Plastic Steering Wheels Technology in Waiting

TECHNOLOGY SteeRing PLASTIC STEERING WHEELS TECHNOLOGY IN WAITING

the steering wheel is one of the essential components of the human-machine interface in the car. even though current standard steering wheels have frames made of die cast magnesium or aluminium, tests carried out by takata show that it is also possible to use plastics for steering wheels or their frames that meet their many and varied requirements.

46 www.autotechreview.com AUtHORS structure of a steering Wheel The requirements according to ECE- R12 are still a major challenge to the The core of the steering wheel is the strength of the steering wheel frame. frame, which is mainly responsible for the These include not only the above-men- transmission of force from the driver to tioned body block test, but also the so- the steering column, as well as for vibra- called head impact test, in which a tion behaviour and for the structural hemisphere strikes various points of the heiko ruck strength of the steering wheel. Current, steering wheel at the same speed of is Head of Development Steering common materials used for the frame are 24.1 km/h [2]. The ECE-R12 standard Wheels and Frontal Airbags at takata-Petri Ag in Aschaffenburg die cast magnesium or aluminium. The describes the legal conditions for the (germany). frame is encapsulated by PUR (Polyure - approval of a steering wheel. In addition thane) foam, which is also used to create to ECE-R12, Customers have their own the design. The foam is painted and forms structural, static and dynamic test the surface of the steering wheel. In most requirements that must also be met. cases, the airbag and the steering wheel Today, the mass of the steering wheel are connected by direct snap-in fixtures usually ranges from 1,200 to 2,000 g. between the frame and the airbag hous- Comfort criteria for the steering wheel norbert mÜller ing. ❶ shows a schematic representation include the resonance frequency and is Head of Pre-Development of the components of a steering wheel. moment of inertia. The resonance fre- Steering Wheels at takata-Petri Ag in Aschaffenburg (germany). The term “plastic steering wheel” in quency describes the vibration behaviour modern technical language means that of the steering wheel in the hands of a that the frame materials currently used driver. The moment of inertia (MOI) have been replaced by plastic. defines the rotational recovery behaviour of the steering wheel during steering manoeuvres. The MOI can be influenced reQuirements for a by the frame rim mass. bernharD kempf steering Wheel As standard, customer specifications is Project Leader Plastic Steering also include the static torsion test. During Wheel in Pre-Development Steering Wheels at takata-Petri Ag in Until the 1960s, the steering wheel had this test, the torsional stiffness of the Aschaffenburg (germany). the function of the direction sensor in steering wheel is tested for plastic defor- the car. In addition, it was also used as mation. In this test, driving against an a design element. Discussions concern- obstacle, for example diagonal driving ing the introduction of the ECE-R12 against the kerb, is simulated in the labo- standard and its final introduction in ratory. The torsional fatigue strength of 1969 changed the perspective regarding the frame is highly relevant for the dura- the steering wheel. Due to the ECE bility test, as it has to simulate the steer- claus ruDolf standard, the steering wheel became a ing motions over the vehicle’s service life. is Director Development takata europe at takata-Petri Ag in safety component. The body block test Other tests on steering wheels are more Aschaffenburg (germany). according to ECE-R12, which is carried customer-specific. ❷ shows the many out at 24.1 km/h on a stationary steer- requirements that a steering wheel has to ing wheel, required a new design of the meet as the interface between the driver steering wheel frame [1, 2]. and the vehicle.

❶ Components of a steering wheel autotechreview May 2012 Volume 1 i issue 5 47 TECHNOLOGY Steering

All of these laboratory tests have one feature in common: the tests are all car- ried out at ambient temperature in the laboratory. Tests in the limit temperature range are not performed.

Development objectives

The advantages of using plastic for the steering wheel skeleton include a possible mass reduction, new design opportunities and, not least, reduced production costs, while still fulfilling all the existing requirements. ❷ Requirements for the development of a steering wheel As the technical properties of polymer materials directly depend on the ambient temperature, Takata went one step further with respect to its internal list of target Filled PUR foams were also rejected in the PUR technology were developed with functions: the basis for the varied func- pre-selection phase for strength reasons. external partners who had the required tional and quality tests is the temperature In order to test the remaining number technological competence and who also range of an airbag. of variants, the predevelopment depart- had their own injection and foaming tools. ment launched an extensive sampling Takata carried out in-house sampling programme. of the standard materials polyamides, Solution strategies For evaluation of plastic injection polypropylenes and blends with different moulding techniques and materials, Takata fibres on their injection tools. In cooperation with the development, created an injection mold comparable to The plastic steering wheels produced tool engineering and production depart- the current magnesium frame tools. using auxiliary tools and different pro­ ments, various possible solutions for the PUR technology was examined with cesses and materials were mainly tested design of a plastic frame were devel- the aid of a foaming mould. ❸ shows the under dynamic conditions. Differences in oped. The range of concepts included different approaches. the test results were observed. the use of standard plastic materials on The results of these preliminary tests standard machines, the use of new along with the financial assessment of the injection moulding processes and the Concept and material selection production processes in which different use of various PUR technologies. Within materials were used led to the decision to the application of PUR technology, the Both the IMC technology and the LFI foam further develop Takata plastic frames production of a plastic skeleton using LFI (Long Fibre Injection) technology or a PUR foam with short glass fibres was to be examined. For the standard processes, research focused on the use of a wide range of plastic materials. Optionally, single-shell and two-shell concepts were considered. From a processing perspective new altem­ate manufacturing processes such as IMC (Injection Moulding Compounding) technology were considered. In addition to glass-fibre reinforced plastic, other alternatives such as bio materials and car- bon-fibre reinforced plastics were taken into consideration. In a first stage, all these possible solu- tions were subjected to a technical and financial assessment. The version of a two-part frame in which the two shells are welded or adhesively bonded to each ❸ Possible solutions for the other was eliminated for cost reasons. manufacture of a plastic frame

48 www.autotechreview.com ❹ Transfer of the topology optimisation into three-dimensional CAD data

using a standardised injection process These load cases, which mainly involve tions of the car manufacturers. For this using glass-fibre reinforced polyamides. bending and torsional stress, were brought purpose, static, statically non-linear and together in a combined data record for dynamic calculations are carried out with optimisation. Maximum stiffness was current explicit and implicit FE solvers. Development methods defined as the target condition and vol- The aim is to achieve a balanced behav- ume as a secondary condition. The opti- iour with regard to durability and vibra- The ideal way of meeting the require- mum determined after several loops tion, as well as to adapt the deformation ments of the carrier structure of a steering serves the design engineer as a draft and crash behaviour precisely to the vehi- wheel regarding strength, crash behaviour design for the creation of the respective cle’s requirement. and fatigue strength in a specified design CAD model, ❹. In order to optimise the process and space is to use structural and topological The design of the carrier structure the mould at an early stage, detailed pro­ optimisation. In this respect, different illustrated in the CAD model must be cess simulation was carried out using load cases were chosen from the technical obtained by means of FEM simulations in Moldflow. The virtual mould-filling stud- specifications of the car manufacturers. accordance with the technical specifica- ies help to optimise the injection concept, to lay joining lines in load-uncritical ranges and to offer an initial trend for the glass fibre distribution in the component. As strength is of decisive importance for this component, and this mainly depends on the fibre length on the one hand and on the fibre distribution on the other, spe- cial attention was paid to the consideration of the glass fibre distribution in the exist- ing FEM models. The BASF software Ultra­ sim makes it possible to transfer the glass fibre distribution to existing FEM networks. Thus, it is possible to discretise the local fibre orientation, the type of loading, the extension rate and the temperature, in extreme cases up to element level [3]. The ❺ Development history for the plastic frame settings chosen by Takata finally result in autotechreview May 2012 Volume 1 I Issue 5 49 TECHNOLOGY Steering

❻ Comparison of the deceleration curves for simulation and testing during a body block impact according to ECE-R12

a compromise between industrially accept- not only in the static but also in the dynamic steering wheel by screws or snap-in fix- able computing times and the model cor- test series. During static durability testing, tures, seems to be possible. The injection relation achieved. To validate the model the test results were convincing even from of switch shells or volute springs is feasi- correlation, test items were created in an the first sampling series. It was possible to ble. Only by the application of such meas- existing tool and tested in various test meet the ECE-R 12 requirements through ures will cost and weight benefits of a configurations. The results of these tests several development loops and further plastic steering wheel be achieved. More- (force-displacement curves, component strength optimisations on the frame, ❻. over, a plastic steering wheel will also failure and deformation) were compared The biggest challenge in the list of tar- generate new degrees of freedom for the to the calculated results. The result shows get functions was the fulfilment of the design of a steering wheel. an almost complete concurrence between dynamic and static requirements in the simulation and testing. Due to this corre- low and high temperature range. This goal References lation, the method was also used for fur- was also achieved. [1] Zehner, T.: Richtungsänderung – Das Lenkrad im Wandel der Zeit. (Change in Direction – The ther component develop­ment. Steering Wheel Through the Ages) Köln, KISD – Initial dynamic crash calculations Köln International School of Design, Diplomarbeit, showed the need for optimisation in the Outlook 2007. (Cologne KISD Thesis 2007) area of the steering wheel hub and spokes. [2] ECE-R12: Einheitliche Bedingungen für die Genehmigung der Kraftfahrzeuge hinsichtlich des At the end of the virtual development With this steering wheel development, Schutzes des Fahrzeugführers vor der Lenkanlage process, a design with performance data Takata has proven that it is possible to bei Unfallstößen. Grundfassung vom 01.06.1969. that are comparable to today’s magne- replace the current materials used for (Uniform conditions for the approval of vehicles in regard of the driver‘s protection in front of the sium structures was achieved. Based on frames, such as aluminium, magnesium steering system during accidents. Initial version of these simulation results, the prototype and steel, by modern plastics in the June 21, 1969) mould used for a plastic frame can be put future. A plastic frame can meet today’s [3] Glaser, S.; Wüst, A.: Modellierung am Com­ puter. In: Kunststoffe, 2005, Nr. 3, S. 132-136. ❺ into operation. shows the development standard market requirements for steer- (Modeling at the Computer. In: Plastics, 2005, method for the plastic skeleton. ing wheels. No. 3, pp. 132-136) At first sight, there is no cost advantage, as the source materials magnesium or alu- Result minium and polyamide pellets have simi- lar material prices, although these fluctu- Meanwhile, more than 500 plastic frames ate to a large extent in the global material have been manufactured using the proto- market. New possibilities offered by the type mould and tested in the laboratory in plastic steering wheel, however, lie in the accordance with the list of target functions. design of the steering wheels and attach- The results show conformity with the ments. The integration of separate elements, Read this article on results predicted by computer simulation, which are currently connected to the www.autotechreview.com

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