Aus der Praxis für die Praxis 1 Introduction High-speed injection lubricated gear units are found mainly in power plant technology between the gas or steam turbine and the generator. The solid or hollow shafts of the single-stage gear units are mounted in the housing via slide bearings. They are made from 18CrNiMo7-6 hardened steel and the gear teeth are case-hardened. Single helical gears are used for the low and medium power range. Only the double helical gears are considered for high power range from approx. ˃ 60 MW and circumferential speeds of up to 180 m/ s. High demands are placed on the turbo gear units regarding reliability, operational safety, low noise level and low loss of power. Therefore, the factor of safety from pitting and tooth breakage is also high at S H ˃ 1,6 and S F ˃ 3. Nevertheless, damage to the load flanks can be observed. The power losses determine the degree of efficiency, which is approx. 98 - 99 %. In a gear unit with a nominal power of e. g. 100 MW, the power loss is 1MW, which turns into heat. Brown discoloured oil residues adhering Tribologie + Schmierungstechnik 65. Jahrgang 1/ 2018 33 * Dipl. Ing.-Univ. Erwin Bauer AEB Kompetenzcenter, Garching b. München Aeb-garching@t-online.de Heat in Turbo Transmissions Causes, Effects & Remedies E. Bauer* Wärmeentwicklung in Turbogetrieben, Ursachen, Auswirkungen und Abhilfemaßnahmen In Turbogetrieben mit Umfangsgeschwindigkeiten ab ca. 80 bis zu 180 m/ s entstehen vor allem durch Strömungsverluste erhebliche Wärmemengen. Ist die Kühlleistung nicht ausreichend, kommt es zu Veränderungen des Öls, die sich in braun gefärbten, an den Oberflächen anhaftenden Belägen äußert. Bei zu hohem Temperaturniveau sind die thermisch bedingten Verformungen erheblich größer als die elastischen Biege- und Torsionsverformungen der verzahnten Wellen. Zudem treten im Laufe der Betriebszeit bleibende Verformungen durch irreversible Gefügeveränderungen auf. Dies führt zur Verschlechterung der Verzahnungsqualität, zu Veränderungen im Tragbild, und zu deutlich höheren Beanspruchungen als in der Tragfähigkeitsberechnung ausgewiesen. Die Folge sind Ermüdungsschäden. Fallbeispiele werden vorgestellt und Abhilfemaßnahmen diskutiert. Schlüsselwörter Turbogetriebe, Wärmehaushalt, Belagbildung, thermische Verformung, Restaustenit, Flankenkorrektur, Ermüdungsschaden, Gegenmaßnahmen Under atmospheric conditions, the no-load losses in turbo gear units with high pitch line velocities of > 80 m/ s, generate such high thermal loads that permanent adhering oil residues are formed and irreversible changes in the shape of the teeth can be observed. This results in local overload at the locations with the highest thermal load. In spite of more than adequate factor of safety from pitting, damages on the active flanks still occur. In the examples presented here, they start not at the surface but in the volume of the teeth below the case depth. The largest portion of the no-load losses are due to windage losses. If turbo gear units are operated in a vacuum at approx. 0.1 bar, the no-load losses can be reduced by approx. 30 % compared to atmospheric operation. Nevertheless, the thermal distortions may not be disregarded when determining tooth corrections. A structural transformation can be counteracted through higher annealing temperatures after case-hardening. If, during atmospheric operation of turbo gear units, an inspection reveals that an irreversible change in shape has occurred, the original condition can be reestablished through regrinding. However, an Ultrasonic Inspection should show beforehand that there are no impermissible large discontinuities in the volume of the teeth. The contact pattern should be inspected at regular intervals so that a new gear unit - if possible with vacuum technology - can be purchased in a timely manner if necessary. Keywords high-speed gear units, heat balance, varnishing, thermal distortion, retained austenite, lead modification, fatigue, corrective measures Kurzfassung Abstract T+S_1_18 06.12.17 12: 19 Seite 33 Aus der Praxis für die Praxis permanently to the teeth and the bearings were found on the turbo gear units that were inspected. Based on case studies this article describes the distribution of the oil residue along the face width and the position and type of flank damages. Results of Ultrasonic inspections, the measurement of profile and tooth trace, as well as the X-ray measurement of the content of retained austenite are presented. The causes and effects of the high thermal load are discussed, based on fundamental observations with respect to heat balance. 2 Heat Balance In gear units with relatively low circumferential speeds, the friction losses are generated mainly in the mesh and in the sliding bearings. These losses consist of the loadindependent proportions, the so-called no-load losses, and the load-dependent proportions. In the mesh, the gear friction losses determine the load-dependent proportion. They increase proportionally with nominal power. The no-load losses are low compared to the tooth friction losses. The situation is different in the case of the injection-lubricated turbo gear units considered here, with high pitch line velocities of greater than 80 m/ s. Here, the total loss of power consists almost exclusively of the no-load losses. According to [1] these are composed of losses due to squeezing of the oil film in the mesh, losses due to acceleration and deflection of the injected oil, and losses due to windage. The more oil is injected, the higher the no-load losses. Not only the quantity but also the direction in which the oil is injected has an effect. Injection after mesh has a favorable effect on cooling. As a rule, oil is injected before and after the mesh, the larger quantity being injected after the mesh. Losses due to squeezing occur when the injected oil is moving through the mesh from the leading to the trailing end. The generated heat is dissipated mainly via the oil mist with the rest through thermal dissipation into the pinion and the wheel. The windage losses are caused by the swirling of the oil mist in the housing. They make up the largest proportion of the no-load losses. In [2], a single helical pinion was fitted with thermocouples at several points along the face width. The temperatures were measured at different pitch line velocities under idling conditions. The result is shown in Figure 1 (from [3]). The temperature curves rise in parallel with increasing speed. At all circumferential speeds, the temperature at the leading end of the face width is constant up to approx. half of the face width, increases significantly and then reaches the maximum temperature shortly before the trailing end at approx. b/ 6. Thereafter, the temperature drops down. This temperature distribution results in different levels of thermal expansion along the tooth trace. 3 Varnishing ISO VG Class 46 turbine oils are used for turbo gear units. They have a high degree of thermal stability and high resistance to the formation of deposits. The injection temperature is approx. 40 to 50 °C. Accelerated aging occurs in the oil at very high temperatures. The oil becomes depleted of antioxidants and the base oil decomposes. Insoluble brown-coloured deposits are formed, which adhere permanently to the components. They cannot be removed with solvents. The aging process of the oil must be checked by an oil analysis at regular intervals and appropriate measures taken if necessary. The permanent adhering deposits on the tooth flanks increase losses due to friction, when the oil is squeezed through the mesh. They also have a heat-insulating effect. As a result, heat dissipation into the pinion and wheel is inhibited. 4 Case Studies In all the examples presented here, the turbo gear units are operated under atmospheric conditions. 4.1 Example 1 In the case of a single helical gear unit with a low power range but with a high pitch line velocity of 110 m/ s, damage to the teeth of the pinion was observed during an 34 Tribologie + Schmierungstechnik 65. Jahrgang 1/ 2018 Figure 1: Distribution of bulk temperature on a single helical pinion T+S_1_18 06.12.17 12: 19 Seite 34 Aus der Praxis für die Praxis inspection after approx. 65,000 operating hours. The pinion shaft has an unfavorable ratio of b/ d 1 > 1.1. Figure 2 shows the pinion with the adhering browncoloured deposits, which are formed more intensively on the warmer exit side NDE than on the cooler leading end DE. The injected oil runs along the tooth trace in the direction of NDE and gradually heats up. This results in a qualitative temperature distribution as shown in Figure 1. Due to the non-uniform thermal expansion occurring over the face width, the flank and profile shape changes, and a high contact stress occurs in the NDE half, leading to ruptures over the tooth height (Figure 3). However, the ruptures do not start on the surface, as by classic pitting. The crack starter is located in the tooth volume shortly below the case depth. The fatigue fracture surface is aligned parallel to the active flank A (Figure 4). Consequently, it propagates towards the tooth tip and the no active flank NA, forming lines of rest. If the bulk temperature during operation is higher than the annealing temperature selected after casehardening, an irreversible increase in volume occurs due to the structural transformation of retained austenite into martensite. Therefore, in the case of single-helical gears, a change from the cylindrical into the conical shape can be observed. 4.2 Example 2 In the case of a double-helical gear unit with a medium power range and a high circumferential speed of 150 m/ s and an adequate ratio of B/ d1 < 2, adhering residue has formed on the gear teeth due to excessively high temperatures (Figure 5). Based on the discolorations in the root surface of the tooth space it can be seen on both tooth halves TS (turbine side) and GS (generator side) that the highest temperatures are located near the exit side. In the TS tooth half (Figure 6), the zone with the highest thermal load is located between 0.5*b and 0.8*b. In the Tribologie + Schmierungstechnik 65. Jahrgang 1/ 2018 35 Fig. 1: Distribution of bulk temperature on a single helical pinion Fig. 2: Pinion with deposits on the teeth NDE exit side Fig. 3 L DE Figure 2: Pinion with deposits on the teeth Fig. 3: Ruptions on the active flanks A near exit side Fig. 4: Crack direction towards tooth tip and no active flank NA A Figure 3: Ruptures on the active flanks A near exit side Fig. 3: Ruptions on the active flanks A near exit side Fig. 4: Crack direction towards tooth tip and no active flank NA NA A Figure 4: Crack direction towards tooth tip and no active flank NA Fig. 5: Double helical pinion with deposits Fig. 6: TS-tooth half; overheated zone between 0,5b and 0,8b TS GS exit side exit side DE Figure 5: Double helical pinion with deposits Fig. 5: Double helical pinion with deposits Fig. 6: TS-tooth half; overheated zone between 0,5b and 0,8b exit side ~0,5b ~0,8b Figure 6: TS-tooth half; overheated zone between 0,5b and 0,8b T+S_1_18 06.12.17 12: 19 Seite 35 Aus der Praxis für die Praxis area of the tip relief, the flank is damaged over nearly the entire face width. The maximum damage is located close to the exit side (Figure 7). In the GS tooth half, the zone with the highest thermal load (Figure 8) is located between 0,5*b and 0,9*b. The damage to the tooth tip is at a maximum also close to the exit side. In the same zone, line-shaped damage in the tooth root area can be detected at the exit side (arrow, Figure 9). The zones with the highest thermal load qualitatively correspond to the temperature curve shown in Figure 1. The damage to the tooth tip suggests that an irreversible change in the profile has taken place. The thermal load on the wheel (Figure 10) is lower than on the pinion. Based on the visual findings, it was decided to carry out an Ultrasonic Inspection according to [4]. On the pinion, 20 UT-indications within the volume of several teeth were detected in the TS tooth half and 4 in the GS tooth half. All are located in the range of the pitch circle and above. They occur frequently at a distance of 0.6 to 0.7*b from the gap S, consequently in the area of the highest thermal (see Figure 6, 8) and therefore also the highest mechanical load. On the wheel, 3 indications were detected in the TS tooth half and 33 in the GS tooth half. They are located in the range of the pitch circle and below. In the TS tooth half, they are located approx. in the middle of the face width, whereas in the GS tooth half, they are at a distance of 0.6 to 0.9*b from the leading end S. All UT-indications in the volume of the teeth of the pinion and the wheel are located slightly below the case depth. On one tooth from the GS tooth half of the wheel an internal discontinuity with a lengthwise expansion of approx. 75 mm was detected. It is located close to the exit side at a distance of 0.6 to 0.85*b from the gap. The discontinuity was opened in the laboratory (Figures 11, 12). The elliptically shaped smooth and bright shining primary fracture surface is surrounded by a honeycomb fracture (W) that resulted during opening in the laboratory. The ellipse has a size of 2a*2b = 75* 9 mm. The area of the ellipse corresponds to a circle with a diameter of approx. 26 mm. The primary fracture surface was induced during operation by a fatigue fracture in vacuum. The V-shaped striations (Figure 13) identify a starting point below the case depth (arrow, Figure 14). The fatigue fracture is located between the base and the 36 Tribologie + Schmierungstechnik 65. Jahrgang 1/ 2018 Fig. 7: Damage at tooth tip (arrow) near exit side Fig. 8: GS-tooth half; overheated zone between 0,5b and 0,9b Figure 7: Damage at tooth tip (arrow) near exit side Fig. 9: Damage at tooth tip and near tooth root (arrow) Fig. 10: Wheel with brown coloured deposits GS TS Figure 10: Wheel with brown coloured deposits Fig. 7: Damage at tooth tip (arrow) near exit side Fig. 8: GS-tooth half; overheated zone between 0,5b and 0,9b exit side ~0,5b ~0,9b Figure 8: GS-tooth half; overheated zone between 0,5b and 0,9b Fig. 9: Damage at tooth tip and near tooth root (arrow) Fig. 10: Wheel with brown coloured deposits Figure 9: Damage at tooth tip and near tooth root (arrow) T+S_1_18 06.12.17 12: 19 Seite 36 Aus der Praxis für die Praxis pitch circle and is sloped at an angle towards the active flank A. 4.3 Example 3 On a single helical gear unit with a medium power range, injection-lubrication and a pitch line velocity of 90 m/ s, a flank breakage [5] has occurred on one tooth of the pinion after several years of operation. The ratio b/ d1 is marginal. The temperature distribution over the face width is comparable to that in Example 1. An Ultrasonic Inspection showed an accumulation of UT-indications in the area of the highest thermal load. For the pinion, the comparison of the gear teeth measurement data from the initial state and after the damage on the active and no-active flanks shows a change in the tooth trace and in the profile. On the active flanks the tooth trace error has changed from quality 3 to 10 and the profile error from quality 4 to 9. The change in the profile indicates that the base circle has widened and that the tooth tip protrudes. The irreversible change in the tooth trace and profile indicates a structural transformation of retained austenite Tribologie + Schmierungstechnik 65. Jahrgang 1/ 2018 37 Abb. 11: Opened UT-indication-fatigue fracture; W: ductile fracture Fig. 12: Microsection S W A ~9 mm W Figure 11: Opened UT-indication-fatigue fracture; W: ductile fracture Figure 14: Microsection S; crack starter (arrow) Abb. 11: Opened UT-indication-fatigue fracture; W: ductile fracture Fig. 12: Microsection S S x Fig 13 Figure 12: Microsection S Fig. 13: Crack propagation crack starter 0 (arrow) Fig. 14: Microsection S; crack starter (arrow) Figure 13: Crack propagation crack starter 0 (arrow) Fig. 15: Distribution of retained austenite over the face width Fig. 16: Single helical gear; deflection, thermal distortions and lead modification exit side Fig. 15: Distribution of retained austenite over the face width Fig. 16: Single helical gear; deflection, thermal distortions and lead modification Fig. 15: Distribution of retained austenite over the face width Fig. 16: Single helical gear; deflection, thermal distortions and lead modification Figure 15: Distribution of retained austenite over the face width T+S_1_18 06.12.17 12: 19 Seite 37 Aus der Praxis für die Praxis into martensite. For this purpose, the content of retained austenite was determined by X-ray measurement. The measurement was carried out directly on the tooth tip as well as on the active and no-active flanks at several points along the face width. Due to restricted accessibility, the measurement on the surfaces of both flanks was only possible in an area approx. 2 mm below the tooth tip. The result of the measurement on one tooth of the pinion is shown in Figure 15. Assuming that the content of retained austenite is constant everywhere in the new state, the curves of both flanks indicate a drop to about 6 - 8 % shortly before the exit side. Therefore, most of the retained austenite was converted into martensite in this area. This conversion results in an irreversible increase in volume. The conversion is greatest in the area of the highest thermal and mechanical load. The UTindications occur here more frequently and this is also the area where the flank breakage is located. 5 Corrective Measures 5.1 New gear unit to be manufactured The calculation of the elastic bending and torsional deformations is well known. At high pitch line velocities of more than about 80 m/ s, considerable thermal distortions also occur, which may no longer be disregarded. Figures 16, 17 from [3] show the significant influence the thermal distortions have on the lead modification being performed. The profile correction must also be adjusted accordingly. The goal is to keep the thermally induced deformations as low as possible. The windage losses make up the largest proportion of the no-load losses. To reduce this, newer turbo gear units are operated in a vacuum at about 0,1 bar. This makes it possible to reduce the no-load losses by approx. 30 % compared to atmospheric conditions. One option for counteracting the structural transformation of retained austenite into martensite is the selection of a higher annealing temperature after case-hardening. 5.2 Gear unit after prolonged run-time The examples in chapter 4 show gear units which have been operated under atmospheric conditions. During an inspection, it is possible to decide whether an irreversible change of the shape has taken place through visual inspection of the contact pattern and the distribution of the oil residues over the face width as well as by measuring the back lash. It also makes sense to re-establish the initial state by regrinding. It is to be assumed that a large proportion of the retained austenite has already been converted before regrinding and therefore the likelihood of a further structural transformation is low during subsequent operation. However, this measure only makes sense if an Ultrasonic Inspection proves that there are no impermissible discontinuities in the volume of the teeth. The contact pattern should be inspected at regular intervals so that a new gear unit can be purchased in a timely manner if necessary. 38 Tribologie + Schmierungstechnik 65. Jahrgang 1/ 2018 Figure 16: Single helical gear; deflection, thermal distortions and lead modification Figure 17: double helical gear; deflection, thermal distortions and lead modification T+S_1_18 06.12.17 12: 19 Seite 38 Aus der Praxis für die Praxis References [1] G. Niemann, H. Winter: Maschinenelemente Band II, 2. Auflage; Springer Verlag 1983 [2] L. Martinaglia: Thermal Behaviour of High-Speed Gears and Tooth Corrections for Such Gears; Mechanism and Machine Theory, 1973. Vol. 8 P. 293-303 [3] MAAG: Tooth flank modifications for involute high speed gears and gears of similar requirements; MG-TFModifications 1000.09.03 [4] B. Metzner*, E. Bauer, K. Graf, A. Bohl, D. Lang: Ultraschallprüfung der Verzahnungen von Gasturbinengetrieben; VGB Power Tech 11/ 2003 [5] E. Bauer*, A. Bohl: Flank Breakage on gears for energy systems; VDI Bericht 2108.2 S. 1039-1052; GEARTECH- NOLOGY November/ Dezember 2011 p. 36-42 Tribologie + Schmierungstechnik 65. Jahrgang 1/ 2018 39 Anzeige Dipl.-Ing. (FH) Thomas Merkle, M.Eng. Kreiselpumpen und Pumpensysteme Betrieb, Instandhaltung und Schadensvermeidung 3. Auflage 2017, 140 S., 104 Farbabb., 20 Tab., 10 Grafiken, 29 Diagramme, 39,80 €, 52,00 CHF (Kontakt & Studium, 702) ISBN 978-3-8169-3396-0 Zum Buch: Hier erhalten Planer, Anlagenbauer und Betreiber wichtige Informationen zum Betrieb von Kreiselpumpen und darüber, wie sich Schäden an Pumpen und Pumpensystemen minimieren oder vermeiden lassen. Das Buch vermittelt Hinweise und Vorschläge für Maßnahmen zu Fehlervermeidung, Fehlererkennung (Überwachung) und Fehlermanagement. Anhand von praktischen Beispielen werden Schadensmechanismen und Zusammenhänge aufgezeigt und bewertet. 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(FH) Thomas Merkle, M.Eng., verfügt über jahrzehntelange Erfahrungen in den Bereichen Strömungstechnik und Energietechnik – von Entwicklung und Konstruktion bis hin zu Planung und Betrieb von Pumpen und Pumpensystemen. Seit 15 Jahren arbeitet er in leitender Position eines Industrieunternehmens in der Pumpenbranche. Im Rahmen dieser Tätigkeit wurden zahlreiche Untersuchungen zum Thema »Pumpenverschleiß« und »Effizienzsteigerung« durchgeführt und Lösungen erarbeitet. Blätterbare Leseprobe und einfache Bestellung unter: www.expertverlag.de/ 3396 Bestellhotline: Tel: 07159 / 92 65-0 • Fax: -20 E-Mail: expert@expertverlag.de T+S_1_18 06.12.17 12: 19 Seite 39