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Fibre-reinforced Positioning Lever for Manufacturing Equipment When developing manufacturing equipment, short cycle times are the key issue. Elastic deformation of working bodies can limit the top speed of such systems. Since the deformation is caused by the acceleration of these components, the mass and stiffness of the moving parts are crucial. CFRP offer significant advantages over metallic materials in this respect.

The use of ultra-modern processing machines is one of the key requirements for a globally interconnected consumer society. In addition to plastic, paper, printing and other special machines, packaging machines in particular are a key component for the provision of commodities and consumables. While reliability and ecological aspects are important, the working speed of the systems deployed is the most important criterion. In recent years, productivity requirements cou-

pled with ever-intensifying market competition have led to a significant reduction of the required cycle times for designing such processing machines. Despite primarily nonuniform movement routines of working bodies of packaging machines, certain applications achieve speeds of more than 1000 cycles per minute [1, 2]. However, there is very little scope for even greater productivity given the inherent material properties of the metallic substances used.

Figure 1

Simplified, representative CAD model of the positioning lever assembly examined (©LZS)

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The use of more modern materials thus seems one option to boost performance. Due to a combination of high stiffness, low density and considerable damping capacity fibre-reinforced plastics (FRP) are already deployed in many highly dynamically configured constructions. However, while the process of continuous filament reinforcement is already applied to plastic for use in highly stressed components deployed in a range of industries, its potential has been scarcely exploited in the packaging machine sector [3]. In order to tap into this potential to boost efficiency, Leichtbau-Zentrum Sachsen (LZS), supported by the Institute for Lightweight Engineering and Polymer Technology (ILK) of the Technische Universität Dresden has developed a fibre-reinforced positioning lever for use in packaging machines.

Challenge Implementing a processing step often requires the constant rotatory movement of the drive to be converted via a gear into a translatory movement. Figure 1 shows the representative structure (cam disc, drive shaft, lever arm and interface element) of such an assembly within a packaging machine. The cyclic acceleration of inert masses initiated by this set-up periodically gives rise to forces which generate oscillations of the stressed components. Excessive

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Authors DIPL.ING. RALPH BOCHYNEK is Division Manager for Material, Component and System Testing at Leichtbau-Zentrum Sachsen GmbH (Germany).

DR.ING. BIRGIT PAUL is Manager Innovations at Leichtbau-Zentrum Sachsen GmbH (Germany).

FRANZ BILKENROTH is Degree Student in the Material, Component and System Testing Department at Leichtbau-Zentrum Sachsen GmbH (Germany).

PROF. DR.ING. NIELS MODLER is Member of the Board of the Institute for Lightweight Engineering and Polymer Technology at Technische Universität Dresden and Chair Holder of Function-integrative Lightweight Engineering (Germany).

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amplitude of the deflection at the time of contact of the interface element with the packaging material may mean the stability of the entire process can no longer be guaranteed. Reducing the inert masses or boosting the rigidities of the stressed structure, however, is a way of reducing the resulting deformation and allowing the maximum feasible cycle count to be increased. Their high rigidity parameters and low mass make carbonfibre-reinforced plastics (CFRP) the ideal choice for this task.

Conceptual Development For the purpose of conceptual development, the positioning lever examined is further subclassified into the base body and both lever arms. The base body is fixed to the drive shaft via a fixed bolted connection. The first step involves collating the framework conditions for designing the positioning lever, then developing a specific positioning lever concept using weighted assessments based on application. The iterative detailing and calculation of the preferred variant is performed in accordance with the design shown in Figure 2. In addition to mechanical needs imposed, the conceptual development of new components also has to meet additional requirements such as mounting accuracy and installation space. Positioning levers intended for use in the food processing industry are subject to specific requirements for hygiene and safety for human health of the materials used [4, 5]. To prevent any contamination of foodstuff, any corners in which production residues could accumulate must be eliminated early on in the design stage. If critical areas cannot be avoided for design-related reasons (for example, in the area of joints when positioning levers feature a hybrid construction), these must be either covered by additional components or designed so as to be easily accessible and thus easy to clean. In accordance with Regulation No. 10/2011 of the European Commission, additional specific provisions apply to fibre composite components (“multilayer composites”), which could come into contact with foodstuffs during the handling process.

To guarantee the safety of the materials used for human health, epoxy-resin based polymers including a certified food-safe coating must be used. This coating process involves applying a colourless homogeneous surface treatment to CFRP components, which makes it easier to detect any contamination or damage. In accordance with current legislation, the status of materials as safe for use with foods is also regulated in standards, for example DIN EN 1672-2: Food processing machinery – Basic concepts – Part 1: Safety requirements and Part 2: Hygiene requirements. The loads exerted from joints and actual operation impose a complex stress pattern on the base body, which prompts the use of isotropic materials. Moreover, the close proximity to the rotation axis reduces the influence of the base body mass on the overall load-bearing capacity of the component. The lever arm, conversely, given its centre of gravity far away from this axis, is the primary element determining the inertia of the positioning lever while in operation and the resulting process-critical deformation. Using a lighter and more rigid material is key in this field. The lever arm itself is mainly subject to bending and torsional stresses while in operation. The result – a relatively simple stress state – paves the way to optimally exploiting the strongly anisotropic rigidity properties of CFRP materials. To obtain the targeted resisting moment against bending and torsional stresses with a low component mass, the structure of the lever arm should preferably be a sealed hollow profile cross-section. This design also reduces the accumulation of residues. Three main concepts are compared for the component in question with regard to material selection, Figure 3: base body and lever arm respectively made from an aluminium alloy (Al; conventional design) or CFRP (CFRP design), and a third variant featuring a hybrid design with a base body made from an aluminium alloy and lever arms made from CFRP. As far as mechanical properties are concerned, the hybrid design is the preferred choice from those mentioned. With rigidity in mind, an aluminium alloy is used for the base body, and CFRP for the lever arm. This hybrid application also allows a

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weight reduction of 32 % and boosts the cycle time by 143 % compared to conventional Al construction. When designing innovative fibre-composite components, nothing is more important than selecting the right production method. As well as making a tentative decision on what scope of serial production is economically viable, the manufacturing method also has to anticipate the attainable mechanical properties of the fibre-reinforced components. Compared to metallic materials, it is particularly imperative when using CFRP to take the potential manufacturing process and individual advantages and downsides of the same into account during the early phases of conceptual development. For the component described here, three methods were considered, Table 1: } winding followed by resin-transfer moulding (RTM) method } autoclaving of correspondingly shaped prepregs } pressing of organic sheets. In the case in question, the second method is preferred, since pre-impregnated carbon fibres (prepregs) guarantee uniform component quality with a high fibre volume ratio. This method allows economical prototype production due to the relatively low expenditure on tool manufacture. Prototypes guarantee rapid experimental validation of numeric models, which are key to the component design stage. The use of UD prepregs allows the greatest level of design freedom in terms of the fibre angle of individual layers, Figure 4, and thus the targeted adjustment of desired rigidities for the overall composite element, since the UD prepregs can be aligned in the desired direction arbitrarily.

Figure 2 Schematic illustration of the procedure for development of CFRP components (© LZS)

Production When designing components for use in the food processing industry, flat surfaces free of corners and edges in which dirt can accumulate are required [6]. Fibre-reinforced hollow sections that meet this requirement can either be produced by gluing together two individually manufactured half-shells or using the film bubble process. Given the small size of the lever arm being produced in this case, a glued joint would prove dis-

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Figure 3 Comparison of the three positioning lever concepts in terms of cycle frequency, mass and rigidity (© LZS) WORLDWIDE

Table 1 Specific advantages and disadvantages of selected production methods for manufacturing CFRP components (© LZS)

advantageous, since it would lead to a significant decline in torsional strength. The film bubble process, on the other hand, offers additional advantages. The metallic load application elements can be positioned before applying the fibre layers on the silicone tubing, Figure 5. The elasticity of silicone rubber means undercut inserts can be used while the demouldable nature of the core remains intact. Since any exchange of materials between silicone and epoxy resin can be ruled out during the linkage process, this procedure also eliminates the need to apply release agents to the core. The silicone tube is then sealed by mounting a clamping plate to consolidate the fibre composite. Once the fully configured core has been inserted into the external tool and it is closed, pressure is applied within the tube via a connection in the base plate. As well as the advantage of uniform and easily adjustable compressive force via air pressure, this approach also eliminates the need for an autoclave. The entire process can be performed in a heating chamber. Unlike conventional vacuum structures, the silicone tube is reusable. Besides the technical feasibility of the production process, the focus also is on man-

+ uncomplicated function + good mechanical proper ties + par tially automatable

Winding + RTM

- cost-intensive prototype production - limited shelf angle when winding + uncomplicated functional integration + good mechanical proper ties + low production costs for prototypes

Prepreg + Autoclave

- dif ficult pressing of individual layers - high propor tion of manual procedures

Organic sheet + Presses

+ inexpensive materials + shor t cycle times + automatable - ver y cost-intensive prototype production - high development expense - lower lightweight limit

Figure 4

Detailed view of the fully configured inner core (© LZS)

Figure 5

Cutaway view of the tool group, including a component for producing the CFRP lever arm (© LZS)

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ufacturing costs, Figure 6. Assuming that 100 positioning levers are manufactured each year, the manufacture of hybrid positioning levers is up to 25 % more expensive compared to conventional aluminium equivalents. But only a year or so is needed to recoup the procurement costs incurred.

Numerical Evaluation The individual components of the positioning lever are dimensioned in accordance with stiffness requirements. The design aims to configure a component with working elements which are subject to as little deformation as possible while in operation. The vast majority of the load exerted does not occur as a result of the impact from the packaged item, but as a result of the very frequent acceleration of the component in question. The displacement simulated in the finite elements (FE) model at the interface element mainly comprises bending and torsion of the lever arm. The layer structure of the CFRP components is optimised numerically to minimise the level of deformation. In one further FE model, an eigenfrequency analysis is performed while using an optimised layer structure. When the system-specific eigenfrequencies are compared with the excitation frequencies, the speeds at which resonances may occur can be determined. Where applicable, adjusting

Figure 6

Comparing the material and production costs for concepts of conventional Al and hybrid design (* assumption: milled aluminium parts; ** part-specific tooling costs included (assumption: 100 parts/a)) (© LZS)

the laminate structure and the associated change in rigidities can prevent upward surging of the structure in the desired speed ranges. The first eigenfrequency of the assembly at 365 Hz is far higher than the basic excitation frequency generated by the cam disc (17 Hz). Since the function denoted by the curved path is not a simple sinusoidal line, however, resonances may still occur at integer multiples of the base frequency [7]. Applying discreet Fourier transformation (DFT) to the excitation function allows these elements to be broken down into their harmonic frequency components. As applied in

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the Campbell diagram, the intersections of these harmonics with the eigenfrequencies of the lever show the speeds at which resonances are likely to occur, Figure 7 (right). The DFT proves that proportions of the harmonic frequencies above sixfold base frequency are only included in the excitation signal to a marginal extent. Due to the fact that the first eigenfrequency in the output range of up to 1000/min only intersects with the 18th harmonic, resonances are unlikely to occur while the positioning lever is in normal operation. Additional numeric investigations of the resonance behaviour confirmed this finding.

Procedure for determining possible resonance frequencies (© LZS)

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Figure 8 Deviation of the interfacing element peak from the target movement over two cycles, comparison of the positioning lever in aluminium (red) and hybrid design (blue) (© LZS)

The displacement of the interface element peak shows the scope for a performance boost through the use of fibre-reinforced components. Figure 8 shows the deviation of

vention, a reduction in displacement of 75 % was achieved. With this in mind, the positioning lever in a hybrid construction is suited for operation in higher speed ranges.

This hybrid application also boosts the cycle time by 143 % compared to conventional Al construction. the outer-most node at the interface element from the desired target motion over two full rotations of the cam disc as a simulation calculation. Compared to the motion path of a metallic positioning lever featuring an integral design, a significantly improved assembly behaviour is clearly apparent. Accordingly, at the process-critical point of inter-

Figure 9 Illustration of the proposed positioning lever in a hybrid design (© LZS)

Conclusion and Outlook The use of carbon-fibre-reinforced plastics in processing machines was examined with the aim of maximising the potential performance boost. Using the example of a high-frequency drive positioning lever, LZS, supported by ILK, developed a concept for manufacturing a positioning lever assembly featuring an

innovative hybrid CFRP-aluminium design, Figure 9. In addition to numerically optimising the laminate structure, simulations showed the oscillation behaviour and the deformation under load. Comparing simulation results with measurements conducted on a conventional positioning lever made of aluminium showed that the maximum positional deviation of the working elements from their target position could be reduced by more than half by using CFRP. To allow these simulation results to be validated, a prototype of the hybrid positioning lever will be manufactured. Motion measurements and a permanent operation test are going to be performed on a test rig. The results of the planned experimental investigations will be used for further modifications of the FE models. |

References [1] Bleisch, G.; Majschak, J.-P.; Weiß, U.: Verpackungstechnische Prozesse: Lebensmittel-, Pharmaund Chemieindustrie. 1st edition, Hamburg: Behr’s Verlag, 2011 [2] Hirsch, A.: Werkzeugmaschinen: Grundlagen, Auslegung, Ausführungsbeispiele. 2nd edition, Wiesbaden: Vieweg-Teubner-Verlag, 2012 [3] Weck, M.: Werkzeugmaschinen 2: Konstruktion und Berechnung. Springer-Verlag, 2006 [4] Commission Regulation (EU) Nr. 10/2011 on plastic materials and articles intended to come into contact with food. 2011 [5] Matissek, R.: Unerwünschte Stoffe, Kontaminationen und Prozesskontaminationen in Lebensmitteln. Lebensmittelchemie, Springer-Verlag, 2016, pp. 281-371 [6] Hauser, G. et al.: Hygienic Equipment Design Criteria. Frankfurt: European Hygienic Engineering and Design Group, 2004 [7] Dresig, H.: Schwingungen mechanischer Antriebssysteme: Modellbildung, Berechnung, Analyse, Synthese. 2nd edition, Berlin: Springer-Verlag, 2005

Thanks The authors would like to thank engineers Felix Dehmel, Stefan Hoschützky and Achim Mertel for assisting with this article.

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