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Optimization of Highly Stressed Joints in Uneven Mechanisms by Using a Toggle Clamping Unit as Reference

Uneven mechanisms are used in a plurality of applications in which the transmission of motion or force is required. For instance, a common application of such a mechanism with a high non-linear transmission is a toggle-clamping unit of injection molding machines. In comparison with other mechanisms the operating principle for the force reproduction of these mechanisms is especially occur during the transition in the stretched position. However, highly dynamic motions and mass inertia of the movable platen lead to high loads and an oscillating movement of toggle joints having differential (variable) radial force vectors. In contrast to that, during the build-up phase of the clamping force generation radial forces are solely caused by the clamping force and vary by small angle values. As a result, the magnitude of these radial forces induce high stresses in the contact area of the bolt joints as well as exhibit the capability of contact surfaces damage. As one probable consequence, functionality and service life of joints are reduced.

Based on a 5-point double-toggle clamping unit as a reference the fundamental optimization approaches for the load-conformable design were identified. In order to describe the stresses basically, the dynamic properties as well as the structural properties are investigated by the use of numerical simulation methods (Multibody-Simulation, Finite-Element-Analysis). The Analysis results showed that the small pivoting movement of the toggle results in a relative movement between the bolt and joint bores and in addition high and local stresses in the areas of the bore edge (edge pressure). A verification of the results of the FEM-analysis enabled the identification of clearance fits as a primary factor on the stress values. For the reduction of these tribological stresses, a joint geometry has been developed which consider the requirements for the kinematic phase as well as the build-up phase of the clamping force. This joint geometry is designed as a flexure hinge and enable the mechanical separation of the closing procedure into the dynamic phase and the build-up phase of the clamping force by using additional contact surfaces. The change of these phases is realized by blocking the bolt rotation to the additional contact surfaces. A computer-aided topology optimization was used to develop a load-conformable joint geometry. The bending of the flexure hinge allows the elastic build-up phase of the clamping force by the movement of the crosshead. High stresses in the components of the toggle mechanism are the result of the elastic build-up phase of the clamping force. Modifications of the simulation model allowed the definition of an investigation area for the optimization of the parameters which describe the geometry and finally to determine joint design. The manufacture of the joint geometry enable the experimental evaluation of the effect mechanisms.Further optimization approaches focus on high and local stress gradients on the bolts in areas of edge pressure. Especially in these areas the bolt contour are described by geometric parameters. A load-conformable bolt design has been developed by using the computer-aided parameter optimization. Regarding the contact pressure, a comparison of the optimized and the cylindrical bolt geometry showed a significant effect of improvement. However, a higher bending of the bolts and thus an increase of the von Mises stresses are the result of the local reduction of the bolt diameter.For the experimental verification of the effect mechanisms of the flexure hinge, a testing device was developed. By a servo-hydraulic linear cylinder a defined force is applied to a toggle system. The high test loads and the required oscillating movement to bend the flexure hinge are realized by the transmission of the toggle system. The monitoring of the stresses on the test specimen was done based on measurement by using strain gauges. An influence on the realization of the test loads has the orientation of the specimen to the main force direction. This was confirm by the analysis of the measurement data. In principle, the effect mechanisms of the flexure hinges are suitable for the realization of the clamping forces in the reference toggle clamping unit. This is shown by the experimental evaluation.

In order to reduce the contact pressure on the bolt joints in a clamping unit during the build-up phase of the clamping force, a new concept for a clamping unit was developed. The aim was to use flexure hinges for the elastic build-up phase of the clamping force taking into account low stress levels in the components of the clamping unit. For that purpose, two parametric models of flexure hinges were developed. By using the parameter optimization, the flexure hinges were optimized regarding the build-up phase of the clamping force and low stress levels. Based on this results a positive-locking connection without the use of bolts in clamping units was realized by structural development. A final FEM-analysis showed that the connection enabled the elastic build-up phase exhibiting a lower stress level in the contact areas and in the areas of the flexure hinges.

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Author
 Burkhard Jürgen Gronhoff

Burkhard Jürgen Gronhoff
Lehrstuhl für Konstruktion und Kunststoffmaschinen
Universität Duisburg-Essen

Information

Free keywords: uneven mechanisms, non-linear transmission, highly stressed joints, clamping unit, finite-element-analysis, flexure hinge, topology optimization, elastic build-up phase of the clamping force, reducing the contact pressure, parameter optimization, load-conformable bolt design, experimental verification
Institute / chair: Fakultät für Ingenieurwissenschaften, Abteilung Maschinenbau und Verfahrenstechnik der Universität Duisburg-Essen
Language: German
Technical consultant for expert services: Prof. Dr.-Ing. Johannes Wortberg (Betreuer), Prof. Dr.-Ing. Paul Josef Mauk
Publication year: 2015
Provider: Wissenschaftlicher Arbeitskreis Kunststofftechnik (WAK) / Kunststoffe.de

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