With advances in material technology, allowing, for instance, engines to withstand higher combustion pressure and consequently improving performance, comes challenges to productivity. These materials are, in fact, more difficult to machine with regards to tool wear and especially machine tool stability. Machining vibrations have historically been one of the major limitations to productivity and product quality and the cost of machining vibration for cylinder head manufacturing has been estimated at 0.35 euro per part. The literature review shows that most of the research on cutting stability has been concentrating on the use of the stability limits diagram (SLD), addressing the limitations of this approach. On the other hand, research dedicated to development of machine tool components designed for chatter avoidance has been concentrating solely on one component at the time. This thesis proposes therefore to extend the stability limits of the machining system by enhancing the structure’s damping capability via a unified concept based on the distribution of damping within the machining system exploiting the joints composing the machine tool structure. The design solution proposed is based on the enhancement of damping of joint through the exploitation of viscoelastic polymers’ damping properties consciously designed as High Damping Interfaces (HDI). The tool-turret joint and the turret-lathe joint have been analysed. The computational models for dimensioning the HDI’s within these joints are presented in the thesis and validated by the experiments. The models offer the possibility of consciously design damping in the machining system structure and balance it with regards to the needed stiffness. These models and the experimental results demonstrate that the approach of enhancing joint damping is viable and effective. The unified concept of the full chain of redesigned components enables the generation of the lowest surface roughness over the whole range of tested cutting parameters. The improved machining system is not affected by instability at any of the tested cutting parameters and offers an outstanding surface quality. The major scientific contribution of this thesis is therefore represented by the proposed unified concept for designing damping in a machining system alongside the models for computation and optimisation of the HDIs. From the industrial application point of view, the presented approach allows the end user to select the most suitable parameters in terms of productivity as the enhanced machine tool system becomes less sensitive to stability issues provoked by difficult-to-machine materials or fluctuations of the work material properties that may occur in ordinary production processes. / <p>QC 20120413</p> / DampComat / Production 4 micro / FFI Robust Machining

Bloch analysis was originally developed by Felix Bloch to solve Schrödinger's equation for the electron wave function in a periodic potential field, such as that found in a pristine crystalline solid. His method has since been adapted to study elastic wave propagation in periodic structures. The absence of a rigorous mathematical analysis of the approach, as applied to periodic structures, has resulted in mistreatment of internal forces and misapplication to nonlinear media. In this thesis, we detail a mathematical basis for Bloch analysis and thereby shed important light on the proper application of the technique. We show conclusively that translational invariance is not a proper justification for invoking the existence of a "propagation constant," and that in nonlinear media this results in a flawed analysis. Next, we propose a general framework for applying Bloch analysis in damped systems and investigate the effect of damping on dispersion curves. In the context of Schrödinger's equation, damping is absent and energy is conserved. In the damped setting, application of Bloch analysis is not straight-forward and requires additional considerations in order to obtain valid results. Results are presented in which the approach is applied to example structures. These results reveal that damping may introduce wavenumber band gaps and bending of dispersion curves such that two or more temporal frequencies exist for each dispersion curve and wavenumber. We close the thesis by deriving conditions which predict the number of wavevectors at each frequency in a dispersion relation. This has important implications for the number of nearest neighbor interactions that must be included in a model in order to obtain dispersion predictions which match experiment.

No description available.

Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references.

Wheel loaders are under the influence of low frequency vibrations that may be harmful for the health of the operator, and for the productivity of the machine. The strong vibrations can significantly impact the operation of the machine since they are not equipped with wheel suspension systems and due to their work environment in rough terrains with uneven surfaces. The risk of spilling or dropping the load is also increased and they can introduce higher mechanical stress that can wear out parts faster. The focus of this thesis work is to develop an approach for damping these vibrations dynamically by improving the functionality of the electrohydraulic system in Volvo's prototype wheel loader LX1. The dynamic damping system controls the cylinder of the lift framework to make it behave as a damper. The system utilizes the lift cylinder pressure and piston position as feedback to adjust the oil flow in the cylinder using the proposed control system. Results indicate that the proposed technique is capable of attenuation that is comparable with the existing boom suspension system based on accumulators through simulations and experimental tests.

Vibrating machinery like rock drills and compactors are becoming more prominent in modem industry. The vibrations of these machines can damage surrounding structures and foundations and be harmful to their operators. Hand arm vibration syndrome is one example of serious injuries suffered by operators of these machines. Due to the fact that these machines need to vibrate, vibration absorbers that minimise the vibrations of the machines cannot be used. In such cases vibration isolators are necessary to isolate the vibration between the vibrating machine and other bodies like the handle or foundations. A tuned vibration isolator is a type of isolator that is able to isolate a certain frequency very effectively. These isolators can retain low mass and high stiffness compared to traditional isolators and can obtain complete isolation at the isolation frequency if no damping is present. The liquid inertia vibration eliminator (LIVE) is such a tuned vibration isolator that makes use of hydraulic amplification, which result in a very compact design. A LIVE isolator was designed incorporating the variable stiffness spring and a variable damping mechanism. Equations for the damped natural and isolation frequency of the LIVE isolator were also derived. The reason for changing the stiffness was to be able to adjust the isolation frequency of the isolator to coincide with the excitation frequency that resulted in a more effective isolator. The variable stiffness spring consisted of two leaf springs mounted on top of each other and separated at the centre to stiffen the whole spring assembly. The leaf springs were separated by a wax actuator that was controlled with a closed loop displacement control system to form a smart actuator. A stiffness change of 2.7 times the original stiffness was obtained by separating the springs. The variable damping mechanism was to be able to control the amount of amplification of noise at the natural frequency. An experimental isolator was built and tested and resulted in a tunable vibration isolator. The isolation frequency of the isolator could be shifted from 22.8 Hz to 36.2 Hz and a transmissibility of 10% was achieved over that whole range. The variable damping mechanism increased the viscous damping ratio from 0.001 to 0.033. A control system was designed and implemented that tuned the isolator automatically to the excitation conditions. It incorporated an optimisation algorithm to determine the optimum settings and then kept the isolator at that setting until the excitation conditions change. The whole process was then repeated. A tunable vibration isolator was therefore successfully developed that can be used to isolate tonal vibrations very effectively. The isolation frequency and damping of the isolator can be changed while in operation and a transmissibility of 10% can be achieved at the isolation frequency. / Thesis (M Eng (Mechanical Engineering))--University of Pretoria, 2006. / Mechanical and Aeronautical Engineering / unrestricted

The aim of the submitted diploma thesis is the magnitude determination of the headlamp response to the dynamic excitation. Currently it is necessary to measure this response, because it is one of the assessed criteria of quality of the headlamp. When comparing results of the experimental measurement and the computational modelling, a good consensus of resonant frequencies is observed, but there are significant differences in acceleration amplitudes. The cause of this is the variable damping value. The theoretical part of the thesis deals with polymeric materials, basics of oscillation and damping. It serves to understand the mechanisms, which influence the results of the experimental measurement. The practical part is dedicated to the procedure of the computational modelling and the experimental measurement implementation. The analysis of the experimental measurement results leads to an ascertainment of nonlinearities, which influence the headlamp behaviour. The results of a sensitivity analysis of the computational modelling are used to create curves, on the basis of which the damping is estimated. The inclusion of the estimated damping into computational modelling leads to reduction of the compared differences of the acceleration amplitudes. The conclusion of the thesis contains the recommendation for the damping determination during the computational modelling of the headlamp.

No description available.

No description available.

The optical power of satellites such as the Hubble telescope is directly related to the size of the primary mirror. However, due to the limited capacity of the shuttle bay, progress towards the development of more powerful satellites using traditional construction methods has come to a standstill. Therefore, to allow larger satellites to be launched into space significant interest has been shown in the development of ultra large inflatable structures that can be packaged inside the shuttle bay and then deployed once in space. To facilitate the packaging of the inflated device in its launch configuration, most structures utilize a thin film membrane as the optical or antenna surface. Once the inflated structure is deployed in space, it is subject to vibrations induced mechanically by guidance systems and space debris as well as thermally induced vibrations from variable amounts of direct sunlight. For the optimal performance of the satellite, it is crucial that the vibration of the membrane be quickly suppressed. However, due to the extremely flexible nature of the membrane structure, few actuation methods exist that avoid local deformation and surface aberrations. One potential method of applying damping to the membrane structure is to use magnetic damping. Magnetic dampers function through the eddy currents that are generated in a conductive material that experiences a time varying magnetic field. However, following the generation of these currents, the internal resistance of the conductor causes them to dissipate into heat. Because a portion of the moving conductor's kinetic energy is used to generate the eddy currents, which are then dissipated, a damping effect occurs. This damping force can be described as a viscous force due to the dependence on the velocity of the conductor. While eddy currents form an effective method of applying damping, they have normally been used for magnetic braking applications. Furthermore, the dampers that have been designed for vibration suppression have typically been ineffective at suppressing structural vibration, incompatible with practical systems, and cumbersome to the structure resulting in significant mass loading and changes to the dynamic response. To alleviate these issues, three previously unrealized damping mechanisms that function through eddy currents have been developed, modeled and tested. The dampers do not contact the structure, thus, allowing them to add damping to the system without inducing the mass loading and added stiffness that are typically common with other forms of damping. The first damping concept is completely passive and functions solely due to the conductor's motion in a static magnetic field. The second damping system is semi-active and improves the passive damper by allowing the magnet's position to be actively controlled, thus, maximizing the magnet's velocity relative to the beam and enhancing the damping force. The final system is completely active using an electromagnet, through which the current can be actively modified to induce a time changing magnetic flux on the structure and a damping effect. The three innovative damping mechanisms that have resulted from this research apply control forces to the structure without contacting it, which cannot be done by any other passive vibration control system. Furthermore, the non-contact nature of these dampers makes them compatible with the flexible membranes needed to advance the performance of optical satellites. / Ph. D.