Analysis of a shrink-fit failure on a gear hub/shaft assembly

The paper presents results and conclusions arising from an investigation into a shrink-fit failure on a gear hub/shaft assembly. The work formed one element of a full failure investigation and set out specifically to determine whether or not fretting or micro-slipping was the cause of failure. The basic rationale of the study was to start with assumed operating conditions and progressively refine the model in order to accurately take account of the true operating conditions. This approach permitted conclusions to be drawn after every simulation concerning the propensity to slip along the gear hub/shaft interface. Two modelling techniques were used in parallel: a novel analytical approach and a finite element study. Results from the two approaches were consistent and in relatively good agreement. The principal conclusion made was that the observed slip and subsequent fretting damage could only have occurred as a result of another, principal, failure mechanism which was responsible for increasing the internal diameter of the gear hub.     

Investigation of causes of fan shaft failure

Energy efficiency of thermal power plants depends on the vital plant elements’ reliability levels. In order to preserve high level of energy efficiency, the vital elements’ operational capability is continually monitored via observing the working temperature, vibration and noise levels. Yet, however, in spite of this, and usually due to inadequate maintenance, inadequate assembly, errors in the design or manufacturing processes, or due to accidents, abrupt failures of the plant’s vital elements can occur. An example for this is the volumetric destruction of the fresh air fan shaft due to melting of the shaft at the shaft’s stationary support. In addition to shaft destruction, the fan bearing and wheel blades have undergone destruction as well. This breakdown has occurred in the course of continual monitoring of the bearing service temperature and vibrations levels. Former studies, aimed at determining the breakdown causes, were based on the detailed examination of the damaged elements’ materials and detailed analysis of the bearing setting arrangement. Considering the fact that the shaft stationary support was realized by means of a double-pressed joint, a thorough analysis has been made in this paper of the pressed joint operational capability in the boundary service conditions, in order to determine the extent to which the pressed joint participated in the breakdown.     

Investigation into recurring military helicopter landing gear failure

The skid landing gear of a military helicopter failed while the helicopter was moored on the ground at the airport. The rear cross tube of the landing gear assembly was found fractured into two separated pieces. The fracture occurred in the right flange of the rear cross tube, where a connection with the spring tube is established using a clamp. The initial visual inspection of the fracture zone revealed a presence of heavy corrosion and significant damage of the anti-corrosion protective layer on the outer surface of the flange of the rear cross tube. Fractographic analysis highlighted corrosion as the main cause of the failure. Evidence was found to show that the fracture was initiated from corrosion pits located on the exterior, underside of the cylindrical part of the flange. Metallographic examination discovered corrosion pits with micro cracks and multiple branched secondary cracks in the crack origin area, indicating the occurrence of stress corrosion cracking. Chemical analysis of the corrosion deposits showed the presence of sodium, chlorine, calcium and sulfur. The stress analysis of the helicopter landing gear assembly, carried out by means of finite element method, confirmed that the crack origin was located at the area with the maximum tensile stress in the flange.     

Investigation of a Boeing 747 wing main landing gear trunnion failure

A loud noise was heard from the vicinity of the port wing landing gear during pushback of a Boeing 747-300 from the terminal at Sydney (Australia) airport. Inspection showed that one of the wing landing gear trunnion fork assemblies had failed. Detailed investigation revealed that the trunnion had failed by fatigue cracking. Deep machining grooves were found at the root of an internal radius that had not been shot-peened as required, and a chemical surface process during manufacture had resulted in shallow intergranular attack at the bottom of these grooves. It is probable that the critical cracking started from some of these grooves. In addition, the wall thickness at the failure location was significantly less than the minimum required in the drawings.Since the deep machining grooves, the lack of peening and the intergranular attack were all consequences of manufacturing, the fatigue cracking probably started shortly after the component entered service. This implies that fatigue cracking was present during all the trunnion overhauls, but was not detected by non-destructive inspections during the overhauls. Quantitative fractography was used to produce a crack growth curve based on fracture surface markings thought to represent the overhaul timings. The crack growth curve suggested that the fatigue cracking was large enough to be detected by inspection during the last overhaul, if not the one before. However, it was probably not easy to detect the cracking. This investigation therefore highlights the difficulties that can be encountered when inspection is the last (or only) line of defence against failure owing to unexpected manufacturing deficiencies.     

Analysis of fatigue failure on the keyway of the reduction gear input shaft connecting a diesel engine caused by torsional vibration

The vibratory torque of a diesel engine caused by the reciprocating motion of the mass and gas pressure force of a cylinder is one of the main causes of the failure of the driving shaft of the diesel engine and the connecting shaft to the reduction gear. Because high cycle torsional fatigue can occur in the reduction gear shaft connecting the engine under vibratory torsional stress, the US Navy restricts it under the MIL G 17859D(SH) standard and suggests a procedure for evaluating the safety of the shaft for the reduction gear. In this study, the structural safety of the reduction gear input shaft in which fatigue failure occurs in typical naval vessels is investigated in accordance with the VDI 3822 RCA (root cause analysis) procedure based on the MIL G 17859D(SH) standard. When evaluating the safety factor in accordance with the MIL G 17859D(SH) standard, the alternating bending moment from the lateral vibration and the stress concentration factor under static load are considered. In addition, an improved design is suggested by CAE to satisfy the safety factor suggested by the MIL G 17859D(SH) standard.     

带齿轮副船舶推进轴系动态响应仿真研究

为更好分析及准确判断船用齿轮箱故障,以某江海直达货船推进轴系为研究对象,建立带齿轮副推进轴系的扭转振动模型,运用齿轮副时变啮合刚度理论计算方法,分析齿轮副在故障态下啮合刚度的变化,拓展传统推进轴系瞬态响应计算。基于有限元分析与信号处理软件得到考虑时变啮合刚度时正常态与故障态下带齿轮箱推进轴系瞬态响应频谱,并分析频谱中故障频率的成分与特点,为完整推进轴系齿轮副故障分析提供依据。     

HXD1机车小齿轮轴断裂失效应力分析

HX_D1机车牵引电机小齿轮轴的两个疲劳源位于齿轮轴大端油槽-油孔交界处,相对于油槽谷底直径平面呈反对称分布,且不在油槽谷底.为深入研究应力在疲劳裂纹源萌生位置和裂纹萌生过程中的作用,本文基于有限元法建立了HX_D1机车牵引电机转轴组件有限元细节应力分析模型,分析了在齿轮副啮合力作用下小齿轮轴的细节应力及分布状态.计算结果表明:小齿轮轴大端油孔两侧的两个应力集中点关于油槽谷底直径平面呈现反对称分布,这与裂纹源的实际位置吻合;从小齿轮轴锥端向齿端观察,油孔左侧应力集中点第一主应力值较右侧大(约26 MPa),这一区别导致左侧首先萌生疲劳裂纹的概率增大,该分析结果与失效小齿轮轴失效样本统计分析结果吻合.计算结果证实,油槽-油孔交界处呈反对称分布的应力集中在小齿轮轴的疲劳裂纹萌生过程中起决定性作用,建议采用表面机械强化的方法在两个疲劳危险点引入适当的残余压应力,以改善小齿轮轴的抗疲劳性能.     

Tribological failure analysis of gear contacts of Exciter Sieve gear boxes

In the present paper the premature failure of gear contact encountered in Exciter Sieve gear boxes has been analyzed. The cause of gear contact failure is identified by simulating the load bearing capacity of lubricants and conducting controlled experiments on an Amsler disk-on-disk tribo tester. The results of performance behavior (i.e., load carrying capacity of lubricants, contact friction and weight loss of test specimens) of the simulated gear contacts have been reported. The theoretical and experimental results indicate presence of mixed to partial elastohydrodynamic lubrication conditions in the gear contact. To mitigate the problem of scuffing and scoring in the gear contacts, lubricating oils with extreme pressure additives and Base oil without additive have been tested and performance results are reported.     

Investigation of gear performance of MLNGPs as an additive on polyamide 6 spur gear

Molded plastic gears have long provided an alternative to metal gears in lightly loaded drives. They transmit power quietly and often without lubrication in numerous applications, furthermore decrease the quantity of parts and oppose chemicals in numerous applications. Previously, plastic gears were restricted to to 0.25 hp because of varieties in their properties and uncertainties about how they react to natural conditions such as moisture, temperature and chemical. Today, better molding controls combined with design practices that more accurately encompass environmental factors have boosted plastic gear drive capacity to 1.5 hp. Using reinforcement this is standout amongst the most practices to enhance the gear performance.

This study estimated the effects of multilayer graphene nanoplatelets (MLNGPs) as an additive on polyamide 6 (PA6) spur gear performance. These include strength, elastic modulus, thermal stability, dynamic mechanical analysis, moisture absorption, and wear characteristics.The nanocomposite gear was made by melt mixing method and injection moulded into thick flanges. The flanges were machined using CNC milling machine to produce spur gear. The wear experiments were performed at a running speed of 1400 rpm and at torques of 13 and 16 Nm with different concentration 0, 0.1, 0.3 and 0.5 wt% MLNGPs using test rig. The result showed that 0.3% of MLGNPs is the optimum concentration. Young's modulus increased up to 40%, Vickers microhardness value increased up to 25%, storage modulus E’ is increased up to 37% and glass transition temperature is increased up to 14%. On the other hand TGA result shows that the Tonest increased up to 7.5% and Td increased up to 2%, and wear decreased by 35% at 16 Nm and 54% at 13 Nm.     

Pitting failure of truck spiral bevel gear

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.     

Fatigue failure of a bar of a twin-screw extruder for plastics

The paper deals with the analysis of the failures regarding one of the bars of a twin screw extruder for plastics and reinforced plastics. The extruder was equipped with an 50 kW electric motor and very soon showed failure problems, always on the same bar, that was substituted many times. The analysis of the material, tensile tests and fractographical observations of some broken bars helped to understand that failures were due to fatigue and to define some design improvements of the bar. In-service strain gauge measurements were executed to assess the actual load acting on the bars and the data were elaborated by using the spectral analysis approach and were used to assess the fatigue strength of the bars.     

Analysis of a vehicle wheel shaft failure

This paper describes an investigation which was carried out on a failed wheeldrive shaft component used on an unmanned, remotely operated vehicle for manoeuvring military targets. As many vehicles had been manufactured and delivered to customers it was necessary to establish whether it was thought likely that more failures might occur. A study of the broken shaft shows how vulnerable such a rotating component can be to failure by fatigue, even when operating under steady conditions, if basic preventative design actions are not taken. The analysis considers the effects of both transmission torque and weight (thus bending) upon stress levels and assesses their individual affect on the breakage and upon any subsequent modifications needed to improve the design. The drive shaft arrangement is compared with the feasible alternative of using a driven wheel arrangement rotating on a stationary axle. Findings confirm the importance of recognizing in advance the salient factors leading to fatigue and the necessity in paying adequate attention to detail during design and manufacture if long service life is to be achieved.     

Investigation on the failure of air compressor

The cause for the failure of an air compressor has been investigated. It was found that a pre-existing fatigue crack was present at the root of the impeller blade. Transients and unsteady operation of the equipment prior to the accident are thought to have grown the fatigue crack to its critical size, thereby causing an imbalance in the impeller rotation and leading to failure.     

Effect of shaft misalignment and friction force on time varying mesh stiffness of spur gear pair

Shaft misalignment and sliding friction between meshing teeth are considered as primary excitation to generate vibrations and extra dynamic loads on transmitting gear teeth. Time varying mesh stiffness (TVMS) is an important parameter to understand the dynamics of meshing gear pair. Potential energy method is one of the most acceptable methods to calculate TVMS. This paper proposes a computer simulation based approach to study the effect of shaft misalignment and friction on total effective mesh stiffness for spur gear pair. The results showed clearly that misalignment and friction affect TVMS of gear pair. The effect of misalignment and friction has also been studied for cracked gear pair and results are discussed.     

Investigation of the influence of asymmetrical profiles on the wear behavior in gear hobbing

Asymmetrical gears have advantages in applications with a preferred load direction, such as automotive transmissions or wind turbines. The operational behavior of gears with asymmetrical profiles has already been thoroughly investigated. However, there are very few studies on the manufacturability of gears with asymmetric profiles regarding the possible interactions between pressure angle composition, tool and process design. In this report, the influence of asymmetrical profiles on the wear behavior of hobs is analyzed for the first time.

For this purpose, eight different tool variants have been defined for an investigation on tool wear behavior based upon fly-cutting trials. These variants consist of two symmetric and four asymmetric variants. In addition, two asymmetric variants with additional protuberance have been defined. Tool performance decreased within the fly-cutting trials with a low pressure angle on the leading flank and also in the presence of a protuberance. As the pressure angle increases on the leading flank, an increase on the tool life was observed.

     

Tribo-dynamics model of a spur gear pair with gyroscopic effect and flexible shaft

Forschung im Ingenieurwesen - This study proposes a tribo-dynamic model of a spur gear by coupling an elastic dynamic model and an isothermal elastohydrodynamic lubrication (EHL)...     

Fatigue failure of a boiler feed pump rotor shaft

This paper deals with a detailed failure investigation of a rotor shaft in a boiler feed pump of a thermal power plant. The investigation mainly included chemical analysis, microscopy, fractography, hardness measurement and residual stress measurement. Analysis revealed that the metallization processes was primarily responsible for the failure. There were a large number of defects at the interface where the metallization process was terminated near the fillet. The crack had initiated from one or more such defect areas and then further propagated by fatigue. The mode of fracture was fatigue and the presence of oxides at the base of the cavities at the interface regions indicated that surface preparation was not made properly prior to metallization.     

Metallurgical failure analysis of a BWR recirculation pump shaft

This paper presents the metallurgical analysis of a Recirculation Pump from a Nuclear Power Plant. The damaged component was the austenitic stainless steel shaft. As a result of a preliminary visual examination, some axial cracks were found in the threaded area of the shaft in the thermal barrier area. The analyses performed include liquid penetrant testing, metallography and deposits analysis. Based on the obtained results, it was concluded that the origin of the axial cracks was thermal fatigue caused by mixing of cold seal injection water and hot reactor coolant in the thermal barrier area.     

Fatigue failure of a rear axle shaft of an automobile

This paper describes the failure analysis of a rear axle shaft used in an automobile which had been involved in an accident. The axle shaft was found to break into two pieces. The investigation was carried out in order to establish whether the failure was the cause or a consequence of the accident. An evaluation of the failed axle shaft was undertaken to assess its integrity that included a visual examination, photo documentation, chemical analysis, micro-hardness measurement, tensile testing, and metallographic examination. The failure zones were examined with the help of a scanning electron microscope equipped with EDX facility. Results indicate that the axle shaft fractured in reversed bending fatigue as a result of improper welding.