Josu Aguirrebeitia

Calculation of General Static Load-Carrying Capacity for the Design of Four-Contact-Point Slewing Bearings

This paper presents a calculation of the general static load-carrying capacity of four-contact-point slewing bearings under axial, radial, and tilting-moment loads. This calculation is based on a generalization of Sjovall and Rumbarger’s equations and provides an acceptance surface in the load space. This acceptance surface provides a solid basis to compute acceptance curves for the design and selection of bearings of this kind.

A Tetraparametric Metamodel for The Analysis and Design of Bolting Sequences for Wind Generator Flanges

This paper presents a metamodel that enables estimation of the elastic interaction that occurs in the bolted joints of a wind generator tower during the tightening sequence. In this kind of joint, there is a gap between the contact surfaces of the flanges. The metamodel is composed of four parameters, which are enough to simulate the response of the flange under the tightening loads of the bolts. Even though the behavior of the joint is nonlinear because of the gap, the parameters are obtained from two simple linear elastic analyses of a finite element (FE) model of the flange. The corresponding loss of load in the bolts has been estimated for various sequences with minimum computational cost. Thus, there is no need for costly experimental measurements or nonlinear FE simulations.

Dental implants with conical implant-abutment interface: influence of the conical angle difference on the mechanical behavior of the implant.

PURPOSE Misfit in the conical implant-abutment interface plays an important role on the mechanical behavior of the implant when masticatory forces are applied. The origin of the misfit adopted in this work is a conical angle difference between implant and abutment, which can be due to a combination of design decisions and manufacturing tolerances. The goal of this work was to investigate the effects of the implant-abutment conical angle difference in the following mechanical features: interfacial microgap, preload loss on the bolt, stress level in the bone, and abutment removal force and/or torque. MATERIALS AND METHODS A simplified three-dimensional nonlinear monoparametric finite element model of an OsseoSpeed TX 4.5 S 9-mm implant (Astra Tech) with a tapered implant-abutment interface was built to evaluate the variability of the mechanical features cited above with the conical angle difference, keeping constant the overall geometry, load and boundary conditions, material properties, frictional behavior, and mesh structure. RESULTS As the conical angle difference increased, the following effects were observed: the microgap decreased and remained almost constant for values over a given positive angle difference, the stress level in the bone increased sensitively, the removal force and/or torque needed to separate the abutment from the implant varied slightly, and the bolt preload loss increased. CONCLUSIONS In light of the results provided, the conical angle difference in the implant-abutment interface had a significant influence on the overall mechanical behavior of the implant. Among the four mechanical features considered, the interfacial microgap and the bone stress were demonstrated to be the most sensitive to the conical angle difference, and therefore the most relevant when selecting an optimum value in the design process of a conical interface.

Eigensensitivity-Based Optimal Damper Location in Variable Geometry Trusses

In this paper, two procedures are described to identify the optimal damping element locations in variable geometry trusses and to improve their behavior when dynamic loads are applied. In simple structures subjected to well-identified actions, obtaining an optimal location for damping elements can be relatively easy. However, as the geometry of the structures becomes more complex, the number of elements increases, and the frequency range of external actions is wider, finding the optimal location of dampers along the system becomes a difficult task. Furthermore, if the structure varies its geometry in successive positions, such as folding, unfolding, or any other movement, another level of complexity is added to the problem. The two procedures presented in this paper are based on calculating the effectiveness indices obtained from the derivatives of the eigenfrequencies of the dynamic eigenproblem with regard to parameters like stiffness or damping.

General static load capacity in four contact point slewing bearings

This work tries to represent the general static load-carrying capacity of fourcontact-point slewing bearings under general loading. This representation is based on a generalization of Sjovall and Rumbarger’s equations and provides an acceptance surface in the load space. This acceptance surface provides the key set of acceptance curves for the design and selection of bearings.

Experimental study of the removal force in tapered implant-abutment interfaces: a pilot study.

STATEMENT OF PROBLEM Conically tapered interface fits (TIF) provide a reliable and strong self-locking mechanism between a dental implant and its matching abutment. On occasion, it may be necessary to remove the abutment for maintenance purposes. The removal of an indexed implant with a TIF-type connection requires the application of a (removal) force to overcome the friction force due to preload. PURPOSE The purpose of this study was to measure the removal force needed to extract the abutment from the implant in TIF-type connections. MATERIAL AND METHODS A workbench was designed and built to measure the forces involved in the abutment removal process. Experiments were conducted to test the removal force (F(R)) for 20 conical interfaces specifically built for the study. The effects of the preload magnitude (F(P)) and the difference between the taper angles of the implant and the abutment (taper mismatch) were investigated experimentally and theoretically. A 2-way factorial ANOVA and regression analysis was used to evaluate the variability in the process and the influence of the 2 variables considered in the experiments (α=.05). RESULTS Experiments revealed that the (F(R)-F(P)) ratio decreases with the preload F(P), whereas the influence of the taper mismatch cannot be clearly stated. CONCLUSIONS The removal force increases with increasing preload and the F(R)-F(P) ratio varies widely. This variability is attributed to the variability of the friction coefficient, and it can influence implant-removal applications because the removal force can be, in some restorations, as large as 40% of the preload.

Eigensensitivity Analysis in Variable Geometry Trusses

T HE adaptive structures of variable geometry are light mechanical systems capable of modifying their geometry and mechanical properties to adapt to different operating conditions. There are different kinds of mechanical variable geometry systems [1], amongwhich stand out the variable geometry trusses (VGTs) [2– 5]. These structures are a subset of the former, generally comprising a large number of biarticulated bars to form a complex truss. Some of these bars are active elements; that is, their length can vary in a controlled way to enable the actuation of the VGT. One of the most important problems to be solved in VGTs is vibration, essentially due to two factors. On the one hand, they are generally slimline structures, making them easily excitable at low frequencieswith large amplitudes, whichmight interferewith correct VGT operation, thereby harming its accuracy, and even resulting in collisions with environmental obstacles [5–7]. On the other hand, as VGTs may have highly different configurations throughout their operation, the dynamic properties (natural frequencies and vibration modes) also change to a large extent. There have been numerous contributions onvibratory dynamics of VGTs. There are a few works, like those of Keane and Bright [8], Keane [9], andNair andKeane [10], which opt for optimum redesign of the structure geometry using evolutionary methods to determine structural geometries presenting a better dynamic response; the most widely accepted procedure for improving and/or controlling dynamic properties is the inclusion of active, semiactive, or passive elements to control its dynamic response. In this sense, Bilbao et al. [11] carried out a detailed revision of the state of the art and proposed a new methodology for the optimal location of damping elements. This Note further contributes to vibratory dynamics of VGTs, describing a tool developed to efficiently estimate the dynamic properties of VGTs (frequencies and modes) throughout their movement, by responding to two needs. First, the information that frequencies and modes supply is often sufficient to foresee the dynamic behavior without having to perform costly direct dynamic analyses, as verified in [12]. Second, should said analyses be necessary, they can be approached from the modal superposition viewpoint, for which a tool that efficiently estimates natural frequencies and vibration modes is necessary. A linear estimation is proposed tomodel the variation of these dynamic properties throughout the VGTevolution, so that there is no need to calculate them per position but only for a fraction of these positions, whichmay result in considerable computational saving. Thus, a methodology estimating the value of natural frequencies and vibration modes in a new position from their value in the immediately prior position is employed. This is done by differentiating natural frequencies and vibration modes with respect to nodal coordinates at the starting position and developing a first-order series around the starting position to extrapolate the values of natural frequencies and vibration modes for a new position.

Calculation of the Ball Raceway Interferences Due to Manufacturing Errors and Their Influence on the Friction Moment in Four-Contact-Point Slewing Bearings

This work proposes a procedure for the determination of the interferences between balls and raceways in four-contact-point slewing bearings due to manufacturing errors. The procedure is applied to a particular case and finite element analyses are performed for the friction moment calculation, considering different preloads. The results are used to evaluate the influence of manufacturing errors in the friction torque of the bearing.

Effect Of Misalignment On Rod LipSeal Behaviour

The most common failure mechanism of pneumatic cylinders is related to wear of head seals. It usually occurs due to excessive radial loads or vibrations (dynamic run-out) or as consequence of an incorrect mounting (rod to bore misalignment). Sealing behaviour has been the object of several analytical and numerical studies orientated to reproduce actual working conditions but perfect rod alignment has been often considered. A tri-dimensional quasi-static numerical model of rod radial misalignment and its effect on facing seal behaviour is herein presented as a first step of a study concerning mounting effects on head seal performances. In the specific, a thermoplastic polyurethane (TPU) rod lip seal has been considered. An analytical model was also developed considering the characteristics of the rubber-like material and stiffness variation of the lip seal versus deformation. A non-linear dependence of stiffness on deformation of rubber-like material was found. A cross validation between analytical and numerical models is provided.

A Metamodeling Technique for Variable Geometry Trusses Design via Equivalent Parametric Macroelements

A metamodeling methodology is applied to reduce the large computational cost required in the design of variable geometry trusses (VGTs). Using this methodology, submodels of finite elements within a complete FE model of the VGT are substituted by groups of fewer elements called equivalent parametric macroelements (EPMs). The EPM optimum parameters are obtained using equivalence criteria based on elastic energy and inertial properties. The optimization process is performed using nonlinear least square minimization and genetic algorithms.