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

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.

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

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

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.

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

Table II: Structures Team Concept Comparison Table

Table III: Fabrication Team Concept Comparison Table

Table IV: Controls Team Concept Comparison Table

Table V: Concept Comparison Totals

Figure II: QFD Diagram (Next Page)

APPENDIX B: Overall Design Solid Models

Figure I: Initial Concept Solid Model

Figure II: Second Concept Solid Model Figure II: Second Concept Cup/Collet Set­Up

Figure IV: Final Design Solid Model

APPENDIX C: Collet Assembly Progression

Figure I: Initial Collet Design

Figure II: Initial Collet Design ­ Exploded View

Figure III: Second Collet Design Model Figure IV: Second Collet Design ­ Exploded View

Figure V: Second Collet Design Parts Detail Drawing

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

Figure XI: Final Collet Design ­ Stopper Detail Drawing

Figure XII: Final Collet Design ­ Chuck Detail Drawing

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

Figure IV: Second Cup Design Model (Full Plate Assembly)

Figure V: Second Cup Design Mid­Plane View

Figure VI: Second Cup Design ­ Cup Detail Drawing

Figure VII: Second Cup Design ­ Cap Detail Drawing

Figure VIII: Second Cup Design Fabrication Image

Figure IX: Second Cup Design Fabrication Image (Disassembled)

Figure X: Final Cup Design ­ Cap Detail Drawing

Figure XI: Final Cup Design ­ Retainer Detail Drawing

Figure XII: Final Cup Design ­ Cup Detail Drawing

Figure XIII: Final Cup Design ­ Centering Plate Detail Drawing

Figure XIV: Final Cup Design ­ Thermal FEA Results

Table I: Final Cup Design ­ Thermal FEA Mesh/Element Details

Figure XV: Final Cup Design ­ Full Assembly Side View

Figure XVI: Final Cup Design ­ View Into Cup from Top

APPENDIX E: Pneumatic Cylinder Set­Up Information

Figure I: Pneumatic Cylinder Set­Up Model

Figure II: Pneumatic Cylinder Set­Up 2D Dimensions Drawing

Table I: Pneumatic Cylinder Calculation Results for Various Scenarios

Table II: Pneumatic Cylinder Details

Figure III: Pneumatic Cylinder Set­Up Image

Figure IV: Pneumatic Cylinder Air Filter and Regulator Set­Up Image

APPENDIX F: Controls Team Diagrams

Figure I: Instrumentation Block Diagram

Figure II: Force/Torque Controls Diagram

Figure III: Temperature Controls Diagram

Figure IV: Power Supply Controls Diagram

APPENDIX G: Budgets

Table I: Second Design Fabrication Budget

Table II: Second Design Pneumatic Cylinder Set­Up Budget

Table III: Second Design Controls System Budget

Table IV: Second Design Total Budget

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