Feasibility of cold roll forming of external involute-helical gears for automatic transmission

Helical-involute gears have been in wide use for more than a century. They are important machine elements in many types of equipment. Machining by creep-type hobbing has been the main manufacturing technique for these gears. However, in recent years, net-shape forming has come under investigation by a number of researchers. Cold-forming of gears would reduce waste through chip elimination. This method is expected to have the additional advantage of producing stronger gears. The present research investigated flat-rolling as a new high-volume manufacturing technique for automatic transmission precision gears.; Geometric properties of flat-rolled (FR) gears were benchmarked against machined gears through metrological evaluation. Process boundaries were established through capability studies. The beneficial effects of work-hardening were established through metallurgical testing and hardness-mapping. Critical process parameters were identified through design of experiments (DOE). Two-dimensional ABAQUS and three-dimensional DEFORM finite element analyses (FEA) were performed. The simulation models were calibrated against designed experiments with a specific gear design. The analytical and experimental results identified a number of improvements to the process. Validation of FEA has opened the way for optimization of the gear-rolling process through virtual DOE.; The rolling process presented many FEA challenges. Full-length die contours had to be modeled since the stressed area of the die changed as the forming operation progressed. Nodal coordinates had to be updated constantly because of blank rotation. Friction and slippage had to be considered in order to correctly model the three-dimensional material flow. Finally, very high mesh densities were required because of tight part tolerances. These FR characteristics increased the number of calculations for the simulation and pushed FEA capability to its limit. Furthermore, high-volume manufacturing of precision external -involute helical gears using flat-rolling is currently impeded by a number of quality issues. However, financial and operational analyses have indicated that barring technical and quality problems, flat-roll forming is capable of producing pinion gears at lower cost and without any adverse impact on productivity. The economic benefits provide sufficient motivation for further research into this field.