 2d2 did reach a slightly lower order of magnitude
than the other three pins, somewhat closer to the highest bushing wear rates. Sample 4d4
of the POD testing did appear to have markedly increased wear, however a review of the
disks in Figure 21 did not show any abnormal effects on the 4d4 mating disk, and the
raw load and speed data output by the POD rig does not indicate an increase of either
variable beyond the defined values. It is not clear why this sample, experienced 5 to 6
times more wear than the samples 3d3 and 5d5. Overall, the average specific wear for
the pin-on-disc test was calculated to be nearly 110 times greater than the average
specific wear of the oscillating test using Delrin® Acetal.
23
3.2 UHMWPE Results
Each of the UHMWPE test pins showed visible signs of wear through varying levels
of polishing of the mating 440C test disks and through the accumulation of UHMWPE
wear debris powder around the base of the pin, though to a lesser extent than the Acetal
samples. An example of such wear debris is shown in the Figure below. Note that the
image on the left was taken at the start of the test, while the image on the right was
captured at the completion of the test. The discs were not polished or worn by the
UHMWPE as much as they were by the Acetal, but there appeared to be a more distinct
transfer film present. It could be wiped away with moderate pressure from a clean rag.
Figure 24 – UHMWPE POD Sample 8u3 Start and Finish Comparison – Pin Debris
Figure 25 – UHMWPE POD Pins and Disks After Testing – Variable Impact, Mostly Transfer
24
Each of the UHMWPE bushings also showed visible signs of wear, leaving a
slightly discolored, burnished finish on the mating 440C sleeve in the area representing
the load zone. This likely represents a combination of transfer film material and very
slight polishing of the sleeve material. The material that appeared to be the transfer film
also wiped away, similar to the film on the POD disks. Each bushing showed signs of
material extrusion out of the sides of the housing during testing, indicating that the test
may have at least temporarily reached the thermal operating limit of UHMWPE. In other
bearing tests performed at similar speeds in the Kamatics’ T&D laboratory, temperatures
approaching 180°F have been observed, though the loads and resulting bearing pressures
were over 10 times higher than they were in this test, which would have resulted in more
heat generation. UHMWPE can operate at 200°F for short durations, but some factor
appears to have pushed this material beyond this limit. When exposed to elevated
temperatures beyond its design limit, UHMWPE will lose mechanical properties and
abrasion resistance, though based on the wear values recorded, it appears that only the
mechanical properties may have been compromised, allowing the material to deform.
Temperature is not normally monitored on the test rig utilized for this oscillating test.
Figure 26 – UHMWPE Bushings and Mating 440C Sleeves – Minor Burnishing and Transfer
25
Figure 27 – UHMWPE Sample Showing Extrusion/Warping (L), Wear/Temperature Indications (R)
When the results were collected, the mass loss per sample and the specific wear
rate for each test was calculated for the UHMWPE samples. The following table was
generated.
Table 7 – UHMWPE Sample Wear Results
POD
UHMWPE
Oscillating
Sample
Sample
Start
Mass, g
Sample
Finish
Mass, g
6u1
0.2605
0.2599
Specific
Wear
Rate
in3/lbs•ft
4.86E-10
7u2
0.2640
0.2639
8.11E-11
8u3
0.2596
0.2594
1.62E-10
9u4
0.2640
0.2636
3.24E-10
10u5
0.2614
0.2611
2.43E-10
66
2.4691
2.4690
4.50E-12
77
2.4279
2.4272
3.15E-11
88
2.4722
2.4720
9.01E-12
99
2.5123
2.5120
1.35E-11
1010
2.4241
2.4240
4.50E-12
Average
Specific
Wear,
in3/lbs•ft
2.59E-10
1.26E-11
Once again an initial review of the calculated data for the UHMWPE samples
indicates that the POD test samples experienced an average rate of wear about 1 order of
magnitude larger than the oscillating test samples. The measured mass loss for some of
26
the bushings samples, 66 and 1010 particularly, approached zero, registering at 0.0001
grams, which is the limit of accuracy for the digital scale used. As noted earlier, it was
interesting that the bushings did not register more measurable wear given that they may
have had their mechanical properties and abrasion resistance compromised by
temperature effects. Pin sample 6u1 experienced about 3 times as much wear as the
other samples at the same order of magnitude, and 6 times as much wear as sample 7u2,
but no notable variations from the prescribed test parameters were observed. Overall,
the POD test sample average specific wear was approximately 20 times greater than the
average specific wear of the oscillating test.
3.3 Molded Self-Lubricating Bearing Material Results
Each of the Molded Self-lubricating bearing test pins showed visible signs of wear
through varying levels of polishing of the mating 440C test disks and through the
accumulation of wear debris powder around the base of the pin. An example of such
wear debris is shown in the Figure below. Note that the image on the left was taken at
the start of the test, while the image on the right was captured at the completion of the
test.
Figure 28 – Molded Liner POD Sample 8u3 Start and Finish Comparison – Pin Debris
27
Figure 29 – Molded Liner POD Pins and Disks After Testing – Variable Impact, Transfer
Each of the Molded Liner bushings also showed visible signs of wear, leaving a
discolored, polished finish on the mating 440C sleeve in the area representing the load
zone. This is representative of the typical transfer film for this material and the usual
polishing of the sleeve material due to the slightly abrasive nature of the thermosetting
resin and fillers that make up a portion of the bearing material. No other adverse affects
could be seen on the bushings or the mating sleeves.
Figure 30 – Molded Liner Bushings and Mating 440C Sleeves – Minor Burnishing and Transfer
28
Figure 31 – Molded Liner Sample Showing Typical Wear Zone Burnishing; Mating Sleeve Transfer
When the results were collected, the mass loss per sample and the specific wear
rate for each test was calculated for the Molded Liner samples. The following table was
generated.
Table 8 – Molded Liner Sample Wear Results
POD
Molded
Liner
Oscillating
Sample
Sample
Start
Mass, g
Sample
Finish
Mass, g
11m1
0.4315
0.4306
Specific
Wear
Rate
in3/lbs•ft
1.69E-11
12m2
0.4334
0.4320
2.03E-11
13m3
0.4405
0.4399
3.04E-11
14m4
0.4208
0.4200
1.69E-11
15m5
0.4246
0.4241
1.35E-11
11
18.3451
18.3446
4.50E-12
22
18.3238
18.3232
3.15E-11
33
18.3571
18.3562
9.01E-12
44
18.3171
18.3166
1.35E-11
55
18.3411
18.3407
4.50E-12
29
Average
Specific
Wear,
in3/lbs•ft
5.11E-10
1.96E-11
An initial review of the calculated data for the Molded Liner samples indicates
that the POD test samples experienced an average rate of wear almost 1 order of
magnitude larger than the oscillating test samples. The molded liner performed
consistently in the oscillating test, while the POD test results were slightly less
consistent with a peak mass loss of 0.0014 grams, the only one of five samples to break
into the 10-5 order of magnitude. As with the other high mark wear rates and mass loss
measurements seen during this testing, there was no clear indication as to why the
variation occurred, and without further investigation is likely within the natural variation
of the material’s performance at these operating conditions. Overall, the POD test
sample average specific wear was approximately 26 times greater than the average
specific wear of the oscillating test for the Molded Liner.
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4. CONCLUSIONS
Through the data collected in this project, it appears that for these given materials
and operating parameters that the pin-on-disk test may provide a slightly more
conservative wear rate than the oscillating test. Though both are used frequently to
provide characterizations of the tribological properties of new bearing materials or
combinations of materials in tribological systems, the simpler POD test may provide
appropriately conservative values to eliminate at least some risk in defining the
operating capabilities of certain bearing materials.
One of the main benefits of the standard ASTM G99 pin-on-disc tribometer that
is frequently referenced in other works is the short turn around, low cost, and simplicity
of the tests. Procuring or manufacturing pins and the mating disks of varying materials
is generally simpler and more economical than procuring or manufacturing entire
journal, spherical, or roller bearing assemblies. A new Falex ISX brand tribometer,
based on the ASTM G99 parameters, may cost upwards of $40,000, but a new, similarly
functioning system can be manufactured inside of 6 weeks for a fraction of the cost [5],
making the pin-on-disc test system accessible to most companies and academic
institutions. On the other hand, a system similar to the oscillating test rig utilized in this
project would require significantly more resources to design and manufacture, as would
the bearing samples if not readily available from inventory.
For all three materials; the Delrin® Acetal, the UHMWPE, and the Molded Selflubricating Bearing Liner; the results clearly trended towards one conclusion. Despite
the occasional near zero mass loss or high mark wear rate, the averages consistently
pointed towards the Oscillating Bearing test generating less overall wear under the
established loads and speeds. Considering that the three products chosen for this test are
widely used for a greater number of applications that subject the materials to
contamination, much higher loads, varying speeds, and greater equivalent linear
distances traveled, this observation may not hold up under further testing. The data,
however limited it may be in scope and quantity, does indicate that there may in fact be
an advantage to using the pin-on-disc method in more instances where replicating the
actual application is traditionally stressed. It is recommended that more materials and
conditions be evaluated to further support or refute this claim.
31
5. REFERENCES
[1]
Foster, J., MATERIALS
FOR
WEAR RESISTANT SURFACES, Nuclear Engineering
and Design 17, (1971), p. 205-246
[2]
Biswas, S.K., Vijayan, Kalyani., FRICTION
AND WEAR OF
PTFE –
A REVIEW.,
Elsevier – Wear, (1992)
[3]
Unal, H., Mimaroglu, A., Ekiz, H., SLIDING FRICTION AND WEAR BEHAVIOR OF
POLYTETRAFLUOROETHYLENE AND ITS COMPOSITES UNDER DRY CONDITIONS,
Elsevier – Wear, (2003)
[4]
SAE International, AS81934 – BEARINGS, SLEEVE, PLAIN
AND
FLANGED,
SELF-LUBRICATING, (1998)
[5]
Lessard, M., IMPROVED TRIBOLOGICAL C HARACTERISTICS OF TI-6AL-4V
REACTIVE SURFACE TREATMENTS , (2011)
BY
32