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外 文 翻 譯
班級：09機(jī)制4班
學(xué)號：B09300418
姓名:劉曉冕
指導(dǎo)教師：俞高紅 教授
Pitting failure of truck spiral bevel gear
Abstract
Spiral bevel gears are some of the most important elements used in truck differential. In this study, the fracture of spiral bevel gear for truck differential produced from case hardening steel is investigated. In order to study the causes of the failure,specimens prepared from the damaged spiral bevel gears were subjected to experiments, such as visual inspection,hardness, chemical analysis and metallurgical tests. Pitting occurrence on gear surfaces was observed. The effect of microstructure on the fracture was considered. Low surface hardness values were found. The calculated contact stress was higher than the allowable contact stress which is emphasized in literature.
1. Introduction
Differential drives are packaged units used for a wide range of power-transmission applications. The spiral bevel gears are beginning to supersede straight-bevel gears in differential drives. They have curved oblique teeth that contact each other gradually and smoothly from one end of the tooth to the other, meshing with a rolling contact similar to helical gears (Fig. 1). They have the advantage of ensuring evenly distributed tooth loads and carry more loads without surface fatigue. Thrust loading depends on the direction of rotation and whether the spiral angle of the teeth is positive or negative [1,2]. The investigated spiral bevel gears are made of two different case hardening steel. The case hardening steel (20MnCr5, EN10084) has a low carbon–chromium and the other steel (17NiCrMo6-4, EN10084) has a low nickel–chromium–molybdenum with medium hardenability, generally supplied in the as rolled condition with a maximum brinell hardness of 280 (30 HRC). It is characterized by good core strength and toughness in small to medium sections with case hardness up to 62 HRC when carburized, hardened and tempered. These steels can also be used (uncarburized) as high tensile steel, which when suitably hardened and tempered can be utilized for various applications requiring good tensile strength and reasonable toughness. Almost three gears are damaged every month in truck service. Therefore, the damaged spiral bevel gears of truck were evaluated, and the causes of fracture of a gear manufactured from case hardening steel were carried out. Some properties of truck differential are given in Table 1. Also, the main dimensions of the gears are shown in Fig. 2. A number of mechanical and microstructure analyses are carried out to determine the causes of fracture.
2. Techniques used in fracture analysis
From one point of view, causes of gear failure may include a design error, an application error, or a manufacturing error. Design errors include such factors as improper gear geometry as well as the wrong materials, quality levels, lubrication systems, or other specifications. Application errors can be caused by a number of problems, including mounting and installation, vibration, cooling, lubrication, and maintenance. Manufacturing errors may show up in the field as errors in machining or heat treating [3]. In this analysis, the four damaged spiral bevel gear specimens were subjected to various tests. The following experimental works and stress calculations were done:
visual inspection and fractography;
hardness tests;
chemical analysis;
metallographic analysis;
contact stress calculation.
3. Analysis and results
3.1. Visual inspection and fractography
The investigated gears are shown in Fig. 3. The failed gears showed similar failure and did bear indication of fatigue crack growth when the fracture surface was examined, indicating that the failure was of a brittle type of fracture. The pitting on gear teeth surfaces assisted the failure. Pitting is caused by excessive surface stress due to high normal loads, a high local temperature due to high rubbing speeds, or inadequate lubricant. The pitting occurrence and the fractured surfaces of gears are shown in Fig. 4. According to the fractured surfaces, it was said that the failure was due to pitting.
3.2. Hardness analysis
Case-hardened gears are hardened only on the surface of the gear teeth, to a predetermined depth, to about 58 to 62 Rockwell C, or roughly as hard as a bearing race. The increased hardness improves the gear’s durability rating by providing greater resistance to pitting and greater strength, or resistance to
breakage [4–6]. Hardness analysis of fractured gear materials was carried out using a Rockwell hardness test machine. The measurements were carried out on three different surface areas. The core and surface hardness values are given in Tables 2 and 3. Core hardness over 40 HRC is not recommended due to potential for distortion, residual stresses, and brittleness but the gear 1 core hardness value is higher than the recommended values. The surface hardness of gears was observed as 50–54 HRC which is lower than the values stated in the literature.
3.3. Chemical analysis
Chemical analyses of 20MnCr5 and 17NiCrMo6-4 case hardening steels according to EN 10084 are shown in Table 4. The chemical composition of the piston materials was determined by spectroscopy chemical analysis. The chemical compositions of gear material are listed in Table 5. It was understood from the chemical composition that the material was case hardening steel. The gear 1 is 17NiCrMo6-4 and 2, 3 and 4 are 20MnCr5. The composition of gear materials contains low C and Cr, Ni and Mo content, which cause the structure to quench in a tough mode. The alloying additions improve the hardenability of the steel. Chromium improves corrosion resistance, while manganese contributes to deoxidation of the melt and also improves machinability. Nickel reduces distortion and cracking upon quenching.
3.4. Metallographic analysis
The metallographic specimens were first ground, polished and etched using standard techniques in order to examine the inner structure. A light optical microscope was used in the investigations. It can be understood from the figures that the gears were carburized and then cooled in the oil ambient. The microstructures of the failed gear materials show that they are similar structures. From the observation, it is concluded that the case hardening process was not properly done. Also, because of the application of improper heat treatment, gears core structure have a wholly martensite which is depicted Fig. 5. The core structure should be tough in gears not martensite and brittle.
3.5. Stress calculation
Since the pitting occurrence was observed at visual inspection, the contact stress on gear teeth was calculated.The stress experienced by the spiral bevel tooth during operation was estimated using the design torque of 250 Nm. The contact stress on the loaded tooth can be calculated using the equation [7]。
The terms used in equation are explained in Table 6. Using Eq. (1) and Table 6, the contact stress was calculated to be 1994 MPa. According to literature [6,7], allowable contact stress is 1550 MPa. This value is lower than the calculated value. In this case, gears have about 0.77 safety factors and they have not contact strength. Thus, the pitting failure was observed on gear teeth surface. The occurring pits have contributed to
the failure of gears.
4. Conclusion
In this research, the influences of microstructure, chemical composition and hardness of the gears were investigated and contact stress was calculated. From the experimental observations and calculations, the following conclusions may be made:
1. In order to obtain same hardness and microstructure, the gear materials should be of same chemical composition.
2. The surface hardness of gears is low. In order to obtain maximum pitting resistance, the gears outer surface hardness should be increased to 58–60 HRC.
3. In order to obtain different microstructure between core and surface, carburising heat treatment should be made proper conditions, such as time, case depth. The case depth should be under control.
4. Due to the high tooth-contact pressures, oil film thickness may not be enough. The lubrication could be difficult. Therefore, the pitting occurrence increases. On the examination of fractured parts, it can be concluded that the gears expose to overloading. In order to decreasing contact pressure, the gears geometry can be optimized in design stage or the pinion design torque can be decreased.
卡車螺旋錐齒輪的點(diǎn)蝕故障
摘要：
螺旋錐齒輪是卡車差動齒輪中的重要組成部分。在這個研究當(dāng)中，對因表面硬化鋼齒輪而導(dǎo)致卡車差動齒輪中錐齒輪的斷裂進(jìn)行了調(diào)查。為了研究引起失效的原因，專家們從損壞的錐齒輪樣品中進(jìn)行實(shí)驗，如外觀檢查，硬度、化學(xué)分析和冶金測試。齒輪表面的點(diǎn)蝕是可以被觀察到的。微觀結(jié)構(gòu)的效應(yīng)在斷裂中被考慮了進(jìn)去。低表面硬度的價值被發(fā)現(xiàn)。被計算的接觸應(yīng)力高于可允許的接觸應(yīng)力是這篇文章介紹的重點(diǎn)。
1、 介紹
差分驅(qū)動器廣泛應(yīng)用于動力傳輸?shù)膯卧?。螺旋錐齒輪開始在差分驅(qū)動器中優(yōu)于直錐齒輪。它們有彎曲的斜齒，并且逐漸接觸從一端過渡到另一端，嚙合的螺旋齒輪類似于滾動接觸。它們的優(yōu)點(diǎn)是確保負(fù)載均勻的分布在齒上，從而使其攜帶更多的載荷且不發(fā)生表面疲勞。推力載荷取決于旋轉(zhuǎn)的方向和螺旋角的正負(fù)，調(diào)查的螺旋錐齒輪是由倆種不同的表面硬化鋼構(gòu)成的，表面硬化鋼（20MnCr5,EN10084)具有低的碳-鉻元素，其他鋼（17NiCrMo6-4,EN10084)具有低的鎳-鉻-鉬元素和中等的淬透性，在一般的軋制條件下，供給的最大布氏硬度為280（30HRC）。它的特點(diǎn)是在經(jīng)過滲碳、淬火和回火后，中型材表面硬度提升至62HRC時，可以承受較高的應(yīng)力并且具有較小的韌性。這些鋼（非滲碳）也可用于作為高強(qiáng)度鋼，并且通過適當(dāng)?shù)拇慊鸷突鼗鸷?，產(chǎn)生較好的拉伸強(qiáng)度和韌性，可滿足多種應(yīng)用?？ㄜ囘\(yùn)行的每個月中大約都有三個齒輪損壞。因此，對卡車中受損的螺旋錐齒輪進(jìn)行了評估，并且分析了表面硬化鋼制造的齒輪斷裂的原因。
2、 斷裂分析中應(yīng)用的技術(shù)
從企業(yè)的角度來說，齒輪發(fā)生故障的原因可能有設(shè)計錯誤、程序錯誤或者制造錯誤。設(shè)計錯誤包括齒輪幾何形狀不當(dāng)，材料不當(dāng)，質(zhì)量水平不夠或是潤滑系統(tǒng)不完善。程序錯誤包括安裝、振動、冷卻和維護(hù)多個因素構(gòu)成。制造錯誤可能會發(fā)生在現(xiàn)場的熱處理或是作業(yè)中的不當(dāng)處理。
在這個分析中，四個損壞的螺旋錐齒輪樣本進(jìn)行各種實(shí)驗。進(jìn)行的實(shí)驗以及測量結(jié)果如下：
1、外觀和斷口檢驗
2、硬度實(shí)驗
3、化學(xué)分析
4、金相分析
5、接觸應(yīng)力的計算
3、 分析方法和結(jié)果
3.1 外觀和斷口檢驗
在圖3所示調(diào)查的齒輪中。失效的齒輪都表現(xiàn)出了類似的故 障，對疲勞裂紋擴(kuò)展的斷裂面進(jìn)行了檢查，表明故障時脆性的折斷。
齒牙上的表面點(diǎn)蝕促進(jìn)了齒輪的失效。點(diǎn)蝕是由于過多的表面承受高載荷，由于過高的摩擦速度導(dǎo)致局部溫度過高，或是不充分潤滑導(dǎo)致的。示于圖4的齒輪發(fā)生點(diǎn)蝕的斷裂表面，通過其斷面表面，可以說是由于點(diǎn)蝕導(dǎo)致的。
3.2 硬度分析
表面硬化的齒輪的硬化只發(fā)生在齒輪表面，達(dá)到預(yù)定深度，達(dá)到58到62洛氏溫度。通過增加硬度來提高齒輪的耐用性可以通過增加抗點(diǎn)蝕能力和提高耐斷裂強(qiáng)度來達(dá)到。使用洛氏硬度試驗機(jī)對斷裂的齒輪材料進(jìn)行了硬度分析，進(jìn)行了三個不同表面區(qū)域的測量。其芯部和表面的硬度值分別在表2和表3中給出。由于潛在的失真，剩余應(yīng)力和脆性，硬度高于40HRC的材料是不推薦的，但是齒輪1的硬度值是高于推薦值的。被觀察到的50-54HRC表面硬度的材料是低于文獻(xiàn)中所提到的數(shù)值的。
3.3 化學(xué)分析
對表面材料20MnCr5和17NiCrMo6-4的齒輪進(jìn)行化學(xué)分析，通過EN10084在表4中給出。由光譜化學(xué)分析確定材料的化學(xué)組成。齒輪材料的化學(xué)成分在表5中給出。通過觀察該材料的化學(xué)成分，確定該材料為硬化鋼。齒輪1的材料為17NiCrMo6-4，齒輪2、3和4的材料為20MnCr5。齒輪材料的化學(xué)成分含有量較低的C和Cr,Ni和Mo元素，通過急速冷卻后可形成特定的結(jié)構(gòu)。合金添加劑可以提高鋼的淬透性。鉻可以提高耐腐蝕性，而錳有助于脫氧的熔融，同時提高了可加工性。鎳減少淬火開裂后的變形。
3.4 金相分析
失效的齒輪材料有著相類似的結(jié)構(gòu)。從觀察中可以得到結(jié)論。該情況下，硬化過程不完全。此外由于熱處理應(yīng)用不當(dāng)，齒輪材料中的馬氏體在圖5中呈現(xiàn)。
3.5 應(yīng)力計算
通過可視觀察齒輪的點(diǎn)蝕，發(fā)生在輪齒上的接觸應(yīng)力是可以被計算的。在對螺旋錐齒輪試驗中，對齒輪附加扭矩為250NM。對于附加在輪齒上的接觸應(yīng)力可以由公式7計算。表6中有對該公式的術(shù)語解釋。使用公式1和表6可以計算出接觸應(yīng)力為1994MPa。通過文本給出，可允許的接觸應(yīng)力為1550MPa。此值低于計算給出的數(shù)值。在這種情況下，齒輪大概有0.77安全系數(shù)且沒有達(dá)到其接觸強(qiáng)度。因此，在輪齒表面上可觀察到點(diǎn)蝕現(xiàn)象。點(diǎn)蝕的發(fā)生是齒輪失效的原因。
4、 總結(jié)
在這次的實(shí)驗中，微觀結(jié)構(gòu)、化學(xué)組成和齒輪硬度被考慮了進(jìn)來，同時計算出接觸應(yīng)力。從實(shí)驗觀測和應(yīng)力計算中，得出以下結(jié)論：
1、 為了獲得相同的硬度和微觀結(jié)構(gòu)，齒輪材料應(yīng)該有相同的化 學(xué)組成。
2、 齒輪表面硬度過低。為了獲得最大的耐腐蝕性，齒輪的表面 硬度應(yīng)提高至58-60HRC。
3、 為了獲得不同的芯部和表面組織，滲碳熱處理應(yīng)給出適當(dāng)?shù)? 條件，如時間、硬化層深度等等，深度應(yīng)在控制之下。
4、 由于高的齒接觸壓力，可能達(dá)不到足夠的油膜厚度。潤滑可 能非常困難，導(dǎo)致點(diǎn)蝕發(fā)生增加。通過裂隙部位的檢查，可 以得出結(jié)論，齒輪承受重載荷。為了降低接觸應(yīng)力，可以對 齒輪的幾何形狀進(jìn)行優(yōu)化。在設(shè)計階段中，小齒輪的設(shè)計可 以降低扭矩。
Engineering failure analysis
Abstract
The scale and complexity of computer-based safety critical systems, like those used in the transport and manufacturing industries, pose significant challenges for failure analysis.
Over the last decade, research has focused on automating this task. In one approach, predictive models of system failure are constructed from the topology of the system and local component failure models using a process of composition. An alternative approach employs model-checking of state automata to study the effects of failure and verify system safety properties. In this paper, we discuss these two approaches to failure analysis. We then focus on Hierarchically Performed Hazard Origin & Propagation Studies (HiP-HOPS) – one of the more advanced compositional approaches – and discuss its capabilities for automatic synthesis of fault trees, combinatorial Failure Modes and Effects Analyses, and reliability versus cost optimisation of systems via application of automatic model transformations.We summarise these contributions and demonstrate the application of HiP-HOPS on a simplified fuel oil system for a ship engine. In light of this example, we discuss strengths and limitations of the method in relation to other state-of-the-art techniques. In particular,because HiP-HOPS is deductive in nature, relating system failures back to their causes, it is less prone to combinatorial explosion and can more readily be iterated. For this reason, it enables exhaustive assessment of combinations of failures and design optimisation using computationally expensive meta-heuristics.
1. Introduction
Increasing complexity in the design of modern engineering systems challenges the applicability of rule-based design and
classical safety and reliability analysis techniques. As new technologies introduce complex failure modes, classical manual
analysis of systems becomes increasingly difficult and error prone.To address these difficulties, we have developed a computerised tool called ‘HiP-HOPS’ (Hierarchically Performed Hazard Origin & Propagation Studies) that simplifies aspects of the engineering and analysis process. The central capability of this tool is the automatic synthesis of Fault Trees and Failure Modes and Effects Analyses (FMEAs) by interpreting reusable specifications of component failure in the context of a system model. The analysis is largely automated,requiring only the initial component failure data to be provided, therefore reducing the manual effort required to examine safety; at the same time,the underlying algorithms can scale up to analyse complex systems relatively quickly, enabling the analysis of systems that would otherwise require partial or fragmented manual analyses.More recently, we have extended the above concept to solve a design optimisation problem: reliability versus cost optimisation via selection and replication of components and alternative subsystem architectures. HiP-HOPS employs genetic algorithms to evolve initial non-optimal designs into new designs that better achieve reliability requirements with minimal cost. By selecting different component implementations with different reliability and cost characteristics, or substituting alternative subsystem architectures with more robust patterns of failure behaviour, many solutions from a large design space can be explored and evaluated quickly. Our hope is that these capabilities, used in conjunction with computer-aided design and modelling tools, allow HiP-HOPS to facilitate the useful integration of a largely automated and simplified form of safety and reliability analysis in the context of an improved design process. This in turn will, we hope, address the broader issue of how to make safety a more controlled facet of the design so as to enable early detection of potential hazards and to direct the design of preventative measures. The utilisation of the approach and tools has been shown to be beneficial in case studies on engineering systems in the shipping [1] and offshore industries [2]. This paper outlines these safety analysis and reliability optimisation technologies and their application in an advanced and largely automated engineering process.
2. Safety analysis and reliability optimisation
3. Safety analysis using HiP-HOPS
HiP-HOPS is a compositional safety analysis tool that takes a set of local component failure data, which describes how output failures of those components are generated from combinations of internal failure modes and deviations received at the components’ inputs, and then synthesises fault trees that reflect the propagation of failures throughout the whole system.From those fault trees, it can generate both qualitative and quantitative results as well as a multiple failure mode FMEA
[35].A HiP-HOPS study of a system design typically has three main phases:
Modelling phase: system modelling & failure annotation.
Synthesis phase: fault tree synthesis.
Analysis phase: fault tree analysis & FMEA synthesis.
Although the first phase remains primarily manual in nature, the other phases are fully automated. The general process in
HiP-HOPS is illustrated in Fig. 2 below: The first phase – system modelling & failure annotation – consists of developing a model of the system (including hydraulic, electrical or electronic, mechanical systems, as well as conceptual block and data flow diagrams) and then annotating the components in that model with failure data. This phase is carried out using an external modelling tool or package compatible with HiP-HOPS. HiP-HOPS has interfaces to a number of different modelling tools, including Matlab Simulink, Eclipse-based UML tools, and particularly SimulationX. The latter is an engineering modelling & simulation tool developed by ITI GmbH[36] with a fully integrated interface to HiP-HOPS. This has the advantage that existing system models, or at least models that would have been developed anyway in the course of the design process, can also be re-used for safety analysis purposes rather than having to develop a new model specific to safety. The second phase is the fault tree synthesis process. In this phase, HiP-HOPS automatically traces the paths of failure propagation through the model by combining the local failure data for individual components and subsystems. The result is a network of interconnected fault trees defining the relationships between failures of system outputs and their root causes in the failure modes of individual components. It is a deductive process, working backwards from the system outputs to determine which components caused those failures and in what logical combinations.The final phase involves the analysis of those fault trees and the generation of an FMEA. The fault trees are first minimised to obtain the minimal cut sets – the smallest possible combinations of failures capable of causing any given system failure –and these are then used as the basis of both quantitative analysis (to determine the probability of a system failure) and the FMEA, which directly relates individual component failures to their effects on the rest of the system. The FMEA takes the form of a table indicating which system failures are caused by each component failure.The various phases of a HiP-HOPS safety analysis will now be described in more detail.
4. Design optimisation using HiP-HOPS
HiP-HOPS analysis may show that safety, reliability and cost requirements have been met, in which case the proposed system design can be realised. In practice, though, this analysis will often indicate that certain requirements cannot be met by the current design, in which case the design will need to be revised.This is a problem commonly encountered in the design of reliable or safety critical systems. Designers of such systems usually have to achieve certain levels of safety and reliability while working within cost constraints. Design is a creative exercise that relies on the technical skills of the design team and also on experience and lessons learnt from successful earlier projects, and thus the bulk of design work
is creative. However, we believe that further automation can assist the process of iterating the design by aiding in the selection of alternative components or subsystem architectures as well as in the replication of components in the model, all of which may be required to ensure that the system ultimately meets its safety and reliability requirements with minimal cost.A higher degree of reliability and safety can often be achieved by using a more reliable and expensive component, an
alternative subsystem design (e.g. A primary/standby architecture), or by using replicated components or subsystems to achieve redundancy and therefore ensure that functions are still provided when components or subsystems fail. In a typical
system design, however, there are many options for substitution and replication at different places in the system and different
levels of the design hierarchy. It may be possible, for example, to achieve the same reli