The next level of axial forming for a sustainable component and process chain design
Based on the state of the art, the known manufacturing processes for producing external helical gears generally possess specific disadvantages, e.g., high material waste, extremely high tool wear, energy inefficiency, required assembly processes, or insufficient gearing quality. As a result of further development of the Felss core technology – the axial forming process – the mentioned negative effects can be partially or completely avoided. The axial forming process and the new associated machine concept developed as part of a feasibility study allow the manufacture of helical gears with a significant improvement of achievable gear quality to IT5-6 with a total helix deviation of up to 10 µm. Furthermore, the application of an energy and material-efficient cold-forming process, compared to forging or machining processes, offers a significant improvement of the PCF (Product Carbon Footprint) and can provide momentous advantages concerning the sustainability of the automotive industry in series production.
Introduction
There are over 1.3 billion motor vehicles worldwide today, of which one billion alone are passenger cars. By 2035, this number is expected to rise to around two billion. Thias enormous increase in the mobility sector poses major challenges not only for Germany, but for almost all industrialized and emerging countries. Traffic- and production-related emissions of CO2, air pollutants, and noise are causing problems, and dependence on oil imports is growing. Thus, effective climate and environmental protection targets can only be achieved if road traffic and the automotive industry also make a significant contribution. However, because many people still depend on the car, it is not enough to avoid solely on traffic and to focus on short distances and bicycles. Road traffic as well as the production of power-driven vehicles themselves, must become more environmentally friendly with less negative impact on climate and health, and for a better quality of life in the city of tomorrow. Cold metal forming plays a major role worldwide in the production of vehicle components regarding lightweight construction, sustainability, accuracy, and productivity. By utilizing material and applying a lightweight structural design, cold metal forming can significantly contribute to reducing the CO2 footprint and emissions of the entire process chain at acceptable costs through innovative solutions [1, 2].
The Felss Group is a globally well-known solution provider in the field of cold forming for the automotive industry. With 100 years of experience in niche technologies, Felss has been able to target rotary swaging and axial forming for the reduction of component weights. As an established lightweight design expert, Felss focuses on identifying and implementing individual, optimal, sustainable customer-oriented solutions and applying them for product development, from the machines to the finished component. Considering the increasing importance of environmental aspects, Felss concentrated its development resources in recent years strongly on generating customer benefits, such as reducing the CO2 footprint. These efforts resulted in entirely new forming processes and the further development of the existing core technologies, rotary swaging, and axial forming. The achieved extension of axial forming process limits, which now enables the production of helical gears, has already been published in [9]. Therefore, the focus of this publication represents the development of the new axial forming machine.
Dr.-Ing. Nadezda Missal
Stefanie Schwertel
Maximilian Ludwig
Veröffentlicht
State of the art
Axial forming belongs to incremental forming processes, and the principle is shown in Fig. 1a. A gear forming tool, e.g., a die, forms the teeth in the axial direction. The gearing is thus highly precise because all teeth are generated simultaneously by a one-piece tool directly on the component. Felss axial forming is generally carried out by the process of frequency modulation or recursive movement of the forming tool. Thereby, the forming process consists of a continuous repeat of a forward stroke and a subsequent, significantly shorter backward stroke. During the backward stroke, there is no contact between the forming die and the forming zone; thus, the forming zone can be relubricated. Therefore, the typical lubrication film breakage for the cold metal forming process caused by the high contact stresses and significant surface enlargement can be completely avoided. The frictional forces can thereby be reduced by up to 30% compared to conventional axial forming.
Fig. 1b shows the typical force curve of an incremental axial forming process. During the forward stroke, a forming process with a positive force or pressure component of the forming force occurs. Within the backward stroke, when the die is removed from the forming zone, only the frictional forces between the die and the component exist, which cause the negative components of the force curve.
As part of a feasibility study, alternative forming manufacturing processes for the production of external helical gears were first considered so that the technological disadvantages of these processes against axial forming could finally be identified. The results of this literature review showed that axial forming combines the advantages of the impact extrusion and rolling processes compared to the conventional metal forming manufacturing processes. Through the application of axial forming with frequency modulation as an incremental process, high contact stresses and high tool wear can be avoided. The main competitor to axial forming is the hobbing process. Axial forming can be considered as a new forming alternative to gear hobbing, which completely avoids material waste due to the full utilization of material and, thus, reduces material costs and the CO2 footprint. Moreover, forming in a one-piece tool results in an excellent total cumulative pitch deviation so that gear qualities of IT5-6 can be achieved compared to IT7 by hobbing. Furthermore, the axial forming process enables a compact design of the toothing because a distance of only 1.5-2 mm after the gear to the next shaft shoulder is necessitated. The long run-outs after the gear teeth, which are a technological requirement in gear hobbing, are therefore no longer necessary within axial forming [3 through 8]
Summarized, the analysis of state of the art showed that the described forming and machining processes are generally only suitable for external gearing. The production of internal helical gears is mainly realized by further machining processes, such as broaching, skiving, or flow-forming. However, these processes can only be applied to through holes and don’t offer any application possibility for gears in blind holes. The axial forming also, in this case, offers an economical alternative to machining and additionally provides the possibility for the manufacturing of internal helical gears in blind holes. A detailed analysis of the state of the art can be found in [9].
Production of helical gears by axial forming
A feasibility study was initiated based on the presented state of the art and the consideration of the significant advantages of axial forming over other helical gear manufacturing processes. The primary objective was to confirm the applicability of axial forming for manufacturing external helical gears. In cooperation with various partners, including Hofer powertrain GmbH, two components with different gear parameters were identified (Table 1). This experimental trial was carried out according to [9] on an existing Felss Aximus H02 axial forming machine which was retrofitted with a driven tool carrier with a worm gear unit and servomotor. Thereby, a standard frequency modulation and a standard oil for highly loaded forming processes were applied for the forming of the helical gear components.
The tolerances achieved in the trial (Fig. 3a) demonstrate that forming an external gear through axial forming with a helix angle of 22° using the driven tool carrier is feasible. Moreover, similar quality ratios were determined for all gear components independent of gear heights and helix angles.
The gear component tolerances specified by the customer were achieved with the exception of the total helix deviation Fβ, and thus, axial forming was verified as a forming alternative to hobbing. The tolerance of total helix deviations Fβ of the helical gear components produced on the Aximus H02 by axial forming showed a very specific trend over the length of the gearing which correlates with the occurrent varying load in the gearing die during forming. This observation indicates that there is a relationship between the finished part quality of the gear component and the occurrent different load-dependent machine torsion.
The forming of the gearing can be divided into three different load areas or forming areas on the component (cf. Fig. 3a, red lines). The start of gear forming represents the first area where the required axial and torsional forces increase continuously to their maximum until the die-filling of the gear is reached. The subsequent main forming area extends over almost the entire length of the gear and offers very constant forming conditions due to the complete contact between the component and the forming die. In the remainig forming area at the end of the gearing, the forces drop again by reducing the contact between the component and the forming die. In order to reduce the dependence of the gear quality on the unavoidable load changes of the forming process, a study was carried out on the economic realization of the greatest possible torsional stiffness of different axial forming machines and their tool guidance design concepts. A FEM-based software Meshparts was used for this study. This offers the possibility to efficiently analyze the deformation of various machine designs via structural-mechanical simulations under the forming load and to evaluate their potential influence on the gear quality.
Due to the complex and component-intensive machine design, the simulation model is reduced to the main supports of the machine frame within the forming process (cf. Fig. 2). For the simulation, unnecessary construction elements, such as smaller radii and bores, were neglected. The reduction of the simulation model and the simplification of the contours made a drastic reduction of the calculation effort possible due to the lower number of required meshing elements of approx. 1.2 million without any significant influence on the investigated result. In accordance to the standard machine design, the bearing and contact surfaces were respectively defined, and the machine frame was preloaded via the tension rods. The other specified boundary conditions describe the maximum machine load during the forming process of a helical gear. This is represented by the application of a constant torsional force by the driven tool carrier corresponding to 9000 Nm and the application of the associated reaction torque to the clamping device for component fixing. Moreover, the axial load of the forming process is also calculated by 400 kN, which is applied through the connecting surfaces of the hydraulic feed cylinder and the clamping device.
Fig. 2: Simulation model of the horizontal forming machine with applied forces and torques
The standard design of the die guidance system of a conventional, horizontal axial forming machine, which was installed to produce the first helical gears shown in Fig. 3a, was defined as the reference condition for this investigation. This die guide consists of four recirculating ball-bearing carriages which guide the forming tool in the press space by two diagonally positioned guide elements. The numerically determined total torsion of this machine type amounts to a torsion angle of 0.05588 degrees, whereby approx. 98% of the total torsion occurs due to the torsion of the driven tool carrier and its guides. The first optimization stage of the horizontal axial forming machine was the installation of two additional guide elements, each with two recirculating ball-bearing carriages. The addition of the guide elements now stabilizes the forming tool on four posts of the machine frame preventing displacement due to torsion. This modification already resulted in an improvement of the torsional machine stiffness of approx. 41% compared to the reference condition and corresponds to a torsional angle of only 0.033 degrees.
Based on the calculated results concerning increased machine stiffness and its potential positive influence on gear quality, the 4-element guide concept was implemented on the Aximus H02 axial forming machine. After the upgrade was completed, the previous experimental test was repeated. Compared to the gear results from (Fig. 3a) of the conventional guiding concept, the low machine torsional stiffness against torsional forming loads could be verified as the main influence on the gear quality. Due to the new stiffened machine concept, the reduction of the previously insufficient total helix deviations Fβ from over 20 μm to below 10 μm (Fig. 3b) was achieved to be within the specified tolerance limits.
Fig. 3: Helix corrected measurements of the gearing produced by a) the conventional forming machine, b) the stiffened forming machine
After additional validation tests, the actual torsion angle of 0.052 degrees in the reference condition of the axial forming machine at an applied torque of 9000 Nm was measured. There is a deviation of 5% between the calculated value and the actual value to determine the torsion angle. So, the simulation model provides a realistic representation of the determined torsional stiffness. After validation of the simulation model, the correlation between the new stiffened machine concepts and gear quality by the production of helical gears on vertical axial forming machines was evaluated.
As both horizontal and vertical axial forming machines must be applied for the production of helical gears, a numerical investigation of the machine frame’s resilience under identical load and boundary conditions had to be carried out for the vertical machine design, as described above. The investigation of the already stiffened machine concept by integrating the previously optimized tool guidance showed a torsional angle of 0.0407 degrees. This represents an improvement in torsional stiffness of only 27% compared to the horizontal reference state. This significant drop in torsional stiffness compared to the stiffened horizontal axial forming machine results from the omission of support for the upper and lower base plates of the machine frame (cf. Fig. 4). Whereas the mounting points of both base plates via the common machine base counteracts the torsion of the horizontal press frame, further optimization steps are required for the vertical machine design. Due to the solid ram design and the installation of four or optionally eight recirculating ball-bearing carriages, a significant stiffening could be achieved. This results in a stiffness improvement of the vertical axial forming machines of approx. 70% compared to the reference state, so the torsion angle amounts to only 0.17 degrees.
Fig. 4: Simulated deformation of the loaded axial forming machine with a) the stiffened design with 4 guide elements and b) with the optimized machine design
Machine torsion occurs from insufficient tool guidance. Therefore, the new simplified vertical machine frame was developed for a complete machine concept based on the data obtained from the numerical investigation. The main focus was on implementing the greatest possible machine stiffness so that acceptable gear quality within the production of greater helical gears in the future could also be achievable. The optimized design of the machine frame and the ram unit is shown in Fig. 5. According to the simulation model. Thus, the ram unit consists of a solid ram plate (yellow) axially supported by eight recirculating ball-bearing carriages along the guide rails. The bearing package for absorbing the axial forming forces during the turning of the forming tool is situated inside this ram plate. It has been designed for a minimum space requirement. In this design, the driven tool carrier has been extended by a second servo motor in order to generate greater torques with simultaneously higher positioning accuracy and, thus, to improve the achievable gear tolerances. Moreover, the forming tool can now be decoupled from the driven tool carrier by a short-stroke cylinder. Therefore, damage of the already formed gearing during the ejection by the uninhibited torsion of the forming tool can be completely prevented. Furthermore, a direct measuring system can be installed on the rotary bearing at a later date in order to be able to track the actual positions and actual velocity of the driven tool carrier during the axial forming process with maximum precision. Although the optimized design of the ram unit results in a significant dimension increase, the maximum ram stroke remains constant or is even longer due to the simultaneous extension of the press frame. Therefore, this new machine concept can manufacture longer helical gears in the future.
Fig. 5: Optimized design of the vertical axial forming machine
This new machine concept for the manufacture of high-precision helical gears was developed on the basis of investigations by structural-mechanical FEM simulations and designed for high rigidity, especially with regard to torsional loads. In the future, this machine concept will be applied to manufacture a wide range of helical gears according to the achievable gearing parameters, component dimensions, and the corresponding tolerances. In order to get the newly developed machine concept ready for series production, the simulation results concerning numerically determined torsion angle will be evaluated by carrying out experimental trials. An overview of the achieved simulation accuracy as well as the influence of the new machine concept on the gear qualities and tolerances will be represented in a further publication after completion of the experimental trial.
Conclusion
The Felss axial forming is not only capable of manufacturing simple spur gears but is also suitable for the production of helical gears due to the modified machine design. In order to achieve gearing qualities of IT5-6 and to comply with marketable gearing tolerances, the conventional axial forming machine had to be extended by a driven tool carrier and stiffened to support the occurring torque. By a complex synchronization of the turning drive and the recursive stroke, the manufacturing of external helical gears with a helix angle of up to 22° has already been achieved with process stability. Through additional stiffening of a horizontal axial forming machine, the reduction of the previously high total helix deviation up to 10 μm could be realized, which makes an economical application of axial forming for the production of helical gears feasible. The vertical axial forming machines possess significantly lower torsional stiffness due to their design, so a completely new ram guide concept had to be developed for these forming machines. Using the numerical optimization process (FEM-based software Meshparts), the simulated machine deformation under the maximum permissible load was reduced by 70% compared with the reference condition. In combination with a high-powered drive and high-precision measuring technology, the production of helical gears on vertical axial forming machines can be not only feasible to achieve precise gear tolerances but also a significant expansion of the gear parameters and dimensions that can be generated.
Sources
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[6] Neugebauer, R.; Hellfritzsch, U.; Lahl, M.: Advanced process limits by rolling of helical gears. International Journal of Material Forming. 2008, p 1183-1186
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[9] Missal, N.; Schwertel, S: Innovative cold metal forming processes for a sustainable future. International Conference on Gears 2023
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