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Analysis and design against pitting of gears in power transmissions for off-highway vehicles
Get PDFThe work described in this thesis has been developed within the scope of a higher education apprenticeship program in cooperation with Carraro S.p.A., global player in the market of power drivelines for vehicles employed in industrial and agricultural applications, also addressed as off-highway vehicles. The study is aimed to the analysis of the mechanisms leading to contact fatigue failures of gears and to the development of reliable and efficient methods to characterize gears materials. Pitting is the most hazardous among rolling contact fatigue phenomena occurring in gears. It manifests with the formation of craters on the surfaces leading to noise, vibration and loss of efficiency, and eventually may constitute the initiation cause for secondary damages such as tooth fractures. The sun gears of the planetary gear sets, which constitute the final stage of reduction in off-highway drivelines, are particularly susceptible to pitting failures, being subjected to high torques under poor lubricating conditions determined by the low speed of rotation.
In this work, contact fatigue phenomena are studied by the observation of their occurrence in sun gears failed during endurance tests performed on the whole axles. The failure mechanisms as well as the most common approaches proposed in the literature for the evaluation of the load carrying capacity and the prevision of the lifetime of gears are commented through an analysis of the existing literature on the subject. Then the problem of the design against pitting of case hardened gears is faced from two different perspectives: on the one hand the determination of reliable endurance limits for the materials used by the suppliers of Carraro S.p.A. in the manufacturing of case hardened gears, based on a reanalysis of the database of validation tests performed in the past on the whole axles; on the other hand an experimental analysis of the pressure distribution on gear teeth during operation is reported. This latter study was useful to assess the major discrepancies between theoretical models and actual systems, in order to improve the awareness in the use of calculation software packages dedicated to the prevision of the lifetimes of gears. Finally, the development of a test rig for the contact fatigue characterization of gears materials by means of relatively fast tests on simple disc specimens is described. The first attempts to resemble the operating conditions of a particular gear pair with a dedicated design of specimens and to correlate the fatigue performances of gears and discs are discussed as well
ROLLING CONTACT FATIGUE CHARACTERIZATION OF GEAR MATERIALS USING TWIN-DISC TESTS
No full textThe ISO standard 6336 for the calculation of the load carrying capacity of gears provides design curves against contact fatigue, which allow a rough estimation of the gear durability based on a coarse classification of materials. For a more accurate assessment, the derivation of the permissible values of Hertzian pressure on the tooth flanks by means of tests is suggested. In order to evaluate the endurable stress of a material in a fast and cost-effective way, the ISO standard mentions the use of tests on pairs of discs in contact, choosing the geometries, loads and speeds in order to simulate as close as possible the working conditions of the actual gear pair. However, no information is given about the determination of gears durability from the results of tests on discs, and about an appropriate method for specimens design. In this paper a twin-disc test rig is presented along with the results of preliminary tests on through hardened steels. Finally, the criteria used to design the specimens in order to recreate the working condition experienced by sun gears of off-highway axles are discussed
CONTACT FATIGUE CARACTERIZATION OF 17NiCrMo6-4 SPECIMENS USING A TWIN-DISC TEST BENCH
No full textIn this work, a twin-disc test rig for the assessment of the contact fatigue performances of gear materials is presented. Tests were performed on 17NiCrMo6-4 discs with two different slide-to-roll ratios. The specimen were designed to recreate the meshing conditions between sun and planet gears of the planetary gear set located in the wheel hub of off-highway vehicles. Sun gears indeed, are particularly prone to manifest pitting damages because of the high frequency of contacts due to the meshing with three or four planets, and because of the small teeth dimensions which results in high contact pressures even with moderate applied loads. The damaging mode observed in disc specimens is then compared with the actual failure mode of gears
DESIGN CURVES AGAINST CONTACT FATIGUE FOR CASE HARDENED GEARS OF OFF-HIGHWAY DRIVELINES
No full textIl pitting è uno dei meccanismi di cedimento più frequenti nelle ruote dentate di assali per veicoli off-highway. A causa della molteplicità di parametri che influenzano il fenomeno, i risultati di test a pitting mostrano una dispersione elevata. Di conseguenza sono necessarie campagne di test dispendiose su un elevato numero di ruote campione per la determinazione delle curve di fatica a pitting. Tuttavia, poiché le aziende che progettano sistemi di trasmissione di potenza sottopongono a prove di fatica i nuovi prodotti, in questo lavoro sono state derivate curve di resistenza a pitting partendo da 13 test eseguiti da Carraro S.p.A. sull’intero assale. La pressione agente sul fianco del dente è stata calcolata dapprima secondo il metodo proposto dalla normativa ISO 6336 e, successivamente, attraverso una più accurata analisi del contatto. Sono quindi state ricavate le curve di fatica a pitting e stimate le bande di dispersione per gli acciai cementati impiegati nei test
A twin disc test rig for contact fatigue characterization of gear materials
No full textPitting on gear tooth flanks is one of the major causes of failure in power transmissions. Cracks originate at the surface and propagate at a small depth causing the detachment of material debris, which results in craters. Pitting is detrimental as it leads to vibration, noise, loss of efficiency and eventually to the gear un-serviceability. The contact fatigue characterization of gear materials requires a great number of endurance tests on reference gears and is rarely affordable for industries. For this reason, the ISO standard 6336 suggests that tests on rolling pair of disks may be performed in order to compare the pitting durability of either different materials or manufacturing processes. However, the standard does not provide guidance about the geometry of the specimens and the correlation between the results of disc tests and actual gears durability.
In this paper a twin-disc test rig is presented, that was conceived to reproduce the contact pressure and the sliding velocity of gears at one particular point along the tooth profile. A criterion for specimens design is also described. The discs were sized to resemble the working conditions experienced by sun gears mounted in the final drive of an axle for medium power Off-Highway vehicles. In particular, the Lower Point of Single Tooth Contact (LPSTC) was considered to design the tests, being the most favourable location for pitting occurrence because of the high contact pressures and unfavourable kinematic conditions
Experimental analysis of tooth-root strains in a sun gear of the final drive of an off-highway axle
No full textThe force acting on gear teeth can be influenced by several factors such as profile modifications, stiffness variations during meshing, inversion of the sliding direction at the pitch line, tip-to-root interferences, gears and shaft deflections and bearings clearances. Moreover, in planetary gear sets the load can be shared unevenly among the planet gears due to manufacturing inaccuracies of the system. An accurate evaluation of the real load-time history experienced by the teeth is not straightforward and is affected by strong approximations even when advanced simulation software packages are used to create the theoretical model. Therefore, experimental analysis of the behavior of gears under in-service load still constitutes a major step in the development of new transmission systems. In this work, three strain gauges were applied at different positions along the tooth root width of the sun gear mounted in the final drive of an off-highway axle. Strain measurements where then performed during a bench test of the complete axle and the signal was acquired by means of a telemetry system. Finally, the acquired data were used to assess the accuracy of software calculations and to identify the causes of overloads
Experimental analysis of contact fatigue damage in case hardened gears for off-highway axles
No full textPitting is one of the main causes of failure in planetary gear sets of axles designed for agricultural
vehicles. The high torque and the low wheel speed typical of such machines result in poor
lubrication and promote the onset of contact fatigue failure by pitting, which generally occurs
earlier in the sun gear than in both the planets and the ring of wheel hub planetary drives. In
fact, sun gears are subjected to the highest contact pressure and, at the same time,
unfavourable rolling-sliding working conditions. In this paper, six case hardened sun gears
damaged by pitting during endurance tests were analysed. The aim of the analysis was to highlight
the key aspects of the morphology and the evolution of pitting damage on the case hardened
sun gears
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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