Engineering Failure Analysis 18 (2011) 450–454

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Engineering Failure Analysis

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Failure of the pinion from the drive of a cement mill

Gorazd Kosec a, Aleš Nagode b, Igor Budak c, Aco Antic c, Borut Kosec b,⇑a Acroni, d.o.o, Cesta Borisa Kidrica 44, 4270 Jesenice, Sloveniab University of Ljubljana, Faculty of Natural Sciences and Engineering, Askerceva cesta 12, 1000 Ljubljana, Sloveniac University of Novi Sad, Faculty of Technical Sciences, Trg D. Obradovica 6, 21000 Novi Sad, Serbia

a r t i c l e i n f o

Article history:Received 8 July 2010Received in revised form 27 September 2010Accepted 28 September 2010Available online 27 October 2010

Keywords:Cement millPinionFailureHardening

1350-6307/$ - see front matter � 2010 Elsevier Ltddoi:10.1016/j.engfailanal.2010.09.033

⇑ Corresponding author. Tel.: +386 1 2000 410; faE-mail address: borut.kosec@omm.ntf.uni-lj.si (B

a b s t r a c t

The pinion from the drive of the cement mill was failed; the teeth cracked and spalloccurred on the sides of several teeth. The failure was only located on one side of thepinion. This type of failure is common with surface-hardened gears.

We have found that the failure of the pinion is a direct consequence of the incorrectgeometry of the surface hardened layer. The lifespan of the pinion could have beenextended if the whole surface of the faces and roots of the teeth had been hardened andif the hardening had been deeper.

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1. Introduction

In this article we describe an example of the rupture of gear teeth of a relatively large module of large dimensionsthat – built into reduction gears of large machinery and devices in process industries (e.g. cement mill) – also endures largeloads, forces and torques [1,2].

When manufacturing gears for large modules, wear of the gear teeth faces is often prevented by surface hardening [3,4].Little attention is usually paid to the resistance of gears against fatigue. When it comes to gear fatigue, the division, signs andamount of internal stresses acquired specifically through surface hardening are very important. The incorrect geometry ofthe hardened surface is the cause of improper internal stress distribution and inadequate structural strength [5].

With gears, the hardened surface area of the teeth faces often ends near the root of the teeth where the maximum tensile(positive) residual stresses occur. This is normally also the area of the largest changes of external tensile stresses due to oper-ation of the gear. The superposition of positive stresses from both sources, in connection with additional eventual geometricstress concentrators, contributes to the formation and spread of fatigue cracks. However, since the gears frequently rotate inboth directions, cracks appear in both roots of a tooth, of which one crack is usually longer.

The failure of the investigated pinion of the cement mill drive (No. 354881, tooth 28, module 36, diameter 1640 mm,width 1800 mm) occurred in the form of fatigue cracks and the spall of steel on the faces of several teeth. The failure wasonly located on one side of the pinion (Fig. 1). The teeth breakage began with cracking, which typically started at the rootsof the teeth faces and spread outwards. The breakage resulted in transverse ruptures along the length of the teeth.

The other failure that occurred was the spall of the steel on the faces of the teeth. Such failure is caused by excessiveHertzian pressure applied to the faces, or is a consequence of the lack of compressive strength of the steel at a critical depth

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x: +386 1 4704 560.. Kosec).

Fig. 1. Failure of the pinion: part of the broken off tooth, and the spall of steel on the faces of the teeth.

G. Kosec et al. / Engineering Failure Analysis 18 (2011) 450–454 451

of the teeth surface. In this way, the unbroken teeth are also damaged, but the extent of this type of damage was significantlysmaller in our case.

2. Macroscopic characteristics of failures

Visible lines composed of temper colours formed on both side faces of the individual teeth and their roots. They are aconsequence of the thermal effects of surface hardening [6]. These lines were wider and more distinctive at the undamagedside of the gear [7,8]. The hardened areas were along the faces and at the roots, and were interrupted at the top of the teeth(Figs. 2 and 3).

The teeth faces on the failed side were macroscopically etched [9]. This revealed the surface hardened layer, the macro-scopic average thickness of which is approximately 1 mm, which generally starts at the top of the tooth and ends approx-imately 10 mm above its root.

The macroscopic profile of the teeth’s surface hardened layer is not satisfactory. It has two disadvantages: it only covers apart of the teeth faces, and it is very thin. The entire surface of the faces and roots should have been hardened; however, it isnot necessary to harden the surface at the top of the teeth. The macroscopic characteristics of the surface areas of the frac-tures show that the fractures are a consequence of the fatigue of the steel which is capable of achieving high hardness by aproper heat treatment [10,11].

The chemical composition of the steel of the pinion is shown in Table 1. According to its chemical composition, the steel ofthe pinion corresponds to the high-strength steel used for improving VCMo140 of the Slovene steel manufacturer MetalRavne [12].

The microstructure of the steel of pinion was observed by optical (light) microscope (OM) Olympus 815X as well as withscanning electron microscope (SEM) Jeol 5610 at Faculty of Natural Sciences and Engineering, University of Ljubljana. Thesamples were metallographically prepared by grinding and polishing, and then etched by 2% Nital.

Fig. 2. A broken tooth and two unbroken teeth with a fatigue crack. Drawn lines on the metal surface show the area where the material would be cut off forthe further investigations.

Fig. 3. Two failured teeth: the lines indicating the heated surface.

Table 1Chemical composition of steel pinion [12].

Element C Si Mn Cr Mo Ni P S

Mass.% 0.40 0.34 0.69 1.16 0.27 0.28 0.01 0.03

452 G. Kosec et al. / Engineering Failure Analysis 18 (2011) 450–454

The microstructure of the pinion steel reveals that the pinion was previously strengthened and its surface was hardened(Fig. 4).

The pinion was only flank hardened and, therefore, the microstructure of the steel in faces consists of martensite (Fig. 5),while the microstructure of the steel in core consists of tempered bainite and ferrite (Fig. 6). In the transition area (betweenthe hardened and non-hardened area) the microstructure transforms through martensite to the temper bainite and ferrite.

The constant hardness (approximately 650 HV) is characteristic of the hardened surface of the pinion, and decreases overa transition zone to the hardness at the tooth core (approximately 275 HV) (Fig. 7).

3. Cause of failure

The investigated pinion from the drive of the mill was only flank hardened. The hardened pattern occupies the tooth flankarea and stops prior to the tooth fillet. This provides the required wear resistance. However, in the hardened/non-hardenedtransition region, the residual stresses change from compressive in the hardened area (flank) to the tensile in non-hardenedarea (root). A combination of applied tensile stresses due to operation of the pinion with tensile residual stresses createsfavourable conditions for early crack development in the root/area [13].

Such a geometry (pattern) of the hardened area is prone to fatigue due to repeated loading/unloading operation. Since thepinion was moving only in one direction, the cracks initiated in the teeth root/fillet areas only on the one side of the tooth.

Fig. 4. The area where the hardened surface of the tooth ends (OM).

Fig. 5. Microstructure of the steel at the hardened surface: martensite (SEM).

Fig. 6. Microstructure of the steel at the core of the tooth: bainite (light), ferrite (dark) (SEM).

Fig. 7. Microhardness in the hardened surface and transition to the core.

G. Kosec et al. / Engineering Failure Analysis 18 (2011) 450–454 453

Thus, the correct geometry of the hardening would be flank and root hardening which provides excellent combination offatigue and wear strength, as well as resistance to shock loading and scuffing.

The areas of the heated surface at the faces of the pinion teeth show that the planner and performer of the surface hard-ening knew this fact, but failed to strengthen the gear teeth correctly [14]. The spall of the steel on the teeth faces is a con-sequence of excessive Hertzian pressures, which exceed the compressive strength of the steel. The critical area where thecracks and spall first occurred is at the point of transition between the hardened surface (martensite) and the core of thetooth, where the mechanical properties of steel (strength) begin to decrease rapidly (Figs. 8 and 9). As in the previous case,the problem could be solved with flank and root hardening, which has to be deep enough.

Fig. 8. Crack at the point of transition from the hardened surface to the core (OM).

Fig. 9. Microstructure at the point of transition from the hardened surface to the core (SEM).

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4. Conclusions

The damage of the pinion (fatigue fracture of the teeth and the spall of the steel on the faces of the teeth) are a conse-quence of the incorrect geometry (pattern) of the surface hardened layer.

The teeth broke off due to the fatigue. The cracks first appeared in the tooth/fillet area and spread outwards, while thebreakage resulted in a cross-break along the height of the teeth. The other failure that occurred was the spall of the steelat the faces of the teeth. Thus, in addition to the ruptured teeth, the remaining teeth were also damaged. However, the extentof this failure was smaller.

We could, therefore, extend the lifespan of the pinion if the entire faces and roots of the teeth were hardened, and if thesurface hardening was deeper.

References

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