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小齿轮
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齿轮
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小齿轮和锥齿轮的失效分析[中文5000字],小齿轮,以及,齿轮,失效,分析,中文
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Failure investigation of crown wheel and pinion A. Benselya, , , S. Stephen Jayakumara, D. Mohan Lala, G. Nagarajana and A. Rajaduraib Received 14 September 2005; accepted 31 October 2005. Available online 9 February 2006. Abstract The crown wheel and pinion are the critical components in the transmission system of an automobile. Failure of these components has drastic effect on the vehicular movement. This in turn leads to increased downtime for repairs. The cost of these components adds to the criticality in addition to its function. A fractured gear was subjected to detailed analysis using standard metallurgical techniques to identify the cause for failure. The study concludes that the failure is due to the compromise made in raw material composition by the manufacturer, which is evident by the presence of high manganese content and non-existence of nickel and molybdenum. This resulted in high core hardness (458 HV) leading to premature failure of the vital component of transmission system in a vehicle. Keywords: Carburization; Crown wheel; Gear -tooth failures; Failure investigation fractography; Retained austenite Article Outline 1. Introduction 2. Manufacturing process 3. Experimental investigation 4. Visual examination 5. Pitting 6. Chemical analysis 7. Microhardness survey 8. Microstructure 外文翻译共 12 页 第 1 页9. Tooth contact studies 10. Conclusions Acknowledgements References1. Introduction Life expectancy of mechanical systems is always dependent on the most critical component of the system 1. In power transmission system this is usually the gear. Gear design is commonly bounded by the requirements that gear should carry high loads at high speeds with minimal size and weight. A typical crown wheel and pinion used in heavy vehicles is shown in Fig. 1. Display Full Size version of this image (54K)Fig. 1. Crown wheel and pinion. They are the most stress prone parts of a vehicle and demands high wear resistance, high contact fatigue strength. An ideal crown wheel and pinion should have uniform and optimum metallurgical quality, excellent heat distortion control, maximum impact strength, stiff wear resistance, optimal transmission efficiency, less noise, vibration-free operation and gear geometry in accordance with American Gear Manufacturers Association (AGMA) 11 qualities. Gas carburizing is a process employed to achieve some of these properties. It produces a very high wear resistant case and a soft tough core 2. The manufacturer should make the critical components durable and efficient through accurate and consistent manufacturing standards by selecting appropriate 外文翻译共 12 页 第 2 页material and correct heat treatment parameters. En 353 (15 Ni Cr 1 Mo 12) and En 207 (20 Mn Cr 1) are the two widely used fine-grained steel billet materials used in manufacturing of these critical components. Typical applications of En 353 being heavy-duty gears, shaft, pinions, camshafts, gudgeon pins, heavy vehicles transmission components while En 207 being used widely for medium sized gear wheels and shafts 3. This paper deals with a failure investigation of a crown wheel and pinion. 2. Manufacturing process Crown wheel and pinion are manufactured from forged blanks that are isothermally annealed at 850880 C to obtain uniform properties after heat treatment. The forgings are precision machined by computer numerical control gear generators to high dimensional accuracy. It is followed by gas carburizing at 900930 C to have uniform case, which can vary from 1.5241.905 mm in its depth. Hardening is done at 780820 C in controlled atmospheric temperature and press quenched in oil to avoid distortion. In the case of pinion, selective case hardening is done to impart maximum strength to the pinion to maximize wear resistance. The pinion thread is specially treated to soft conditions to withstand higher shock loading and yielding arising out of torque tightening. After hardening, the crown wheel and pinion are tempered at 150200 C to remove thermal stresses. Finally, the crown wheel and pinion are checked thoroughly for hard spots to prevent premature failure and also to ensure noise-free operation. The backlash is kept within 0.20.3 mm band to keep noise and vibration to a bare minimum 4. 3. Experimental investigation The research methodology adopted in the present investigation is shown in Fig. 2. 外文翻译共 12 页 第 3 页Display Full Size version of this image (26K)Fig. 2. Research methodology for failure investigation. 4. Visual examination The failed crown wheel and pinion taken for the investigation is from a medium type commercial vehicle. This vehicle has a 120 kW engine and can transmit a payload of 13,000 kg. It is a more stable and ideal vehicle for construction and off road applications. If a crown wheel or pinion fails, always it is necessary that both have to be replaced completely as a matching pair; otherwise the life of the unit will be greatly reduced. The number of teeth in the crown wheel varies from passenger vehicles to heavy vehicles. Heavy vehicles have less number of teeth when compared to passenger vehicles and also the thickness of the teeth is larger than passenger vehicles teeth thickness. From the visual examination it was found that the number of teeth in the crown wheel and pinion is 45 and 7, respectively. The expected normal life of the component will approximately range between 1,50,000 and 2,00,000 km. The fractured surfaces of the failed crown wheel and pinion showed the presence of burn marks (cold weld) on the edge of a teeth, partial uprooting on 8 number of tooth and teeth chipping all along the outer edge of the crown wheel. It was further observed that the gear teeth had no worn-out surface either on the front or on the rear side indicating that the crown wheel and pinion are relatively new (see Fig. 3). Display Full Size version of this image (35K)Fig. 3. Crown wheel failure. 外文翻译共 12 页 第 4 页The companion gear (i.e. pinion) shows sub case fatigue fracture initiated by fine cracks. Large fragments have spalled away from the tooth. Fatigue beach marks can also be seen in Fig. 4. Sub case fatigue is fracture of case hardened components by the formation of crack below the contact surface within the hertzian stress field. However, the depth at which the crack forms is much greater than the macro pitting fatigue and it is a function of material strength in conjunction with the alternating hertzian shear stress. It is also sometimes referred as case crushing but since it results from fatigue crack that initiates below the effective case depth or in the lower carbon portion of the case, the former nomenclature will be used frequently. Thin case depth relative to radius of curvature is the factor that controls the occurrence of sub case fatigue. Display Full Size version of this image (34K)Fig. 4. Pinion failure. 5. Pitting Generally, gears fail due to several mechanisms but most often due to surface pitting of gear teeth. Surface pitting is in fact the principal mode of failure of mechanical elements that are subjected to rolling contacts and governs the surface life of a component under applied load 5. Hence, the failed components were subjected to macro examination using a stereomicroscope for pitting failure. The pitting of gear teeth is characterized by the occurrence of small pits on the contact surfaces, as visible in Fig. 4. The process of surface pitting can be visualized as formation of surface-breaking or sub surface initial cracks, which grow under repeated contact loading. Eventually the crack becomes large enough for unstable growth to occur, which results in a part of the surface material layer breaking away. The resulting void is a pit. The availability of pits in both the components was very less. Relatively, the number 外文翻译共 12 页 第 5 页of pits in pinion is larger as the number of revolutions of pinion is higher than crown wheel. This confirms to the fact that the failure is premature and not due to pitting. 6. Chemical analysis As no information with respect to the chemical composition and the heat treatment condition of the pinion material was available, the next task in the failure analysis was the material identification. A small size specimen was cut using abrasive cut off wheel from location A of the crown wheel as shown in Fig. 3 and subjected for optical emission spectrometer studies and metallographic examination. Chemical analysis was carried out at two different locations, one at the surface (case) and another at the central portion (core) of the component. The chemical analysis helps to identify the basic composition of the raw material selected for the component, carbon potential used for carburizing process and any compromise on the basic composition with respect to the component that made by the manufacturer. The results of the chemical analysis along with the nominal composition of En 353 and En 207 are given in the Table 1. Table 1. Chemical composition Elements Chemical composition (wt%) En 353 En 207 Failed component Case CoreCarbon 0.120.18 0.170.22 0.91 0.24Chromium 0.751.25 1.01.3 1.46 1.44Manganese 0.61.0 1.11.4 1.44 1.43Nickel 1.01.5 0.01 0.01外文翻译共 12 页 第 6 页Elements Chemical composition (wt%) En 353 En 207 Failed component Case CoreMolybdenum 0.080.15 0.01 0.01Silicon 0.10.35 0.4 (max) 0.24 0.26It was found that the material selected for the preparation of the crown wheel and pinion was not exactly matching with either En 353 or En 207, but it was nearer to En 207 with variation in the composition of certain elements. En 353 is a better material for heavy-duty application than En 207 and it is costlier. Also elements like C, and Mn were found to be excess than the required level expected for heavy-duty applications. C, Mn, Cr, Ni and Mo increase hardenability but the influence is higher for manganese, and nickel on comparing with other alloying elements 3. The presence of Si also increases hardenability a little but retains hardness of the component during tempering. Nickel strengthens ferrite and increases hardenability, refines the grain, increases elastic limit and tensile strength with no practical loss in ductility. It also improves the resistance to fatigue and impact. Since nickel is almost nil in the failed material it is devoid of all the above said characteristics. A higher percentage of molybdenum inhibits grain boundary segregation as well as help in reducing temper brittleness. Generally molybdenum is used in combination with nickel and chromium as it enters into ferrite to form carbides. It also inhibits grain growth. However, as seen in Table 1. The molybdenum percentage was also quite marginal resulting in deficiency of temper brittleness and grain boundary segregation. In order to check the hardenability of the material the carbon equivalent (CE) was estimated as follows:(1)外文翻译共 12 页 第 7 页The carbon equivalent (CE) for the failed material was found to be 0.77, whereas it was only 0.713 and 0.77 for En 207 and En 353, respectively. The chemical analysis confirms that the failure is due to a selection of less withstanding materials. Not only that the manufacturer has made a compromise on the base composition by adding manganese in higher proportion, which is a cheaper substitute for costly nickel. The carbon content is also high. This resulted in high core hardness, finally ending up in premature failure of component 6. 7. Microhardness survey Being a case carburized material a gradient of decreasing hardness exists from the case to the core. Hence, microhardness survey was carried out and the result is shown in Fig. 5. In general, the cut off value taken for calculating the effective case depth (ECD) is 540 HV and it is expected to be between 1.524 and 1.905 mm. The depth at which the original carbon content (hardness remains the same with respect to the depth) of the material is reached is called the total case depth. Hence, the total case depth is more than the effective case depth. The case hardness, total case depth and effective case depth were found to be 737 HV, 1.4 and 1.22 mm, respectively. For the failed component the core hardness was measured at the center of tooth base. It was found to be 458 HV. Normally, the desired core hardness is between 317 and 401 HV and is tolerable up to 430 HV, beyond that the component is highly prone to failure. This is very high for heavy-duty application and is due to high manganese content in the raw material. The high core hardness results in sub case fatigue and poor resistance to impact. It is also the reason for the premature failure. In this study, the ECD was only 1.22 mm, which confirms inadequate case depth. Insufficient case depth resulted in spalling of a pinion tooth as shown in Fig. 4 and in turn reduces durability of the crown wheel. This is due to low temperature employed during carburization or may be due to inadequate gas feed. 外文翻译共 12 页 第 8 页Display Full Size version of this image (12K)Fig. 5. Microhardness survey. 8. Microstructure Although retained austenite has been claimed to benefit contact fatigue life, there are situations when austenite can be determinantal to dimensional stability and surface hardness. During operation metastable retained austenite will transform under stress and strain to untempered martensite, which result in a volume expansion. This volume expansion can create distortion, induce stress and may result in a decreased life through misalignment and noise. Excess retained austenite will also lower material hardness and resistance to fatigue initiation. Basically, steel must have sufficient quantities of correct alloying elements to produce component with proper surface and core hardness in addition to avoiding unwanted non-martensitic transformation products (NMTP), such as quenched-in pearlite 7. Carbon content controls surface hardness and other alloying elements aid in controlling the core hardness and the amount of NMTP. The NMTP microconstituents like ferrite, pearlite and bainite are softer than the martensite and reduce the contact fatigue resistance of steel. Hence, it should be avoided. Alloying element, such as carbon, manganese, nickel, molybdenum and chromium lower the martensite start temperature of iron and thus produce greater levels of retained austenite. A small specimen cut from the failed component is further subjected to microstructural study using optical microscope. The case and the core micrograph of the failed component are shown in Fig. 6. Display Full Size version of this image (78K)Fig. 6. Microstructure of failed component. (a) Case micrograph; (b) core micrograph. 外文翻译共 12 页 第 9 页The case micrograph shows the presence of 70% martensite and 2025% retained austenite and a very little ferrite, whereas the core micrograph shows banded bainite along with evenly distributed ferrite. Instead of formation of fine pearlite the presence of banded bainite could be attributed to the presence of increased amount of manganese, which is confirmed by the high core hardness value of 458 HV. This is approximately equivalent to the hardness of bainite (410 HV). Important information about the nature of fracture can be obtained from microscopic examination of fracture surface 8. Fractography is done using scanning electron microscope (SEM). The large depth of focus and the fact that the actual surface can be examined make the SEM an important tool for failure analysis. A portion of the fractured teeth was studied using SEM and is shown in Fig. 7. The mode in which the fracture has occurred is highly brittle in nature as evident by the cleavage facets. Display Full Size version of this image (52K)Fig. 7. SEM picture of the failed specimen. 9. Tooth contact studies The failed component is subjected to contact studies in order to know the contact details and sequence of failure. Tooth contact analysis was carried out by revolving the failed pinion on crown wheel by referring the index number given in the back cone face of the crown wheel and the shank of the pinion. The index number of failed teeth of crown wheel is identified as 17, 18, 19, 27, 29, 30, 31, 32 and 38, whereas for the pinion it is 3. As the gear ratio is 6.5, for every revolution the failed teeth of pinion does not come and mate at the same teeth in the next revolution. The sequence with which the failed tooth (third tooth) of pinion mating with crown wheel is given in Table 2 along with the level of damage observed through visual inspection. The 外文翻译共 12 页 第 10 页sequence in which the fracture in crown wheel has occurred is 17, 31, 38, 18, 32, 29, 19, 30 and 27. From Table 2, it is observed that the failed pinion teeth does not affect all the teeth of crown it mate. It also confirms that failure has not taken place in one revolution of crown wheel. For the failure to occur in all the identified teeth, definitely the crown wheel has revolved seven times after the initial failure. The sequence indicates fairly a gradual progression of the damage and ends with the cold weld at the end. This confirms that cold welding has occurred in seventh revolution of crown and all other fracture in the preceding 6 revolutions. The mode of failure of crown wheel is by partial uprooting. Table 2. Tooth contact analysis of crown wheel and pinion Revolution of crown wheelSequence of contact of failed pinion teeth with crown wheel indicating the level of damage1 3 10 17 Small 24 31 Medium 38 Medium 452 7 14 21 28 35 423 4 11 18 Medium 25 32 Large 394 1 8 15 22 29 Small 36 435 5 12 19 Small 26 33 406 2 19 16 23 30 Large 37 447 6 13 20 27 Cold weld 34 41The teeth chipping have occurred all around the edges of crown wheel tooth contact analysis was carried out with the help of marking medium (a yellow paint) on one of the pinion tooth. Then it is rotated over the crown wheel. The pattern of contact registered by the marking medium on the crown wheel matches with
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