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Failure analysis of an automobile diff erential pinion shaft H. Bayrakceken * Afyon Kocatepe University, Technical Education Faculty, 03200 Afyon, Turkey Received 8 July 2005; accepted 14 July 2005 Available online 2 September 2005 Abstract Diff erential is used to decrease the speed and to provide moment increase for transmitting the movement coming from the engine to the wheels by turning it according to the suitable angle in vehicles and to provide that inner and outer wheels turn diff erently. Pinion gear and shaft at the entrance are manufactured as a single part whereas they are in diff erent forms according to automobile types. Mirror gear which will work with this gear should become familiar before the assembly. In case of any breakdown, they should be changed as a pair. Generally, in these systems there are wear damages in gears. The gear inspected in this study has damage as a form of shaft fracture. In this study, failure analysis of the diff erential pinion shaft is carried out. Mechanical characteristics of the material are obtained fi rst. Then, the microstructure and chemical compositions are determined. Some fractographic studies are carried out to asses the fatigue and fracture conditions. ? 2005 Elsevier Ltd. All rights reserved. Keywords: Diff erential; Fracture; Power transfer; Pinion shaft 1. Introduction The fi nal-drive gears may be directly or indirectly driven from the output gearing of the gearbox. Di- rectly driven fi nal drives are used when the engine and transmission units are combined together to form an integral construction. Indirectly driven fi nal drives are used at the rear of the vehicle being either sprung and attached to the body structure or unsprung and incorporated in the rear-axle casing. The fi nal-drive gears are used in the transmission system for the following reasons 1: (a) to redirect the drive from the gearbox or propeller shaft through 90? and, (b) to provide a permanent gear reduction between the engine and the driving road-wheels. 1350-6307/$ - see front matter ? 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2005.07.019 * Tel.: +90 272 228 1311; fax: +90 272 228 1319. E-mail address: .tr. Engineering Failure Analysis 13 (2006) 14221428 /locate/engfailanal In vehicles, diff erential is the main part which transmits the movement coming from the engine to the wheels. On a smooth road, the movement comes to both wheels evenly. The inner wheel should turn less and the outer wheel should turn more to do the turning without lateral slipping and being fl ung. Diff eren- tial, which is generally placed in the middle part of the rear bridge, consists of pinion gear, mirror gear, diff erential box, two axle gear and two pinion spider gears. A schematic illustration of a diff erential is given in Fig. 1. The technical drawing of the fractured pinion shaft is also given in Fig. 2. Fig. 3 shows the photograph of the fractured pinion shaft and the fracture sec- tion is indicated. In diff erentials, mirror and pinion gear are made to get used to each other during manufacturing and the same serial number is given. Both of them are changed on condition that there are any problems. In these systems, the common damage is the wear of gears 24. In this study, the pinion shaft of the diff erential of a minibus has been inspected. The minibus is a diesel vehicle driven at the rear axle and has a passenger capacity of 15 people. Maximum engine power is 90/4000 HP/rpm, and maximum torque is 205/1600 Nm/rpm. Its transmission box has manual system (5 forward, 1 back). The damage was caused by stopping and starting the minibus at a traffi c lights. In this diff erential, entrance shaft which carries the pinion gear was broken. Various studies have been made to determine the type and possible reasons of the damage. These are: ? studies carried out to determine the material of the shaft; ? studies carried out to determine the micro-structure; ? studies related to the fracture surface. There is a closer photograph of the fractured surfaces and fracture area in Fig. 4. The fracture was caused by taking out circular mark gear seen in the middle of surfaces. Fig. 1. Schematic of the analysed diff erential. H. Bayrakceken / Engineering Failure Analysis 13 (2006) 142214281423 Fig. 2. Technical drawing of the analysed pinion shaft. Fig. 3. The picture of the undamaged diff erential pinion analysed in the study. Fig. 4. Photographs of failed shaft and fracture surfaces. 1424H. Bayrakceken / Engineering Failure Analysis 13 (2006) 14221428 2. Experimental procedure Specimens extracted from the shaft were subjected to various tests including hardness tests and metal- lographic and scanning electron microscopy as well as the determination of chemical composition. All tests were carried out at room temperature. 2.1. Chemical and metallurgical analysis Chemical analysis of the fractured diff erential material was carried out using a spectrometer. The chem- ical composition of the material is given in Table 1. Chemical composition shows that the material is a low alloy carburising steel of the AISI 8620 type. Hardenability of this steel is very low because of low carbon proportion. Therefore, surface area be- comes hard and highly enduring, and inner areas becomes tough by increasing carbon proportion on the surface area with cementation operation. This is the kind of steel which is generally used in mechanical parts subjected do torsion and bending. High resistance is obtained on the surface and high fatigue endur- ance value can be obtained with compressive residual stress by making the surface harder 57. In which alloy elements distribute themselves in carbon steels depends primarily on the compound- and carbide-forming tendencies of each element. Nickel dissolves in the a ferrite of the steel since it has less ten- dency to form carbides than iron. Silicon combines to a limited extent with the oxygen present in the steel to form nonmetallic inclusions but otherwise dissolves in the ferrite. Most of the manganese added to carbon steels dissolves in the ferrite. Chromium, which has a somewhat stronger carbide-forming tendency than iron, partitions between the ferrite and carbide phases. The distribution of chromium depends on the amount of carbon present and if other stronger carbide-forming elements such as titanium and columbium are absent. Tungsten and molybdenum combine with carbon to form carbides if there is suffi cient carbon present and if other stronger carbide-forming elements such as titanium and columbium are absent. Man- ganese and nickel lower the eutectoid temperature 8. Preliminary micro structural examination of the failed diff erential material is shown in Fig. 5. It can be seen that the material has a mixed structure in which some ferrite exist probably as a result of slow cooling and high Si content. High Si content in this type of steel improves the heat treatment susceptibility as well as Table 1 Chemical analysis of the pinion gear material (wt%) FeCSiMnPSCrMoNi 96.920.2350.2520.7860.0440.0160.4810.1510.517 Fig. 5. Micro structure of the material (200). H. Bayrakceken / Engineering Failure Analysis 13 (2006) 142214281425 an improvement of yield strength and maximum stress without any reduction of ductility 9. If the micro- structure cannot be inverted to martensite by quenching, a reduction of fatigue limit is observed. There are areas with carbon phase in Fig. 5(a). There is the transition boundary of carburisation in Fig. 5(b) and (c) shows the matrix region without carburisation. As far as it is seen in these photographs, the piece was fi rst carburised, then the quenching operation was done and than tempered. This situation can be understood from blind martensite plates. 2.2. Hardness tests The hardness measurements are carried out by a MetTest-HT type computer integrated hardness tester. The load is 1471 N. The medium hardness value of the interior regions is obtained as 43 HRC. Micro hard- ness measurements have been made to determine the chance of hardness values along the cross-section be- cause of the hardening of surface area due to carburisation. The results of Vickers hardness measurement under a load of 4.903 N are illustrated in Table 2. 2.3. Inspection of the fracture The direct observations of the piece with fractured surfaces and SEM analyses are given in this chapter. The crack started because of a possible problem in the bottom of notch caused the shaft to be broken com- pletely. The crack started on the outer part, after some time it continued beyond the centre and there was only a little part left. And this part was broken statically during sudden starting of the vehicle at the traffi c lights. As a characteristic of the fatigue fracture, there are two regions in the fractured surface. These are a smooth surface created by crack propagation and a rough surface created by sudden fracture. These two regions can be seen clearly for the entire problem as in Fig. 4. The fatigue crack propagation region covers more than 80% of the cross-section. Table 2 Micro hardness values Distance from surface (lm)50100200400Center Values HV (4903N)588410293286263 Fig. 6. SEM image of the fracture surface showing the ductile shear. 1426H. Bayrakceken / Engineering Failure Analysis 13 (2006) 14221428 Shaft works under the eff ect of bending, torsion and axial forces which aff ect repeatedly depending on the usage place. There is a sharp fi llet at level on the fractured section. For this reason, stress concentration factors of the area have been determined. Kt= 2.4 value (for bending and tension) and Kt= 1.9 value (for torsion) have been acquired according to calculations. These are quite high values for areas exposed to combined loading. These observations and analysis show that the piece was broken under the infl uence of torsion with low nominal stresses and medium stress concentration 10. The scanning electron microscopy shows that the fracture has taken place in a ductile manner (Fig. 6). There are some shear lips in the crack propagation region which is a glue of the plastic shear deformations. Fig. 7 shows the beach marks of the fatigue crack propagation. The distance between any two lines is nearly 133 nm. 3. Conclusions A failed diff erential pinion shaft is analysed in this study. The pinion shaft is produced from AISI 8620 low carbon carburising steel which had a carburising, quenching and tempering heat treatment process. Mechanical properties, micro structural properties, chemical compositions and fractographic analyses are carried out to determine the possible fracture reasons of the component. As a conclusion, the following statements can be drawn: ? The fracture has taken place at a region having a high stress concentration by a fatigue procedure under a combined bending, torsion and axial stresses having highly reversible nature. ? The crack of the fracture is initiated probably at a material defect region at the critical location. ? The fracture is taken place in a ductile manner. ? Possible later failures may easily be prevented by reducing the stress concentration at the critical location. Acknowledgement The author is very indebted to Prof. S. Tasgetiren for his advice and recommendations during the study. Fig. 7. SEM image of the fracture surface showing the beach marks of the fatigue crack propagation. H. Bayrakceken / Engineering Failure Analysis 13 (2006) 142214281427 References 1 Heisler H. Vehicle and engine