A failure investigation has been conducted on a diesel-engine gear-shaft used in a truck, which is made from 45# steel.

A diesel engine gear-shaft used in a truck fractured during running. It was reported that the oil hole onthe cylinder of the failed gear-shaft was stopped up by over-long centre bolt. The failed gear-shaft is made

The microstructures of the gear-shaft were observed by scanning electron microscopy (SEM) on a Philips

* Corresponding author. Tel.: +86 0411 8472 9613; fax: +86 0411 8472 8670.E-mail address: xxiaolei@mail.dlmu.edu.cn (X.L. Xu).

Engineering Failure Analysis 13 (2006) 13511357

www.elsevier.com/locate/engfailanal1350-6307/$ - see front matter 2005 Elsevier Ltd. All rights reserved.of 45# steel. The cylinder surface and the platform of the xed plate of the gear-shaft is demanded to beinduction-quenched according to the technical specication. The paper describes the detailed metallurgicalinvestigation and a careful fractographic study on the failed gear-shaft. The possible failure reasons wereassessed.

2. Experimental methods

The chemical composition of the failed gear-shaft was analyzed by spectroscopy chemical analysis method.The crack initiated from the root transition region between the cylinder and the platform of the gear-shaft, and propagatedtoward the direction with an angle of 45 to the axial direction rst, then toward the direction normal to the axial direction.Multiply-origin fatigue fracture is the dominant failure mechanism. The oil hole on the cylinder of the gear-shaft wasstopped up by the overlong bolt to lead to the absence of oil lubrication between the cylinder surface and the internal circlesurface of the gear so that the friction force between them increased, which is mainly responsible for the failure of the gear-shaft. The absence of the induction-hardened layer in the root transition region between the cylinder and the platform ofthe xed plate of the gear-shaft makes the fatigue strength decrease in this region. The fatigue crack initiated and prop-agated in this region easily under the abnormal stress. 2005 Elsevier Ltd. All rights reserved.

Keywords: Gear-shaft failure; Multiple-origin fatigue; Induction-hardening

1. IntroductionFailure analysis of a diesel engine gear-shaft

Xiaolei Xu *, Zhiwei Yu, Hongxin Ding

Electromechanics and Material Engineering College, Dalian Maritime University, Dalian 116026, PR China

Received 2 September 2005; accepted 24 October 2005Available online 13 February 2006

Abstractdoi:10.1016/j.engfailanal.2005.10.015

The failed gear-shaft is shown in Fig. 1. It can be seen that the gear-shaft fractured into two pieces to form

Wear appears on the platform of the xed plate and the region close to the mark A was worn seriously

3.2.1. Fracture morphology

induction-hardening. This leads to the lower whole strength of the gear-shaft. The microstructure of the cyl-

1352 X.L. Xu et al. / Engineering Failure Analysis 13 (2006) 13511357inder in the induction-hardened region is composed of ne martensite (Fig. 6b). However, it is important thatthe microstructure in the crack origin region is composed troostite and ferrite (Fig. 6c), which is suggested thatSEM observations on the crack origin region of the fracture P1 are shown in Fig. 2. Two fatigue steps canbe observed, which indicates multiple-origin fatigue fracture is the dominant failure mechanism [1] of the gear-shaft. The fracture surface of the crack origin region is smooth and no obvious inclusions can be observed.The crack propagated rapidly after the crack crossed the platform (Fig. 3). In the stable crack propagationregion on the fracture P2, beach marks or fatigue striations can be observed on both sides of the centre hole(Fig. 4). It indicates further that the fatigue fracture occurred on the gear-shaft.

3.2.2. Wear morphology

SEM observations on the wear morphology of the cylinder surface show that the serious wear occurred onthe cylinder surface opposite oil hole, which attributes to the abrasive wear [2] (Fig. 5a). It is suggested that thecontact compressive force is greater between the internal circle surface of the gear and the cylinder surface inthe region. It should be mentioned that the contact compressive force is agreement with the drive force result-ing in the crack propagation on the fracture P1, which would be discussed in Section 4 in detail. Lighter wearoccurred on the cylinder surface opposite the crack origin region, or on the same side as the oil hole, whichattributes to the adhesive wear [2] (Fig. 5b).

3.2.3. Microstructure examinationThe microstructure in various zone of the failed gear-shaft was observed by SEM. It indicates that the core

microstructure of the failed gear-shaft is composed of the ne pearlite and ferrite other than tempered sorbite(Fig. 6a). It is suggested that the material was not conducted by quenching and tempering treatment before(marked in Fig. 1b). Detailed observations indicate that the wear marks around the crack on the platformexhibit continuously. It is suggested that the surface wear of the platform had occurred before the gear-shaftfractured. Serious wear occurred on the cylinder surface opposite the oil hole (by arrow in Fig. 1a). The wallthickness of the cylinder opposite the oil hole is 7.28 mm, but another side is 8.27 mm. It is suggested that theinclined wear took place on the cylinder surface. It is noted that the cylinder surface worn seriously and thecrack origin region are situated at the same orientation.

3.2. Micro-observationstwo fractures, fractures P1 and P2. Surface of fracture P1 has a 45 angle to the axial direction of the cylinderand surface of fracture P2 is vertical to the axial direction. From the crack propagation paths on the fractures,it can be inferred that the fractures P1 and P2 formed by two crack propagation processes. Fracture P1 pro-duced before the formation of the fracture P2. The crack initiated from the root meeting region between theplatform of the xed plate and the cylinder (by arrows in Fig. 1c and d). The crack propagated toward thedirection with an angle of about 45 to the axial direction rst, then propagating toward the direction normalto the axial direction after crossing the xed plate. The surface of fracture P1 is relatively smooth and thebeach marks can be observed on the surface of fractures P1 and P2, which are fatigue features [1].XL-30 scanning electron microscope. The fractured surfaces were analyzed by visual and SEM observations tostudy the failure mechanism.

3. Results

3.1. Visual examinationthe transition region between the cylinder and the platform was not induction-hardened.

Fig. 1. Failed gear-shaft and fracture surfaces: (a) failed gear-shaft, (b) macrograph of xed plate surface, (c) matched fractures P1 and(d) matched fractures P2.

X.L. Xu et al. / Engineering Failure Analysis 13 (2006) 13511357 1353

1354 X.L. Xu et al. / Engineering Failure Analysis 13 (2006) 13511357Fig. 2. SEM fractography of the crack origin region on the fracture P1.3.3. Chemical composition of the gear-shaft material

Table 1 gives the chemical composition of the core material of the failed gear-shaft, compared with thespecied chemical composition of the gear-shaft material. It can be seen that the chemical composition ofthe material for the failed gear-shaft is within the specied range.

4. Analysis on failure causes

The diagrammatic sketch of force acting on the gear-shaft is shown in Fig. 7, two forces are acted on thegear-shaft when practical servicing: the vertical compressive force on the cylinder surface by the gear, F1; andthe tightening force by the xed bolt, F2. The main stress resulting from the two forces is along the bisector ofthe root transition llet between the cylinder and the platform, which corresponds to the drive force for crackinitiation and propagation along the fracture P1. In normal servicing, the compressive force acted on the gear-shaft by gear should be smaller. However, the oil hole on the cylinder of the failed gear-shaft was stop up byoverlong bolt to lead to the absence of the oil lubrication between the internal circle surface of the gear and thecylinder surface of the gear-shaft and to make the friction force between them increase. From the wear marks,the compressive force on the cylinder surface opposite the oil hole is really greater. Microstructure observa-tions indicate that crack origin region was not quenching-hardened, which make the fatigue strength of the

Fig. 3. SEM fractography of the crack propagation region on fracture P1.

Fig. 5. SEM observations on worn surface of the cylinder: (a) seriously worn surface showing abrasive feature and (b) lightly worn surfaceshowing adhesive feature.

Fig. 4. SEM fractography of fracture P2 showing fatigue striations: (a) low-power morphology and (b) high-powered morphology.

X.L. Xu et al. / Engineering Failure Analysis 13 (2006) 13511357 1355

Fig. 6. Microstructure of failed gear-shaft: (a) core region, (b) induction hardened region at the cylinder surface and (c) root transitionregion between the cylinder and the xed plate.

Table 1Chemical composition of the gear-shaft material (wt.%)

Element C Si Mn P S Fe

Analysed 0.43 0.29 0.51 0.028 0.021 BalanceAs specied 0.420.45 0.170.37 0.50 60.035 60.035 Balance

1356 X.L. Xu et al. / Engineering Failure Analysis 13 (2006) 13511357

X.L. Xu et al. / Engineering Failure Analysis 13 (2006) 13511357 1357crack origin region decrease. Additionally, the root transitional region is usually the stress concentration posi-tion. So fatigue crack initiated at the root transitional region, and propagated along the 45 direction in theregion easily by the stress, especially by the greater abnormal stress described above, forming fracture P1.When crack crossed the whole platform, the action force F2 contributed to the crack propagation along frac-ture P1 would decrease and the stress acted on the gear shaft corresponds to the bending stress. Crack prop-agated along the direction normal to the axel to fracture at last, forming fracture P2.

5. Conclusions

(1) The overlong bolt stop up the oil hole on the cylinder of the gear-shaft to lead to the absence of the oillubrication between the internal circle surface of the gear and the cylinder surface of the gear-shaft, which

oil hole

F2

seriously worn cylinder surfaceF1

P2

P145

main stress plane

Fig. 7. Diagrammatic sketch of force acting on the gear-shaft.make the friction force between them increase. This is mainly responsible for the failure of the gear-shaft.(2) Absence of the hardened layer in the root transition region between the platform and the cylinder of the

gear-shaft makes the fatigue strength of the region decrease. The fatigue crack can initiate and propagateeasily in the region under the act of the greater abnormal force. Multiple origin fatigue is the dominantfailure mechanism of the gear-shaft.

6. Recommendations

(1) Decrease the length of the centre bolt the in order not to stop up the oil hole of the gear-shaft.(2) Improve the shape of the inductor and the technology parameter of the induction-quenching to make

sure that the root transition region between the cylinder and the platform of the xed plate can be induc-tion-hardened.

References

[1] Metal handbook. In: Fractograph and atlas of fractograph. 8th ed. vol. 9. Metals Park (OH): American Society for Metals; 1974.[2] Bhushan B. Principles and applications of tribology. New York: A Wiley-Interscience Publication; 1999.

Failure analysis of a diesel engine gear-shaftIntroductionExperimental methodsResultsVisual examinationMicro-observationsFracture morphologyWear morphologyMicrostructure examination

Chemical composition of the gear-shaft material

Analysis on failure causesConclusionsRecommendationsReferences