Failure analysis of an automobile differential pinion shaft
Abstract
Differential 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 differently. Pinion gear and shaft at the entrance are manufactured as a single part whereas they are in different 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 differential pinion shaft is carried out. Mechanical characteristics of the material are obtained first. Then, the microstructure and chemical compositions are determined. Some fractographic studies are 2005 Elsevier Ltd. All rights reserved.
Keywords: Differential; Fracture; Power transfer; Pinion shaft
1. Introduction
The final-drive gears may be directly or indirectly driven from the output gearing of the gearbox. Directly driven final drives are used when the engine and transmission units are combined together to form an integral construction. Indirectly driven final 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 final-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.
In vehicles, differential 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 flung. Differential, which is generally placed in the middle part of the rear bridge, consists of pinion gear, mirror gear, differential box, two axle gear and two pinion spider gears.
A schematic illustration of a differential is given in Fig, 1. The technical drawing of pinion the fractured pinion shaft is also given in Fig, 2, Fig. 3 shows the photograph of the fractured pinion shaft and the fracture section is indicated.
In differentials, 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 [2-4]. In this study, the pinion shaft of the differential 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 traffic lights. In this differential, 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.
2. Experimental procedure
Specimens extracted from the shaft were subjected to various tests including hardness tests and metallographic 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 differential material was carried out using a spectrometer. The chemical composition of the material is given in Table 1. Chemical composition shows that the material is a lowalloy carburizing steel of the AISI 8620 type.
Hardenability of this steel is very low because of low carbon proportion. Therefore, surface area becomes 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 endurance value can be obtained with compressive residual stress by making the surface harder [5-7].
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 α ferrite of the steel since it has less tendency 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 depends on the 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 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 is there is sufficient carbon present and if other stronger carbide-forming elements such da titanium and columbium are absent. Manganese and nickel lower the eutectoid temperature [8].
Preliminary micro structural examination of the failed differential 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
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 carburization in Fig. 5(b) and (c) shows the matrix region without carburization. As far as it is seen in there photographs, the piece was first carburized, then the quenching operation was done 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 obtained as 43 HRC. Micro hard-ness measurements have been made to determine the chance of hardness values along cross-section be-cause of the hardening of surface area due to carburization. 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 completely. 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 traffic lights. As a characteristic of the fatigue , 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.
Shaft works under the effect of bending, torsion and axial forces which affect repeatedly depending on the usage place. There is a sharp fillet 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 influence of torsion with low nominal stresses 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 lines is nearly 133 nm.
3. Conclusions
A failed differential pinion shaft is analysed in this study. The pinion shaft is produced from AISI
8620 low carbon carburising steel which had a carbursing, 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 srudy.
References
[1] Heisler H. Vehicle and engine technology. 2nd ed. London: SAE International; 1999.
[2] Makevet E, Roman I. Failure analysis of a final drive transmission in off-road vehicles. Eng
Failure Anal 2002;9:579-92.
[3] Orhan S, Aktu ¨rk N. Determination of physical faults in gearbox through vibration analysis. J Fac Eng Arch Gazi University 2003;18(3):97–106..
[4] Tasgetiren S, Aslantas ? K, Ucun I. Effect of press-fitting pressure on the fatigue damages of root in spur gears. Technol Res: EJMT 2004;2:21–9.
[5] Nanawarea GK, Pableb MJ. Failures of rear axle shafts of 575 DI tractors. Eng Failure Anal 2003;10:719–24.
[6] Aslantas K, Tasgetiren S. A study of spur gear pitting formation and life prediction. Wear 2004;257:1167–75.
[7] Savas V, O ¨ zek C. Investigation of the distribution of temperature on a shaft with respect to the deflection. Technol Res: EJMT 2005;1:33–8.
[8] Smith FW. Principles of materials science and engineering. 3rd ed. USA: McGraw-Hill Series; 1996. p. 517–18.
[9] ASM metal handbook, vol. 1. Properties and selection, irons, steels, and high performance alloys; 1991.
[10] Voort GFV. Visual examination and light microscopy. ASM handbook metallography and microstructures. Materials Park (OH): ASM International; 1991. p. 100–65.
汽车差速器小齿轮轴的失效分析
摘要
差速器是用来降低速度增加扭矩并根据合适的角度向两轮传递动力。小齿轮和其所安装的轴是一体的。在装配前应熟悉这一齿轮结构。不管发生任何故障，小齿轮和其所安装的轴都要一起更换。一般而言，在这些系统中，齿轮的损坏形式为磨损损坏。在这项研究中检查的齿轮，损坏形式为轴断裂。在这项研究为差速器小齿轮轴的故障分析。首先获得的材料的机械特性。然后，确定微观结构和化学组合物。
关键词：差速器；断裂；动力传递；小齿轮轴
1．简介
最终的驱动齿轮可以直接或间接地从变速器的输出齿轮驱动。当发动机和传动装置结合在一起形成一个整体结构时，需使用直接驱动的最终驱动齿轮。间接驱动末级驱动器或借助一些辅助装置敷在汽车后方或者纳入驱动桥。最后的传动系统中使用该齿轮如以下原因：
（1）将传动轴从变速器或传动轴上定向到90度。
（2）在发动机和驱动轮之间提供永久减速。
在车辆中，差速器是传递发动机和车轮之间运动的主要部分，在平滑的路面上，运动是由两个车轮均匀传动的。内轮应转向少，外轮应多转向，不然转向时会发生滑移。差速器，一般放在后桥的中间，由星形齿轮架、差速器箱、半轴齿轮和星形齿轮组成。
图1是一个示意图。图2、图3显示了小齿轮轴的技术图和小齿轮轴的照片，并指出了断裂的部分。
在差速器制造过程中，从动轮和小齿轮的使用相同的序列号。出现问题二者都需更换。在这些系统中，常见的损伤是齿轮[2-4]磨损。在这项研究中，对一辆面包车的差速器小齿轮轴进行了检查。该面包车是一辆后轮驱动的柴油车
并有15人的载客能力。发动机最大功率为90 / 4000马力/转速，最大扭矩为205 / 1600纳米/转/分。它的变速箱有手动系统（5向前，1回）。损害是由停在交通灯下启动面包车引起。在这差速器中，带有小齿轮的入口轴被打破了。各种各样的研究已经确定的类型和可能的损坏原因如下：
•进行研究，以确定轴的材料；
•进行研究，确定了微结构；
•与断裂面相关的研究。
图4 裂隙面和断裂面积的近距离照片。该断裂是由在表面的中间看到的圆形标志齿轮去除造成的。
2.实验程序
将轴上提取的试样进行各种测试，包括硬度测试和金相和扫描电子显微镜以及化学成分的测定。所有的测试都在室温下进行。
2.1化学和冶金分析
使用光谱仪进行了断裂材料的化学分析。材料的化学成分在表1中给出。化学成分表明该材料是一种低合金渗碳钢AISI 8620型。因为低碳的比例，钢的淬透性很低。因此，在表面增加碳的比例与胶结操作，表面将变得坚硬，持久耐用，并使内部变得强硬，。这是一种常用的钢结构，用在受扭弯的机械零件中。通过残余压应力和加强硬度可获得高疲劳强度的高性能表面。
在碳钢中合金元素的分布主要取决于各元素的化合物和碳化物的形成倾向。镍在钢中的铁素体中溶解，因为它没有比铁形成碳化物的倾向更大。硅与刚中的少部分氧反应形成非金属化合物，不然则分解与铁素体。与铁相比，铬更易与碳反应。掺入铬取决于碳含量。
失效差速器材料的初步微结构检查示于图5。它可以看出，该材料具有混合结构，其中可能存在某些铁素体。这种钢的高硅含量，提高了热处理的敏感性，以及
屈服强度且提高最大应力而不减少塑性[ 9 ]。如果微观结构无法通过淬火向马氏体转变，则观察到了疲劳极限的降低。
图5（1）有碳相区。在图5的渗碳过渡边界（b）和（c）显示矩阵区域无渗碳。只要是在那里的照片看，这件作品是第一渗碳淬火，然后回火操作。这种情况可以理解，从不以观察到的马氏体板。
2.2硬度试验
开展的一mettest HT型计算机集成硬度计硬度测量。负载是1471；中等硬度值的内部区得到43 HRC。显微硬度测量已确定硬度值沿截面加大由于渗碳。4.903 n所示表2负荷下的维氏硬度测量结果。
2.3 断裂处的检查
本章中给出直接观测结果与断裂面扫描电镜分析。裂纹开始，一个可能是，底部的裂缝导致轴断裂。裂缝开始在外的部分，经过一段时间后，它继续超越中心，只有小部分为断裂。这部分是在等交通灯的车辆突然启动时被打破的。作为疲劳的一个特征，断裂面有2个区域。这是一个光滑的表面裂纹扩展和粗糙的表面创建的突然断裂。这2个区域可以清楚地看到整个问题如图4。疲劳裂纹区覆盖80%以上的横截面。
轴在弯曲，扭转，轴向力的作用下，受影响的地方反复使用。有一个锋利的薄面在水平上的裂缝性剖面上。这个原因使该区的应力集中系数确定。KT = 2.4价值（弯曲和张力），和KT = 1.9价值（扭转）是根据计算获得。这些
都是相当高的数值，区域暴露在联合载荷下。这些观察和分析显示，在扭转应力的作用下，轴断裂且在延展的状态下（fig.6）。裂纹扩展区内有一定的剪切裂痕，这是塑性剪切变形。图7显示了疲劳裂纹扩展的海滩纹。任何线之间的距离是近133纳米。
3.结论
在这项研究中，对一个失效的差速器小齿轮轴进行了分析。小齿轮轴产生于AISI 8620低碳渗碳钢渗碳、淬火和回火热处理工艺。力学性能、微观结构特性、化学成分和断口分析可以确定以下断裂原因：
•在一个具有高应力集中受弯曲、扭转和轴向应力作用下且具有高度可逆性的区域发生断裂。
•在关键部位的材料缺陷区域，可能会引发断裂。
•在一种韧性的方式中发生断裂。
•在关键位置减少应力集中，可以防止可能出现的故障发生。
参考
[1] Heisler H. Vehicle and engine technology. 2nd ed. London: SAE International; 1999.
[2] Makevet E, Roman I. Failure analysis of a final drive transmission in off-road vehicles. Eng Failure Anal 2002;9:579-92.
[3] Orhan S, Aktu ¨rk N. Determination of physical faults in gearbox through vibration analysis. J Fac Eng Arch Gazi University 2003;18(3):97–106..
[4] Tasgetiren S, Aslantas ? K, Ucun I. Effect of press-fitting pressure on the fatigue damages of root in spur gears. Technol Res: EJMT 2004;2:21–9.
[5] Nanawarea GK, Pableb MJ. Failures of rear axle shafts of 575 DI tractors. Eng Failure Anal 2003;10:719–24.
[6] Aslantas K, Tasgetiren S. A study of spur gear pitting formation and life prediction. Wear 2004;257:1167–75.
[7] Savas V, O ¨ zek C. Investigation of the distribution of temperature on a shaft with respect to the deflection. Technol Res: EJMT 2005;1:33–8.
[8] Smith FW. Principles of materials science and engineering. 3rd ed. USA: McGraw-Hill Series; 1996. p. 517–18.
[9] ASM metal handbook, vol. 1. Properties and selection, irons, steels, and high performance alloys; 1991.
[10] Voort GFV. Visual examination and light microscopy. ASM handbook metallography and microstructures. Materials Park (OH): ASM International; 1991. p. 100–65.
