1,720,974 research outputs found
Suspension forces on a tri-axle air suspended semi-trailer
Get PDFDissertation (MEng)--University of Pretoria, 2009.The aim of this study is to investigate the use of multi-body vehicle simulation models to predict the suspension forces acting on the chassis of the vehicle, in order to perform durability analyses. Traditionally, durability of vehicles is evaluated with proving ground tests. This implies that a physical prototype of the vehicle is required before its durability can be evaluated. If we were able to evaluate the durability of the vehicle without any physical part or a full prototype of the vehicle available, great cost and time savings may be gained. These possible gains have lead to the use of computer aided engineering (CAE) tools. These tools have supplemented the proving ground durability test by using historical measured data and/or predicted data from vehicle simulation models, as input to the durability analyses i.e. Finite Element Analyses (FEA). The usefulness of the historical test data is limited and many of the vehicle simulation models that are used to predict the input data, have not been validated. In this study a validated mathematical model of a 40 ton flat bed tri-axle semi-trailer, able to predict the suspension forces, is created. The validation of the full vehicle model includes correlations for displacements, velocities, accelerations and forces of various vehicle parameters. A validated mathematical model of the air springs, that includes mass transfer and flow effects for use in full vehicle dynamic simulations, is also developed. The results obtained indicate that the air spring model, integrated into the full vehicle model, is able to give relative accurate predictions of displacements, velocities, accelerations and forces of various vehicle parameters, over a discrete road event and over a rough road.Mechanical and Aeronautical EngineeringUnrestricte
Validated leaf spring suspension models
Get PDFThesis (PhD(Eng))--University of Pretoria, 2012.Mathematical and computer modelling have been playing an increasingly important role in the Computer Aided Engineering (CAE) process. Simulation offers great advantages in the development and analysis phase of products and offers a faster, better and more cost effective way than using physical prototypes alone. The ever increasing demand for new and improved products in the vehicle industry has decreased the time available for the development of new vehicles, but at the same time the demands on quality, reliability and mass that are set for the vehicle are becoming ever more stringent. These requirements have lead to the investigation of procedures and methodologies such as virtual prototyping that will reduce the development time of new vehicles without inhibiting the quality of the vehicle. In order to perform effective and reliable simulations in the CAE process, accurate simulation models of the vehicle and its associated systems, subsystems and components are required. In the vehicle dynamics context simulation models of the tyres, suspension, springs, damper, etc, are needed. This study will look at creating a validated model of a leaf spring suspension system used on commercial vehicles. The primary goal set for the model is to be able to predict the forces at the points where the suspension system is attached to the vehicle chassis as the model is to be used in full vehicle durability simulations. The component which will receive a considerable amount of attention in this study is the leaf spring. Leaf springs have been used in vehicle suspensions for many years. Even though leaf springs are frequently used in practice they still hold great challenges in creating accurate mathematical models. It is needless to say that an accurate model of a leaf spring is required if accurate full vehicle models are to be created. As all simulation models in this study are required to be validated against experimental measurements a thorough experimental characterisation of the suspension system of interest, as well as two different leaf springs, are performed. In order to measure the forces between the suspension attachment points and the chassis, two six component load cells were developed, calibrated, verified and validated. This study will primarily focus on the modelling of a multi-leaf spring as well as a parabolic leaf spring. The study starts with a literature study into the various existing modelling techniques for leaf springs. A novel leaf spring model, which is based on a macro modelling view point similar to that used for modelling material behaviour, is developed. One of the modelling techniques found in the literature, i.e. neural networks, is also used to model the leaf spring. The use of neural networks is applied and some of the challenges associated with the method are indicated. The accuracy and efficiency of the physics-based elasto-plastic leaf spring model and the non physics-based neural network model are compared. The modified percentage relative error metric is compared to two other quantitative validation metrics that were identified from the literature study. It is concluded that the modified percentage relative error has certain limitations but that it is able to give an accurate and representative account of the agreement/disagreement between two periodic signals around zero. The modified percentage relative error is used to obtain the accuracies of the elasto-plastic leaf spring models and the neural network model. Both models give good results with the neural network being almost 3 times more computationally efficient. The elasto-plastic leaf spring model, for the multi-leaf spring, is further extended to model the behaviour of a parabolic leaf spring. Qualitative validation using experimental data shows that the elasto-plastic leaf spring model is able to accurately predict the vertical behaviour of both the multi-leaf spring as well as the parabolic leaf spring. The elasto-plastic leaf spring model was also combined with a method that is able to capture the effect of changes in the spring stiffness due to changes in the loaded length. Quantitative validation shows that the method proposed for accounting for the change in stiffness due to changes in the loaded length is able to capture this characteristic of the physical leaf spring. Following a systematic modelling approach the elasto-plastic multi-leaf spring model is incorporated into a model of a simplified version of the physical suspension system. The qualitative validation results from this model show that the model is able to accurately predict the forces that are transmitted from the suspension system to the chassis. The models created in this study can be used in future work and, with the addition of more detail the models, can be extended to create a model of the complete suspension system.Mechanical and Aeronautical Engineeringunrestricte
Canine lumbosacral fracture-luxation stabilised with the String of Pearls interlocking plate system or pins combined with polymethylmethacrylate : a biomechanical comparison
No full textDissertation (MSc)--University of Pretoria, 2016.A biomechanical comparison was conducted on two internal spinal fixation techniques: pins
and Polymethylmethacrylate, and the String of Pearls Interlocking Plate System. Both
techniques were applied to a surgically simulated level L7-S1 complete spinal injury, with the
objective of the study being to compare the stability of the two techniques. Cadaver
specimens from 18 skeletally mature large breed dogs were used. These specimens were
randomly divided into two equal groups and fixated using one of the two internal spinal
fixation techniques.
The lumbosacral spine specimens (L5-S3) were subjected to a mechanically applied
bending moment, which was applied to the caudal and cranial ends of the specimen.
Biomechanical parameters including range of motion (RoM), neutral zone (NZ), and elastic
zone stiffness (EZS) were used to compare the stability of the two fixation techniques. No
significant difference between the means of the NZ in flexion (p-value=0.3458), extension (pvalue=
0.1255), and the total value (p-value = 0.3458) of the injured lumbosacral (L7-S1) joint
fixated with the two fixation techniques was found. Similarly, no significant difference
between the means of the RoM in flexion (p-value = 0.2386) and extension (p-value =
0.1255), or between the means of the EZS in extension (p-value = 0.4094) was noted.
Therefore, it can be concluded that the stability of the injured joint between the two fixation
techniques is similar.tm2016Companion Animal Clinical StudiesMScUnrestricte
Modelling of a passive hydraulic steering for locomotives
Get PDFDissertation (MEng)--University of Pretoria, 2015.During the past few decades, substantial improvements were made to rail infrastructure worldwide. This was necessary to accommodate the ever increasing transportation demand and requirements. Nowadays, trains are required to transport heavier loads and to travel at higher speeds.
One of the major improvements was achieved by the development of the off-flange curving bogie designs to reduce wheel and rail wear. Off-flange designs include passive steering and actively controlled steering. The development and implementation of self-steering bogies on locomotives was promoted in the early 1980’s by two major locomotive manufacturers. Up to date, thousands of these locomotives, with built-in self-steering bogies, have been manufactured and taken into service (Swenson, 1999).
Most self-steering bogies have mechanical linkage systems to steer the wheel sets. As an alternative to the mechanical linkage system, the DCD Group (a South African manufacturer of rail and mining equipment) initiated the development of a Passive Hydraulic Steering (PHS) system.
First PHS prototype systems, developed by DCD, have proven that huge wear reduction possibilities exist on both, rails and wheels. In addition the prototype systems also significantly decreased noise and vibration levels when negotiating tight corners (Swenson & Scott, 1996 and DCD Rolling Stock, 2012). However, existing prototype solutions require further improvements and development for optimisation. To be able to identify and implement improvements, the need exists to perform modelling and testing of the systems to obtain a better understanding of the operation and suitability of a complete unit. The aim of this research project is thus, to mathematically model an existing prototype PHS system and validate the model with data from experiments and tests. This model can then be used in order to improve and optimise performance, cost and reliability of the system, before mass production is considered.
A literature survey was conducted, focusing on general wheel and rail wear mechanisms, techniques to improve wheel and rail life and on existing techniques for modelling the hydraulics and multi-body dynamics of locomotive systems.
The literature survey was followed by extensive laboratory tests on component basis, a quarter PHS system and on the full PHS system. From these tests all parameters needed for the characterisation of the PHS and the mathematical model were determined. These tests also provided data for the validation of the PHS model.
Finally, a mathematical model of the PHS system was successfully generated and validated. This model can now be used in a multi-body dynamic locomotive simulation to evaluate its effectiveness.
The results and findings of the literature survey, experiments and modelling are reported on and discussed in this report.tm2015Mechanical and Aeronautical EngineeringMEngUnrestricte
Ride comfort difference thresholds for a vehicle on a 4-poster test rig
Get PDFDissertation (MEng)--University of Pretoria, 2017.To improve ride comfort a reduction in the acceleration experienced by occupants is required. Simulation software and test equipment are able to measure reductions in acceleration that are too small for humans to perceive. It is therefore important to know how large the reduction in vibration should be for occupants to perceive an improvement in comfort. This study determined difference thresholds (DTs) for ten automotive engineers seated in a vehicle on a 4-poster test rig. Participants were exposed to all six axes of vibration. DTs were determined for two road profiles using vertical acceleration measured on the seat and seat rail. The two road profiles were obtained by scaling the magnitude of the vertical displacements of a test track used for ride comfort evaluations. The two roads had different magnitudes, but the same spectral shape, and were therefore used to investigate the validity of Weber's Law. The BS 6841 weighted r.m.s. magnitude of the vertical acceleration measured on the seat were 0.58 and 1.01 m/s2 for the two roads. An up-down-transformed-response (UDTR) test procedure was used with a three-down-one-up rule to determine DTs. There was no statistically significant difference found in the medians of the relative difference threshold (RDT), calculated from the vertical seat acceleration, over the two roads. The median RDT for the two roads were 10.1 % and 8.6 % respectively. Results were consistent with Weber's law.Mechanical and Aeronautical EngineeringMEngUnrestricte
EFFECT OF FOOT POSITION ON THE COMPRESSION AND LATERAL FORCE PRODUCTION OF A PLAYER IN A RUGBY SCRUM
Get PDFKnowledge of the force production of an individual player with different foot positions, and ultimately of the full pack, will provide coaches and trainers with valuable information in order to improve their team\u27s ability to manipulate the scrum forces and moments as well as making the scrum more stable thereby improving safety. This study aimed to examine the differences in the scrum compression force and lateral force generated by an individual player during the sustained pushing phase of the scrum with parallel and nonparallel foot positions. Nineteen front row rugby players scrummed against an instrumented scrum machine with three different foot positions. The results showed that the parallel foot position produces a higher pushing force than either of the other two nonparallel conditions. It was also shown that the nonparallel conditions produces a greater lateral force than the parallel foot condition with the lateral force being directed towards the side of the front foot.
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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