Tribological Investigation of the Polymer- Based ...

PEER - R E V I E W E D

Tribological Investigation of the Polymer-Based Lubrication System Using a Laboratory Reciprocating Bench TestSimon C. Tung (STLE fellow), General Motors R&D Center, Warren, Mich., Yong Huang, Clemson

University, Dept. of Mechanical Engineering and Dennis C. Karczynski, General Motors Quality

Network (STLE member), Detroit, Mich.

Published in STLE Tribology Transactions (Vol. 50, No. 4/October-December 2007, pp. 458-465). Presented at the World Tribology Con-gress in Washington, D.C., Sept. 12-16, 2005. Final manuscript approved June 27, 2007. Review led by Dong Zhu. Copyright© STLE

40 F E B R U A R Y 2 0 0 8 T R I B O L O G Y & L U B R I C A T I O N T E C H N O L O G Y

AbstractFlooded oil lubrication systems are commonly utilized for many manufacturing processes. However,there is an increasing concern with the flooded lubrication systems due to their biological and envi-ronmental impact as well as their maintenance and disposal cost. Polymer-based material, a sponge-like solid lubricant allowing the oil to weep out while in use, provides a promising alternative environ-mentally benign lubrication solution to satisfy the lubrication requirements for some manufacturingprocesses. In addition, it can minimize contamination of the coolant system, reduce waste processingand disposal costs, and alleviate the in-plant safety hazard from oil leakage on floor surfaces.

The goal of this study is to develop and improve the current lubrication system by applying a poly-mer-based near-dry lubrication system in order to replace the oil flooded transfer line lubrication sys-tems currently in production use. Using a laboratory reciprocating bench test based on the design ofexperiments, this study characterized the tribological performance of the polymer-based lubricationsystem under a variety of operating conditions, identified the main factor that impacts the tribologi-cal performance of the polymer-based lubrication system, and developed a model to predict the wearloss of the polymer-based lubricant plate. These results provide the guidelines for possible applicationsof the polymer-based lubrication system used in the manufacturing process.

IntroductionFlooded oil lubrication systems are commonly utilized for some manufacturingprocesses. A primary concern with the flooded lubrication system is the biologicaland environmental impact. In addition to possible fumes, smoke and odors, the fluidmay also cause severe reactions on the skin and to various parts of the operator’sbody. Maintenance due to oil degradation and ultimate disposal also cause addi-tional cost and reduce the manufacturing efficiency. Polymer-based material, a solidlubricant that is a mixture of polymers, oil,1 and selected additives, whose matrix isin a sponge-like form allowing the oil to weep out while in use, provides a promisingalternative environmentally benign lubrication solution to satisfy the requirements ofsome manufacturing processes. It minimizes contamination of the coolant system,reduces waste processing and disposal costs, and alleviates the in-plant safety haz-ard from oil leakage on floor surfaces.

This study aims to develop and improve a polymer-based near-dry lubrication sys-tem to replace the oil flooded transfer line lubrication systems currently in use. Inthis study, the feasibility and durability of the proposed polymer-based lubricationsystem was investigated using a reciprocating bench durability test based on anexperimental design. The experimental observations were further analyzed statisti-cally. A weight loss model of the polymer-based lubricant plate was also developed,and the related worn surface characteristics were determined.

Editor’s Note: Product in-novation is the result ofbringing to life new tech-nology, and it’s usuallytied to solving a problem,a need or a desire. If welook around the world to-day, we see that everyoneis becoming more safety-,environmentally- and cost-conscious. In this month’sEditor’s Choice paper, au-thors Simon Tung, YongHuang and Dennis Kar-czynski have constructedand evaluated an innova-tive polymer-based near-dry lubricant delivery sys-tem that could replace oil-flooded systems whilemeeting these concerns.Their research provides aguideline for possible ap-plications in manufactur-ing processes. Product in-novation isn’t easy. And inany business–it’s innova-tion that distinguishes aleader from a follower. Iencourage you to read thisarticle and be informed.

– Dr. Maureen HunterTLT editor

Experimental designExperimental setupTo investigate the tribological performance of the proposedpolymer-based lubrication system, a polymer-based leadedbronze plate (hardness HB 32.8) as shown in Figure 1 wasused in this study. The polymer-based lubricant plate was38.1 mm by 50.8 mm (1.5 inch x 2.0 inch), and it containedtwelve polymer-impregnated holes. Inside each hole was aproprietary mixture of polymers, oil and selected additives,and the oil oozed out from the hole during the serviceperiod. The non-uniform hole distribution was expected toreduce the number of holes while achieving the same lubri-cation performance. The 83.8 mm by 132.1 mm (3.3 inch x5.2 inch) coupon was made of P20 steel with a hardness ofHRc 55. The surface roughness (Ra) of the steel coupon thatslid against the polymer-based plate was 0.2250 µm.

To evaluate the tribological performance of the polymer-based system, a Cameron Plint high frequency and weartester was modified to serve as a test bed as shown in Fig-ure 2. This Cameron Plint tester provided: (1.) a reciprocat-ing contact motion, similar to what exists in a manufactur-ing process and/or transfer line; (2.) a range of load andsliding speed; (3.) accurate friction coefficient information;and (4.) contact potential information to represent thelubrication film thickness along the sliding interface. Aphotograph of the whole test setup is shown in Figure 3.

To keep the polymer-based plate in the same slidingdirection, two rails were manufactured to maintain thepolymer-based plate moving direction during testing. Thefriction force between the polymer-based plate and therails was assumed negligible compared to that between thepolymer-based plate and the steel plate under the appliednormal loads (100 N and 250 N).

Test conditionsTest conditions were determined based on the typical oper-ating conditions for a transfer line system provided by amanufacturing factory in Michigan. The operating tempera-ture was controlled at 30ºC using a heater block as shownin Figure 2. This temperature was the same as that in thefactory. For real transfer line systems, the interface temper-ature will be a little higher than 30ºC because of frictionalheating. This temperature rise along the sliding interface isa function of load and sliding speed and was considerednegligible under lubricated conditions.

The applied pressure in the factory ranges from 7.5 psi to25 psi. Simulation of this condition in this test required aload range of 100 N to 333 N for a 38.1 mm x 50.8 mm (1.5inch x 2.0 inch) plate. Since the highest load which can beprovided by the Cameron Plint machine used is 250 N(equal to 18.75 psi), both 250 N (the highest load) and 100

CONTINUED ON PAGE 42

T R I B O L O G Y & L U B R I C A T I O N T E C H N O L O G Y F E B R U A R Y 2 0 0 8 41

Figure 1. Photo of the polymer-based bronze plate.

Figure 2. Schematic of the reciprocating bench test.

Figure 3. Photo of the reciprocating bench test.

PEER - R E V I E W E D

N were chosen as the two load conditions to be simulated.The transfer speed in the factory ranges from about 150

inch/min to 350 inch/min. Since the amplitude of thestroke of the Cameron Plint machine used was set at15mm, the frequencies of 2.5 Hz and 5 Hz were chosenhere, which are equal to average sliding speeds of 75mm/s (177.2 inch/min) and 150 mm/s (354.3 inch/min),respectively.

Design of experimentsTo determine the effects of operating conditions on thepolymer-based plate weight loss, a two-factor, two-level,full-factorial design of experiments was performed. Asshown in Table 1, the two factors were the load and thesliding speed. Each factor had two levels. Each conditionwas run for 15 hours. Three replications of each factorlevel combination were conducted, resulting in a total of12 runs. The response variable was the polymer-basedplate weight loss. The polymer-based plate weight losswas measured by a Mettler Instrument B5C 1000 scale.To further investigate the tribological performance of thepolymer-based lubrication system as a function of timeunder severe conditions (high pressure and high slidingspeed, that is, 250 N and 5 Hz), additional tests for thedurations of 60, 150 and 250 hours as shown in Table 2were run for comparison. Due to resource constraints,these longer life tests had only two replications.All testing runs were performed in a random order. The runorder as well as weight loss results are shown in Table 3.

Experimental ResultsStudy of friction coefficient and contact potentialThe oil impregnated in the polymer-based plate oozes outduring the life span of the plate under normal operatingconditions. This improves the lubrication along the slidinginterfaces by helping reach a steady state condition. Thelubrication effect will last for a long time (several years fora transfer line system) under normal operating conditionsuntil the impregnated oil is exhausted.

During the initial break-in phase of the polymer-basedplate, a mainly dry condition exists along the interface. Asoil oozes out, it reduces the friction force and helps theforce reach a steady state value. However, friction coeffi-cient measurements from the Cameron Plint tester werenot repeatable even under the same operating conditionin this study. As a typical case, friction coefficient mayrange from 0.015 to 0.035 under the same operating con-dition (2.5 Hz and 250 N) as shown in Figure 4. This varia-tion was attributed to the surface finishing variation of thesamples, the poor alignment of the sliding pair, and thesensitivity of the measurement since the friction coeffi-cients were all quite low (0.01~0.035) and the accuracy at


CONTINUED FROM PAGE 41

Table 2. Investigation of performance overlonger durations

Table 3. Polymer-based bronze plate weight lossdata

Table 1. Two-factor, Two-level, full-factorial DOE

Figure 4. Friction coefficient under theoperating condition (2.5Hz and 250N).

these low levels was poor. Regardless of the differences inthe friction coefficient measurements, the measured fric-tion coefficients are much lower than that of a regularsteel-bronze pair, which is around 0.1-0.5.2

The electrical contact resistance method is also benefi-cial in studying metallic contact and friction betweenlubricated sliding surfaces. During testing, the contactresistance was measured using the Cameron Plint testerand observed as a contact voltage. A voltage of 50 mV wasapplied across the sliding contact using a potentialdivider. The contact voltage was observed on an oscillo-scope as a root-mean-square (R.M.S.) voltage signal.“Zero volts” implies that the entire voltage drop is zeroacross the contact, i.e., the contact resistance is effectively“zero” (or only a few ohms). “50 mV” implies that there is a50 mv voltage drop across the contact and a very large or“infinite” resistance. A voltage signal directly related to thecontact resistance is obtained by monitoring the voltageacross a resistor connected in parallel with the slidingcontact. In this study, a 50 mV contact voltage means thefull film thickness has been formed. The dimensionlesscontact voltage obtained by dividing the measured con-tact voltage by 50 mV was used to express the develop-ment of film thickness.

The value of contact potential is a function of metalcontact area and film thickness.3-4 The greater the contactarea, the lower the contact potential and the thicker thetribological film, the higher the contact potential. A typi-cal contact potential result from the reproducible meas-urements is shown in Figure 5. The contact potentialstarted out very low and then increased as a layer of lubri-cation (lubricity) was built up causing the electrical resist-ance and contact potential to increase. From Figure 5, itappears that a steady-state lubrication film between thesteel plate and the polymer-based plate was formed afterapproximately 2000 minutes of testing. Once the lubrica-tion film was established, the contact potential reachedan asymptotic value (50 mV). This steady-state lubricantfilm prevented the surface from wear or scuffing.

Polymer-based bronze plate weight lossCompared with a traditional lubrication system, one majoradvantage of the proposed polymer-based lubrication sys-tem is its long life span, which significantly reduces themaintenance required and possible downtime.5 It is impor-tant to predict the oil oozing rate in order to estimate thepolymer-based lubrication system life span. The total poly-mer-based bronze plate weight loss (before and after everyrun) was contributed by both the polymer-based lubricantand bronze plate. As seen from the following wear residueanalysis, carbon (C) and oxygen (O) accounted for approx-imately 80% of the total weight loss, so here it is consid-

ered reasonable to relatively evaluate the oil oozing ratebased on the total weight loss. Given the total weight ofthe oil impregnated, the life span can be approximatelyestimated based on the weight loss rate.

The technique of analysis of variance (ANOVA, bal-anced design)6 is conducted to investigate the statisticallysignificant effects from two independent variables (loadand sliding speed) and the interaction between the loadand the sliding speed. The response variable is the weightloss. The output of ANOVA is shown in Table 4. In additionto the degrees of freedom (DF), the mean square (MS),and the F-distribution value, the table shows the P-valueassociated with each factor and interaction. The P-valuecan be interpreted as the probability that the factor orinteraction is not significant. Therefore, a low P-value pro-vides an indication that a given parameter is significantand the confidence associated with that assessment.

It is shown from the F-distribution values and the P-val-ues in Table 4 that, if the one-sided confidence level is setas 95% (the commonly used alpha level), both the slidingspeed and the load have a significant effect on the weightloss, while there is no significant effect from the interac-tion of the load and the sliding speed. If the one-sidedconfidence level is set as 90%, all these factors have a sig-



Figure 5. A typical contact potential measurementvs. time.

Table 4. Analysis of variance for weight loss


nificant effect on the polymer-based plate weight loss.The boxplots of weight loss by load and weight loss by

sliding speed are also shown in Figures 6 and 7, respec-tively. The line drawn across each box indicates themedian of the data. The bottom and the top edges of thebox mark the first (25th percentile) and third (75th per-centile) quartiles, respectively. These boxplots suggestthat heavier load or higher sliding speed produces moreweight loss. This is expected because heavier loads orhigher sliding speeds lead to higher friction forces andmore severe frictional heating.

A full bilinear model described as Equation (1) is usedto model the weight loss rate of the polymer-based plateafter 15 hours of testing. The resulting 3-D surface plot isshown in Figure 8 to illustrate the trends as load and/orsliding speed increase.

Δw = 41.7778 - 0.0844x - 2.311y + 0.0471xy (1)

where, Δw (mg) is the weight loss of the polymer-basedplate after 15 hours of testing, x (N) is the load, and y (Hz)is the sliding speed (frequency). The constants in Equa-tion (1) are determined from a least squares fit regressionanalysis of the data in Table 3. In terms of applied pres-sure and sliding velocity, the model can be expressed asfollows:

Δw = 41.7778 - 1.1253p - 0.0326v + 0.0089pv (2)

where, Δw (mg) is the polymer-based plate weight loss ofthe polymer-based plate after 15 hours of testing, p (psi)is the applied pressure, and v (inch/min) is the slidingvelocity.

The weight loss vs. the duration of testing is shown inFigure 9. After the initial break-in period, the weight lossrate approaches a steady rate. The trend of the weight losscan be approximated by the following logarithm function,which is also shown in Figure 9.

Δw = 16.508lnt + 20.666 (3)

where, Δw (mg) is the polymer-based plate weight lossafter t hours of testing. The constants in Equation (3) aredetermined from a least squares fit regression analysis ofthe data in Figure 9.

SEM and EDS analysisTypical finished surfaces of the polymer-based plate andthe steel plate under two different conditions are shownin Figures 10 and 11. There were residues on the tops ofthe plates. The color of the residues on the surfaces ofthese finished plates may be green or brown. The brown

PEER - R E V I E W E DCONTINUED FROM PAGE 45

Figure 6. Boxplot of weight loss by load.

Figure 7. Boxplot of weight loss by sliding speed.

Figure 8. 3-D surface plot (the unit of weight lossis mg).

Figure 9. Polymer-based plate weight loss vs. testduration


residue occurred only with the No. 11 run.The chemical composition of the residues was ana-

lyzed by EDS (Energy Dispersive Spectroscopy). Since anEDS analysis needs a sizable sample, the chemical com-position of the green residue from the No. 9 run was ana-lyzed and is shown in Figure 12 and Table 5. It is believedthat Pb and Cu are from the bronze plate, C and part of theO from the oil used in the polymer-based structure, andthe remainder of the O from the environment due to theoxidation of copper.

The brown residue which occurred only during the No.11 run is believed to be due to chemical degradation ofthe polymer-based system. The detailed chemical compo-sition of the brown residue is shown in Figure 13 andTable 6. It is believed that the green residue came fromthe oxidation of copper because the atom percentage ofCu and O of the green residue was much higher than thatof the brown one. Since C, which is the main componentof the impregnated lubricant, accounted for a larger per-centage of the brown residue than that of the greenresidue, it is suggested that the oil structure deterioratedfaster for sample No. 11 with the brown residue. Actually,based on the weight loss data, it can be seen that the No.11 run, which led to the brown residue, lost about 25%more weight than the other two replications under thesame conditions (See Table 3)

In addition, a scanning electron microscope (SEM) wasused to analyze the surface integrity of the finished sur-faces of the steel plate. The worn steel plate was cleanedwith acetone before the measurement. Figures 14 and 15


Figure 10. Photo of the finished surfaces of No. 13test.

Figure 11. Photo of the finished surfaces of No.

11 test (the differences in the color may not be

easily distinguished when comparing the

printout of Figures 10 and 11).

Figure 12. Chemical composition of green residueon the steel plate (No. 9 run).

Table 5. Chemical composition of green residue on thesteel plate (No. 9 run)

Table 6. Chemical composition of brown residue on thesteel plate (No. 11 run)

show (see page xx) deposited particles over the steel platefrom the No. 11 test. Although the results from only onetest are presented here, similar results have beenobserved with the other tests. These deposited materialshad a chemical composition similar to that observed inthe wear residue discussed above. This deposited mate-rial phenomenon may be important for transfer line appli-cations since the contamination due to the depositionfrom the polymer-based system may be critical to theworkpiece surface integrity. No wear mark has beenobserved on the surface of the steel plate, which isexpected since the steel plate was under the lubricatedcondition during testing.

Table 7 shows the changes of surface roughness of thesteel plate measured along the sliding direction with aRank Taylor Hobson Talysurf Analyzer (using a 2CR PhaseCorrected filter) for three different operating conditions.For all cases, the surface of the steel plate after testing issmoother than that before testing. The reason for this isthat the valleys of the original machining marks may havebeen filled by the polymer-based residue instead of beingdeepened by abrasive particle(s). Similar results werefound in other runs.

Summary and conclusionsThe tribological performance of the polymer-based lubri-cation system has been investigated, and a weight lossmodel of the polymer-based plate has been developed.Based on experimental observations and surface analysis,the research results can be summarized as follows:

1 Friction coefficients of the tested system fell in therange of 0.01~0.035 under the typical transfer lineoperating conditions. The low friction coefficientwill help reduce wear in manufacturing processes.

2. After the lubrication film was built-up, the contactpotential approached an asymptotic value (50 mV).This steady-state lubricant film prevented the sur-face from wear or scuffing. It was also observed thatthe weight loss approached an asymptotic valueafter about 250 hours of sliding. Therefore, it is con-cluded the wear rate was gradually reduced afterlong sliding durations.

3. Based on the output of an ANOVA table, if the one-sided confidence level is set as 95%, both slidingspeed and load have significant effects on theweight loss while there is no significant effect at thislevel from the interaction of load and sliding speed.


PEER - R E V I E W E DCONTINUED FROM PAGE 47

Figure 14. SEM picture of the finished surfaces ofthe No. 11 test (50X).

Figure 15. SEM picture of the finished surface ofthe No. 11 test (500X)

Figure 13. Chemical composition of brown residueon the steel plate (No. 11 run).


If the one-sided confidencelevel is set as 90%, load, slidingspeed and their interaction allhave significant effects on thepolymer-based plate weightloss.

4. A weight loss model for thepolymer-based plate has beenproposed based on a bilinearapproach.

5. Both green and brown residueswere observed on the surfaces of both the steelplate and the polymer-based plate after testing. Thegreen residue was mainly composed of C, O, and Cu,while the brown residue contained mainly C. Theorganic brown residue is considered to be the resultof abnormal degradation of the polymer-basedstructure because it was observed only in one runout of 16.

6. A small amount of deposited residue was observedon the worn surface of the finished steel plate. Thisdeposition phenomenon may be important for

transfer line applications since possible contamina-tion due to the polymer-based system is critical tothe workpiece surface integrity.

7. A decrease in surface roughness of the steel platewas observed at the end of testing.

AcknowledgmentsThe authors wish to thank Dr. Michael L. McMillan, Dr.James A. Spearot, Angelo Quintana, Brian Taylor, BobKubic and Curtis Wong for their support and help in per-forming the experimental studies.

1. Kroschwitz, J., Ed. (2001), Polymer-Based LubricationSystem, Encyclopedia Series, John Wiley and Sons,New York.

2. Ashby, M.F. and Jones, D.R.H. (1980), EngineeringMaterials: An Introduction to Their Properties and Applica-tions, Pergamon, New York.

3. Polishuk, A.T. (1983), Automotive Chassis Lubrication,CRC Handbook of Lubrication— Theory and Prac-tice of Tribology, Volume 1, Booser, R.E., Ed., CRCPress, Boca Raton, Fla.

4. Blau, P., Ed. (2003), ASM Handbook, Friction, Lubrica-tion, and Wear, Vol. 18, ASM Handbook Series, 3rdEd.

5. ASTM, (2002), ASTM 2003 Annual Books of ASTMStandards, Petroleum Products, Lubricants, andFossil Fuels, Section 5.

6. Neter, J., Kutner, M. H., Nachtsheim, C. J. and-Wasserman,W. (1996), Applied Linear Statistical Mod-els, 4th Ed., Irwin, Chicago, Ill.

References

Table 7. Surface roughness of the steel plate before and after testing

Tribological Investigation of the Polymer- Based ...

Documents

Transcript of Tribological Investigation of the Polymer- Based ...