Failure analysis of a crane gear shaft

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StructuralIntegrity Procedia 00 (2019) 000–000 Available online at www.sciencedirect.com StructuralIntegrity Procedia 00 (2019) 000–000

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Procedia Structural Integrity 18 (2019) 406–412

25th International Conference on Fracture and Structural Integrity 25th International Conference on Fracture and Structural Integrity

Failure analysis of a crane gear shaft Failure analysis of a crane gear shaft

a a

Goran Vukelicaa, D. Pastorcicbb*, G. Vizentinaa Goran Vukelic , D. Pastorcic *, G. Vizentin

University of Rijeka, Faculty of Maritime Studies, Marine Engineering Department, Studentska 2, 51000 Rijeka, Croatia University of Rijeka, Faculty of Maritime Studies, Engineering Department, Studentska 2, 51000 Rijeka, Croatia bUniversity of Zadar,Marine Maritime Department, Zadar, Croatia bUniversity of Zadar, Maritime Department, Zadar, Croatia

Abstract Abstract This research deals with a teeth failure of a gear shaft that served as a part of shipyard crane drive train. Almost all of the teeth of This research withof a teeth failure of fractured a gear shaft that normal served as a part ofofshipyard crane driveanalysis train. Almost all ofexperimental the teeth of a spiral bevel deals gear, part a larger shaft, during operation the crane. Failure combined aand spiral bevel gear, part of a larger shaft, fractured during normal operation of the crane. Failure analysis combined experimental numerical research. Visual inspection was employed to determine the fracture location and crack propagation paths. and numerical research. Visual inspection was employed to inspect determine the fracture and crack paths. Microscopy, optical and scanning electron (SEM), was used to damaged surfaceslocation of the teeth, reveal propagation possible flaws and Microscopy, opticalofand wasfor used to inspect damaged surfaces of the on teeth, reveal composition possible flaws fine microstructure thescanning material.electron Type of(SEM), steel used manufacturing was determined based chemical of and the fine microstructure of theoptical material. Type of steel used for manufacturing wassource determined based on chemical composition of the material obtained using emission spectrometer with glow discharge (GDS) sample stimulation. Further, tensile material obtained using optical emission spectrometer with glow discharge sourcehardness (GDS) test sample Further, tensile specimens were extracted from the shaft to test the strength of the steel. Additionally, was stimulation. performed. All experimental specimens were extracted from the shaft test the strength the steel. hardness test was performed. All experimental research suggest it is a case of gear teethtospalling, probablyofcaused byAdditionally, excessive contact stresses. To gain better understanding of research suggest it isa a3D case of gear teeth probably caused byand excessive contact (FE) stresses. Toanalysis gain better understanding of the failure process, numerical modelspalling, of the gear shaft was built finite element stress performed. Analysis the failure process,contact a 3D numerical of thecontact gear shaft built and finite element (FE) stress analysis performed. Analysis showed excessive stresses atmodel the teeth area.was Time-varying meshing stiffness (TVMS), an important gear health showed contact stresses at contact Time-varying (TVMS), an gear. important gear results health conditionexcessive parameter, is determined viathe FEteeth procedure forarea. several examples ofmeshing healthy stiffness to gradually damaged Obtained condition is determined via FE procedure for several examples of healthy to gradually damaged gear. Obtained results show howparameter, spall propagation influences TVMS and, thus, affects gear performance leading to potential failure. show how spall propagation influences TVMS and, thus, affects gear performance leading to potential failure. © 2019 The Authors. Published by Elsevier B.V. © 2019 The Authors. Published by Elsevier B.V. © 2019 The Authors. Published by B.V.Italiano Frattura (IGF) ExCo. Peer-review under responsibility of Elsevier the Gruppo Peer-review under responsibility of the Gruppo Italiano Frattura (IGF) ExCo. Peer-review under responsibility of the Gruppo Italiano Frattura (IGF) ExCo. Keywords:gear; bevel gear; crane gear shaft; failure analysis Keywords:gear; bevel gear; crane gear shaft; failure analysis

* Corresponding author. Tel.: 0038598891042. * Corresponding author. Tel.: 0038598891042. E-mail address:[email protected] E-mail address:[email protected] 2452-3216© 2019 The Authors. Published by Elsevier B.V. 2452-3216© 2019 The Authors. Published by Elsevier Peer-review under responsibility of the Gruppo ItalianoB.V. Frattura (IGF) ExCo. Peer-review under responsibility of the Gruppo Italiano Frattura (IGF) ExCo.

2452-3216  2019 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Gruppo Italiano Frattura (IGF) ExCo. 10.1016/j.prostr.2019.08.182

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1. Introduction Gears are almost unavoidable components of every machine. Their application ranges from transmission systems in automotive, plane and marine industry, to special applications in power generating and processing plants. Gears design ranges from simple to sophisticated gear pairs and, along with their complexity, rises the need for thorough understanding of acting stresses and, consequently, possible failures. It is especially important as the data show that gear failures make up about 10% of all rotating machinery failures[1]. As for the gear failures, two common types of localized tooth defects are recognized [2]. One is the tooth fillet crack and the other tooth surface spalling. As previous research shows, fillet cracks usually develop in the tooth fillet region. They are caused by deficiencies in the gear tooth which result in stress concentration points that act as a root of subsequent damage. On the other hand, gear tooth spalling tends to develop in the region near the pitch circle of the tooth surface where the tooth pair is subjected to a higher mesh force. Gear tooth spallings are a product of extremely high localized contact stresses that act as damage initiators. There has been a considerable effort in researching the causes of gear failures and offering the propositions for design improvements. Some of the recent work on gear failures includes investigation of a failed axle of a reduction gearbox where, using experimental procedures, was concluded that at the initiation site during the case carburization quenching cracks were formed [3]. Failure analysis of a helical gear used in a bus gearbox indicated that teeth of the helical gear failed by fatigue with a fatigue crack initiation from destructive pitting and spalling region at one end of tooth in the vicinity of the pitch line because of misalignment [4]. Experimental and numerical study of microstructural degradation of a failed pinion gear at a cement plant has proven that the concentration of tensile residual stresses due to untempered core at the tip aided the micropitting and micro-cracking due to the rolling contact surface fatigue was responsible for the initiation of surface cracks and final failure of the gear [5]. Understanding the causes of gear damage and failures is important since it can contribute to the prevention of catastrophic machinery failures. This paper deals with a teeth failure of a gear shaft that serves as a part of shipyard crane drive train. Almost all of the teeth of a spiral bevel gear, part of a larger shaft, fractured during normal operation of the crane. Failure analysis presented here combines experimental and numerical research giving insight how did the damage propagated and influenced the performance of the gear pair. 2. Experimental procedures 2.1. Visual observations A gear shaft that serves as a part of shipyard crane drive train failed with 15 of its 17 helical teeth damaged. It is a spiral bevel gear, part of a larger shaft. Spiral bevel gears find their main application vehicle differentials, where the direction of drive from the drive shaft must be turned 90 degrees to drive the wheels. Design that is characterized by the helical teeth is capable of providing less vibration and noise than conventional gears with straight teeth. Spiral bevel gears come in pairs and once damaged, they should both be changed. Geometry and dimensions of considered spiral bevel pinion are shown in Fig. 1.

Fig. 1. Geometry and dimensions (in mm) of the considered pinion shaft.

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The pinion shaft was a part of the first stage of the crane gearbox., with the engine power of 20 kW at 1200 rpm. Total weight of the pinion shaft is 7.06 kg. Fig. 2 presents a close-up of the gear with damaged teeth. A damage pattern can easily be detected with teeth gradually being brought to excessive contact and damage evolving over the gear. Little or no corrosion occurred on the gear shaft.

Fig. 2. Fractured spiral bevel gear. Dashed line shows the evolution of damage contact.

Fig. 3 presents the teeth damage at the characteristic positions along each tooth, the start of excessive contact, middle point of damage area and final point of damage area. Figures are taken at suitable magnification using Olympus SZX10 stereo optical microscope.

a)

b)

c)

Fig. 3. Fractured spiral bevel gear with marked locations of performed optical micrographyat 12.5x magnification: a) location 1a showing initiation section of damage area, b) location 1b showing middle section of damage area, c) location 1c showing end section of damage area.

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2.2. Material Chemical composition of the gear material was determined using glow discharge spectrometer (GDS) LECO GDS500A, Tab. 1. Table 1. Chemical composition of the gear material (wt%). C

Mn

Si

P

S

Mo

Ni

Cr

V

Rest

0.25

0.18

0.24

0.024

0.021

0.35

2.2

1.00

0.06

95.725

Composition of tested material is adequate to the special alloy structural steel 28NiCrMoV8-5. The alloy structural steels are widely used in automotive, plane and marine industry, guided missile, weapons, railway, bridges, pressure vessels, machine tools, mechanical components with a bigger sectional size, etc. It is a spring steel typically used in manufacturing of light and heavy vehicle leaf springs and coil springs, safety valve springs, shock absorbers on heavy machinery, instrument springs, friction plates, etc. Comparing the composition of the tested steel to standard EN 10250-3:2000, it can be noted that the percentage of nickel is just over the standard range (1.8-2.1 %), while chromium is just at the minimum value (1-1.5 %). As there was enough material, three standard tensile test specimens were machined out of shaft part and tensile test performed to determine material yield strength of 473 MPa, tensile strength of 820 MPa and Young modulus of 188 GPa. All of the values are within standard range for steel 28NiCrMoV8-5. 2.3. SEM analysis Fracture surfaces on the specimens cut from gears, Fig. 4, were examined using scanning electron microscope (SEM) FEI Quanta 250 under suitable magnification. Spalling damage can be detected along the damage area.

a)

b)

c)

Fig. 4. SEM of fractured spiral bevel gearat locations corresponding to Fig. 3: a) location 1a, 50x magnification, b) location 1b, 31x magnification, c) location 1c, 60x magnification.

3. Calculation Finite element (FE) model of considered gear pair was built in order to perform a numerical analysis of the stress and the contact pressure. Scanned dimensions of the gear pair were used to build 3D FE model of tetrahedron elements in Ansys, with mesh refinement of teeth and contact surfaces. In order to verify FE model, first, a stress analysis, according to DIN 3991[6], is performed. The gear ratio is 4, with pinion and gear number of teeth 17 and 68, respectively. The gear pair was loaded with 700 Nm on the output. Maximum equivalent stress obtained numerically

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is 495.26 MPa, Fig. 5. In accordance with DIN 3991, analytically calculated tooth root stress, σF, and permissible tooth root stress, σFP, for the pinion are σF = 215,74 MPa and σFP = 481,63 MPa. The permissible value of contact stress, σHP, and existing surface pressure, σPHP, are σHP = 574,96 MPa and σPHP = 1011,76 MPa , respectively. Maximum surface pressure obtained numerically is 914,16 MPa, Fig 6. Results differ by no more than 3% for the equivalent stress and 9% for the existing surface pressure giving confidence in further use of FE model.

a)

b)

Fig. 5. Spiral bevel gear pair: a) FE model with mesh, b) pinion equivalent stress

Fig. 6. Pinion surface contact pressure

Besides the FE model of undamaged gear pair, the 3D model of the damaged pinion flanks, as well as the model of the damaged pinion teeth tip were also considered, Fig 7., in order to obtain time varying mesh stiffness (TVMS)[7], Fig. 8., asan important parameter of the system condition.The quasi-static algorithm (QSA) is used for computing the TVMS[9].

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b)

Fig. 7. Spiral bevel gear pair: a) pinion damaged flanks, b) pinion damaged tooth tip.

105

1.47

undamaged flank damage teeth tip damage

1.46 1.45 1.44 1.43 1.42 1.41 1.4

0

5

10

15

Pinion angular position (°)

20

25

Fig. 8. Time varying mesh stiffness (TVMS)

4. Discussion and conclusion Visual examination of the failed spiral bevel gear damage evolving around gear teeth. This damage was probably caused by the mismatch between the gear and the pinion designed axial lines causing unwanted contact between the bodies. This contact resulted in raising the stress points along the gear teeth and excessive stress gradually worn out the surface of the teeth. This tends to be the case of gear tooth spalling. Spallings are a product of extremely high localized contact stresses that act as damage initiators and they tend to develop in the region near the pitch circle of the tooth surface where the tooth pair is subjected to a higher mesh force. As the analysis showed, gear was made of special alloy structural steel 28NiCrMoV8-5. It is a common choice for manufacturing gears exposed to heavy duty tasks. This steel with somewhat elevated content of nickel and chromium. Chromium at steels tends to increase tensile strength, hardness, toughness, resistance to wear and corrosion, while nickel increases strength and hardness without sacrificing ductility and toughness. It can also increase resistance to

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corrosion and scaling at elevated temperatures. Measured yield and maximum tensile strength are in line with expected for steel 28NiCrMoV8-5. Optical and scanning electron microscopy images revealed damage to the teeth surface. Damage evolving around teeth can clearly be tracked, from the introduction point to the end of contact. The origin of the damage can be tracked to the point where the sum of all applied and misapplied stresses intersect the net strength of the gear. Spalling originated below the surface, near the case/core transition surface, leading to final failure of the gear shaft. Comprehensive data of the research performed include adequate images of fractured surface at suitable magnification with details of damage growth and advancement. Numerical analysis indicated localized high contact stresses and TVMS reduction for damaged gear teeth. In order to reduce the contact stress and the effects of high contact stress, material of pinion should be case hardened steel and positive profile shift on the pinion should be performed in manufacturing. Obtained results can be used in understanding the damage, wear and failure behavior of spiral bevel gears in heavy duty gearboxes and further improvements of gear design, manufacturing, finishing and assembly could be made based on this analysis. Acknowledgements This work has been supported by the University of Rijeka within the project uniri-technic-18-200 “Failure analysis of materials in marine environment”. References [1]

R. Ma, Y.S. Chen, Q.J. Cao, Research on dynamics and fault mechanism of spur gear pair with spalling defect, J. Sound Vib. 331 (2012) 2097–2109. [2] J.R. Davis, Gear Materials, Properties, and Manufacture, American Society for Materials, Materials Park, 2005. [3] W. Ost, P. De Baets, J. Quintelier, Investigation of a failed axle of a reduction gearbox, 14 (2007) 1194–1203. doi:10.1016/j.engfailanal.2006.11.030. [4] O. Asi, Fatigue failure of a helical gear in a gearbox, 13 (2006) 1116–1125. doi:10.1016/j.engfailanal.2005.07.020. [5] V. Rajinikanth, M.K. Soni, B. Mahato, M.A. Rao, Study of microstructural degradation of a failed pinion gear at a cement plant, Eng. Fail. Anal. 95 (2019) 117–126. doi:10.1016/j.engfailanal.2018.08.031. [6] Deutsche Norm DIN 3991, Teil 1-4 [7] C.G. Cooley, C. Liu, X. Dai, R.G. Parker, Gear tooth mesh stiffness : A comparison of calculation approaches, MAMT. 105 (2016) 540–553. doi:10.1016/j.mechmachtheory.2016.07.021. [8] Y. Luoa, N. Baddoura, G. Hanb, F. Jiangc, M. Lianga, Evaluation of the time-varying mesh stiffness for gears with tooth spalls with curvedbottom features. Eng. Fail. Anal. 92 (2018) 430–442. doi: 10.1016/j.engfailanal.2018.06.010. [9] J. Zhan , M. Fard, R. Jazar, A CAD-FEM-QSA integration technique for determining the time-varying [10] meshing stiffness of gear pairs, Measurement 100 (2017) 139–149, doi: 10.1016/j.measurement.2016.12.056