Materials Having a High Degree of Adhesion for Gripping Elements Designing

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1 Applied Mechanics and Materials Online: ISSN: , Vol. 613, pp doi: / Trans Tech Publications, Switzerland Materials Having a High Degree of Adhesion for Gripping Elements Designing Marcel Horák 1, a, František Novotný 2, b 1, 2 Technical University of Liberec Institute for Nanomaterials, Advanced Technologies and Innovation Faculty of Mechanical Engineering, Department of Glass Producing Machines and Robotics Studentská 2, Liberec 1, Czech Republic a marcel.horak@tul.cz, b frantisek.novotny@tul.cz Keywords: Gripping element, adhesion, GSA material, rheological model, clean surface. Abstract. The paper analyses one of possibilities to use new materials based on polyurethane, polypropylene or silicone having highly adhesive contact surfaces for gripping elements designing. In concrete terms, it is a case of an alternative approach to the solution of designing standard multielements vacuum gripping heads with an active control system (vacuum level control) controlling gripping forces in processes of automatic handling flat objects of the plate type. The aim of this new design solution is to replace individual gripping elements (suction cups) by elements, the surfaces of which coming to the contact with the object handled are provided with adhesion materials. The system minimizing energy demands stemmed from this solution, namely in the way that it decreases or even eliminates the pressure air consumption when combined vacuum-adhesive or only adhesive gripping elements are used. Moreover, it is possible to use the adhesive gripping principle profitably in technological processes susceptible to the environment contamination with airflow when manufacturing new products with many functional layers. Besides a general analysis of problems, the paper presents outputs of laboratory tests. Also a computer model of the contact respecting rheological behaviour of the adhesive material basic matrix is given. Introduction Nowadays there are many so-called GSA materials, being inspirited by the biologic gecko effect and produced or modified afterwards (Fig. 1) so as a high degree of surface adhesion energy was obtained [1]. Fig. 1 Structure of polypropylene microfibres, on the right the side view All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (# , Pennsylvania State University, University Park, USA-18/09/16,10:41:35)

2 Applied Mechanics and Materials Vol These materials can be divided into some basic groups [2] characterized by hard polymers, soft polymers and nanomaterials using e.g. a system of surface carbon nanotubes. Because of a good availability a relatively mechanically rugged soft silicone polymer was analyzed for the given purpose. In comparison with microfibrous structures its outstanding feature is the smooth or extremely smooth surface structure with irregular projections (Fig. 2) up to maximum height 80 nm. Fig. 2 Surface structure of silicone rubber (60,000x magnified) This type of material was applied subsequently as the adhesion layer for the specific combined gripping element described in detail in [3, 4]. This element can work in two operating regimes having either passive or combined character, when moreover vacuum is supplied into the sealed space between the element and the object handled. The resultant gripping force is given by a level of adhesion and vacuum in relation to the element effective contact surface. In general it is possible to state that in modified design solutions of vacuum gripping elements any material with an adhesion layer increases carrying capacity both in a radial and axial directions substantially in order of tens per cent [5]. Fig. 3 Modification of the adhesion layer contact surface (2A segmented plane surface, 2B flat radially divided plane surface, 2C height profiled plane surface, 2D cylindrically flat divided plane surface)

3 222 Industrial and Service Robotics When combining with a bearing plate, it makes possible to handle thin plates characterized by low lateral rigidity, which finds expression in excessive deformations that can lead to the handled object damage. The contact plane surface of the gripping element (Fig.3), initially compact, being constituted by the bearing plate 1 and the adhesive layer 2, can be, depending on the surface structure and material properties of the object handled, at once segmented 2A, flat (area) divided 2B, cylindrically divided 2D, altitude (height) profiled 2C, and heterogeneous as far as the material as concerned, which increases or vice versa decreases a level of resultant coefficients of adhesion and friction in the final consequence. Laboratory tests of the adhesive layer carrying capacity The layer function was observed during radial and axial external cycles of load application [6] in passive and combined operations. The great attention was concentrated on cases of the exclusively passive operation and the layer axial loading, when the active surface is reduced step by step under the influence of the layer material rheological behaviour owing to the growing axial load or the contact (gripping) time. Thus, a degree of the contact profile displacement rises, the stable contact passes to the instable one, and subsequently the handled object loosening (collapse) occurs. Assuming static holding, the measured results are resumed in the diagram in Fig. 4. They present maximum gripping times depending on values of the adhesive layer axial load. Fig. 4 Maximum the layer axial carrying capacity at 25ºC It can be stated that with the chosen handling time (e.g. 150 s) the element carrying capacity 0.12 N.cm -2 corresponds, which represents limit equilibrium. When bringing a suitable degree of safety at range k = 3 5, the real base for the gripping element dimensioning is available. During laboratory tests also an impact the first contact size and time of its effect on the object handled was evaluated. It was showed that the down-pressure increase above 0.3 N.cm -2 and the time of effect longer than 10 s although enlarge the adhesion layer active surface, but it does not extend substantially a resultant gripping time.

4 Applied Mechanics and Materials Vol Computer analysis of the adhesive layer behaviour Results of laboratory tests of the adhesive layer behaviour were used during a computer model parametrizing so as to make possible to reach a high agreement of simulation outputs with the real solution. The aim is the model prospective use in the technical practice for predicting functions of special gripping elements with adhesive layers having complicated shapes and geometry under various operational conditions. Within the bounds of the computer simulation in the SW product MSC.Marc 2013, the Mooney-Rivlin material model [7, 8] was used that expresses in general mechanical energy of distortion W as the sum of stress invariants I 1 and I 2 so that where C ij are material constants. The five-parametric model is in common usage for most of application when solving nonlinear problems. The equation (1) can be transcribed to the form (1) (2) In compliance with the premises that the real engineering stress is given by the relation, the equation (2), on obtaining a derivative, can be, for the case of uniaxial loading, when it is valid for relative deformations that 1 = and also 2 = 3 = -1/2, and stress 3 = 0, transposed to the relation (3) It is obvious that the equation (3) is relatively complicated. In the engineering practice the form, being inclusive of only the first two material constants C 10 and C 01, is used predominantly which can be determined e.g. by the least squares method in cases that a dependence of the engineering stress on relative deformation is found experimentally. Therefore it is valid that In general there will be the function for the set of n measured and experimentally found points [ 1, 1 Eng ],, [ n, n Eng ] as follows (4) (5) from which it is possible to obtain a system of two equations after determining partial derivatives according to C 10 and C 01, and subsequent zeroing. By resolving them, material constants in the equation (4) can be determined. Based on cyclic uniaxial tensile tests, the material model of the adhesion layer was characterized by the constants C 10 = Pa and C 01 = Pa for the purposes of this project. So as to describe the system at the contact line, where a gradual separation from the handled object occurs under the influence of external loading, edge conditions, and time-dependent rheological behaviour of the adhesion layer, it is possible to use some approaches and techniques. The VCCT (Virtual Crack Closure Technique) is based on the principle of fracture mechanics, i.e. the special approach defining properties of elements from the standpoint of cohesive energy or stress pattern depending on the profile displacement. So-called CZM technique (Cohesive Zone Model) is used at zones in the contact line, and stress criteria are used in BGD (Breaking Glued Contact) models of contact tasks [9, 10]. After the computer model trimming and optimizing, results of simulations, e.g. in Fig. 5 and 6, show the distribution of contact force fields at the contact

5 224 Industrial and Service Robotics start, namely for values of the adhesive layer external load at the interval from 0.9 to 1.5 kpa which correspond to the measured data given in the diagram in Fig. 4. Fig. 5 Character of contact forces during axial loading with stress 0.9 kpa (right view shows the real profile of contact area) Fig. 6 Character of contact forces during axial loading with stress 1.5 kpa (right view shows the real profile of contact area) Summary The paper presents a basic analysis of materials provided with very adherent surface layers being based on the principle of the biologic gecko effect. The material surface structure on a basis of synthetic silicone rubber is analyzed in details. One of possibilities to use these materials as a substitute of standard vacuum active suction cups is presented for handling flat objects of the plate type. The aim of such substitution is to decrease pressure air consumption as well as to increase carrying capacity of gripping elements both in the axial direction, and primarily in the radial direction in the handling processes in which the plates are handled vertically. The patented concept of a combined vacuum-adhesive gripping element is showed, and possibilities to modify the adhesive layer changing geometry are presented. The authors great attention is concentrated to the sphere of laboratory testing the silicone adhesive layer behaviour depending on external load values and gripping time during handling with a low level dynamics along the gripping element axis. Moreover, a computer model of the deformation contact task was realized describing in details the

6 Applied Mechanics and Materials Vol replacement of silicone real behavior by the Mooney-Rivlin rheological model. Essential approaches to the contact line simulation were described. Acknowledgement The results of this project LO1201 were obtained with the financial assistance of the Ministry of Education, Youth, and Sports of the Czech Republic as a part of targeted support from the "Národní program udržitelnosti I" programme. References [1] Information on [2] Information on [3] Horák, M., Novotný, F. Suction Gripping Element. CZ Patent (2011), MPT B 65 G 47/91, B 65 H 5/14, B 60 R 9/058, Industrial Property Office CZ, Information on [4] Horák, M., Novotný, F. The Study of Mechanics of Deformation Behaviour of Service Robots Gripping Systems. Engineering Mechanics 2012, 18th International Conference, May 14-17, 2012, Svratka, Czech Republic, p ISBN [5] Horák, M., Novotný, F. Increases in the Radial Capacity of Vacuum Gripping Elements. MM Science Journal, October 2011, Special Edition - 20th International Workshop on Robotics in Alpe-Adria-Danube Region (RAAD), October , MKČR E 7645, Information on p ISSN (Print), ISSN (On-line) [6] Horák, M., Novotný, F. Special Gripping Elements for Handling with Flat Objects. Applied Mechanics and Materials, Vol. 282 (2013), pp 27-32, 2013, Trans Tech Publications, Switzerland, DOI: / ISSN [7] MSC.Marc Manual Volume A - Theory and User Information. p. 793 [8] Selvadurai, A. P. S. Deflections of a Rubber Membrane. Journal of the Mechanics and Physics of Solids, Vol. 54 (2006), pp , Elsevier Ltd., DOI: /j.jmps , ISSN [9] da Silva, L. F. M. and Campilho, R. D. S. G. Advances in Numerical Modelling of Adhesive Joints, SpringerBriefs in Computational Mechanics, DOI: / _1 [10] Hermes, F. H. Process Zone and Cohesive Element Size in Numerical Simulations of Delamination in Bi-layers. Master Thesis. Eindhoven University of Technology, Department of Mechanical Engineering, 2010, p. 74