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Power Loss in FZG gears lubricated with industrial gear oils: Biodegradable Ester vs. Mineral oil
Life Cycle Tribology D. Dowson et al. (Editors) © 2005 Elsevier B.V. All rights reserved
421
Power Loss in FZG gears lubricated with industrial gear oils: Biodegradable Ester vs. Mineral oil R. Martins a, J. Seabra \ Ch. Seyfertc, R. Luther0, A. Igartuad and A. Britoe a
INEGI, Institute de Engenharia Mecanica e Gestao Industrial, Porto, Portugal.
b
DEMEGI, Faculdade de Engenharia da Universidade do Porto, Portugal.
c
FUCHS Petrolub AG, Mannheim, Germany.
d
Fundacion TEKNIKER, Eibar (Gipuzkoa), Spain.
e
A. Brito, Industria Portuguesa de Engrenagens L.da, Porto, Portugal.
ABSTRACT Two industrial gear oils, a reference paraffinic mineral oil with a special additive package for extra protection against micropitting and a biodegradable non-toxic ester, are compared in terms of their power dissipation in gear applications [1,2]. The physical properties, wear properties and chemical contents of the two lubricants are characterized. The viscosity-temperature behaviors are compared to describe the feasible operation temperature range. Standard tests with the Four-Ball machine and the FZG test rig characterize the wear protection properties. Biodegradability and toxicity tests are performed in order to assess the biodegradability and toxicity of the two lubricants. Friction and wear tests have been performed with a configuration that combines rolling/sliding in a line contact simulating the working conditions on gears. The results for the ester oil presented a lower friction coefficient and operating temperature throughout tthe test in relation to mineral oil. Power loss gear tests are performed on the FZG machine using type C gears, for wide ranges of the applied torque and input speed, in order to compare the energetic performance of the two industrial gear oils [ 3 ]. The results of the power loss gear tests show that the operating temperature of the ester oil is always smaller than that of the mineral oil. Lubricant samples are collected during and at the end of the gear tests [ 4 ]. The lubricant samples are analyzed by Direct Reading Ferrography (DR3) in order to evaluate the wear particles concentration (CPUC) and the index of wear particles severity (ISUC). Both parameters indicate that the gear lubricated with the mineral oil suffered more flank tooth wear than the one lubricated with the biodegradable ester. The influence of each lubricant on the friction coefficient between the gear teeth is discussed taking into consideration the operating torque and speed and the stabilized temperature.
1. INTRODUCTION ..... „ , . . The thermal equilibrium of a gearbox is attained when the power losses are equal to the evacuated heat.
The main mechanisms of power dissipation inside a gearbox are the churning losses and the frictional losses [ 1, 2, 6, 7 1. These are commonly ^ no.|oad d ^ ca,,ed dent losses and dep endent losses, respectively,
422 All The churning losses are related with moving parts immersed in the lubricant and depend on the lubricant properties, speed and parts geometry. The main sources of churning losses are the gears, the bearings and the seals. The frictional losses are related with contacting bodies in relative motion. The main sources of frictional losses are the gears and the rolling bearings. These losses are mainly dependent of the applied load, rolling and sliding speeds, lubricant properties and friction coefficients. There are three main heat evacuation modes: conduction, radiation and convection. These forms of heat evacuation depend mainly on the case temperature, the environment temperature, the case geometry and the coefficients that govern each of these heat exchange forms, like gearbox surface emissivity, etc. The thermal equilibrium of a gearbox is reached when the operating temperature stabilizes. This temperature is dependent of the gearbox characteristics and of the lubricant [ 1, 2, 7 ]. The lubricant has capital influence in all kind of power losses, both friction and churning losses. The higher the power losses the higher will be the equilibrium temperature of the gearbox. Lower stabilization temperatures means higher efficiency, lower friction coefficient, smaller oil oxidation and longer oil life [ 8 ]. The main objective of this study is to compare the energetic and wear performance of two industrial gear oils: a reference mineral oil and a non-toxic biodegradable ester. For that purpose screening tests simulating gear operating conditions and gear tests performed on the FZG machine using type C gears were done. On the screening tests the friction coefficient and the temperatures were measured, on the FZG gear tests the final stabilized oil operating temperature is measured for different values of the input torque and input speed. The protection against wear provided by the two lubricants is also compared. Lubricant samples, collected during and at the end of the FZG gear tests, are analyzed by Direct Reading Ferrography in order to evaluate the index of wear particles (CPUC and ISUC) and compare the wear behaviour of each lubricant. 2. LUBRICANTS Environmental awareness leads to a growing interest in biodegradable non-toxic lubricants.
However, the key aspect for any industrial application is technical performance and technical advantages proved in dedicated tests. Environmental compatibility is usually viewed in respect to biodegradability and toxicity. While the first issue is reached by using a suitable biodegradable base fluid, low toxicity requires an additivation that is environmentally friendly [ 10 ], too. However, lubricant performance (friction, wear, lifetime, load bearing, efficiency etc.) has a major impact on its overall environmental compatibility. Premature wear, high energy needs are as well harmful to the environment. Two industrial gear lubricants are tested and compared: a reference ISO VG 150 mineral oil, containing an additive package to improve micropitting resistance, and an ISO VG 100 biodegradable ester lubricant with a low toxicity additivation. Both oils are specified as CLP gear oils according to DIN 51517. The main properties of the two lubricants are shown in Table 1. 2.1. Chemical properties The reference gear oil is based on a paraffinic mineral oil with significant residual sulphur content. It contains an ashless antiwear additive package based on phosphorous and sulphur chemistry and metal-organic corrosion preventives. In contrast, the biodegradable product uses a fully saturated ester based on harvestable materials. The absence of unsaturated bonds in this base fluid leads to excellent thermal and oxidative stability. To combine the desired low toxicity with superior gear performance, environmentally compatible, highly efficient additives have been selected. Metal-organic compounds have been completely avoided. 2.2. Physical properties The physical properties of both industrial gear oils are also shown in Table 1. There is a significant difference in rheology between both oils: The two oils have almost identical viscosities at 100°C but the ester gear oil has much lower viscosity at lower temperatures. This is caused by the higher viscosity index of the ester product. This beneficial property leads to a wider range of operational temperature compared to the mineral oil. This can also be seen from the pour point that is 20°C lower with the ester than with the mineral oil. Thermal expansion and density of the ester gear oil are higher than that of the mineral oil.
423 2.3. Standard mechanical tests The VKA weld load is an extreme pressure test to determine the additives response at welding conditions. Both lubricants withstand mean pressure of approximately 5500 N/mm2 (for a four-ball load of 2200 N) without seizure. The wear scar after one hour at 300 N (2900 N/mm2) is similar for both oils. Considering the extreme pressure performance judged by the Brugger crossed cylinder test, there is a considerable higher value for the mineral oil than for the ester. Both values are higher than the value considered sufficient for hydraulic oils (30 N/mm2) but only the mineral oil exceeds the value usually stated for transmission oils (50 N/mm2). In the FZG scuffing test both lubricants pass FZG load stage 12 (contact pressure of 1.8 GPa) without any seizure. Summarizing these mechanical tests (see Table 1), both lubricants have almost the same performance and can both be considered suitable for a wide range of gear applications. 2.4. Biodegradability and toxicity properties Standardised, internationally recognised test methods are available for determining the biodegradability and environmental toxicity of lubricants and their components. The "ultimate" biodegradability of lubricants is best assessed using a "ready" biodegradability test as Table 1 - Lubricants properties. Parameter [unit]
Desig.
Base oil Physical properties Density @15°C [g/cm3] Kinematic Viscosity @ 40 °C [cSt] Kinematic Viscosity @ 100 °C [cSt] Viscosity Index [/] Pour point [°C] Thermal expansion [ 5 ] [g/cm3 °C] Wear properties VKA weld load [N] VKA wear scar (1 h/300N) [mm] Brugger crossed cylinder test [N/mm2] FZG rating [/] Biodegrability and toxicity properties Ready biodegradability [%] Aquatic toxicity with Daphnia [ppm] Aquatic toxicity with Alga [ppm]
published by the OECD and adopted by European Union. 2.4.1
OECD 301B, Evolution test
A measured volume of inoculated mineral medium, containing a known concentration of test substance (10-20 mg DOC or TOC/1) as the nominal sole source of organic carbon, is aerated by the passage of carbon dioxide free air at a controlled rate in the dark or diffuse light. The CO2 is trapped in barium or sodium hydroxide and is measured by titration of the residual hydroxide or as inorganic carbon. The amount of carbon dioxide produced from the test substance (corrected for that derived from the blank inoculum) is expressed as a percentage of ThCO2. The degree of biodegradation may also be calculated from supplemental DOC analysis at the beginning and end of incubation. 2.4.2
OECD 201 "Alga growth inhibition test"
A 72h algal growth inhibition test is performed in long cell test vials, with "Selenastrum capricornutum". Optical Density (at 670 nm) is used as the parameter for algal growth/inhibition. The tests are performed in disposable cells of 10 cm path-length which allow direct and rapid scoring of the OD in the "long cell" test vials, using any conventional colorimeter equipped with a holder for 10 cm cells. Method DIN 51451
Pl5 V40
v1Oo VI
DIN 51757 DIN 51562 DIN 51562 DIN ISO 2909 DIN ISO 3016
0.897 146 14.0 92 -21 7.16x10"
0.925 99.4 14.6 152 -42 8.05x10"'
DIN 51350-2 DIN 51350-3 DIN 51347-2 DIN 513540
2200 0.32 68 >12
2200 0.35 37 >12
OECD, 301 B OECD, 202 OECD, 201
<60 -
>60 >100 >100
av
K-FZG
EL50
EL50
Lubricating Oils paraffin ic saturated ester mineral oil
424
2.4.3
OECD 202, "Daphnia Magna" The 24h EC50 Acute Immobilisation
In the acute immobilisations test a range of concentrations of the substance investigated exerts different degree of toxic effects on the swimming capability of Daphnia under otherwise identical testing conditions. Certain concentrations result in certain percentages of Daphnia being no longer capable of swimming at 24 hours. The Daphnia Magna must be no more than 24 hours old at the beginning of the test, laboratory bred and apparently healthy. The percentage immobility at 24 hours is determined for each dilution. Normal statistical procedures are then used to calculate the EC50 or EL50 for the appropriate exposure period. 2.4.4
71 Pinion specimen
Wheel specimen
Biodegradability and toxicity results
The mineral oil didn't match the minimum requirements of 60% biodegradability in 28 days, as show in Table 1. Thus, no toxicity tests were performed for this lubricant. The ester based oil exceeded the minimum requirements of 60% biodegradability in 28 days and pass both toxicity tests, OECD 201 "Alga growth inhibition test" and OECD 202 "Daphnia Magna acute immobilization" as show in Table 1. 3. GEAR SIMULATION TESTS
Figure 1 - Gear simulation test configuration. 3.1. Material The material used in these tests is the 34CrNiMo6+NAP (High Pressure Nitriding). The material hardness along depth is represented on Figure 2. The material presents a structure formed by tempered martensite of sorbo-bainitic aspect, as represented in Figure 3. In the surface, a diffusion layer with some thin nitride film can be detected. It has not been detected any combination layer.
In order to simulate the wear of spur gears, a friction and wear test configuration developed in the framework of the previous European BRITE Project TESS has been used (see Figure 1). The sliding/rolling percentage selected is similar to the real gear as well as the operating conditions. With these tests it's intended to predict the oil behaviour in gear applications. The operating conditions are: • Load • Rotating speed • Linear speed • Specific film thickness Sliding/rolling • Initial temperature • Test duration
72.6 kg, 1968 rpm 2.7 m/s 0.16 to 0.2 35/40% 22 °C 480 minutes
300 100
200
300
400 Depth
500
Figure 2 - Hardness measurement along depth (UNE 7363-78). The Nitrided coating thickness is 0.3mm.
425
• COF H Temperature increase 90
u
|
0.03
80
TL
S 0.04
-—
70
59 60
i I
5 0.02 0.01
Mineral
Figure 3 -High pressure nitride steel metallographic structure.
ra
0.028
50
a>
ture incr
0.05 c
£
2L E
40
Ester
Figure 4 - Mean coefficient of friction and temperature increase for both tested lubricants.
3.2. Test results In this gear test simulation, the pitting behaviour of the lubricant and material are analysed. The final root mean square roughness was 1.33um for the specimens lubricated with Ester oil and 1.66um for the specimens lubricated with the mineral oil. The average coefficient of friction during the test and the total increase of lubricant temperature are represented in Figure 4. The friction coefficient measured for the mineral lubricant is 54% larger than that measured for the Ester. The same happens with temperature increase, being the final temperature of mineral lubricant higher 13°C. The mass loss of specimens is presented in Figure 5. The specimen lubricated with the ester oil show a greater mass loss (19% higher) than those lubricated with the mineral oil Both rollers lubricated with mineral or ester oil developed pitting during tests, as show in Figure 6. 4. GEAR POWER LOSS TESTS
I Wear of the disc DWear of the rollers D Total Wear 10.0
10
-
8.4
P DO
B
1" 6 la
?
4 2 0
5.2 4.1
-•
4.3
..
•
^m,—,
Mineral
—
Ester
Figure 5 - Mass loss of the specimens for both lubricants.
Mineral
Ester 34CrNiMo6+NAF
4.1. FZG gear test rig and type C gears Figure 7 shows a schematic view of the FZG back-to-back spur gear test rig. FZG type C gears are used in the tests. They are made of 20MnCr5 steel, carburised, quenched and oil annealed. Both gears are run-in before performing the power loss tests. The characteristics of the gears are shown in Table 2. The gears are dip lubricated with an oil volume of 1.5 1.
Figure 6 - Images of rollers after the tests and both present pitting.
426
Figure 7 - Schematic view of FZG gear test rig. 4.2. Operating conditions Each lubricant is submitted to a set of gear tests performed in a wide range of speed and torque conditions, as well as in input power, as shown in Table 3. The first 3 tests are performed at very low torque {no-load tests) corresponding to FZG stage 1 and three different speeds, in order to evaluate the influence of the oil on the churning losses. The no-load tests lasted for two hours at each operating speed. The initial oil bath temperature is approximately room temperature (s20°C). The other 9 tests {load tests) are performed at three different speeds {1000 rpm, 2000 rpm and 3000 rpm) and three increasing torque levels, corresponding to FZG load stages 5, 7 and 9 and to Hertzian contact pressures of 0.92 GPa, 1.29 GPa, 1.65 GPa, respectively. Each load test began at approximately 40°C and the oil sump temperature is free. Each load test lasted four hours during which the input power remains constant. The four hours test time is defined so that the power dissipated and the evacuated heat are in equilibrium and the oil sump reaches a stabilized temperature. A maximum oil sump temperature of 180°C is imposed for protection of the test rig. The monitored variables during the tests are: oil sump temperature, test gearbox temperature, ambient temperature, speed and torque. During the load tests and after each testing speed {lOOOrpm, 2000rpm and 3000rpm), lubricant samples are collected for direct reading ferrometry, viscosity and density measurements.
Table 2 ~~ Characteristics of FZG type C gears. Parameter /Units/ Pinion Wheel Number of teeths [/] 16 24 Module [mm] 4.5 [mm] Center distance 91.5 Addendum diameter [mm] 82.45 118.35 Pressure angle 20 [°] Addendum modification [/] +0.182 +0.171 [mm] 14 Face width 0.2 Ra (A.c=0.25 mm) [um] Hardness [HRC] 58 a 62 Young Module [Pa] 206x109 Poisson Coefficient [/] 0.3 Specific heat [Nm/(kg°K)] 450 Thermal conductivity [N/(s°K)] 50 [Kg/m3] Specific weight 7850 Table 3 - Operating conditions used in the power loss tests. N"of Speed Torque Input Test type Power cycles [rpm] |Nm| |k\V| 1000 0.52 No load 4.95 0 .72 2000 1.04 tests 3000 1.56 141.15 14.78 1000 0 .72 275.10 28.81 453.00 47.44 141.15 29.56 Load 2000 1.44 275.10 57.62 tests 453.00 94.88 141.15 44.34 3000 2 .16 275.10 86.43 453.00 142.31 5. RESULTS AND DISCUSSION 5.1. Stabilisation temperature Figure 8 shows the evolution of the oil sump temperature during the tests performed at 3000rpm with mineral and ester oils. At low torque (5 Nm) the oil temperature evolution and stabilization is very similar for both oils. In the other tests, at higher input torques, the temperature rise is always greater for the mineral oil.
427 The stabilization temperatures of the mineral oil are higher than those of the ester oil, and the difference between them increases with increasing torque. The largest difference in stabilization temperature occurred for the highest torque (453 Nm), when the mineral oil reached the limiting temperature of 180°C after 112 minutes, while the
ester oil attained 156°C in the same period and reached a stabilized temperature of 170°C after 240 minutes. In Figure 9 the stabilised temperature measured for each test is plotted against the input speed and torque for both oils.
3000rpm
Mineral, 453N.m - Ester, 453N.m Mineral, 275N.m Ester, 275N.m Mineral, 141N.m Ester, 141N.m Mineral, 5N.m Ester, 5N.m
100
150
200
250
Time [min]
Figure 8 - Oil sump temperature for mineral and ester oils at 3000 rpm for different input torque values. Maximum Oil Temperature
*t=112min
Figure 9 - Stabilised oil sump temperature vs. input speed and torque for ester and mineral industrial gear oils.
428 The difference in stabilization temperature between the two oils goes from 2°C to 15°C, depending on the operating conditions. For the highest input power (142 kW) the difference is above 30°C and the mineral oil reached the limiting temperature. In the case of the no-load tests, performed at very low torque (5 N.m), such difference depends mainly on the viscosity and specific weight of the lubricants. The tests performed at 1000 rpm and 2000 rpm generate low stabilization temperatures at which the viscosity of the mineral oil is significantly higher than that of the ester oil, making its stabilised temperature higher. For the test performed at 3000 rpm, the stabilization temperature reaches approximately 75°C. At this temperature the viscosities of both oils are similar, but the ester oil has a higher specific weight leading to a higher stabilization temperature then the mineral oil. These results indicate that the churning losses are similar for both oils at typical gearbox operating temperatures (between 70 °C and 80 °C) and for the same operating speed. For all the other tests at higher torques the stabilized temperature of the mineral oil is always higher then that of the ester oil, suggesting that the friction power loss inside the contact between gear teeth is considerably smaller for the ester oil. These differences in stabilization temperature and in friction power losses between the two oils, at identical operating conditions, are due to the smaller friction coefficient between the gear teeth provided by the ester oil. The increasing differences in stabilisation temperatures with increasing torques, between the mineral and ester oils, suggests that the difference in friction coefficient between the oils also increases with increasing torques.
Dl + Ds as
Dl' - Ds' ISUC =
CPUC = d
where d
d'
stands for the oil sample dilution. Figure 10 and Figure 11 show the evolution of CPUC and ISUC indexes during the power loss tests for both lubricants. CPUC
0
1
2
3
4
[x106] Cictes Figure 10 - Evolution of CPUC index during the power loss tests for both industrial gear oils.
5.2. Oil analysis Three lubricant samples were obtained of each oil after the load tests at 1000 rpm, 2000 rpm and 3000 rpm, respectively. They were analysed by Direct Reading Ferrometry in order to measure the the ferrometric parameters Dl (large wear particles index) and Ds (small wear particles index). The values of Dl and Ds are used to evaluate the concentration of wear particles index - CPUC and the severity of wear particles index - ISUC, defined
0
1
2
3
4
[x106] Cicles Figure 11 - Evolution of ISUC index during the power loss tests for both industrial gear oils.
429 The CPUC grows when the sum of the small and the large particles increase, while the ISUC index grows when the number of particles with a size greater then 5um are present in major number then the smaller then 5 urn. The analysis of both figures show clearly that the min eral oil presents significantly higher CPUC and ISUC values than the ester oil, meaning that the wear particles released with the mineral oil have a larger size and are in major number. There's a huge difference in the CPUC and ISUC values between the mineral and the ester oils, mainly after the tests performed at 2000 rpm and 3000 rpm. Elemental analysis of both oils after the end of the test was performed using RFA. The iron content of both oils was rather low. However, the mineral oil showed 2 to 3 times higher iron contents than the ester product. This confirms the higher wear rate of the mineral oil in this test. An important reason for the higher wear rate of the mineral oil is that its higher friction leads to oil temperatures well above those indicated for mineral oils. The stabilized temperatures reached 131 °C at 2000 rpm and supersedes 180 °C at 3000 rpm for the highest torque applied (453 N.m), as indicated in Figure 4, which is considerably above the maximum temperature this oil can stand. The kinematic viscosity of both lubricants was measured before and after the power loss tests and the results are presented in Table 4.
V100
14.0
14.1
+0.7
VI
92
94
+3
Fe
0
18 ppm
n.d.
V40
99.4
100.2
+0.8
ter
era
Table 4 - Viscosity and viscosity index values, measured before and after power loss tests, for the mineral and the ester industrial gear oils. Iron content of the used oils has been determined by RFA [9]. variation used oil fresh oil % 145.8 +0.9 144.5 V40
V100
14.6
14.6
0
W
VI
150
150
0
Fe
0
5 ppm
n.d.
No significant changes could be observed. This corresponds to the evolution of the neutralisation number as an indicator for oxidation. It also showed no significant increase. This indicates that both oils show no sign for oxidative degradation. 6. CONCLUSIONS The results obtained allow the following conclusions: 1. The performance of biodegradable ester in standard mechanical tests is analogous to that exhibited by the mineral oil. Both present excellent results. 2. The gear simulation test showed that the ester lubricant exhibit a smaller friction coefficient and a smaller operating temperature. 3. The mass loss in the gear simulation tests is larger for ester oil. 4. The no-load FZG gear tests performed with type C gears indicate that the churning losses of the mineral and ester industrial gear oils are identical, generating similar stabilized operating temperatures. 5. The loaded FZG gear tests indicate that the friction losses of the mineral oil are significantly higher than those obtained with the ester oil, generating much higher stabilized operating temperatures. This suggests that the friction coefficient between the gear teeth is smaller with the ester oil, in accordance with gear simulation test. 6. The differences in stabilised temperatures, between the mineral and ester oils, increase with increasing torques. That difference becomes very important for the most severe operating conditions. 7. The Direct Reading Ferrometric analysis of the oil samples collected during the load gear tests show that the wear particles concentration and the wear severity is much higher for the mineral oil than for the ester oil. 8. The mineral oil cannot be operated at temperatures above 100 °C during large periods. At high load and high speed (high input power) the stabilized operating temperature of the mineral is very high (>100 °C) and a large amount of wear particles is produced.
430 9. For the same severe operating conditions the ester oil produces a much smaller amount of wear particles, supporting the high operating temperatures without loosing its anti-wear properties.
[ 3]
B.-R. Hohn, K. Michaelis and R. Dobereiner, "Load Carrying Capacity Properties of Fast Biodegradable Gear Lubricants", Journal of the STLE - Lubrication Engineering, November 1999, 15,16,33-38.
[ 4]
T. M. Hunt, "Handbook of Wear Debris Analysis and Particle Detection in Liquids", Elsevier Science, UK, 1993.
[ 5]
J. Denis, J. Briant, J.-C. Hipeaux, "PhysicoChimie des Lubrifiants - Analyses et Essais", Editions Technip, 1997.
[6]
P. Eschmann, "Ball and roller bearings - Theory, design and applications", Wiley, 1985.
[ 7]
H. Winter, K. Michaelis, "Investigation on the Thermal Balance of Gear Drives", ASME, 1979.
[8]
B.-R. Hohn, K. Michaelis and A. Doleschel, "Frictional behavior of synthetic gear lubricants", Tribology research: From model experiment to Industrial Problem, Elsevier, 2001.
[9]
DIN 51396 part 2 Testing of lubricants Determination of wear elements - Part 2: Analysis by wavelength dispersive X-ray spectrometry
ACKNOWLEDGEMENT The authors express their gratitude to the European Union for the financial support given to this work through the GROWTH Project N°. GRD22001-50119, Contract N° G3RD-CT-2002-00796EREBIO, "EREBIO - Emission reduction from engines and transmissions substituting harmful additives in biolubricants by triboreactive materials".
REFERENCES [ 1]
[ 2]
B.-R. Hohn, K. Michaelis and T. Vollmer, "Thermal Balance of Gear Drives: Balance Between Power Loss and Heat Dissipation", AGMA Technical Paper, October 1996, pp 12, ISBN: 1-55589-675-8. Chr. Changenet and M. Pasquier, "Power Losses and Heat Exchange in Reduction Gears: Numerical and Experimental results", VD1Berichte, NR. 1665, 2002, 603-613.
[ 10 ] M. Week, O. Hurasky-Schonwerth and Ch. Bugiel, "Service Behaviour of PVD-Coated Gearing Lubricated with Biodegradable Synthetic Ester Oils", VDI-BerichteNr.1665, 2002.
421
Power Loss in FZG gears lubricated with industrial gear oils: Biodegradable Ester vs. Mineral oil R. Martins a, J. Seabra \ Ch. Seyfertc, R. Luther0, A. Igartuad and A. Britoe a
INEGI, Institute de Engenharia Mecanica e Gestao Industrial, Porto, Portugal.
b
DEMEGI, Faculdade de Engenharia da Universidade do Porto, Portugal.
c
FUCHS Petrolub AG, Mannheim, Germany.
d
Fundacion TEKNIKER, Eibar (Gipuzkoa), Spain.
e
A. Brito, Industria Portuguesa de Engrenagens L.da, Porto, Portugal.
ABSTRACT Two industrial gear oils, a reference paraffinic mineral oil with a special additive package for extra protection against micropitting and a biodegradable non-toxic ester, are compared in terms of their power dissipation in gear applications [1,2]. The physical properties, wear properties and chemical contents of the two lubricants are characterized. The viscosity-temperature behaviors are compared to describe the feasible operation temperature range. Standard tests with the Four-Ball machine and the FZG test rig characterize the wear protection properties. Biodegradability and toxicity tests are performed in order to assess the biodegradability and toxicity of the two lubricants. Friction and wear tests have been performed with a configuration that combines rolling/sliding in a line contact simulating the working conditions on gears. The results for the ester oil presented a lower friction coefficient and operating temperature throughout tthe test in relation to mineral oil. Power loss gear tests are performed on the FZG machine using type C gears, for wide ranges of the applied torque and input speed, in order to compare the energetic performance of the two industrial gear oils [ 3 ]. The results of the power loss gear tests show that the operating temperature of the ester oil is always smaller than that of the mineral oil. Lubricant samples are collected during and at the end of the gear tests [ 4 ]. The lubricant samples are analyzed by Direct Reading Ferrography (DR3) in order to evaluate the wear particles concentration (CPUC) and the index of wear particles severity (ISUC). Both parameters indicate that the gear lubricated with the mineral oil suffered more flank tooth wear than the one lubricated with the biodegradable ester. The influence of each lubricant on the friction coefficient between the gear teeth is discussed taking into consideration the operating torque and speed and the stabilized temperature.
1. INTRODUCTION ..... „ , . . The thermal equilibrium of a gearbox is attained when the power losses are equal to the evacuated heat.
The main mechanisms of power dissipation inside a gearbox are the churning losses and the frictional losses [ 1, 2, 6, 7 1. These are commonly ^ no.|oad d ^ ca,,ed dent losses and dep endent losses, respectively,
422 All The churning losses are related with moving parts immersed in the lubricant and depend on the lubricant properties, speed and parts geometry. The main sources of churning losses are the gears, the bearings and the seals. The frictional losses are related with contacting bodies in relative motion. The main sources of frictional losses are the gears and the rolling bearings. These losses are mainly dependent of the applied load, rolling and sliding speeds, lubricant properties and friction coefficients. There are three main heat evacuation modes: conduction, radiation and convection. These forms of heat evacuation depend mainly on the case temperature, the environment temperature, the case geometry and the coefficients that govern each of these heat exchange forms, like gearbox surface emissivity, etc. The thermal equilibrium of a gearbox is reached when the operating temperature stabilizes. This temperature is dependent of the gearbox characteristics and of the lubricant [ 1, 2, 7 ]. The lubricant has capital influence in all kind of power losses, both friction and churning losses. The higher the power losses the higher will be the equilibrium temperature of the gearbox. Lower stabilization temperatures means higher efficiency, lower friction coefficient, smaller oil oxidation and longer oil life [ 8 ]. The main objective of this study is to compare the energetic and wear performance of two industrial gear oils: a reference mineral oil and a non-toxic biodegradable ester. For that purpose screening tests simulating gear operating conditions and gear tests performed on the FZG machine using type C gears were done. On the screening tests the friction coefficient and the temperatures were measured, on the FZG gear tests the final stabilized oil operating temperature is measured for different values of the input torque and input speed. The protection against wear provided by the two lubricants is also compared. Lubricant samples, collected during and at the end of the FZG gear tests, are analyzed by Direct Reading Ferrography in order to evaluate the index of wear particles (CPUC and ISUC) and compare the wear behaviour of each lubricant. 2. LUBRICANTS Environmental awareness leads to a growing interest in biodegradable non-toxic lubricants.
However, the key aspect for any industrial application is technical performance and technical advantages proved in dedicated tests. Environmental compatibility is usually viewed in respect to biodegradability and toxicity. While the first issue is reached by using a suitable biodegradable base fluid, low toxicity requires an additivation that is environmentally friendly [ 10 ], too. However, lubricant performance (friction, wear, lifetime, load bearing, efficiency etc.) has a major impact on its overall environmental compatibility. Premature wear, high energy needs are as well harmful to the environment. Two industrial gear lubricants are tested and compared: a reference ISO VG 150 mineral oil, containing an additive package to improve micropitting resistance, and an ISO VG 100 biodegradable ester lubricant with a low toxicity additivation. Both oils are specified as CLP gear oils according to DIN 51517. The main properties of the two lubricants are shown in Table 1. 2.1. Chemical properties The reference gear oil is based on a paraffinic mineral oil with significant residual sulphur content. It contains an ashless antiwear additive package based on phosphorous and sulphur chemistry and metal-organic corrosion preventives. In contrast, the biodegradable product uses a fully saturated ester based on harvestable materials. The absence of unsaturated bonds in this base fluid leads to excellent thermal and oxidative stability. To combine the desired low toxicity with superior gear performance, environmentally compatible, highly efficient additives have been selected. Metal-organic compounds have been completely avoided. 2.2. Physical properties The physical properties of both industrial gear oils are also shown in Table 1. There is a significant difference in rheology between both oils: The two oils have almost identical viscosities at 100°C but the ester gear oil has much lower viscosity at lower temperatures. This is caused by the higher viscosity index of the ester product. This beneficial property leads to a wider range of operational temperature compared to the mineral oil. This can also be seen from the pour point that is 20°C lower with the ester than with the mineral oil. Thermal expansion and density of the ester gear oil are higher than that of the mineral oil.
423 2.3. Standard mechanical tests The VKA weld load is an extreme pressure test to determine the additives response at welding conditions. Both lubricants withstand mean pressure of approximately 5500 N/mm2 (for a four-ball load of 2200 N) without seizure. The wear scar after one hour at 300 N (2900 N/mm2) is similar for both oils. Considering the extreme pressure performance judged by the Brugger crossed cylinder test, there is a considerable higher value for the mineral oil than for the ester. Both values are higher than the value considered sufficient for hydraulic oils (30 N/mm2) but only the mineral oil exceeds the value usually stated for transmission oils (50 N/mm2). In the FZG scuffing test both lubricants pass FZG load stage 12 (contact pressure of 1.8 GPa) without any seizure. Summarizing these mechanical tests (see Table 1), both lubricants have almost the same performance and can both be considered suitable for a wide range of gear applications. 2.4. Biodegradability and toxicity properties Standardised, internationally recognised test methods are available for determining the biodegradability and environmental toxicity of lubricants and their components. The "ultimate" biodegradability of lubricants is best assessed using a "ready" biodegradability test as Table 1 - Lubricants properties. Parameter [unit]
Desig.
Base oil Physical properties Density @15°C [g/cm3] Kinematic Viscosity @ 40 °C [cSt] Kinematic Viscosity @ 100 °C [cSt] Viscosity Index [/] Pour point [°C] Thermal expansion [ 5 ] [g/cm3 °C] Wear properties VKA weld load [N] VKA wear scar (1 h/300N) [mm] Brugger crossed cylinder test [N/mm2] FZG rating [/] Biodegrability and toxicity properties Ready biodegradability [%] Aquatic toxicity with Daphnia [ppm] Aquatic toxicity with Alga [ppm]
published by the OECD and adopted by European Union. 2.4.1
OECD 301B, Evolution test
A measured volume of inoculated mineral medium, containing a known concentration of test substance (10-20 mg DOC or TOC/1) as the nominal sole source of organic carbon, is aerated by the passage of carbon dioxide free air at a controlled rate in the dark or diffuse light. The CO2 is trapped in barium or sodium hydroxide and is measured by titration of the residual hydroxide or as inorganic carbon. The amount of carbon dioxide produced from the test substance (corrected for that derived from the blank inoculum) is expressed as a percentage of ThCO2. The degree of biodegradation may also be calculated from supplemental DOC analysis at the beginning and end of incubation. 2.4.2
OECD 201 "Alga growth inhibition test"
A 72h algal growth inhibition test is performed in long cell test vials, with "Selenastrum capricornutum". Optical Density (at 670 nm) is used as the parameter for algal growth/inhibition. The tests are performed in disposable cells of 10 cm path-length which allow direct and rapid scoring of the OD in the "long cell" test vials, using any conventional colorimeter equipped with a holder for 10 cm cells. Method DIN 51451
Pl5 V40
v1Oo VI
DIN 51757 DIN 51562 DIN 51562 DIN ISO 2909 DIN ISO 3016
0.897 146 14.0 92 -21 7.16x10"
0.925 99.4 14.6 152 -42 8.05x10"'
DIN 51350-2 DIN 51350-3 DIN 51347-2 DIN 513540
2200 0.32 68 >12
2200 0.35 37 >12
OECD, 301 B OECD, 202 OECD, 201
<60 -
>60 >100 >100
av
K-FZG
EL50
EL50
Lubricating Oils paraffin ic saturated ester mineral oil
424
2.4.3
OECD 202, "Daphnia Magna" The 24h EC50 Acute Immobilisation
In the acute immobilisations test a range of concentrations of the substance investigated exerts different degree of toxic effects on the swimming capability of Daphnia under otherwise identical testing conditions. Certain concentrations result in certain percentages of Daphnia being no longer capable of swimming at 24 hours. The Daphnia Magna must be no more than 24 hours old at the beginning of the test, laboratory bred and apparently healthy. The percentage immobility at 24 hours is determined for each dilution. Normal statistical procedures are then used to calculate the EC50 or EL50 for the appropriate exposure period. 2.4.4
71 Pinion specimen
Wheel specimen
Biodegradability and toxicity results
The mineral oil didn't match the minimum requirements of 60% biodegradability in 28 days, as show in Table 1. Thus, no toxicity tests were performed for this lubricant. The ester based oil exceeded the minimum requirements of 60% biodegradability in 28 days and pass both toxicity tests, OECD 201 "Alga growth inhibition test" and OECD 202 "Daphnia Magna acute immobilization" as show in Table 1. 3. GEAR SIMULATION TESTS
Figure 1 - Gear simulation test configuration. 3.1. Material The material used in these tests is the 34CrNiMo6+NAP (High Pressure Nitriding). The material hardness along depth is represented on Figure 2. The material presents a structure formed by tempered martensite of sorbo-bainitic aspect, as represented in Figure 3. In the surface, a diffusion layer with some thin nitride film can be detected. It has not been detected any combination layer.
In order to simulate the wear of spur gears, a friction and wear test configuration developed in the framework of the previous European BRITE Project TESS has been used (see Figure 1). The sliding/rolling percentage selected is similar to the real gear as well as the operating conditions. With these tests it's intended to predict the oil behaviour in gear applications. The operating conditions are: • Load • Rotating speed • Linear speed • Specific film thickness Sliding/rolling • Initial temperature • Test duration
72.6 kg, 1968 rpm 2.7 m/s 0.16 to 0.2 35/40% 22 °C 480 minutes
300 100
200
300
400 Depth
500
Figure 2 - Hardness measurement along depth (UNE 7363-78). The Nitrided coating thickness is 0.3mm.
425
• COF H Temperature increase 90
u
|
0.03
80
TL
S 0.04
-—
70
59 60
i I
5 0.02 0.01
Mineral
Figure 3 -High pressure nitride steel metallographic structure.
ra
0.028
50
a>
ture incr
0.05 c
£
2L E
40
Ester
Figure 4 - Mean coefficient of friction and temperature increase for both tested lubricants.
3.2. Test results In this gear test simulation, the pitting behaviour of the lubricant and material are analysed. The final root mean square roughness was 1.33um for the specimens lubricated with Ester oil and 1.66um for the specimens lubricated with the mineral oil. The average coefficient of friction during the test and the total increase of lubricant temperature are represented in Figure 4. The friction coefficient measured for the mineral lubricant is 54% larger than that measured for the Ester. The same happens with temperature increase, being the final temperature of mineral lubricant higher 13°C. The mass loss of specimens is presented in Figure 5. The specimen lubricated with the ester oil show a greater mass loss (19% higher) than those lubricated with the mineral oil Both rollers lubricated with mineral or ester oil developed pitting during tests, as show in Figure 6. 4. GEAR POWER LOSS TESTS
I Wear of the disc DWear of the rollers D Total Wear 10.0
10
-
8.4
P DO
B
1" 6 la
?
4 2 0
5.2 4.1
-•
4.3
..
•
^m,—,
Mineral
—
Ester
Figure 5 - Mass loss of the specimens for both lubricants.
Mineral
Ester 34CrNiMo6+NAF
4.1. FZG gear test rig and type C gears Figure 7 shows a schematic view of the FZG back-to-back spur gear test rig. FZG type C gears are used in the tests. They are made of 20MnCr5 steel, carburised, quenched and oil annealed. Both gears are run-in before performing the power loss tests. The characteristics of the gears are shown in Table 2. The gears are dip lubricated with an oil volume of 1.5 1.
Figure 6 - Images of rollers after the tests and both present pitting.
426
Figure 7 - Schematic view of FZG gear test rig. 4.2. Operating conditions Each lubricant is submitted to a set of gear tests performed in a wide range of speed and torque conditions, as well as in input power, as shown in Table 3. The first 3 tests are performed at very low torque {no-load tests) corresponding to FZG stage 1 and three different speeds, in order to evaluate the influence of the oil on the churning losses. The no-load tests lasted for two hours at each operating speed. The initial oil bath temperature is approximately room temperature (s20°C). The other 9 tests {load tests) are performed at three different speeds {1000 rpm, 2000 rpm and 3000 rpm) and three increasing torque levels, corresponding to FZG load stages 5, 7 and 9 and to Hertzian contact pressures of 0.92 GPa, 1.29 GPa, 1.65 GPa, respectively. Each load test began at approximately 40°C and the oil sump temperature is free. Each load test lasted four hours during which the input power remains constant. The four hours test time is defined so that the power dissipated and the evacuated heat are in equilibrium and the oil sump reaches a stabilized temperature. A maximum oil sump temperature of 180°C is imposed for protection of the test rig. The monitored variables during the tests are: oil sump temperature, test gearbox temperature, ambient temperature, speed and torque. During the load tests and after each testing speed {lOOOrpm, 2000rpm and 3000rpm), lubricant samples are collected for direct reading ferrometry, viscosity and density measurements.
Table 2 ~~ Characteristics of FZG type C gears. Parameter /Units/ Pinion Wheel Number of teeths [/] 16 24 Module [mm] 4.5 [mm] Center distance 91.5 Addendum diameter [mm] 82.45 118.35 Pressure angle 20 [°] Addendum modification [/] +0.182 +0.171 [mm] 14 Face width 0.2 Ra (A.c=0.25 mm) [um] Hardness [HRC] 58 a 62 Young Module [Pa] 206x109 Poisson Coefficient [/] 0.3 Specific heat [Nm/(kg°K)] 450 Thermal conductivity [N/(s°K)] 50 [Kg/m3] Specific weight 7850 Table 3 - Operating conditions used in the power loss tests. N"of Speed Torque Input Test type Power cycles [rpm] |Nm| |k\V| 1000 0.52 No load 4.95 0 .72 2000 1.04 tests 3000 1.56 141.15 14.78 1000 0 .72 275.10 28.81 453.00 47.44 141.15 29.56 Load 2000 1.44 275.10 57.62 tests 453.00 94.88 141.15 44.34 3000 2 .16 275.10 86.43 453.00 142.31 5. RESULTS AND DISCUSSION 5.1. Stabilisation temperature Figure 8 shows the evolution of the oil sump temperature during the tests performed at 3000rpm with mineral and ester oils. At low torque (5 Nm) the oil temperature evolution and stabilization is very similar for both oils. In the other tests, at higher input torques, the temperature rise is always greater for the mineral oil.
427 The stabilization temperatures of the mineral oil are higher than those of the ester oil, and the difference between them increases with increasing torque. The largest difference in stabilization temperature occurred for the highest torque (453 Nm), when the mineral oil reached the limiting temperature of 180°C after 112 minutes, while the
ester oil attained 156°C in the same period and reached a stabilized temperature of 170°C after 240 minutes. In Figure 9 the stabilised temperature measured for each test is plotted against the input speed and torque for both oils.
3000rpm
Mineral, 453N.m - Ester, 453N.m Mineral, 275N.m Ester, 275N.m Mineral, 141N.m Ester, 141N.m Mineral, 5N.m Ester, 5N.m
100
150
200
250
Time [min]
Figure 8 - Oil sump temperature for mineral and ester oils at 3000 rpm for different input torque values. Maximum Oil Temperature
*t=112min
Figure 9 - Stabilised oil sump temperature vs. input speed and torque for ester and mineral industrial gear oils.
428 The difference in stabilization temperature between the two oils goes from 2°C to 15°C, depending on the operating conditions. For the highest input power (142 kW) the difference is above 30°C and the mineral oil reached the limiting temperature. In the case of the no-load tests, performed at very low torque (5 N.m), such difference depends mainly on the viscosity and specific weight of the lubricants. The tests performed at 1000 rpm and 2000 rpm generate low stabilization temperatures at which the viscosity of the mineral oil is significantly higher than that of the ester oil, making its stabilised temperature higher. For the test performed at 3000 rpm, the stabilization temperature reaches approximately 75°C. At this temperature the viscosities of both oils are similar, but the ester oil has a higher specific weight leading to a higher stabilization temperature then the mineral oil. These results indicate that the churning losses are similar for both oils at typical gearbox operating temperatures (between 70 °C and 80 °C) and for the same operating speed. For all the other tests at higher torques the stabilized temperature of the mineral oil is always higher then that of the ester oil, suggesting that the friction power loss inside the contact between gear teeth is considerably smaller for the ester oil. These differences in stabilization temperature and in friction power losses between the two oils, at identical operating conditions, are due to the smaller friction coefficient between the gear teeth provided by the ester oil. The increasing differences in stabilisation temperatures with increasing torques, between the mineral and ester oils, suggests that the difference in friction coefficient between the oils also increases with increasing torques.
Dl + Ds as
Dl' - Ds' ISUC =
CPUC = d
where d
d'
stands for the oil sample dilution. Figure 10 and Figure 11 show the evolution of CPUC and ISUC indexes during the power loss tests for both lubricants. CPUC
0
1
2
3
4
[x106] Cictes Figure 10 - Evolution of CPUC index during the power loss tests for both industrial gear oils.
5.2. Oil analysis Three lubricant samples were obtained of each oil after the load tests at 1000 rpm, 2000 rpm and 3000 rpm, respectively. They were analysed by Direct Reading Ferrometry in order to measure the the ferrometric parameters Dl (large wear particles index) and Ds (small wear particles index). The values of Dl and Ds are used to evaluate the concentration of wear particles index - CPUC and the severity of wear particles index - ISUC, defined
0
1
2
3
4
[x106] Cicles Figure 11 - Evolution of ISUC index during the power loss tests for both industrial gear oils.
429 The CPUC grows when the sum of the small and the large particles increase, while the ISUC index grows when the number of particles with a size greater then 5um are present in major number then the smaller then 5 urn. The analysis of both figures show clearly that the min eral oil presents significantly higher CPUC and ISUC values than the ester oil, meaning that the wear particles released with the mineral oil have a larger size and are in major number. There's a huge difference in the CPUC and ISUC values between the mineral and the ester oils, mainly after the tests performed at 2000 rpm and 3000 rpm. Elemental analysis of both oils after the end of the test was performed using RFA. The iron content of both oils was rather low. However, the mineral oil showed 2 to 3 times higher iron contents than the ester product. This confirms the higher wear rate of the mineral oil in this test. An important reason for the higher wear rate of the mineral oil is that its higher friction leads to oil temperatures well above those indicated for mineral oils. The stabilized temperatures reached 131 °C at 2000 rpm and supersedes 180 °C at 3000 rpm for the highest torque applied (453 N.m), as indicated in Figure 4, which is considerably above the maximum temperature this oil can stand. The kinematic viscosity of both lubricants was measured before and after the power loss tests and the results are presented in Table 4.
V100
14.0
14.1
+0.7
VI
92
94
+3
Fe
0
18 ppm
n.d.
V40
99.4
100.2
+0.8
ter
era
Table 4 - Viscosity and viscosity index values, measured before and after power loss tests, for the mineral and the ester industrial gear oils. Iron content of the used oils has been determined by RFA [9]. variation used oil fresh oil % 145.8 +0.9 144.5 V40
V100
14.6
14.6
0
W
VI
150
150
0
Fe
0
5 ppm
n.d.
No significant changes could be observed. This corresponds to the evolution of the neutralisation number as an indicator for oxidation. It also showed no significant increase. This indicates that both oils show no sign for oxidative degradation. 6. CONCLUSIONS The results obtained allow the following conclusions: 1. The performance of biodegradable ester in standard mechanical tests is analogous to that exhibited by the mineral oil. Both present excellent results. 2. The gear simulation test showed that the ester lubricant exhibit a smaller friction coefficient and a smaller operating temperature. 3. The mass loss in the gear simulation tests is larger for ester oil. 4. The no-load FZG gear tests performed with type C gears indicate that the churning losses of the mineral and ester industrial gear oils are identical, generating similar stabilized operating temperatures. 5. The loaded FZG gear tests indicate that the friction losses of the mineral oil are significantly higher than those obtained with the ester oil, generating much higher stabilized operating temperatures. This suggests that the friction coefficient between the gear teeth is smaller with the ester oil, in accordance with gear simulation test. 6. The differences in stabilised temperatures, between the mineral and ester oils, increase with increasing torques. That difference becomes very important for the most severe operating conditions. 7. The Direct Reading Ferrometric analysis of the oil samples collected during the load gear tests show that the wear particles concentration and the wear severity is much higher for the mineral oil than for the ester oil. 8. The mineral oil cannot be operated at temperatures above 100 °C during large periods. At high load and high speed (high input power) the stabilized operating temperature of the mineral is very high (>100 °C) and a large amount of wear particles is produced.
430 9. For the same severe operating conditions the ester oil produces a much smaller amount of wear particles, supporting the high operating temperatures without loosing its anti-wear properties.
[ 3]
B.-R. Hohn, K. Michaelis and R. Dobereiner, "Load Carrying Capacity Properties of Fast Biodegradable Gear Lubricants", Journal of the STLE - Lubrication Engineering, November 1999, 15,16,33-38.
[ 4]
T. M. Hunt, "Handbook of Wear Debris Analysis and Particle Detection in Liquids", Elsevier Science, UK, 1993.
[ 5]
J. Denis, J. Briant, J.-C. Hipeaux, "PhysicoChimie des Lubrifiants - Analyses et Essais", Editions Technip, 1997.
[6]
P. Eschmann, "Ball and roller bearings - Theory, design and applications", Wiley, 1985.
[ 7]
H. Winter, K. Michaelis, "Investigation on the Thermal Balance of Gear Drives", ASME, 1979.
[8]
B.-R. Hohn, K. Michaelis and A. Doleschel, "Frictional behavior of synthetic gear lubricants", Tribology research: From model experiment to Industrial Problem, Elsevier, 2001.
[9]
DIN 51396 part 2 Testing of lubricants Determination of wear elements - Part 2: Analysis by wavelength dispersive X-ray spectrometry
ACKNOWLEDGEMENT The authors express their gratitude to the European Union for the financial support given to this work through the GROWTH Project N°. GRD22001-50119, Contract N° G3RD-CT-2002-00796EREBIO, "EREBIO - Emission reduction from engines and transmissions substituting harmful additives in biolubricants by triboreactive materials".
REFERENCES [ 1]
[ 2]
B.-R. Hohn, K. Michaelis and T. Vollmer, "Thermal Balance of Gear Drives: Balance Between Power Loss and Heat Dissipation", AGMA Technical Paper, October 1996, pp 12, ISBN: 1-55589-675-8. Chr. Changenet and M. Pasquier, "Power Losses and Heat Exchange in Reduction Gears: Numerical and Experimental results", VD1Berichte, NR. 1665, 2002, 603-613.
[ 10 ] M. Week, O. Hurasky-Schonwerth and Ch. Bugiel, "Service Behaviour of PVD-Coated Gearing Lubricated with Biodegradable Synthetic Ester Oils", VDI-BerichteNr.1665, 2002.