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Engineering Failure Analysis 14 (2007) 725–730

www.elsevier.com/locate/engfailanal

Failures of single cylinder diesel engines crank shafts

H. Bayrakceken *, S. Tasgetiren, F. Aksoy

Afyon Kocatepe University, Technical Education Faculty, Afyon, Turkey

Received 3 January 2006; accepted 8 January 2006Available online 19 May 2006

Abstract

The single cylinder diesel engines are extensively used in agricultural areas for several purposes such as water pumping.In some cases, these engines are also used for driving some auxiliary agricultural vehicles. This imposes unexpected loadson the components of the engines resulting in the premature failures. In this study, two different failure cases of crankshaftsof these engines are analyzed. Some characterization studies and fractographic analyses are carried out to asses the failurereason. However, the cranks have some miner design differences, both failures are occurred after a fatigue process. One ofthe failed cranks has important heat treatment error.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Crank shaft; Engine; Failure

1. Introduction

The single cylinder diesel engines are extensively used in agricultural areas for several purposes such aswater pumping. The use of the diesel fuel and having only one cylinder make them an economic alternativedue to their low investment cost and limited fuel consumption. As a result, these engines are important aux-iliary agricultural tool for rural areas. In some cases these engines are also used for driving some vehicles.Especially, the peoples in the rural areas in Turkey living on the agricultural activities and stockbreedinguse these vehicles for many reasons out of traffic roads. In the fields and wooded gardens, these low velocityvehicles can carry up to 2 ton of load (Fig. 1). However the use of the vehicles is forbidden in the traffic, thefarmers prefer them in off-road places. In addition, these vehicles can be manufactured in small workshopswith limited possibilities by using scrap car parts such as transmission box and differential even the lightingsystem.

The crankshaft transfers the linear piston movement to curvilinear motion while the force of the connectingrod is transformed to torque. The crankshaft is designed by consideration of number and placements of pis-tons, number and bearings and ignition order etc. The crankshaft also drives the camshaft and some otherelements via a chain or a belt drive system. The single cylinder engine has a disadvantage when compared

1350-6307/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.engfailanal.2006.01.006

* Corresponding author. Tel.: +90 272 228 1311; fax: +90 272 228 1319.E-mail address: [email protected] (H. Bayrakceken).

Fig. 1. Three examples of auxiliary agricultural vehicle driven by the single cylinder diesel engine.

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to 2 and 4 cylinder engines in view of balancing due to the difficulties of passing the dead points during theignition.

Crankshafts are made from materials which can be readily shaped, machined, and heat-treated and whichhave desirable mechanical properties such as adequate strength, toughness, hardness, and high fatiguestrength. Traditionally, these shafts were forged, which promoted all the necessary properties; but, with theevolution of the nodular cast irons and improvements in foundry techniques, cast crankshaft are now favoredfor moderate loads, and only for heavy-duty applications do forged shafts predominate [1]. The dead weightsprovide the static and dynamic balance caused by different ignition sequence during working. The main bear-ings and connecting rod bearings are hardened up to 2–3 mm by means of some surface hardening methods.The lubrication of the bearings is provided by lubrication holes drilled in the crankshaft. In some cases theinner regions are bored out for light design and reduced inertial forces.

One of the key element which works under highly variable loading conditions is the crankshaft. This ismore promoted for the single cylinder engines. Several researchers have been studied on the failed crankshafts[2–6]. Researchers generally states that; mechanical fatigue failures are probably the most common cause ofcrankshaft failures [2,3]. The sources may include misalignment of the shaft, rotating-bending, vibrationdue to some problem with the main bearings and high stress concentrations caused by a sharp fillet. Silva stud-ied on the failure of journal bearing of a crankshaft and concluded that the damage of the journals was orig-inated by the small cracks at the surface of the journals. The small cracks originated by thermal fatigue due tooverheating during the grinding operation [4].

Yu and Xu also studied the breakage of a diesel engine crankshaft observing that the failure initiated fromthe fillet region of the crankpin-web. In their observation, the partial absence of the nitrided layer in the fillet

Fig. 2. Failed crankshafts and their fracture surfaces.

Fig. 3. Technical drawing of the analyzed crankshafts.

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region makes the fatigue strength reduce to initiate and propagate fatigue in the weaker region and to lead topremature fatigue fracture [5].

A study on the computation of dynamic stressing of crankshafts is also carried out. The geometry of suchelements couples the torsional, flexural and axial behavior and, owing to the presence of crank mechanisms,the configuration of the system changes in time, with a period equal to one revolution [6].

In the present study, failure analyses of two crankshafts of single cylinder diesel engines are carried out. Theengines are used under excessive environments and loading conditions other than their design conditions.Fig. 2 gives the photographs of the failed crankshafts and Fig. 3 gives the technical drawing and main sizes.The characterization of the material is done by spectroscopy and stress measurements while the fractographicanalyses are carried out by scanning electron microscopy.

2. Analyses

Chemical analysis of the fractured crank shaft material was carried out using a spectrometer. The chemicalcomposition of the material is given in Table 1. Chemical composition shows that the material is a medium-carbon steel of the type AISI 4140. This steel has high hardenability and good fatigue, abrasion and impactresistance and may be shaped by cold or hot forging. To successfully cold form the steel, the row metal has toundergo spheroidize annealing to facilitate shaping and must subsequently be reheated, quenched, and tem-pered in order to obtain the necessary mechanical properties. The chromium content gives good hardness pen-etration and the molybdenum supply uniformity of hardness and high strength. This grade is especiallysuitable for forging and it responds readily to heat treatment and easy to machine in the heat treated condi-tion. The wear resistance can be increased by surface hardening techniques such as flame hardening or induc-tion hardening [7].

Table 1AISI 4140 chemical analysis of the crank shaft material

Composition (wt%)

Fe C Cr Mn Mo P S Si

Crank 1 96.6 0.414 0.933 0.735 0.175 0.022 0.026 0.293Crank 2 96.9 0.41 0.823 0.831 0.164 0.026 0.017 0.187AISI 4140 Min 96.58 0.37–0.44 0.75–1.2 0.65–1.1 0.15–0.3 Max 0.035 Max 0.04 0.15–0.3

Table 2Hardness values on the cross section of the failed crankshafts

Distance from surface mm 1 2 3 4 5 6

Crank 1 HB 213 208 210 212 212 213Crank 2 HB 327 331 320 230 220 215

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Table 2 gives the hardness values measured at the cross sections of the crankshafts. The values for thecrank1 exhibit the characteristic values of annealed 4140 material. That is, no hardening treatment is appliedto this crankshaft for which the yield strength is 420 MPa and ultimate stress is 670 MPa. For the case ofcrank 2, it can be seen that a surface hardening treatment is applied. The hardened case length reaches upto 3 mm which could be obtained by flame or induction hardening [8].

The microstructures of the crankshaft materials are given in Fig. 4. The difference between the two micro-structures also gives the clue of hardening treatment. As can be seen, crank 2 has a hardened surface layer

Fig. 4. Microstructure of the crankshaft components (a, b near surface, c inner region).

Fig. 5. SEM micrographs of the fracture surfaces.

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(Fig. 4a and b) while crank 1 has not. The crank 2 has a tempered martensite structure near the surfaceregions.

The black points observed in the matrix are possibly carbide inclusions grown to very large sizes due toimproper production and heat treatment conditions. These inclusions are larger in crank 1 than crank 2and the difference can be observed in Fig. 4c. The inclusions in both materials are larger than normal sizes [9].

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The scanning electron microscopy photographs of the both crankshaft fracture surfaces are given in Fig. 5.The black regions in Fig. 4 are visualized as cavities and mounds as indicated by arrows in Fig. 5a. These loca-tions may lead to initiation of fatigue crack. However smaller carbide inclusions may hinder the growth ofcrack, inclusions of the present size cause faster failure of the cross sections [10].

The fracture is occurred after a fatigue process as can be concluded from the benchmark striations in thefracture surfaces (Fig. 5b–d). The fatigue crack initiated at the sharp fillet region as can be seen in Fig. 2. Thediameter of the bearing is about 49 mm while the fillet radius is 3 mm. This cause relatively high stress con-centration especially for the single cylinder crankshaft subjected bending and torsional loading applied as astroke process. When the severe environment in which the engine is utilized for driving the auxiliary agricul-tural vehicle is considered, the fracture has taken place as a premature failure.

The diagonal lubrication holes in the Crank 2 might cause to turn the crack propagation direction andspeed up the propagation process. The plastically deformed benchmark lines in Fig. 5c and d exhibit theunhardened feature of the Crank 1.

3. Conclusions

In this study, failure analyses of crankshafts of two single cylinder diesel engines are carried out. Bothengines are used in some auxiliary agricultural vehicles which is not considered using area in the design stageof the engines. However the crankshafts have some miner design differences, both failures are occurred after afatigue process. Both shafts are made form the same material however one of them has undergo a surfacehardening heat treatment and the other is used under the annealed conditions. The failure has began at thesharp fillet region and the lubrication holes influenced the crack growing direction.

Acknowledgement

The authors thank Dr. S. Talas for helpful discussions on the micrographs.

References

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1997;205(2):135–215.[7] Chandler H. Heat treater’s guide, practices and procedures for irons and steels’’. 2nd ed. USA: ASM International; 1995.[8] www.matweb.com; 2005.[9] Yang ZG, Yao G, Li GY, Li SX, Chu ZM, Hui WJ, et al. The effect of inclusions on the fatigue behavior of fine-grained high

strength 42CrMoVNb steel. Int J Fatigue 2004;26:959–67.[10] Talas� S�. Characteristics of inclusions formed in high purity Fe–Ti–O–N system. Technol Res: EJMT 2005(2):13–5.