Introduction

Internal Generating Gear Grinding is a hard finishing process developed by the NIDEC MACHINE TOOL CORPORATION in Japan. It is a grinding process like generating gear grinding but for the internal teeth of a ring gear. The tool used in this process has threads that engage continuously from one tooth after another in the gear. Simultaneously, the grinding wheel moves in axial direction to complete the grinding process along the whole flank.

When introducing a new process, the question is, how significant will the influence on the final produce be? Would the results justify the additional effort to implement hard finishing?

CAE-Analysis

To answer these questions, a study was carried out by the FEV Europe GmbH in Aachen (FEV). A simulation focused on the influence on NVH between conventional manufacturing methods for Planetary Gear Systems (PGS) and the application of generating gear grinding in ring gear production.

For this purpose FEV designed a virtual EV-transmission with a PGS for an electric driving unit (EDU). Using an established transmission development approach, a set with four planetary gears and an input power capacity of 125 kW was chosen, which can be seen as typical for a passenger car. The four planet arrangement tends to show a higher sensitivity of ring-to-planet gear mesh excitation, and can be a good example of a simple structure for an EDU. The PGS gears received a micro-geometry optimization by FEV to achieve reasonable symmetrical contact pattern between planet and ring gear flanks, as well as to show the various possibilities of flank modification during hard finishing with internal generating gear grinding. These parameters included helix crowning Cβ, helix slope fhβ, profile crowning Cα, profile slope fhα and tip relief Cαa.

This simulation was separated into two steps. Step one was to evaluate influences of the manufacturing process on the system excitation mechanism. Step two was a sensitivity evaluation to find out parameters that are most effective to improve NVH performance. In each step, both torque sweeps for peak-to-peak system transmission error as the key excitation measurement and speed sweeps for dynamic responses of the system were executed.

1 – Influences on System Excitation

During step one, three scenarios were investigated for comparison: (1) an ideal gear without manufacturing deviations as reference, (2) a ring gear finished with proven quality results of the internal generating gear grinding process and (3) typical results from conventional, current manufacturing methods, for example broaching and heat treatment without hard finishing.

The simulation started with a full torque sweep from 0 to 100% of maximum input torque 240Nm to see the influence on the transmission error.

As a result, gear grinding scenario (2) showed a constant amount of reduced excitation over the complete torque range and even up to 70% improvement at a torque level of 20% compared to the conventional scenario (3) measured in amplitudes of the transmission error in mrad.

A speed sweep simulation from 0 to 10,000 min-1 on input at a torque level of 20% was performed next to understand the dynamic response of housing and associated components. The simulation measured virtually the surface acceleration at two locations on the transmission housing. In conclusion the speed sweep simulation indicates that acceleration levels on the housing were improved by an average 10.2dB and a maximum of 21.8dB by using internal generating gear grinding.

2 – Sensitivity Evaluation

Step two was aimed towards determining the influence of different gear parameter and their tolerances. Both macro errors, total cumulative pitch error Fp and runout Fr, as well as micro errors, Cβ, fhβ, Cα, fhα, and Cαa were individually investigated. All errors were studied at both ends of the tolerance scale to catch worst case values for the simulation. The torque sweep simulation was executed for each parameter and showed clearly that poor tolerances for Fr and even more significantly for Fp lead to roughly 90% of the excitation. The following speed sweep simulation confirmed these results with the level of acceleration decreased by 12.5dB on average and a 22.3dB maximum in the event of good versus poor Fp values. This means vibration acceleration on the transmission housing can be reduced to 1/4 on average, and to 1/13 at its maximum by improving cumulative pitch error.

Conclusion

Internal generating gear grinding can improve gear quality that improves transmission error and acceleration level on the transmission housing. This is primarily by improving macro geometry errors, such as cumulative pitch Fp and runout Fr. This positive correction function of heat treatment distortion is the key factor for the hard finishing process. Internal generating gear grinding can improve transmission component quality for better NVH performance.

In order to learn more about Internal Generating Gear Grinding, please visit the 4th International Conference on Gear Production 2022 In Garching where we will introduce our latest results on internal grinding tool optimization or see the process in operation by yourself at the WZL in Aachen.