1.
Four­Ball Tribometer
Fall 2014
Group Members
CONTROLS TEAM STRUCTURES TEAM FABRICATION TEAM
Ana Dungan Whitney Stregles Daniel Griffin
Kelsey Kaht Robert Nalecz Colin Holliday
Steve Soto Riley Shay Casey Sheppard
John Willis Jordan Ansley Michael Spaulding
Ibrahim Ahmed
Instructor: Dr. Molina Submitted: December 11, 2014

2. Tribology is the study of how various materials interact in relative motion. A tribometer is a
machine that has been designed to test some specific aspect of these interactions.There are many
different potential set­ups for tribometers depending on what type of contact is being studied. The
objective of this project was to design and fabricate a four­ball tribometer for the lab at Georgia
Southern University. Four­ball tribometers implement a set­up in which three balls are placed in a cup
while a fourth ball is added from the top with a specified pressure and rotated at a set speed. The two
basic variations of the four­ball machine are based off of whether the bottom three balls are allowed to
rotate freely or are locked into place. These two configurations can be used to gather different data and
information about the materials in question. For this project the cup design will lock the balls in place
for purely sliding contact tests.
The first step for the project was establishing a timeline for the semester in order to set goals and
milestones to help keep everybody on time and focused so that the project could be completed by the
end of the semester. A Gantt Chart was created giving each phase of the design process a pre­determined
amount of time with each of the three Milestone presentations marked as well. This chart can be seen in
Appendix A, with both the original schedule (marked in grey) and the actual schedule of how the
semester went (marked in green). Once the schedule was in place, the first stage of the plan was
researching existing products in this field, the major components of the machine and the official
standards which must be met by the new design. The following three ASTM standards were found
concerning four­ball tribometers: D2266, D4172 and D5183. While most of the basic requirements for
the standards were pretty similar, each one describes a different way of using the four­ball set­up.
ASTM Standard D2266 covers the determination of the wear preventive characteristics of greases in
sliding steel­on­steel applications, D4172 is the procedure for making a preliminary evaluation of the
anti­wear properties of fluid lubricants in sliding contact and D5183 describes the procedure for
determining the coefficient of friction.[1] These standards were used to set minimum and maximum
capabilities for the new design as well as tolerances for the accuracy of some features. After reviewing
the standards and researching existing four­ball tribometers, the group came together to do some
brainstorming about potential solutions for the major components of the overall design. During the
brainstorming stage a concept comparison table was developed for rating each potential solution based
off of a set list of requirements. The major aspects of the design which were evaluated in the comparison
tables were load application, torque measurement, wear measurement and how the top ball would be
attached to the shaft of the motor. The comparison constraints included cost, ease of use, size safety,
accuracy, maintenance and repeatability. Each team took a copy of the table and scored the various
concepts on a scale of 1­10 based off of their efficiency in each constraint. The totals for each team were
added up and the concept with the highest score for each component was pursued in the first project
design. At this point a QFD Diagram was also implemented in an effort to determine whether the design
would meet the customer requirements as well as the technical standards. As part of the QFD an attempt
was made at comparing the new design with two of the existing machines available on the market. The
concept comparison tables for each team, a summary table of the requirements set by each standard and
the QFD diagram can be seen in Appendix A. After collecting and discussing the data from the various
research tools used, a rough model of the initial design was drafted up into SolidWorks.

3. The initial design was a very simple set­up without any advanced technological features. A basic
table top stand was designed to support an inline motor which would rotate a drive with a ball attached
at the end. The load would be applied from the bottom via a lever system with a strategically placed
fulcrum and known weights attached at the end. A hydraulic press and screw operated jack were also
considered for this purpose, but it was decided that both of these options made it difficult to calculate the
exact pressure being applied and maintain a steady pressure. The torque measurement methods
considered were either the direct use of strain gauges or a torque transducer. The original design
implemented the use of strain gauges largely due to the high cost and large size of the torque transducer.
The gauges would be applied to the support beam below the cup and calibrated by applying a known
force. The top ball would be mounted to the drive shaft from the motor via a custom collet design.
Pressing or screwing the ball onto the shaft were both considered, however press fitting would make the
ball hard to remove without altering the wear scars and screwing would require drilling and threading
every single ball, a very tedious and time consuming task that would be very inefficient.Two different
cup designs were developed to allow for both rolling and sliding contact tests. The free rolling design
consisted of a cup with a simple retainer to keep the balls from moving in the vertical direction and a
screw on top to keep the lubricant from splashing out. The fixed bottom balls design would simply
include an additional bottom that could be placed in the original cup with three indentations set to hold
the balls in place and stop them from rolling. The concept of how to measure the wear on each ball was
also considered and the final decision was to use a graduated cylinder to determine the volume of each
ball before and after testing. Volume was chosen largely because of the lower cost associated with the
graduated cylinders versus a scale. Solid models for the various aspects of this design were drafted up
and presented in the first milestone presentation.
At the end of the presentation Dr. Soloiu expressed concern that the strain gages would be
difficult to work with and that measuring the torque with them would not be as straightforward as we
anticipated. It was also pointed out that the level of accuracy that would be necessary for measuring the
wear would be almost impossible to attain via the volumetric measuring system that was planned. Using
mass scales to determine the weight of each ball was presented as a potential alternative, but more
research needed to be done in order to determine the range of ball weights used for these tests and
whether or not there is a scale to match both the range and accuracy that is needed for the wear
measurements. Dr. Vlcek also reminded the group that Georgia Southern University has various high
tech microscopes here on campus that can be used to measure the wear on each ball. Another question
raised was how the group intended to control the temperature of the lubricant during testing. The
original plan was to heat the lubricant to the specified temperature listed in the standards before the test
began, but after further research it became clear that the starting temperature also had to be maintained
throughout the test.
After the first milestone, the group revisited the issues pointed out by the professors and began to
alter the design to make use of the drill press Dr. Vlcek said could be used as a base. When meeting with
professors about attempting to implement the use of the lever arm with the drill press it was pointed out
that although the lever arm allows the force to be held constant (by applying a known weight) it does not
have any way of accounting and adjusting for the change in friction as the balls become worn and the

4. contact area is extended which will directly affect the torque measurements. In an effort to avoid this
issue and also to make use of the existing structure of the drill press, some of the group members were
tasked with determining what size pneumatic cylinder would be required to apply the necessary force
via the lever on the press. Another concern that had to be addressed was the alignment of the bottom
balls and cup with the drive shaft. These alignments need to be both extremely accurate and repeatable
to allow for multiple test comparisons. The first solution implemented was a thrust bearing located
below the cup which would allow it to move around on the plate and self align as the top ball was being
lowered.
At this time the controls group was largely focused on researching solutions for the problems of
temperature control at the test site, wear measurement options, RPM control and measuring or
calculating the coefficient of friction (as seen in standard D5138). The temperature control problem was
initially solved by applying heating elements to the sides of the cup in order to keep the lubricant heated
to the required temperature. A thermocouple with the necessary range and accuracy as dictated in the
standards was also found for measuring and tracking the temperature during the test. The main method
of tracking wear scars seen in the standards was the use of a wear measurement camera, a “microscope
capable of measuring the diameters of the scars produced on the three balls to an accuracy of 0.01mm
without removing the ball from the test cup.”[2] So for the second milestone a wear measurement camera
was added to the design expenses. After studying the set­up of the motor in the drill press, it was
determined that an encoder could easily be set­up on the drive shaft to feed rotation data to LabView.
The encoder itself will only count the rotations, however LabView was easily programmed to track time
as well and calculate the RPM of the motor. Force sensors were also added to the design in an effort to
avoid the complicated set­up of strain gauges without turning to a large torque transducer. The plan was
to attach tabs which would come off of the inner cup containing the balls and press against slots cut into
the outer cup that would be attached to the unmoving base plate. The forces read by the sensors at the
tabs could then be used to calculate the coefficient of friction. The controls group also found and priced
appropriate Data Acquisition Systems and power sources for all of the electrical components. All
changes leading up to the second design were then presented in the second milestone presentation. See
Appendix section for detailed data, solid models, calculations and budgets.
While the redesign had solved many problems, there were still changes that needed to be made
and more information to be gathered before the approval would be given to start ordering parts for
fabrication. The professors’ comments made it clear that the design had improved, but that the group
needed more technical data to support some of the general statements that were being made during the
presentation. Some of the major concerns after the second milestone were how the motor speed would
be controlled throughout the test, how the normal load would be calculated or measured, whether or not
the heat being supplied to the cup would be lost into the rest of the machine, the accuracy of the manual
regulator on the pneumatic system and how the air for the pneumatic piston would be cleaned. The wear
measurement camera was also vetoed for the project due to the high cost and the fact that the
microscopes currently in the tribology lab are capable of making the same measurements.
The most significant design change after the second milestone was a restructuring of the controls
system for the project in order to implement an axial transducer with data relays to the DAQ for direct

5. normal force measurement. This update to the design added fairly significantly to the overall budget, but
reduced the amount of equations and calibration needed in the controls system which improves accuracy
and reduces the chance for human error in the data gathering. The addition of the axial transducer also
required an adjustment of the self­alignment system. Since the dual cup design with a thrust bearing
underneath created an extra moment that could not easily be accounted for, a self­alignment plate below
the axial transducer was implemented instead. In an attempt to increase the accuracy and steadiness of
the applied force from the cylinder, an electronic pressure valve was added to the pneumatic system
instead of the manual regulator. The pressure valve could then be placed on a controls loop which
constantly fed data into the computer and adjusted the valve based off of the readings from the
transducer. A temperature controller was added in order to cut apply heating or cooling based off of the
temperature of the lubricant being read by the thermocouple. A Finite Element Analysis of the thermal
behavior of the design indicated that there was not a significant amount of heat loss into the plates from
the cup and that the temperature at the bottom of the cup would not exceed the operating temperature of
the transducer. When researching options for controlling the motor speed, it became clear that the
single­phase motor that came in the drill press would not suffice. For the final design, a three­phase
motor with a Variable Speed Drive would be necessary. In an effort to improve safety, an E­Stop system
was also put into place which would cut power to the drill press if either the lubricant temperature or
torque exceeded a set limit.
At the end of the semester the only parts that were still completely missing from the design were
the Three Phase Motor and VFD Controller. Both of these parts had been ordered, but did not arrive
before the last milestone. All of the fabrication was completed for the testing set­up in this design;
however if the customer would like to alter the design in order to perform free rolling tests as well, the
cup will need to be heat treated in order to keep it from wearing out as the balls rotate. All of the
physical controls components were in place at the end of the semester and most of the calibration for the
various components had been completed, but the DAQ was still not programmed to output the data and
the auto­stop loop was not fully functional. See Appendix at end for early research results, budgets, solid
models, engineering drawings and fabrication images.

6. APPENDIX A: Early Research and Project Planning
Figure I: Gantt Chart Schedule
Table I: Targets from ASTM Standards
ASTM D2266 ASTM D4172 ASTM D5183
Value Tolerance Value Tolerance Value Tolerance
Force
Applied
40 kgf 0.2 kgf 15 kgf 0.2 kgf 40 kgf Not Given
392 N 2 N 147 N 2 N 392 N Not Given
Lubricant
Temp
75 C 2 C 75 C 2 C 75 C 2 C
167 F 4 F 167 F 4 F 167 F 4 F
Speed 1200 rpm 60 rpm 1200 rpm 60 rpm 600 rpm Not Given
Duration 60 min 1 min 60 min 1 min 60 min Not Given
Oil Level very top of cup 3mm above top of balls 3mm above top of balls

7.
Table II: Structures Team Concept Comparison Table
Table III: Fabrication Team Concept Comparison Table

8. Table IV: Controls Team Concept Comparison Table
Table V: Concept Comparison Totals
Figure II: QFD Diagram
(Next Page)

9. APPENDIX B: Overall Design Solid Models
Figure I: Initial Concept Solid Model

10.
Figure II: Second Concept Solid Model Figure II: Second Concept Cup/Collet Set­Up
Figure IV: Final Design Solid Model

11. APPENDIX C: Collet Assembly Progression
Figure I: Initial Collet Design
Figure II: Initial Collet Design ­ Exploded View

12.
Figure III: Second Collet Design Model Figure IV: Second Collet Design ­ Exploded View
Figure V: Second Collet Design Parts Detail Drawing

13.
Figure VI: Final Collet Design Figure VII: Final Collet Design
Total Deformation FEA Results Equivalent Stress FEA Results
Figures VIII ­ X: Final Collet Design ­ Fabrication Images

14.
Figure XI: Final Collet Design ­ Stopper Detail Drawing

15.
Figure XII: Final Collet Design ­ Chuck Detail Drawing

16. APPENDIX D: Cup Design Progression
Figure I: Initial Cup Design Model
Figure II: Initial Cup Design Figure III: Initial Cup Design
Free Rolling Ball Insert Fixed Ball Insert

17.
Figure IV: Second Cup Design Model (Full Plate Assembly)
Figure V: Second Cup Design Mid­Plane View

18.
Figure VI: Second Cup Design ­ Cup Detail Drawing
Figure VII: Second Cup Design ­ Cap Detail Drawing

19.
Figure VIII: Second Cup Design Fabrication Image
Figure IX: Second Cup Design Fabrication Image (Disassembled)

20.
Figure X: Final Cup Design ­ Cap Detail Drawing

21.
Figure XI: Final Cup Design ­ Retainer Detail Drawing

22.
Figure XII: Final Cup Design ­ Cup Detail Drawing

23.
Figure XIII: Final Cup Design ­ Centering Plate Detail Drawing

24.
Figure XIV: Final Cup Design ­ Thermal FEA Results
Table I: Final Cup Design ­ Thermal FEA Mesh/Element Details

25.
Figure XV: Final Cup Design ­ Full Assembly Side View
Figure XVI: Final Cup Design ­ View Into Cup from Top

26. APPENDIX E: Pneumatic Cylinder Set­Up Information
Figure I: Pneumatic Cylinder Set­Up Model
Figure II: Pneumatic Cylinder Set­Up 2D Dimensions Drawing

27.
Table I: Pneumatic Cylinder Calculation Results for Various Scenarios
Table II: Pneumatic Cylinder Details

28.
Figure III: Pneumatic Cylinder Set­Up Image
Figure IV: Pneumatic Cylinder Air Filter and Regulator Set­Up Image

29. APPENDIX F: Controls Team Diagrams
Figure I: Instrumentation Block Diagram

30.
Figure II: Force/Torque Controls Diagram
Figure III: Temperature Controls Diagram
Figure IV: Power Supply Controls Diagram

31. APPENDIX G: Budgets
Table I: Second Design Fabrication Budget
Table II: Second Design Pneumatic Cylinder Set­Up Budget

32. Table III: Second Design Controls System Budget
Table IV: Second Design Total Budget

33. Tables V­X: Final Design Budget Tables (Broken Up by Component)