Mechanical Engineering

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Dynamic modelling of a bolted disc rotor assembly
(2008) Blignaut, Gert; Roberts, Johan
A project investigating the behaviour of an assembled preloaded rotor was performed for an M-Tech qualification in the Mechanical Engineering Department. Pre-Stressing of mechanical structures is widely applied to improve their performance, and in this project the behaviour of an assembled preloaded rotor was investigated. An Impact Test was done on the structure to see if induced stresses originated by a set of bolts which keep the discs system together, would influence the natural dynamic response or the rotor. Tendencies in the natural response were investigated. Analytical models like the Finite Element Beam model and the Solid Finite Element model were studied in order to find a represntative description of this particular structure's dynamic behaviour after pre-tension. From the experimental results it was apparent that the slenderness of the pre-tensioned sector influences the natural frequency. The solid finite element model appears to be the most applicable model to present the assembled rotor-disk system as a continuous shaft. Furthermore, modelling and predictions for a typical rotor and similar assembled structures can be generated from the findings.
Vibration condition monitoring and fault classification of rolling element bearings utilising Kohonen's self-organising maps
(2013-08-21) Nkuna, Jay Shipalani Rhulani; Enslin, J.; Van der Merwe, D. F.
Bearing condition monitoring and fault diagnosis have been studied for many years. Popular techniques are applied through advanced signal processing and pattern recognition technologies. The subject of the research was vibration condition monitoring of incipient damage in rolling element bearings. The research was confined to deep-groove ball bearings because of their common applications in industry. The aim of the research was to apply neural networks to vibration condition monitoring of rolling element bearings. Kohonen's Self-Organising Feature Map is the neural network that was used to enable an automatic condition monitoring system. Bearing vibration is induced during bearing operation and the main cause is bearing friction, which ultimately causes wear and incipient spalling in a rolling element bearing. To obtain rolling element bearing vibrations a condition monitoring test rig for rolling element bearings had to be designed and built. A digital vibration measurement acquisition environment was created in Labview and Matlab. Data from the bearing test rig was recorded with a piezoelectric accelerometer, and an S-type load cell connected to dynamic signal analysis cards. The vibration measurement instrumentation was cost-effective and yielded accurate and repeatable measurements. Defects on rolling element bearings were artificially inflicted so that a pattern of bearing defects could be established. An input data format of vibration statistical parameters was created using the time and frequency domain signals. Kohonen's Self-Organising Feature Maps were trained in the input data, utilising an unsupervised, competitive learning algorithm and vector quantisation to cluster the bearing defects on a two-dimensional topographical map. A new practical dimension to condition monitoring of rolling element bearings was developed. The use of time domain and frequency domain analysis of bearing vibration has been combined with a visual and classification analysis of distinct bearing defects through the application of the Self-Organising Feature Map. This is a suitable technique for rolling element bearing defect detection, remaining bearing life estimation and to assist in planning maintenance schedules.
Characterisation and flowability of titanium grade 5 alloy powders
(Vaal University of Technology, 2013-09) Nziu, P. K.; Mendonidis, P., Prof.; Labuschagne, D., Dr.; Masu, L. M., Prof.
Flowability is one of the essential physical characteristics considered during the use of any powder in a manufacturing process. However, very little research on flowability of titanium powder has been conducted. To this end, this study dealt with global market survey of titanium powder manufacturers and suppliers. In addition, the effects of various physical parameters such particle size, shape, chemical analysis, density and soundness on flowability of titanium grade 5 alloys powder in additive manufacturing application were investigated. Twelve powder samples of titanium alloy grade 5 (Ti6Al4V) were sourced, tested and analyzed using various methods. The choice of the characterization method used depended on its accuracy, equipment availability and application. Particle size and shape were characterized using laser diffraction and scanning electron microscope techniques, respectively. Quantitative and crystallographic analyses were done to determine the chemical composition as well as alpha and beta phases. Shear cell and dynamic tests were performed to determine bulk density, stability, flow energy and flowability where as particle density was performed by a pcynometer. Research on potential manufacturers was conducted using questionnaires. It was established that high cost of titanium powder is partly driven by titanium powder firms that are not willing to disclose information about the product. It was observed that powder flowability is affected by particle size, shape, chemical composition, density and soundness. The particle density was found to be a function of chemical composition that is the alloying elements and impurities present in the powder. It was noted that bulk density, porosity, cohesion and agglomeration were affected by particle size. Soundness of the powder was also found to improve with sphericity of the particles. Among the physical parameters studied, particle size had the highest effect on powder flowability. The highest flowability was noted at particle size of 41 μm.
Mechanical shock values applied in condition monitoring of bearings operating under variable speed and load conditions
(2014-08) Olivier, Allan Andre; Alugongo, A. A.; Masu, L. M.
Monitoring the condition of equipment in industry is very important to prevent unplanned breakdowns and to prolong their life. This is necessary, since it is not always economically viable to stop equipment at regular intervals to do maintenance. Failure on machines can lead to high repair costs and production losses. It is thus of paramount importance that early failure symptoms be identified by means of condition monitoring. This study in the field of condition monitoring is performed to determine if the mechanical shock values induced in defect bearings could be used to measure the condition of a bearing while operating under variable speed and variable load. Variable speed and variable load is becoming more popular in industry because variable speed drives applications ensure effective process control. Variable speed application, cause fault frequencies to fluctuate and therefore vibration applications for constant speed applications, which are speed-dependent, can no longer apply. Vibration-monitoring techniques that have applied for many years have now become obsolete in these variable speed applications. Methods such as Short Time Fourier Transformation (STFT), time scale like wavelet transform, and Order tracking has been applied in variable speed applications with some success. These methods analyses the vibration phases on the signal buy compensating for the speed changes. In this thesis, the Shock pulse method is selected as the analyses tool to measure the mechanical shock. Shock pulse monitoring does not focus on the vibration phases but measures in a small-time window when mechanical shocks are induced in the bearing material before the vibration phase. There is very little documented research in the field of mechanical shock pulse monitoring for conditions of variable speed and variable loads, and therefore this research focuses on recording these mechanical shock values by empirical tests. The tests were performed on a bearing with an induced defect on the outer race. The rolling element of the bearing strikes the defect and the mechanical shock value (dBsv) is measured. The mechanical shock is measured with the Shock pulse method in a small-time window before vibration occurs. In this time window, the dBsv is recorded over time to provide diagnostic information of the bearing during acceleration, deceleration and various loading conditions. These mechanical shocks are elastic waves that mirror the impact-contact-force's time function and the Shock pulse monitoring accelerometer, which is tuned to 32 kHz, will respond to the elastic wave fronts with transient amplitudes proportional to the square of the impact velocities. The mechanical shock values were analysed and reoccurring fault levels were identified on each empirical test. These recurring events from the empirical tests were used as primary data for analysis in this research. These tests were performed on a bearing with an induced failure and it was found that the dBsv measured over time could not be used to monitor the condition of the bearing under variable speed applications. This was because the dBsv changed as the speed increased. To overcome this problem Sohoel’s theory was applied and the initial mechanical shock value (dBi) was calculated for the bearing. The dbi value was subtracted from the dBsv and a value called the maximum mechanical shock value (dBm) was obtained. The dBm values stayed constant for the duration of the test and this allowed the condition of the bearing to be measured under variable speed and variable load conditions with some exception. The exception to the findings was that the dBm values stayed constant during acceleration phases, but during the deceleration phases the values were erratic and scattered. At speed below 200rpm the dBm values did not stay constant and therefore it was concluded that the dBm value recorded the best results only when thrust on the bearing was maximum. The other exception was under no-load conditions. The values were erratic and scattered, and therefore the results were not a true reflection of the bearing condition. The third exception was that the results on bearings with various loads remained constant during increased load changes unless the loading was erratic. During erratic load changes, the results were affected. The results also indicated that the larger the defect on the bearing raceway, the higher the dBm values were. Multipil defects on the bearing race ways were not part of this thesis and this gives an opertunity for futher research. The Shock pulse monitoring technique was 100% successful in monitoring the bearing condition only while the speed of the bearing was increasing. The results obtained in this work demonstrated that the condition of bearings can be monitored in applications of variable speed and variable load if the exception are eliminated and to obtain conclusive results the mechanical shock pulses should be measured over time and not be used as once-off value.
Advanced modelling of porous screens in aerodynamic diffusers using variable resistance factors
(2014-12) Janse van Rensburg, Jacobus Johannes; Van Staden, M. P.; Jacobs, G. G.
Strict emission legislation has forced industry in general to seriously consider the negative impact it has on the environment, specifically concerning emissions from burning fossil fuel into the atmosphere. In cases where emission levels exceed the allowable limit, companies are forced to operate at lower operating conditions and these load losses can result in a significant loss of revenue. This has forced companies to improve their ash filtering capabilities by optimising electrostatic precipitation systems. One of the main factors impacting on the efficiency of such a system is the distribution of the flow across the collection plates. The design of the inlet diffuser plays a major role in the ultimate distribution of the flow through the precipitator. Porous screens are positioned in the diffuser in order to distribute the flow across the total flow area with the aim to achieve a uniform distribution of the flow. CFD is widely used in industry to simulate the flow through precipitators in order to optimise the flow distribution and thus increase the efficiency of the system. It was found however that the current methods used to simulate these screens in CFD models were not well researched and employed fixed resistance values that could not reliably compensate for changes in the resistance coefficient due to a change in the angle of incidence. This study investigates advanced numerical methods for the simulation of porous screens in applications where the angle of incidence changes continuously across the face of the screen. New methods are introduced where the resistance of the screen is calculated as a function of the changing angle of incidence. The methods currently used are also investigated and compared with results from the new methods. Extensive experimental work was required to supply empirical data for the validation of the numerical methods that are proposed. For this reason, the first part of this study focused on the design construction and commissioning of a low speed wind tunnel. Results are presented and discussed for flow profiles through wide-angle diffusers at different angles and also for a number of different screens positioned in the centre of the diffuser. This study also investigates the sensitivity of a CFD simulation code to factors such as numerical discretisation schemes, turbulence models and solution relaxation specifically for wide-angle diffusers. These factors were tested for diffusers at different angles and included tests on open diffusers and also with screens positioned inside the diffuser. It was concluded that the current methods used are not adequate to capture the true flow profiles for a range of different screen geometries. Although the proposed models did improve on the limitations of the current methods, it was found that the applicability of these models is still limited and that further research would be required to develop numerical methods that are valid for a wide range of applications.
Modelling stain rate sensitive nanomaterials' mechanical properties: the effects of varying definitions
(Vaal University of Technology, 2016-06) Sob, Peter Baonhe; Alugongo, A. A. Prof.; Tengen, T. B., Prof.
Presently there exist a lot of controversies about the mechanical properties of nanomaterials. Several convincing reasons and justifications have been put forward for the controversies. Some of the reasons are varying processing routes, varying ways of defining equations, varying grain sizes, varying internal constituent structures, varying techniques of imposing strain on the specimen etc. It is therefore necessary for scientists, engineers and technologists to come up with a clearer way of defining and dealing with nanomaterials’ mechanical properties. The parameters of the internal constituent structures of nanomaterials are random in nature with random spatial patterns. So they can best be studied using random processes, specifically as stochastic processes. In this dissertation the tools of stochastic processes have been used as they offer a better approach to understand and analyse random processes. This research adopts the approach of ascertaining the correct mathematical models to be used for experimentation and modelling. After a thorough literature survey it was observed that size and temperature are two important parameters that must be considered in selecting the relevant mathematical definitions for nanomaterials’ mechanical properties. Temperature has a vital role to play during grain refinement since all severe plastic deformation involves thermomechanical processes. The second task performed in this research is to develop the mathematical formulations based on the experimental observation of 2-D grains and 3-D grains deformed by Accumulative Roll-Bonding and Equal Channel Angular Pressing. The experimental observations revealed that grains deformed by Accumulative Roll-Bonding and Equal Channel Angular Pressing are elongated when observed from the rolling direction, and transverse direction, and equiaxed when observed from the normal direction. In this dissertation, the different experimental observations for the grain size variants during grain refinement were established for 2-D and 3-D grains. This led to the development of a stochastic model of grain-elongation for 2-D and 3-D grains. The third task was experimentations and validation of proposed models. Accumulative Roll-Bonding, Equal Channel Angular Pressing and mechanical testing (tensile test) experiments were performed. The effect of size on elongation and material properties were studied to validate the developed models since size has a major effect on material’s properties. The fourth task was obtaining results and discussion of theoretical developed models and experimental results. The following facts were experimentally observed and also revealed by the models. Different approaches of measuring grain size reveal different strains that cannot be directly obtained from plots of the corresponding grain sizes. Grain elongation evolved as small values for larger grains, but became larger for smaller grains. Material properties increased with elongation reaching a maximum and started decreasing as is evident in the Hall-Petch to the Reverse Hall-Petch Relationship. This was alluded to the fact that extreme plastic straining led to distorted structures where grain boundaries and curvatures were in “non-equilibrium” states. Overall, this dissertation contributed new knowledge to the body of knowledge of nanomaterials’ mechanical properties in a number of ways. The major contributions to the body of knowledge by his study can be summarized as follows: (1) The study has contributed in developing a model of elongation for 2-D grain and 3-D grains. It has been generally reported by researchers that materials deformed by Accumulative Roll-Bonding and Equal Channel Angular Pressing are generally elongated but none of these researchers have developed a model of elongation. Elongation revealed more information about “size” during grain refinement. (2) The Transmission Electron Microscopy revealed the grain shape in three directions. The rolling direction or sliding direction, the normal direction and the transverse direction. Most developed models ignored the different approaches of measuring nanomaterials’ mechanical properties. Most existing models dealt only with the equivalent radius measurement during grain refinement. In this dissertation, the different approaches of measuring nanomaterials’ mechanical properties have been considered in the developed models. From this dissertation an accurate correlation can be made from microscopy results and theoretical results. (3) This research has shown that most of the published results on nanomaterials’ mechanical properties may be correct although controversies exist when comparing the different results. This research has also shown that researchers might have considered different approaches to measure nanomaterials’ mechanical properties. The reason for different results is due to different approaches of measuring nanomaterials’ mechanical properties as revealed in this research. Since different approaches of measuring nanomaterials’ mechanical properties led to different obtained results, this justify that most published results of nanomaterials’ mechanical properties may be correct. This dissertation revealed more properties of nanomaterials that are ignored by the models that considered only the equivalent length. (4) This research has contributed to the understanding of nanomaterials controversies when comparing results from different researchers.
Performance analysis and modelling of diesel engine operational characteristics using pyrolytic oil from scrap tyre
(Vaal University of Technology, 2017-07) Mwanzi, Maube Obadiah; Masu, L. M., Prof.; Alugongo, A. A., Prof.
In this work, an investigation on the fraction of tyre pyrolysis oil with a similar distillation range to that of automotive diesel (150 – 360 oC) was carried out to determine its suitability as an alternative or additive to petro-diesel fuel. The quality of this oil was evaluated by comparing its key properties to the requirements of South African National Standards for Automotive diesel fuel (SANS-342) and to conventional automotive diesel fuel. The viscosity, density, copper strip corrosion of this fuel were found to be within the acceptable limits set by SANS while sulphur content and flash point were out of their respective set limits. In addition, mixing rule equations for predicting viscosity and density for both pure and blends of the oil as a function of temperature were developed and evaluated. The equations were found to be suitable due to their low Absolute Percentage Deviation. Engine performance tests were carried out with blends of Distilled Tyre Pyrolysis Oil (DTPO) and petro-diesel fuel in a single cylinder air cooled diesel engine. The performance, emission and combustion characteristics of the diesel engine while running on these blends were evaluated and subsequently, a comparative analysis was performed with conventional petro-diesel fuel as the reference fuel. It was found that, the engine could run with up to 60% (DTPO) without any problem. Beyond this level the engine became unstable. The power and torque were similar at low and medium speeds. However, at high speeds, the power dropped with increase in DTPO in the blend. Fuel consumption was very comparable for all the test fuels. Carbon monoxide and unburned hydrocarbons were higher for the blends compared to petro-diesel fuel but oxides of Nitrogen were lower. The peak pressure for petro-diesel fuel was marginally higher than that of the blends. Present results indicate that, petro-diesel fuel can be blended with up to 60% DTPO and produce acceptable performance. Testing the diesel engine under different operating conditions is a time consuming and expensive process that also requires the use of specialised equipment which may not be readily available. An Artificial Neural Network (ANN) model based on a back-propagation learning algorithm was developed to predict engine performance and emissions separately, based on fuel blend and speed. The performance and accuracy of the model were evaluated by comparing experimental and ANN predicted results. The ANN was able to predict both engine performance and emissions with acceptable levels of accuracy. The values of correlation coefficient between experimental and predicted data being greater than 0.99. From this work, it can be implied that engine emission and performance can be predicted using neural network-based mode, consequently, it will be able to do further investigations without running laboratory experiments. Energy recovery from waste is an interesting field for engineers and scientists. It is hoped that this work will prompt new research ideals on how tyre pyrolysis oil can be improved for use as diesel engine fuel and building better models for diesel engine performance and emissions
Structural performance of KAT-7's ball screw raceway in the elastic region subjected to varying loads
(Vaal University of Technology, 2017-12) Steenekamp, Nicolaas Kruger; Masu, Leonard, Prof.; Alugongo, Alfayo, Prof.
The structural performance of a KAT-7’s ball screw raceway is not known. No performance data has been generated analytically, numerically nor experimentally. For this study, data was generated in the elastic region of the material. This research was undertaken in three different ways namely analytically, numerically and experimentally. A calibrated load cell was used to validate the analytical solutions. Solid Edge, a parametric software package was used to validate analytically the accumulated rain water mass and structural mass. Abaqus, a finite element analysis software package, was used to model and obtain the ball bearing Hertzian contact stresses numerically. The numerical solution was used to validate the laboratory compression test results on a replica KAT-7 ball screw assembly. The weighted percentage errors between the analytical model data and load cell data were found to be higher for load case scenarios with zero m/s and 10 m/s wind speeds respectively. The parabolic reflector rigid body assumption, exclusion of wind induced hysteresis effects and the quasi-static wind loading site measurement conditions contributed to the weighted percentage error variations. The laboratory and numerical model compression force results revealed a gradual percentage error increase beyond a compression force of 261288 N and up to 572526 N. The percentage error increase had minimum and maximum errors ranging between 6.24 percent and 14.69 percent respectively. The percentage error increase in the numerical model was due to the singular representation of a ball bearing instead of a 212 ball bearings set as experimentally conducted in the laboratory compression test on a replica KAT-7 ball screw assembly. The maximum axial force, F0 , result for load case scenario five was -94469 N with a Hertzian contact stress of 3939 MPa on the raceway surface. The static load rating required to Brinell a deep groove ball bearing raceway was found to be a Hertzian contact stress ranging between 4500 MPa and 4800 MPa. It was evident that the contact stresses incurred under the three considered loads of accumulated rain water, wind loading and structural mass were unable to exceed the 4800 MPa Hertzian contact stress. It was found that a replica KAT-7 ball screw raceway Brinelled under an axial force of 408126 N. The numerical ball screw raceway model Brinelled under an axial force of 380457 N. The Hertzian contact stress at the numerical ball screw raceway surface was determined to be 4898 MPa. Therefore, the Replica KAT-7 ball screw raceway material behaves elastically under an approximate load of up to 38 tonnes.
Quasi-static mechanical properties of treated and untreated sisal fibre reinforced epoxy resin composites
(Vaal University of Technology, 2017-12-15) Webo, Wilson Wachuli; Maringa, Maina, Prof.; Masu, L. M., Prof.
Sisal is a vegetable fibre extracted from the leaves of Agave Sisalana. The fibre is long, bold and creamy white in colour besides being exceptionally strong. It can be used for making agricultural and parcelling twines of various kinds as well as ropes, sacks, carpet and upholstery. The primary purpose of this research was to study and evaluate the use of sisal as a reinforcing fibre in both treated and untreated forms with epoxy resin matrices. The casting process employed during the composite production was the the vacuum infusion. The effects of both the treated sisal fibre-epoxy resin composites and the untreated sisal fibre-epoxy resin composites on the tensile strength and stiffness, flexural strength and stiffness, impact toughness, shear strength, compression strength and hardness were evaluated. Finally, the occurrence of transverse matrix fracture and fibre pull-out were also studied. It was found that the quasi-static mechanical properties of both the treated sisal fibre-epoxy resin composites and the untreated sisal fibre-epoxy resin composites improved with increases in reinforcement weight fractions. Further, fibre surface treatment on the sisal fibres and the attendant increase in the interfacial bond also resulted into improved quasi-static mechanical properties of the treated sisal fibre-epoxy resin composites when compared to untreated sisal fibre-epoxy resin composites.
Optimal geometric configuration of a cross bore in high pressure vessels.
(Vaal University of Technology, 2018-04) Nziu, P. K.; Masu, L. M., Prof.
The purpose of this study was to develop analytical and numerical solutions to be used in the design of thick walled high pressure vessels for optimal location of a cross bore. In addition, the effects of internally applied combined thermo-mechanical loading on Stress Concentration Factor (SCF) on these vessels, was also evaluated. An analytical solution, to predict principal stresses on radial circular cross bore, was developed. The developed analytical solution was verified using finite element analysis methods. An optimisation process, using finite element analysis, was further done to determine the optimal combination of the major cross bore geometry that affect stress concentration. The cross bore geometries that were studied included the size, shape, location, obliquity and thickness ratio. The geometrically optimised cross bore was then subjected to combined thermo-mechanical loading to determine the resulting stress concentration effects. A total of 169 finite element part models were created and analysed. Seven thick walled cylinders having either circular or elliptical shaped cross bore positioned at radial, offset or and inclined were investigated. The analytical solution developed correctly predicted all the radial stresses at the intersection of the cross bore and main bore. However, out of 35 studied models, this analytical solution predicted the magnitude of hoop stresses in 9 models and that of axial stresses in 15 models correctly. The lowest SCF given by the radial circular cross bore was 2.84. Whereas, the SCF due to offsetting of the same cross bore size reduced to 2.31. Radial elliptical shaped cross bore gave the overall lowest SCF at 1.73. In contrast, offsetting of the same elliptical shaped cross bore resulted in tremendous increase in SCF magnitude exceeding 1.971. Additionally, the magnitudes of SCF were observed to increase whenever the circular offset cross bores were inclined along the RZ axis of the cylinder. The hoop stress due to internally applied combined thermo-mechanical loading increased gradually with increase in temperature until it reached a maximum value after which it began to fall sharply. In contrast, the corresponding SCF reduced gradually with increase in temperature until it reached a uniform steady state. After which, any further increase in temperature had insignificant change in stress concentration factor. The optimal SCF magnitude due to combined thermo-mechanical loading was 1.43. This SCF magnitude was slightly lower than that due to the pressure load acting alone.
Optimisation of casting process of sand cast austenitic stainless-steel pump impeller using numerical modelling and additive manufacturing
(Vaal University of Technology, 2018-12) Mugeri, Hudivhamudzimu; Adebiyi, Isaac Damilola, Dr.; Matizamhuka, Wallace R., Dr.
The production of austenitic stainless-steel pump impellers in foundries present a huge challenge mainly due to its thin-walled blades, pouring temperature, presence of junctions and chemical composition. Two different alloys were used namely nodular cast iron and austenitic stainless-steel. Nodular cast iron was used as a comparison alloy due to its excellent flowability whereas austenitic stainless-steel was chosen due to its attractive corrosion and wear resistant properties. Austenitic stainless-steel alloy showed difficulties during casting because of its chemical composition and freezing range. Thin-walled sections are more susceptible to filling defects like misrun and cold-shut. This results in high scrap rate and high processing costs during high production of thin-walled components. High pouring temperature is considered one of the most effective methods to improve filling ability of thin-walled castings. However, there is a major drawback in using this method owing to the high occurrence of shrinkage defects and hot tearing especially at junctions. 1060 aluminium was used as a benchmark to evaluate the effect of wall thickness on the filling and feeding of thin-walled Al components with complex geometry during sand casting. The aim of this dissertation is therefore to optimize casting process of sand cast austenitic stainless-steel pump impeller. Numerical modelling and additive manufacturing were used to optimize the production of this product. The use of casting simulation software combined with three-dimensional (3D) mould printing technology has enabled optimisation of casting parameters to minimise the occurrence of casting defects. Casting parameters of five test samples of complex geometry and varying thicknesses (1.0 mm;1.5 mm;2 mm;2.5 mm and 3.0 mm) were optimised using MAGMAsoft® at a constant pouring temperature of 700 °C and 1060 Aluminium as an alloy. Simulation and casting results showed that complete filling was only possible at a wall thickness of 3 mm. The simulation results showed that as the wall thickness increased from 1 mm to 3 mm the filling ability increased by 67.5 % whereas experimental casting results showed that filling ability increase by 75 %. The combination of MAGMAsoft® simulation and 3D printed moulds proved to be effective tools in predicting filling and feeding of thin-walled aluminium components during sand casting. MAGMAsoft® casting software was used to simulate metal flow and predict the degree of filling at different pouring temperatures. Test samples were cast using 1060 Aluminium alloy at temperatures of 702 °C, 729 °C, 761 °C, 794 °C, 800 °C and 862 °C. Complete mould filling was predicted at 800 °C using the simulation model and 761°C during actual casting. At temperatures above 761°C tearing at the junction was quite pronounced. An optimal of 761°C pouring temperature was found to be appropriate pouring temperature when casting thin-walled aluminum components using sand casting. MAGMAsoft® casting software proved to be an effective tool in optimizing filling and feeding of thin-walled aluminium components during sand casting. Nodular cast iron pump impeller was optimized at 1500 °C using MAGMAsoft® and 3D mould printing technology. Design variables used were feeder radius (17 mm, 18 mm, 19 mm and 20 mm), feeder height (32 mm, 33 mm, 34 mm, 35 mm) and number of feeders of (3, 4 and 5). Simulation and casting results showed a completely-filled casting. The high fluidity of nodular cast iron promotes mould filling ability and prevent any form of misrun defect. Minimum shrinkage was noted at the junctions and top surface of the casting. A new design was proposed to eliminate shrinkage defects at the junctions of the nodular cast iron pump impeller. The design used a tapered circular runner bar with straight ingates. Optimization of nodular cast iron was now done at 1390 °C with the use of MAGMAsoft® and real casting was done 1385 °C. Simulation and casting were in correlation to each other since both showed completely-filled mould cavity with no misrun, cold-shut and shrinkage porosity defect. Simulation proved to be an effective tool in optimizing filling and solidification of nodular cast iron during sand casting. Austenitic stainless-steel pump impeller was optimized at 1500 °C using MAGMAsoft® and 3D mould printing technology. A high quality mould and core print were printed with the use of Voxeljet VX1000 at a minimum period of time. Design variables used were feeder radius (17 mm, 18 mm, 19 mm and 20 mm), feeder height (32 mm, 33 mm, 34 mm, 35 mm) and number of feeders of (3, 4 and 5). An increase in feeder size and the number of feeders greatly reduced hot spot and porosity of the casting but it also reduced the casting yield. The quality of the casting was found to be inversely proportional to the casting yield. Simulation showed a completely-filled casting with actual casting showing only 50 % filling ability. High viscosity of the molten metal and thin walled blades promote quick solidification which caused misrun defects. A new design was proposed to eliminate misrun defects of the first design. MAGMAsoft® was used to optimize this design at 1550 °C. The design used a tapered circular runner bar with tapered ingates. The actual casting showed improved filling ability from 50 % to 80 % while simulation showed completely-filled mould cavity (100 %). Major factors which contributed to low filling ability of austenitic stainless-steel pump impeller were chemistry, runner system and men. Numerical modelling and additive manufacturing did optimize filling and feeding of sand cast austenitic stainless-steel pump impeller.
Identification and analysis of steam temperature maldistribution in superheater tubes via measured and derived parameters
(Vaal University of Technology, 2019-08) Vilakazi, Lethukuthula Nokwazi; Rousseau, Pieter, Prof.; Alugongo, Alfayo, Prof.
Superheater and reheater heat exchangers in power plant boilers can experience temperature excursions and gradients significantly above design values due to cyclic operations. This may result in accelerated life consumption of these components. To understand better the influence of different operating conditions, research is ongoing to develop detailed thermo-fluid process models of the various boiler heat exchangers, and real-plant data are required in the validation of these models. In this study, the final superheater of a 620 MW coal-fired power plant unit was analysed based on real plant measurements taken during steady state operation at 100, 80 and 65 percent of the current boiler capacity. Process parameters routinely measured via the plant distributed control system (DCS), such as the steam temperatures, pressures and mass flow rates, were used as input data to derive other unmeasured parameters using the mass and energy balance (MEB) methodology. Thermocouples were installed previously on the inlet and outlet final superheater stub boxes as well as the outlet manifolds. Thermocouple data were collected from a data logger at the corresponding dates and times of the DCS MEB inputs. Measurement uncertainties were determined by considering instrument and statistical uncertainties, which were then propagated through the MEB calculation to the derived parameters. The MEB methodology was applied to determine the flue gas temperature and flow rates at different operating loads (65, 80 and 100 percent). The good comparison obtained between the values calculated with the MEB and those of the C-schedule for the 100 percent boiler maximum continuous rating (BMCR) provided confidence in the validity of the MEB. The MEB was also compared to real plant data of flue gas temperature. The comparison provided a difference that is less than 26℃. Identification of the measurement uncertainties provided a detailed analysis on each instrument and or measurement and how certain I could be about each measurement. Uncertainties of parameters derived using the MEB methodology were determined. This was achieved by uncertainty propagation through the MEB model. Uncertainty propagation also provided a sensitivity percentage relative to the propagated uncertainties. The extent of temperature maldistribution was determined based on the measured outside tube metal temperatures. The results from the thermocouple measurements on the steam pipes connected to the final super heater inlet and outlet manifold headers show that there is temperature maldistribution between the inlet headers of the four legs. There is also significant maldistribution at the outlet headers resulting in noticeable local temperature gradients. It can also be concluded that the low load of 65 percent resulted in the highest temperature maldistribution compared to the higher loads, of 100 and 80 percent. Super heater tube metal temperatures are exposed to high temperatures at low loads which may lead to tube leaks.
Controllability and stability of selectively wettable nanostructured membrane for oil/water separation
(Vaal University of Technology, 2019-12) Sob, Peter Baonhe; Alugongo, A. A., Prof.; Tengen, T. B., Prof.
Presently, the current membrane technologies used in oil/water separation are inefficient with poor controllability and stability during oil/water separation. The has led to the current problem of membrane fouling and degradation during oil/water separation. Several approaches have been used to modify or design a better wettable surface with limited success since the current problem of membrane fouling is persisting. It is, therefore, necessary for scientists, engineers, and researchers to come up with a new membrane technology that will be more efficient with stable wettability and controllability during oil/water separation. Membranes are made up of nanoparticles on their surface, which are both random in nature. Furthermore, the collection of membrane particles to form mesh membranes are made of pores with further ransom spatial distribution. Thus, it was necessary to use the tools of stochastic processes to theoretically characterize these parameters. These parameters affect both internal and external factors as well as characteristics of random membrane particle and pores on wettability like surface tension and surface energy were established in the current project. Design and production of the membrane material according to established relationships was by both low and high-pressure spay jet coating in a controlled laboratory environment, and microscopic characterization performed using SEM. TEM, EDS, statistical analysis, and Image J particle analyzer. The spread, orientation, morphology, spatial distribution, inter-separation distances, surface roughness, surface smoothness, contact angles, surface density of the particle, mean size of the coated nanoparticle on the membrane surface after different coating rounds were analyzed so as to establish conditions for optimal wettability. The testing of produced membranes under the application of external and internal factors was done. A centrifugal pump was used to pump contaminated oil and water mixture through the membrane under a steady flow rate of 10 L/s with a gauge pressure of 180 kPa at room temperature conditions. The membrane materials from different coating rounds were tested for their abilities to produce pure collected water or oil particles in the collected water. The separated water was analyzed using oil and grease analysis US EPA method 1664B with the SPE-DEX 1000 oil and grease system. As revealed theoretically and validated experimentally, it was found that the random natures of nanoparticle size, the spatial distribution of membrane channels, and their morphology have impacts on surface energy-driven separability of oil and water mixture. It was also observed that the scattering of nanoparticles on the membrane surface during coating lowered surface energy, which enhanced oil/water separation. It was also revealed that there is an optimal nanoparticle size, scattering, morphology, and spatial distribution of membrane channels that offer better separation of water from oil. From the microscopy analysis, different microstructures were revealed for glass, ceramics, and sediment during LP and HP coating. The microstructure characterization showed different surface densities of nanoparticles, mean particle sizes, surface roughness or smoothness, and nanoparticles inter-separation distances. It was also revealed that the materials, which were more stable and efficient with more controlled wettability were glass, sediment, and ceramic HP 3rd rounds of coating. Clusters were observed on the membrane surface during HP and LP coating rounds with more clusters observed in LP coating when compared with HP coating. These clusters increased surface energy, which negatively affected oil/water separation. It was concluded that to improved the wettability surface. membrane clusters must be minimized during coating rounds. This thesis contributed new knowledge to existing body knowledge of membrane technology used in oil/water separation in a number of ways by: (1) Designing a new membrane surface with a more controlled, efficient, and stable wettability process during oil/water separation. (2) Applying the logic of surface energy-driven separability, which has not been previously used extensively to study membrane wettability. (3) Establishing a model for the optimal membrane pore sizes that offer optimal membrane wettability during oil/water separation. (4) Establishing a model for optimal nanoparticle coating that offers optimal membrane wettability during oil /water separation. (5) A great attempt was made in characterizing nanoparticle surface densities, spread, particle coating, and nanoparticles intensity on a wettable membrane surface.
Rub-impact of coupled vibration of vertical rotor-stator system submerged in incompressible fluid
(Vaal University of Technology, 2020-01-21) Sozinando, Desejo Filipeson; Tchomeni, B. X.; Alugongo, A. A., Pof.
Fault diagnosis of a rotor system operating in a fluid is one of the most difficult aspects of rotating machinery. Fluid in machinery plays a significant role in concealing the allowable rubbing stress limit during the impact generated from the rotor-stator rub which may progressively deteriorate the rotating system. Therefore, a numerical and experimental investigation was performed to analyse the influence of the fluid during the rotor-stator contact of a vertical rotor system partially submerged in an incompressible inviscid fluid with a focus on detecting rubbing fault in the presence of axial load. The theoretical model of lateral-torsional rotor consists of a 3-D rub-impact induced parametric excitation, which was assimilated to operate as elastic vertical rotor system by considering the transient vibration of a flexible axial force and energy of the vertical shaft system. The model was established based on Jeffcott rotor, time-varying stiffness and the rotor-stator fluid interaction. The Lagrangian principle was used to establish the governing equation of motion. The hydrodynamic forces acting on the vertical rotor were established and introduced into the system based on the Laplace form of the linearized Navier–Stokes equations under lateral excitation yielding a highly nonlinear 5-DOF system. To evaluate the dynamic response and ensure the accurate acquisition of rubbing features in a fluid, the classical Fast Fourier Transform (FFT) and the vibration waveform have been discretised and illustrated through the frequency components. Furthermore, for effective extraction of some hidden features of rub, the nonlinear features embedded in the vibration waveform have been discretised and illustrated through to the lateral deformation of the rotor and the orbit patterns of the shaft. Qualitative numerical analysis suitable for highly nonlinear and non-stationary signal Time-Frequency strategies, Wavelet Synchrosqueezed Transform (WSST) and Instantaneous Frequency (IF) technique were employed to successfully extract the frequency of oscillating modes and the periodic frequency response of the faulted rotor system. It is demonstrated that the coupled lateral-torsional vibration of the submerged vertical rotor system has the potential to enhance the much-unwanted hidden frequencies of vibration that leads to significant instability of the rotor system. In particular, the responses revealed the existence of unstable regimes with respect to the lateral-torsional deflection as well as the angular velocity. High harmonic peaks were also identified at the critical speed, which can be considered as a monitoring index to detect the rubbing in rotating shafts in a fluid. It was found that even at relatively slow rotating speed fluid elastic forces induced by the co-rotating flow surrounding the shaft significantly affect the transverse natural modes of vibration of the shaft. Despite the interaction between the fluid and the rotor generates self-excitation of low frequencies, obtained results indicated that the fluid-rotor interaction reduces the dynamic vibration response of the faulted system running below the second critical speed. It has been analytically demonstrated that the time-varying stiffness induced is the principal cause of the frequency-modification feature of the dynamic response of an unbalance-rub rotor system at the contact region. The model investigated in this study has potential application for drill string-borehole shaft system used in the oil industry.
Impact of equal channel angular pressing operational parameters on the mechanical properties of characterized Titanium-Based powders
(Vaal University of Technology, 2020-03) Nhlapo, Mirriam; Machio, C., Dr.; Masu, L., Prof.
The powder metallurgy technique Equal Channel Angular Pressing is a severe plastic deformation method. Where desired microstructure and texture can be developed by applying temperature, changing the orientation of the billet through a number of successive passes. It has been extensively researched as a tool for processing solid metal, where it has been shown to provide components with ultra-fine grains that impart superior mechanical properties. The ECAP of titanium-based powder has the potential to greatly aid the cheaper production of titanium-based components. The main objective of this study was to investigate the fundamental interaction between titanium powder characteristics and ECAP process parameters to establish a one on one relationship between powder characteristics, ECAP process parameters and the mechanical properties of the green compacts. Six different titanium powders were used in this study to determine major powder characteristics such as particle size distribution, morphology, density, flowability and chemical composition. The powder was compacted using ECAP technique to produce billets which are also called green compacts. The green compacts were produced based on selective ECAP parameters which were considered effective. Thereafter the green compacts were analysed to check any improvement on mechanical and physical properties. The particle size distribution test results obtained agreed with the supplier’s particle sizes. Four of the powder’s compositions were found to be cohesive, the other two powders, one was freeflowing, the other one flowability could not be measured, due to large particle size distribution. The test results revealed that the morphology for all the powders was irregular with some powders showing angularity, others were dendritic. The tests revealed that the interstitial elements were within the required limits for all the compositions. After the ECAP process, it was found that particle size distribution alone has some effect on the mechanical properties of components. But the morphology, density, flowability and chemical composition, have major effect on mechanical properties of ECAP samples. The relative density was measured after ECAP process, the free-flowing and cohesive powders yielded a relative density of 90% and above, after ECAP first and second pass. The microhardness of the new ECAP billets was found to match that of steels and wrought iron. It was found that application of temperature, backpressure and great number of passes improved the mechanical and physical properties of the billets.
Development of a conducting multiphase polymer composite for fuel cell bipolar plate
(Vaal University of Technology, 2020-06) Alo, Oluwaseun Ayotunde; Pienaar, H. C. vZ., Prof.; Alugongo, A., Prof.; Otunniyi, I. O., Prof.
On account of their lightweight, low-cost, corrosion resistance, and good formability, conductive polymer composites (CPCs) are promising for the production bipolar plate (BP) for polymer electrolyte membrane fuel cell (PEMFC). However, a high conductive filler loading is needed to impart the required level of electrical conductivity to the insulating polymer matrix and as a consequence, the toughness of the plate deteriorates considerably. By using immiscible blend of polymers that have complementary hardness and ductility as matrix, with conducting multi-fillers of different morphologies, it is possible to optimize the matrix strength characteristics and favour the formation of conducting network to produce CPC meeting BP performance standards. Of course, a lot will depend on the formulation of the most favourable composition and production variables. In this regard, polypropylene-epoxy and polyethylene-epoxy blends, filled with zero- and two-dimensional carbon forms – graphite, carbon black (CB), and graphene (Gr) – were investigated over an extensive range of compositions and compression moulding pressures, in this study. Several compounding runs (using melt mixing), at different stages, followed by compression molding, were done. The goal is to obtain combination of composite formulation and processing conditions that will produce the most promising combination of properties for BP application. In the first stage of the investigations, by using thermogravimetric analysis, two-stage decomposition behavior of PP-epoxy and PE-epoxy blends was revealed, which confirms the immiscibility of PP and PE with epoxy resin. Scanning electron microscope (SEM) micrographs of the PP-epoxy and PE-epoxy blends revealed a co-continuous structure, which can be attributed to the close-to-symmetric composition of the blend and compatibilizers added. Preferential localization of synthetic graphite (SG), CB, and Gr in the polymer blends was also revealed by the SEM micrographs. This confirms the fact that CPCs based on PP-epoxy and PE-epoxy blends can be explored further. PP-epoxy and PE-epoxy blends filled with only SG, 30 – 80 wt %, were produced and characterized for their electrical conductivity and flexural properties. In-plane electrical conductivity ranged from 12.09 to 68.03 Scm-1 for PP-epoxy/SG and 11.68 to 72.96 Scm-1 for PE-epoxy/SG composites produced. These are higher than values reported for several single matrix polymer composites at similar filler loadings. With reference to the United States Department of Energy performance targets for BPs, PE-epoxy/SG composites performed better in terms of electrical conductivity, while PP-epoxy/SG composites exhibited better flexural properties. Thereafter, using SG and CB double filler, PE-epoxy/SG/CB composites performed better than PP-epoxy/SG/CB composites in terms of electrical conductivity, while PP-epoxy/SG/CB composites exhibited superior flexural properties than the PE-epoxy/SG/CB composites at similar filler loadings. However, with respect to the DOE targets, composites based on PP-epoxy blend exhibited a more promising combination of electrical conductivity and flexural properties than PE-epoxy blend matrix. PP-epoxy filled with SG/CB was studied further, by using graphene (Gr) as second minor filler. In-plane and through plane electrical conductivities as well as thermal conductivity and thermal diffusivity of the PP-epoxy/SG/CB/Gr composites increased as total filler content was increased from 65 to 85 wt%. It implies that more conductive networks between filler particles were formed. Also, flexural strength, flexural modulus, and impact strength decreased as the total filler content increased from 65 to 85 wt%. The reduced flexural properties could be due to increased agglomeration of CB and Gr, and poor filler wetting at higher filler loadings and low matrix material, which leads to the formation of microvoids and a reduction of the load bearing capacity of composites. With respect to the DOE targets, PP-EP/SG/CB/Gr composite with 80 wt% (i.e., PP/EP/73G/6.2CB/0.8Gr) filler has the best combination of properties. Further improvement in properties of the PP-EP/SG/CB/Gr composite with 80 wt% filler was achieved by molding at higher pressures. As molding pressure was increased from 4.35 to 13.05 MPa, in-plane electrical conductivity increased from 116.31 to 144.99 Scm-1, while flexural strength increased from 29.62 to 42.57 MPa, satisfying the performance requirement targets for bipolar plates.
Topology optimization of a unitary automotive chassis: chassis design through simple structural surfaces and finite element analysis methods
(Vaal University of Technology, 2020-08) Matsimbi, Manuel; Maringa, M., Prof.; Nziu, P. K., Dr.; Masu, L. M., Prof.
The purpose of this study was to develop a design synthesis approach that can be used to reach an optimal design solution (in terms of the strength, stiffness and weight) of automotive body structures during the conceptual stages of the design process. Two conceptual model variants; standard sedan and open-top unitary body structures that were made from the same platform were analysed for their maximum bending moment, stresses, deflections and their maximum load carrying capacity. Topology optimization was also undertaken in order to find a lightweight design of the body structures. The body structures were modelled using three different modelling techniques, namely; the simple beam model, the simple structural surface (SSS) method and the finite element (FE) method. The simple beam model was used to determine the axle reaction forces and the maximum bending moment of a body structure that was subjected to static and dynamic loading conditions. Dynamic load factors and an extra safety factor were used to simulate the dynamic bending loads. The factors were varied from 1.0 to 4.5 with a step of 0.5. It was found that the maximum bending moment under dynamic loading is simply a multiple of the static maximum bending moment and they both occur at a position that is close to the rear part of the front seats. The effects of different geometries on the strength, stiffness and the weight of body structures were studied using the finite element method. The two conceptual models were made into four different plane FE models with each concept having two different FE models. The panels of these models were constructed as simple structural surfaces and were based on the SSS analysis of the standard sedan. The models were subjected to bending and torsion load cases. Each load case was varied similarly for 19 different iterations until the yield point was reached for each FE model. It was also found that the load-displacement graphs were linear for loading within the elastic range, even if there are subassemblies that are missing. However, it was found that this relationship ceases to apply once the body structures are subjected to the torsion loads that are above the yield load. It was also found that the qualitative response to torsion loads was similar for all four body structures. However, the quantitative response was quite observable. It was found that the stiffness can be reduced by at least 37% by omitting subassemblies for the same platform and almost the same mass of the body structure. In addition, the effects of different materials on the strength, stiffness and the weight of body structures were also studied. It was found that lightweight designs can be achieved by using lightweight materials. However, both the bending and torsion stiffness were observed to be reduced or increased in proportion to the Young’s modulus or modulus of elasticity of the material that was used to construct the models. It was also noted that, the stiffness to weight ratio remained almost the same for the same models made from different materials. Topology optimization was undertaken in order to determine alternative load paths of the body structures. The two conceptual models were made into four different solid FE models. It was observed that the load paths remain similar for different volume fraction constraints for similar models under similar loading conditions. It was also noted that at least 20% in weight savings and at least 5% in torsion stiffness improvement can be achieved when topology optimization is used to determine the alternative load paths for a standard sedan model. Besides, the load carrying capacity was found to remain similar. However, the bending stiffness was noted to have reduced due to the reduction in the mass of the structure. In contrast, it was found that for an open-top model, both the bending and torsion stiffnesses were reduced in proportion to the reduction in the mass of the body structure. In addition, it was observed that a further reduction in the mass of the open-top body structure can also significantly reduce its load carrying capacity. Although the stiffness of the optimized open-top model was noted to have reduced due to the reduction in the mass of the structure. The stiffness to weight ratio of the optimized body structure was higher than that of the non-optimal structure.
Lateral-torsional stability for curved 6061-T6 structural aluminium alloys
(Vaal University of Technology, 2020-12-02) Tebo, E-P. T.; Nziu, P. K., Dr.; Masu, L. M., Prof.
Though aluminium (Al) is justifiably described as a green metal with an increasing rate of application in structures, designers still restrain themselves from its applications as a load-bearing skeleton in structure due to insufficient design guidelines. This insufficient information is more with channel sections that might experience lateral-torsional buckling (LTB) when used as a load-bearing skeleton in structures. This study investigates the effects on imperfections on LTB load-carrying stability for 6061-T6 Al alloy channel section arches and proposed design guidelines. The case study focused on freestanding circular fixed end arches subjected to a transverse point load at the shear centre. The software package Abaqus was used to study a total of 110 arch models from three separate channel sections with an additional 16 arch models for validation. Sixty-six channel arches were developed at a constant length, while the remaining 44 arches were formed at constant slender ratios using 11 discrete included angles. The FE analyses methods used for the investigation were validated with existing analytical methods and showed good agreement, despite the assumptions of the bilinear curve used for material nonlinearity, initial geometric imperfections and residual stresses that presented the imperfections of the models. The different investigated factors include slender ratios, change in cross-section area, imperfections, and angles. These factors were found to have substantial impacts on the prebuckling state, which turns to impact LTB behaviour and load-carrying capacity. From arches developed at constant span length, the arches with moderately included angles (50°≤2𝛼≤90°) were found suitable for the designs against LTB, followed by the shallow (2𝛼<50°) and deep arches (90°<2𝛼≤180°) respectively. For arches developed at constant slender ratios, the deep arches were found to be more suitable in the design against LTB, followed by the moderate and shallow arches, respectively. In addition, it was realised that the change in web-flange thickness, section depth and slender ratios, had significant effects on the LTB loads magnitudes and very insignificant effects on the general behaviour across the included angles. The same occurrence was also observed on the prebuckling analyses. All the investigated channel section arches showed the imperfections to have significant impacts on the LTB loads. Arches developed at constant span length showed the maximum elastic LTB loads to have overestimated the expected real LTB loads by approximately 48 percent. While the maximum elastic LTB loads of arches developed at 𝑆𝑟𝑥⁄= 60 and 90 showed that the real LTB loads were overestimated by about 39 and 14 percent, respectively. That said, the elastic LTB loads on average overestimated the real LTB loads by over 50 percent for the arches developed at the constant span length and by only 18 percent for arches developed at the constant slender ratios.
Effect of fault and transmission error on a spur gear meshing stiffness by vibration and time-frequency techniques
(Vaal University of Technology, 2021) Yakeu Happi, Kemajou Herbert; Tchomeni, B. X., Dr.; Alugongo, A. A., Prof.
To meet the ever-increasing demand for maintenance of gear systems, industrial companies have traditionally depended on the shutdown of the machines before processing the fault diagnosis. Nowadays, online monitoring has proven to be effective in terms of machine state analysis and fault prediction. However, the application of such a technique in the analysis of combined multiple nonlinear faults is still a subject of research. The vibration signature of a coexisting nonlinear crack and pit in two-stage gear system is unknown, it can be regarded as one of the most difficult problems to extract and diagnose. Additionally, incorporating both a crack and a pit into numerical models is a time-consuming process that demands a breadth of mechanical understanding. Diagnostics of faulty gears, on the other hand, can be performed in the time and frequency domain or in the Time-Frequency domain, depending on the complexity of the vibration. Non-linear and non-stationary phenomena (Features) occur when repeated pitting and cracking faults occur, reducing the reliability of standard signal processing methods (Gear displacement and Fast Fourier Transform). This thesis solves each of these shortcomings by developing an eight-degree-of-freedom (DOF) gear model with a breathing crack and multiple pitted gear teeth. The identified spur-gear model enabled the investigation of the crack and pitting signatures and their effect on the ensuing vibrations independently of the action of other system components. Additionally, when pitting and cracking coexist, the study was conducted to determine the influence of such a failure on the transmission system. Theoretical results indicated that the presence of pitting and crack in the tooth of the gear resulted in a decrease in mesh stiffness. Additionally, the influence of the breathing pitting and crack results in material fatigue, which results in the generation of a random term in the vibration signal. To corroborate the acquired results, several experimental tests on a spur-gear test rig with a defined pit and crack size range were undertaken under a variety of conditions. In comparison to the presented methodologies, theoretical and experimental results indicate that 3D Frequency-RPM analysis is the most sensitive and resilient method for the early detection and identification of pit and crack faults. Furthermore, when crack or pit faults are studied individually, the STFT analysis yields interesting results. The feature analysis revealed that, when using the Time-Frequency technique, the crack and pit combination in a two-stage gear system is a priori more efficient than the other options.
Optimal geometric configuration of a cross bore in thick compound cylinders
(Vaal University of Technology, 2021-09) Kiplagat, N.; Nziu, P. K., Dr.; Masu, L. M., Prof.
The purpose of this research was to develop optimal numerical solutions that can be employed during the design of cross bored thick-walled compound cylinders. The geometric design parameters of a cross bored compound cylinder that were optimized include shrinkage pressure, cross bore size, shape, location, and obliquity. Finite Element Analysis (FEA) modeling software called Abaqus version 2019 was used to generate numerical solutions. A total of 48 different part models were created and analyzed in this work. The generated FEA results from these models were validated using analytical solutions developed from Lame’s theory. The effects of shrinkage pressure on hoop stresses and Stress Concentration Factor (SCF) were studied to determine the optimal conditions. The optimum shrinkage pressure obtained was henceforth used for further analysis in this work. In addition, using one factor at time optimization technique, an optimization process was carried out to determine the optimal combination of the cross bore configuration geometry that gives minimum SCF. These parameters of cross bore configuration geometry include different sizes of either circular or elliptical-shaped cross bore, positioned at radial, offset, and/or inclined. The analyses of the effects of shrinkage pressure ranging from 4.4733 to 223.662 MPa on 11 different part models, established that the shrinkage pressure of 89.464 MPa generated the minimum SCF magnitude of 3.02. After analyzing 8 different circular cross bore size ratios ranging from 0.1 to 0.8, at the radial position, it was established that the hoop stress increases with an increase in a cross bore size. The smallest cross bore size ratio of 0.1 gave the lowest hoop stress and minimum SCF of 3.02. Whereas the highest stress was developed at the cross-size ratio of 0.8 with an SCF magnitude of 6.75. The minimum magnitude of SCF translates to a reduction of the pressure carrying capacity of the compound cylinder by 67% than a similar plain compound cylinder. Generally, offsetting of the circularly shaped cross bore from the radial position, led to a reduction of the magnitude of SCFs. For instance, from the 8 offset positions analyzed, the minimum SCF occurred at the offset position of 0.006 m with a magnitude of 2.50. This SCF magnitude indicated a reduction of pressure carrying capacity of 60% in comparison to a similar plain compound cylinder. Evaluation of 12 different diameter ratios of elliptical-shaped cross bore ranging from 0.5 to 10, at the radial position, established the lowest SCF magnitude of 1.33 that occurred at a diameter ratio of 5. Henceforth, this optimum diameter ratio was used for further analysis. This aforesaid SCF magnitude translated to a reduction of the pressure carrying capacity of the compound cylinder by 24.81% when compared to a similar plain compound cylinder. Besides, offsetting of elliptically shaped cross bore generally decreased the magnitudes of SCFs. Therefore, for elliptically shaped cross bore, the lowest SCF occurred at radial position with magnitude of 1.33. A general comparison between the effects of circular and elliptical cross bore, established that the elliptical-shaped cross bores generated both lower hoop stresses and SCFs than those of circularly shaped cross bores. On the other hand, oblique elliptical offset cross bores along the Z-axis of the compound cylinder led to an increase in SCFs. As the oblique angle increased from 0 0 to 75 0, the SCFs also increased progressively, however, there was a significant increase in SCF when the inclination angle increased from 60 0 to 75 0. The lowest and highest SCF magnitude was 1.52 and 1.92 at 15 0 and 6.19 at 75 0, respectively. Overall, the optimum geometric configuration of a cross bore in a thick compound cylinder was found to be elliptically shaped, offset at radial position which is an obliquity angle of 0 0 having a diameter ratio a/b of 5.

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