Aus der Praxis für die Praxis 1 Introduction Journal bearings are used to support and guide rotating or pivoting parts within a case. The functionality of journal bearings bases upon the formation of a load bearing fluid film, which separates the rotating part from the bearing. During operation various lubrication regimes can occur, depending on velocity, mechanical loading and viscosity of the lubricant, see Figure 1. For each regime different sliding and failure processes prevail. Conventional operation takes place mostly in hydrodynamic friction regime. Details can be found, inter alia, in [1-3]. Automotive R&D targets higher engine efficiency, based on more stringent environmental regulations such as the CO 2 emission limits [4, 5]. This includes minimization of engine friction, downsizing, light weight design and operational changes such as start stop systems [5, 6]. With regard to journal bearing systems these changes represent new challenges in the field of maintaining sufficient reliability and contribution to engine efficiency. 2 Requirements of journal bearing systems The design process of journal bearing systems is complex. Based on the fact that journal bearing systems are 44 Tribologie + Schmierungstechnik 64. Jahrgang 4/ 2017 * DI Dr. Florian Summer Univ.-Prof. DI Dr. Florian Grün Chair of Mechanical Engineering Montanuniversität Leoben, 8700 Leoben, Austria DI Martin Offenbecher Miba Gleitlager Austria GmbH, 4663 Laakirchen, Austria Dr. Stuart Taylor Dr. Emmanuel Lainé Infineum UK Ltd., Milton Hill, UK Tribology of journal bearings: Start stop operation as life-time factor F. Summer, F. Grün, M. Offenbecher, S. Taylor, E. Lainé* Die Reduktion des Schadstoffausstoßes stellt einen der wichtigsten Treiber im Bereich der Verbrennungsmotorenentwicklung dar. In diesem Zusammenhang werden Start Stopp Systeme eingesetzt um eine weitere Reduktion des Gesamtkraftstoffverbrauchs und folglich auch der Schadstoffemissionen zu erreichen. Jedoch wird durch diesen Einsatz der Gesamtantriebsstrang völlig neuen Betriebsbedingungen ausgesetzt. Insbesondere hydrodynamische Gleitlagersysteme werden durch diese Betriebsveränderungen und den damit verbundenen erhöhten Anteil an Festkörperkontakten intensiver belastet. Um weiterhin den sicheren Lagerbetrieb zu gewährleisten, bedarf es an umfassenden Untersuchungen im Bereich der Start Stopp Dimensionierung von Gleitlagern. Der vorliegende Beitrag umfasst gezielte Untersuchungen von Gleitlagersystemen unter Start Stopp Betriebszuständen. Mithilfe eines neuartigen bauteilnahen Prüfsystems wird dabei insbesondere das Start Stopp Verschleißverhalten von diversen Lagermaterialien erforscht und verglichen. Desweiteren werden der Einfluss der Schmierstoffviskosität und die Additivierung der Motorschmierstoffe auf die Lebensdauer von Lagersystemen unter Start Stopp Betriebsbedingungen aufgezeigt. Schlüsselwörter Gleitlager, Start Stopp Betrieb, bauteilnahe Prüfmethodik, Grenz- und Mischreibung, Verschleiß Reduction of pollutant emissions is one of the key drivers in the field of combustion engine development. In this context, in particular start stop systems are applied in order to further reduce the total fuel consumption and subsequently minimize pollutant emissions. However, this application leads to completely new operating conditions for the powertrain. In particular, hydrodynamic bearing systems are burdened by these operational changes and the increased proportion of solid friction contacts. In order to ensure safe bearing operation, in depth research in the field of start stop bearing dimensioning is required. This paper covers an extensive investigation of journal bearing systems under start stop operation conditions. With the aid of a novel component close test methodology the start stop wear characteristics of various bearing materials are investigated and compared. Furthermore, the impact of the lubricant viscosity and chemical additive formulation of engine lubricants on the start stop lifetime performance of journal bearing systems are discussed. Keywords Journal bearings, start stop operation, component close test methodology, boundary and mixed friction, wear Kurzfassung Abstract T+S_4_17 07.06.17 17: 27 Seite 44 Aus der Praxis für die Praxis essential components of combustion engines for various applications two main tasks take priority, viz. sufficient reliable service life for all operation conditions and frictional efficiency for all contact (lubrication) conditions. In the following subsections main demands of bearing systems are briefly reviewed. 2.1 Longterm system life - Fatigue and wear-fatigue resistance Sufficient fatigue and wear-fatigue performance of journal bearing systems are required in order to ensure non stationary highly loaded operation under hydrodynamic condition. Based on increased firing pressures demands according to high fatigue life represent a mandatory aspect in life time dimensioning of journal bearing systems. 2.2 Tribological system robustness - Resistance against seizure, disturbance variables and start stop Although most of the operation of journal bearing systems takes place in hydrodynamic friction regime, short term boundary and mixed friction events occur frequently. In this regard tribological demands represent a crucial part for reliable functionality of bearing systems. This includes robustness of the whole bearing system of shaft, lubricant and bearing material against seizure or scuffing events or disturbance variables such as dirt particles. In particular, start stop techniques during idling times of the engine, which are applied frequently in order to reduce fuel consumption, represent a fundamental change of the operation characteristic of journal bearings. This change results in frequent passing through of the boundary and mixed friction regime, during which direct contacts of the mating surfaces occur. Hence, wear resistance during start stop operation represents a new tribological demand of journal bearings and, therefore, needs to be considered during the design process of journal bearing systems. 2.3 Friction efficiency Engine efficiency is directly linked with frictional losses of the combustion engine. Environmental considerations increase necessity for a further reduction of engine friction. In case of bearing systems contribution to friction reduction can be realized by decrease of the lubricant viscosity for hydrodynamic regime and optimization of the mating contact between shaft and bearing during operation in boundary and mixed friction regime. However, what tips the scale for significance of each demand for journal bearing systems are the various application fields. Depending on the operation environment, engine family or even specific applications, the requirements on tribological superiority including seizure or start stop performance, friction efficiency or load capability change crucially. 3 Tribological test methodologies for journal bearings Even though simulation of journal bearing systems has progressed far, today testing of journal bearing systems is still an indispensable dimensioning and design tool. In this regard, especially, material design of bearings, shafts and lubricants requires experimental procedure. Testing of journal bearing systems is a manifold task due to the fact that tribological processes are affected by almost innumerable variables including effects deriving from the engine itself. Generally, full scale engine tests are prevalent, which are, however, cost and time consuming. Furthermore, investigations of selective parameters can hardly be realized. In contrast, model test set-ups often do not reproduce the tribological processes or kinematics of real life components. Pondicherry et al [7] did observe insufficient reproducibility of journal bearing contacts with reciprocating pin-on-plate model test configuration. Grün et al [8] reported inadequate damage mechanisms of widely used pin/ ball-on-disc configurations for journal bearing systems. Therefore, damage equivalent and application related testing on reduced realism scale is aimed for. Figure 2 classifies appropriate test methodologies on reduced scale with regard to test possibilities visualized by lubrication and friction regimes (Stribeck depiction) and with regard to visualization possibility of tribological processes. Tribologie + Schmierungstechnik 64. Jahrgang 4/ 2017 45 Figure 1: Lubrication regimes of journal bearing systems and their relation to friction (Stribeck curve) T+S_4_17 07.06.17 17: 27 Seite 45 Aus der Praxis für die Praxis Journal bearing test rigs with hydropulsating or electromechanic actuators are well established [9, 10]. These test configurations mimic lubricant film pressures as found in real-life components, thus offer a high degree of realism. Hence, especially hydrodynamic load bearing capabilities and fatigue properties can be evaluated besides tribological aspects such as wear and seizure behaviour [9]. However, in many cases precise measurement of e. g. coefficient of friction or visualization of contact processes is limited. Furthermore, complex set ups of the test rigs limit the flexibility in varying lubricants or testing parameters. Damage equivalent testing with model test configurations is possible considering ring on disc sliding systems. This test set up is able to reproduce tribological damage mechanisms of operating journal bearing systems such as seizure or wear processes in boundary and mixed friction regime [8, 11]. In combination with deceleration of tribo-processes and failure mechanisms based on low energy input, this test configuration has proven evidence to observe selective sliding interactions [7]. Hence, a ring on disc sliding system is especially applicative to visualise seizure processes, while endurance under hydrodynamic conditions or evaluations of load bearing capabilities of fluid films can be covered with journal bearing test rigs. A component close journal bearing adapter set up, which is implemented within a tribometer test rig, closes the gap between damage equivalent model set ups and full scale bearing test rigs. In this test configuration journal bearing shells are tested under stationary loading conditions. Hence, especially tribological processes in relation to lubrication conditions of the component, such as start stop operation, can be studied in detail with the aid of precise measurements of tribological system parameters, such as coefficient of friction (µ,COF) , contact temperatures, contact resistance or the wear. 4 Journal bearing adapter for start stop evaluation This novel test configuration, which has been established at the Chair of Mechanical Engineering in Leoben, enables component near testing of journal bearing systems in combination with precise visualization of friction characteristics. In this set up two bearing shells are pressed simultaneously against a rotating shaft specimen. The bearing shells are reduced to 120° in this test configuration. 46 Tribologie + Schmierungstechnik 64. Jahrgang 4/ 2017 Figure 2: Classification of damage equivalent test set-ups for journal bearings Figure 3: Journal bearing adapter test set up - (a) schematic, (b) real life [12] T+S_4_17 07.06.17 17: 27 Seite 46 Aus der Praxis für die Praxis The test implementation is realized by the aid of a tribometer test rig TE92 from Phoenix Tribology, see Figure 3. In this depiction the schematic test assembly as well as final test set up within the tribometer can be seen. The shaft specimen is connected with the motor. The 120° bearing shells are mounted in specimen holders with screwed clamps. The normal loading is realised with a pneumatic actuator and is forwarded to the contact by means of lever arms and the bearing shell holders. The tribo-contact is placed within an oil bath, which can be heated up to 200 °C by four heating elements beneath the fluid bath. Several sensors for different variables are mounted. Two thermocouples record the oil bath temperature and the bearing back temperature, respectively. The friction characteristic is recorded by friction torque measurement by means of a load cell on the left side of the pivot mounted oil bath. Additional measurement possibilities for lubricant film pressure, contact resistance or acoustic emission can be used to gather even more details of the ongoing tribo-processes. Further details of the test configuration can be found in [12-14]. 5 Test methodology and materials For this study bearing dimensions with approximately Ø 48 mm have been used. The shaft dimensions have been fitted to the bearing dimensions to reproduce 1.6 per mill clearance of real life bearings. The shaft material of this study is 34CrNiMo6 steel and possesses a surface finish of R a ~ 0.15 μm. The bearing materials cover various bimaterial and trimaterial bearing types as follows: • AlSn20Cu bimetal bearing • Leaded bronze bearings with additional overlays (~16 µm thickness): PVD coated (sputtered) AlSn20, electroplated PbSn and a polymer with MoS 2 and graphite fillers Within the scope of this study also various engine lubricants have been used. Oil A is an experimental EURO III heavy duty diesel engine oil with a viscosity of 10W-40 and contains 68 wt.% of Group III base oil, primary ZDDP (P level of 0.12 %), detergents, dispersants, antioxidants, viscosity modifiers and high ash level. Oil A1 and A2 use rebalanced low viscosity base stocks compared to Oil A. Oil B represents a prototype 10W-40 heavy duty diesel engine oil with a low ash level and is recommended for engines which fulfil latest European emission standard EURO VI. In case of Oil C film forming additives groups such as antiwear/ extreme pressure (AW/ EP) additives are not added compared to Oil A. The designed start stop test program can be seen in Figure 4. The system is constantly loaded with 1.64 MPa. At the beginning a running in period at room temperature (RT) and 0.21 m/ s sliding speed has been defined for 900 seconds. Thereafter, a short heating period to 100 °C oil bath temperature without sliding follows. For selected RT tests the heating period has been skipped. Subsequently, after the heating period, start stop cycles up to 1.2 m/ s are performed. The duration of the start-up ramps is 5 seconds, while stopping the movement lasts 3 seconds. The numbers of the performed start stop cycles have been varied ranging from tests with 100 cycles up to 100,000 cycles. 6 Results The sliding characteristics of journal bearings under start and stop operation show all lubrication regimes. This can be read off from Figure 5, which depicts a 5 second start ramp of a journal bearing system from 0 to 1.2 m/ s sliding velocity. The system behaviour is described precisely by sensor signals for the frictional losses, the contact resistance (contact potential - CP) between the mating surfaces and the formed lubricant pressure with 1 kHz acquisition rate. The system operates at static friction after a short stand still period. At this point μ is at highest level of ~ 0.1. The contact resistance is low and no lubricant pressure exists at this state. Upon start of sliding the system moves towards boundary friction regime. Subsequently, μ decreases slowly. Simultaneously, the lubricant pressure in between the surfaces starts to build up. However, at this state the amount of fluid pressure is not able to fully separate the solid contacts. This can be read off from the CP signal, which is still at the bottom level and indicates metallic contact. Upon further increase of the velocity (→ Line I) µ decreases rapidly and lubricant pressure almost reaches maximum values. Simultaneously, the CP signal starts to rise, which indicates already partial separation of the mating sliding surfaces. The system has tur- Tribologie + Schmierungstechnik 64. Jahrgang 4/ 2017 47 Figure 4: Start stop test program T+S_4_17 07.06.17 17: 27 Seite 47 Aus der Praxis für die Praxis ned into mixed friction condition. Once the CP signal has built up completely (→ Line II) fluid friction dominates. The lubricant pressure stays high and µ starts to reincrease after passing its minimum due to increased shearing of the fluid. In case of stop events the processes are reversed. Start stop operation to reduce fuel consumption during idling times In the past, the total number of engine start events was considerably small. Furthermore, during conventional operation of a well-designed bearing boundary and mixed friction contacts happened seldom. In contrast, current and future engines with start stop systems will have to be designed for a high number of starts [15] with boundary and mixed friction contacts. Furthermore, stops and restarts to reduce engine idling take place at high engine temperatures leading to low oil viscosity, whereas conventional start-ups happen at environmental temperatures and only the corresponding engines stops have performed at elevated temperatures. Hence, journal bearing systems will face frequent numbers of restarts at elevated temperatures. This represents a new operation state. Figure 6 highlights the differences with regard to friction characteristics and wear performance between start stops at room temperature (RT) and elevated engines temperatures around 100 °C. The tested systems consist of a trimetal bearing with a sputtered AlSn overlay, Oil A and the shaft specimen. In Figure 6a the friction curves of start ramps after 1,800 cycles can be read off. It can be seen that the boundary and mixed friction regime is expanded at 100 °C and higher friction is produced compared to the test at RT. The transition between mixed and hydrodynamic friction moves to higher speed with increasing temperature. In hydrodynamic regime lower frictional losses are recorded at 100 °C due to the lower viscosity of the lubricant. Figure 6b visualizes the effects of elevated temperatures on the bearing wear at the highest loaded region. It can be highlighted that a significant difference is seen between wear at RT and at 100 °C. While at RT only a marginal amount of wear is produced, 48 Tribologie + Schmierungstechnik 64. Jahrgang 4/ 2017 Figure 5: Start sliding characteristics of a journal bearing (high speed test data of start ramp at 1 kHz after 1,800 starts, AlSn20 bimetal bearing, Oil A) - (left) µ and CP signal, (right) lubricant pressure Figure 6: Comparison of start stop operation at RT and 100 °C (AlSn20 sputtered overlay on CuPb lining, Oil A) - (a) friction curves (high speed test data at 1 kHz after 1,800 starts), (b) bearing wear of tests at RT and 100 °C and post test cross section depiction of a test at 100 °C (a) (b) T+S_4_17 07.06.17 17: 27 Seite 48 Aus der Praxis für die Praxis the test at 100 °C resulted in 12 µm wear of the overlay. Considering a total overlay thickness of ~16 µm most of the overlay has been worn off after only 1,800 starts. Start stop performance of various bearing materials The bearing material represents the sacrificial part within a journal bearing system. Different bearing material technologies have been developed in accordance with the significance of conventional life time demands for specific applications. For instance, sputtered AlSn20 overlays with high coating hardness have proven high wearfatigue resistance under highest load conditions. Electroplated Lead-Copper overlays or Tin-Copper-Antimony coatings with lower coating hardness give excellent dirt and misalignment capabilities. Polymer coated bearings are known to combine good load bearing capability and high tribological robustness in boundary and mixed friction regime. The ability of various bearing material types to withstand frequent start stop operation as additional performance variable for specific applications is discussed in the following. As seen in Figure 6b the wear resistance under start stop operation at characteristic engine temperatures can result in high wear after a low number of start and stops for the sputtered AlSn20 overlay. Figure 7 compares start stop wear performance at 100 °C after 1,800 starts of the sputtered AlSn overlay with other typical bearing materials. The used lubricant for this test series was Oil A. It can be noted that bearing types with AlSn20 sliding surfaces (bimetal and trimetal) provide the lowest wear resistance under start stop operation. After 1,800 start stops high wear of 12-13 µm outlines the disadvantages for this bearing types to cope with start stop operation. In contrast, electroplated PbSn shows a much better wear performance under similar conditions with only 4 µm bearing wear. The best performance has been found for the spray-coated polymer overlay, which provides almost wear free sliding conditions after 1,800 starts. Similar material rankings have also been observed with ring on disc test configuration [12] and with a journal bearing test rig. In addition, the evolution of wear of the tested bearing materials has been studied in detail. Therefore, start stop tests have been performed with various numbers of restarts from 100 to 100,000. The obtained results are summarized in Figure 8. The trimetal bearing with sputtered AlSn20 overlay has almost lost its overlay after 1,800 cycles. Considering the prerequisite of an existent overlay for ideal functionality of trimaterial bearings the overlay Tribologie + Schmierungstechnik 64. Jahrgang 4/ 2017 49 Figure 7: Comparison of start stop wear performance of various bearing materials after 1,800 starts (Oil A) Figure 8: Comparison of start stop wear evolution of various bearing materials (Oil A) T+S_4_17 07.06.17 17: 27 Seite 49 Aus der Praxis für die Praxis loss represents a sort of end-of-life criteria for these bearing types. For the AlSn20 bimetal bearing tests up to 5,000 starts were performed. Up to 1,800 cycles the wear evolution is similar to that of the trimetal bearing with sputtered AlSn20 overlay. For 5,000 start stops a total wear of 30 µm has been recorded, which exceeds a characteristic bearing clearance. Linear trends have been observed for the evolution of wear after a running in period. This also holds well for all other bearing types tested. The lead based overlay has been tested up to 18,000 starts, at which the bearing coating has almost bordered end of life time with a coating wear of 13 µm. The polymer coating provided superior start stop performance and enabled restarts to a magnitude of 10 5 with only 6 µm maximum bearing wear. In this regard the main wear part derives from running-in wear processes. Hence, the polymer overlay represents the only material, which is able to withstand representative numbers of engine starts with start stop techniques. The differences in start stop wear performance of the tested bearing materials can be traced back to the material design and the contact processes going on. The tribological processes under start stop condition for each material have been elucidated by the aid of detailed surface analysis. The tested AlSn alloys rely on soft phase mechanisms. The action of Sn in a harder Al matrix is based on its low melting point and subsequent solid lubrication function. Sn is known to enhance short term emergency running conditions of Al based materials. In case of recurrent asperity contacts under start stop operation Sn is smeared on the top surface and is consumed quickly; see Figure 9a, which shows a transition area between the highest loaded region and a low loaded region of a test with AlSn20 bimetal bearing. At the highest loaded region the loss of Sn can be noted (Sn is depicted bright in SEM pictures due to the high molecular weight). The sole Al matrix is left and subsequently is subjected to wear processes. In case of the soft lead based overlay, which generally gives high misalignment stability and dirt resistance, hard phase mechanisms are acting in form of intermetallic Cu-Sn particles. During start stop sliding an agglomeration of Cu-Sn phases and formation of a top surface particle structure has been observed, see Figure 9b state I to state II. However, the load bearing structure suffers from particle movement and depletion. Due to easy movement and low embedding stability of the hard particles high wear rates occur. After a total loss of the hard phases the residual Pb layer, Figure 9b state III, wears out constantly with low wear rate. 50 Tribologie + Schmierungstechnik 64. Jahrgang 4/ 2017 Figure 9: Surface analysis of AlSn20 bimetal bearing, electroplated PbSn coating and polymer overlay from start stop tests (Oil A) - (a) loss of Sn in case of AlSn20, (b) PbSn overlay modification, (c) surface evolution of the polymer overlay, (d) load bearing MoS2 lamellae in case of the polymer overlay (a) (b) (c) (d) T+S_4_17 07.06.17 17: 27 Seite 50 Aus der Praxis für die Praxis The superior performance of the polymer overlay can be traced back to optimized heterogeneous material design of the coating; viz. a polyamideimide (PAI) matrix containing graphite and MoS 2 solid lubricant fillers. During running in MoS 2 lamellae and graphite particles get exposed due to running in wear of the polymer (Figure 9c). Henceforth, predominant contact processes are determined by asperity contacts between MoS 2 / graphite and steel. The MoS 2 lamellae and graphite particles serve as solid lubricants and therefore provide low shear resistance sliding against steel. In addition they enable load bearing functionalities, which reduces PAI/ steel contacts and subsequently protects the polymer matrix from further wear, see Figure 9d. Furthermore, the fillers are well adhered within the used PAI polymer. This inhibits uncontrolled particle movement and easy particle loss. These combined effects result in durable surface structures of the polymer overlay during low load start stop sliding leading to a high wear resistance. Viscosity effect on start stop performance Decrease of the lubricant viscosity is able to contribute towards further friction reduction of combustion engines. As seen in Figure 6a frictional losses in hydrodynamic friction regime are decreased due to the reduced lubricant viscosity. Hence, low viscosity lubricants will be frequently used in future in order to reach CO 2 emission limits. However, the reduction of lubricant viscosity also expands boundary and mixed friction regime. The transition between mixed and hydrodynamic friction regime shifts towards higher sliding speeds (see Figure 6a). Hence, overall the amount and the harshness of boundary and mixed friction contacts increase during operation. Therefore, demands of systems with low viscosity lubricants change even more towards frictional efficiency during boundary and mixed friction regime and tribological robustness of bearing systems. Figure 10 depicts the impact of low viscosity oils on the start stop wear performance of journal bearings at similar temperature levels. In Figure 10a the effect of a systematic reduction of the lubricant viscosity on the wear performance of AlSn20 bimetal bearing after 1,800 starts is shown. The chemistry of the lubricants has been held constant. The wear rate increases disproportionally along with the decrease of the lubricant viscosity. From 3.7 high temperature high shear viscosity (HTHS) at 150 °C to 1.8 HTHS at 150 °C the wear amount triples. The evolution of wear based on the viscosity (1.8 HTHS vs. 3.7 HTHS at 150 °C) for AlSn20 bimetal bearing and polymer overlay is shown in Figure 10b. It can be seen that the viscosity decrease resulted in dramatic increase of the start stop wear rates. This holds well for all other bearing types tested in this study. Additive effects on start stop performance Chemical additives are added to the lubricant in order to enhance lubricant performance [16]. Especially AW/ EP additives aim to form gradient layers (so called tribolayers or tribofilms) on the mating surfaces within a lubricated tribological system. As a result the contacting surfaces are protected from tribological damage, such as seizure and wear. This represents common knowledge for Hertzian contact situations. However, also in case of journal bearing systems tribofilm formation has been reported under certain operation conditions such as seizure events Tribologie + Schmierungstechnik 64. Jahrgang 4/ 2017 51 Figure 10: Viscosity effect on start stop wear resistance of journal bearing systems - (a) comparison of three different oil viscosities after 1,800 start stops, (b) start stop wear evolution of selected bearing materials for two different oil viscosities (a) (b) T+S_4_17 07.06.17 17: 27 Seite 51 Aus der Praxis für die Praxis [17, 18]. For start stop operation conditions corresponding investigations have not yet been reported. Figure 11 compares shaft surfaces after seizure and start stop events of tribosystems with AlSn20 bimetal bearing and Oil A, which were tested with the journal bearing adapter. During seizure processes, generated with different test programs at higher loads, Figure 11a, the steel surface is extensively covered with gradient layers consisting of P, S, Zn, O and Ca. These elements derive primarily from the AW additive system of the lubricant such as the ZDDP additive. In contrast, during start stop tests, see Figure 11b, the steel surface is mostly blank without extensive AW tribofilms. Only traces of tribofilm elements have been detected selectively. This difference can be traced back to the different energy input for each sliding situation. Seizure events result from high temperatures, high loads and sliding speed. In such scenarios the high energy enables extensive formation of AW lubricant additive products which react with the nascent steel surface. Whereas, for start stop processes primarily low sliding speed and low loads are characteristic, which 52 Tribologie + Schmierungstechnik 64. Jahrgang 4/ 2017 Figure 11: Lubricant additives/ steel interactions for different operation conditions at journal bearing contacts - (a) seizure events, (b) start stop operation Figure 12: Effect of different lubricant additives chemistries on start stop wear processes of AlSn20 bearing material - (a) comparison of bearing wear, (b) comparison of AlSn20 bearing surface conditions at test end T+S_4_17 07.06.17 17: 27 Seite 52 Aus der Praxis für die Praxis is insufficient to initiate extensive reaction products of the protecting additive system of the used lubricants and restricts the formation to selective areas. The missing of extensive sacrificial tribofilm formation for the tested bearing systems under start stop operation leads to similar friction and wear characteristics for these sliding conditions independently of the used lubricant chemistries, see Figure 12. Lubricants with different AW/ EP additive contents, such as in case of Oil A, B and C, have been tested in combination with the AlSn20 bimetal bearing and result in similar wear values (Figure 12a). In addition, the AlSn bearing surfaces show similar wear pattern after test end (Figure 12b). Also friction characteristic is seen equal, whilst the viscosity of the lubricant and the bearing material are kept constant. 7 Summary and Conclusions Start stop operation is an essential tool to reduce fuel consumption in automotive applications. The current study has proven evidence that start stop processes play a crucial part in journal bearing life time. The following conclusions can be drawn: • Life time of journal bearing systems are determined by many factors and design of journal bearing systems need to be carried out depending on the application. • Frequent start stop processes to reduce engine idling change the operational behaviour of journal bearing systems dramatically. • The used test methodology shows high resolving power and is appropriate to visualize friction and wear processes for start stop operation. • Start stop processes at elevated temperatures are able to result in high wear rates for selected bearing systems. Therefore, if applied, this new operation conditions need to be addressed in journal bearing life time assessments. • With regard to performance of the tested bearing materials, only the tested polymer overlay with MoS 2 and graphite filler structure enabled sufficient life time under start stop operation. • Reduction of the lubricant viscosity results overall in a disproportional increase of the wear rates under start stop operation. • Current lubricant additive technologies are able to enhance the performance of journal bearing systems by means of sacrificial tribofilm formation. These processes are pronounced for seizure events, but have not improved start stop wear resistance of tribosystems with AlSn bimetal bearing material for the given test conditions of this study. Acknowledgement Financial support by the Austrian Federal Government (in particular from Bundesministerium für Verkehr, Innovation und Technologie and Bundesministerium für Wissenschaft, Forschung und Wirtschaft) represented by Österreichische Forschungsförderungsgesellschaft mbH and the Styrian and the Tyrolean Provincial Government, represented by Steirische Wirtschaftsförderungsgesellschaft mbH and Standortagentur Tirol, within the framework of the COMET Funding Programme is gratefully acknowledged. The use of Miba Gleitlager Austria GmbH bearing material products such as Synthec® (Miba 68), sputtered AlSn20 (Miba 37), casted AlSn20 (Miba 14) and electroplated PbSn (Miba 03) and Infineum UK Ltd. lubricants is greatly acknowledged. Nomenclature AW/ EP Antiwear/ extreme pressure COF, µ Coefficient of friction CP Contact potential (contact resistance measurement) EDX Energy dispersive x-ray spectroscopy HTHS High temperature high shear viscosity PAI Polyamideimide PVD Physical vapour deposition RT Room temperature ZDDP Zincdialkyldithiophosphate Literature [1] J. A FFENZELLER AND H. G LÄSER : Lagerung und Schmierung von Verbrennungsmotoren. Springer, Heidelberg, 1996. [2] W.J. B ARTZ: Gleitlager als moderne Maschinenelemente - Konstruktion, Werkstoffauswahl und Schmierung von Radiallagern. Expert-Verl, Ehningen bei Böblingen, 1993. [3] H. C ZICHOS AND K.H. H ABIG : Tribologie-Handbuch: Tribometrie, Tribomaterialien, Tribotechnik. 3 rd edition. Vieweg & Teubner, Wiesbaden, 2010. 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Jahrgang 4/ 2017 Bestellcoupon Tribologie und Schmierungstechnik „Richtungsweisende Informationen aus Forschung und Entwicklung“ Getriebeschmierung - Motorenschmierung - Schmierfette und Schmierstoffe - Kühlschmierstoffe - Schmierung in der Umformtechnik - Tribologisches Verhalten von Werkstoffen - Minimalmengenschmierung - Gebrauchtölanalyse - Mikro- und Nanotribologie - Ökologische Aspekte der Schmierstoffe - Tribologische Prüfverfahren Bestellcoupon Ich möchte Tribologie und Schmierungstechnik näher kennen lernen. Bitte liefern Sie mir ein Probeabonnement (2 Ausgaben), zum Vorzugspreis von 7 39,-. So kann ich die Zeitschrift in Ruhe prüfen. Wenn Sie dann nichts von mir hören, möchte ich Tribologie und Schmierungstechnik weiter beziehen. Zum jährlichen Abo-Preis von 7 189,- Inland bzw. 7 198,- Ausland. Die Rechnungsstellung erfolgt dann jährlich. Das Jahresabonnement ist für ein Jahr gültig; die Kündigungsfrist beträgt sechs Wochen zum Jahresende. Firma, Abteilung Straße, Nr. 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