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Analysis of Possibilities of Using Polymeric Materials for Testing Prototypes of Harmonic Drive

The main objective of the analysis was to assess the suitability of rapid prototyping methods to perform functional prototypes of a harmonic drive. Physical models of a gear wheel for the harmonic drive were made and geometric accuracy control was carried out therefor. Precise measurements of tooth profiles showed differences between models made with the use of 3D printing and corresponding theoretical outlines. The discrepancies were not decisive for the correctness of the work of the analyzed harmonic drive, since the key parameter affecting its kinematic accuracy is the scale's constancy. Introduction Harmonic drives are a special type of gear transmission, mainly due to an extremely complex way of load transfer. Their basic components are flexsplines, in the shape of a thin-walled sleeve with an external toothed ring, a circular spline and a wave generator. The basic diagram of such a gear is shown in Figure 1a. The rotating generator deforms the flexspline, causing its moving in relation to the circular spline with internal teeth. Toothed rims of both wheels meet in the meshing areas, and pass each other outside of these areas [1, 2]. The load is transferred simultaneously by 20 ÷ 50% of the total number of teeth of the flexspline [3, 4]. These exceptional working conditions bring on the need for a very thorough stress analysis [5, 6] to improve the quality of this type of gears. Analytical calculations of this type of gear are not accurate, because the complexity of the problem forces the basing only on general assumptions. Numerical calculations carried out, for example, using the FEM finite element method are based on simplified models, so they also do not give precise results. Only bench tests allow for the exact determination of the strength and durability of a gear, as well as the assessment of the quality of its work or kinematic accuracy. Experiments are carried out with the use of functional prototypes, often by rapid prototyping (RP) methods, from polymer materials. The use of the 3D printing method allows for fast and cheap production of all components of the gear. This is particularly important in the case of a flexspline of a harmonic drive, whose thin-walled construction causes a lot of technological problems during production with classical cutting methods. Therefore, the use of additive methods of rapid prototyping not only facilitates the creation of flexsplines, but opens up new opportunities to improve their design. The key issue, however, is the geometric accuracy of harmonic drives made by 3D printing. Model making The implementation of a functional prototype of a harmonic drive must be preceded by analytical calculations in order to determine its basic geometrical and strength parameters and to model its Terotechnology 2017 Materials Research Forum LLC Materials Research Proceedings 5 (2018) 61-66 doi: http://dx.doi.org/10.21741/9781945291814-11 62 components in the selected CAD program. For the needs of gear production for harmonic drive, the MEM (Melting and Extrusion Modeling) method was chosen, i.e. FDM (Fused Deposition Modeling) with a single nozzle print head [7, 8]. It is an incremental shaping technology that allows for creating physical models with complex geometry, which is especially important in the case of a flexspline of a harmonic drive. The obtained surface quality is not comparable with other methods [9, 10], especially in mass production, but sufficient for prototyping. The ABS copolymer (acrylonitrile butadienestyrene) is one of the most often used materials in 3D printing, with strength and temperature parameters that allow it to be widely used both in industrial production and for prototyping [11, 12]. Measurements A detailed analysis of geometric accuracy of models made with the use of 3D printing was performed using a Roland MDX-40A coordinate milling machine with a special ZSC-1 scanning head mounted (Fig.2b). The measurements were carried out using the contact method due to its high accuracy and the possibility of using models immediately after their manufacture.