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2004 Edition, October 1, 2004

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Noise Optimized Modifications: Renaissance of the Generating Grinders?

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Description / Abstract:

Modern gear calculation programs give us the possibility to determine the load carrying capacity and the noise behavior of a gear transmission considering the shaft and bearing system, the tooth modifications and the production deviations.While load and stress optimized tooth modifications are quite insensible to the normal production deviations; noise and vibration optimized tooth modifications need a much higher production accuracy because of the low values of the transmission errors caused by the changes of the mesh stiffness.

The numerical simulation leads often to more complex modifications which can not be realized with normal standard modifications like crowning and root or tip relief. Topological modifications are required and have to be realized in the grinding process. Differences between the common profile grinding method and the generating grinding method based on the different contact situations between grinding wheel and gear gap will be shown.

More and more companies discover the high improvement potential of topological modifications with which gears can be modified where it is required - the generating engagement relief of a helical gear corresponds to the normal tip and root relief of a spur gear. The teeth are modified in the area of highest pressure (at the beginning and the end of the mesh). A contact area with a lower Herzian pressure can transmit more load.

The topological modifications show big advantages for the noise and vibration behavior also due to the much higher variability in direction of contact pattern. Gears with even overlap ratio should for instance be modified as less as possible, otherwise the basic advantages of the constant contact line is not given anymore. Unfortunately, a load optimized tooth flank modification is not always a noise optimized modification also - a compromise between optimized load distribution and low noise has to be found.

The basic of the modern gear calculation programs is the exact determination of the mesh stiffness. All the secondary influences like shaft bending and deflection in the bearings should also to be considered. For the load distribution calculation a static view is often sufficient. The basic of the dynamic analysis is the load over the time in form of a FFT analysis. The running speed and the behavior of the oscillating system have also to be considered. In a practical example the calculation possibilitieswill be shown. Itwill be demonstrated how an optimized tooth modification can be found. In the calculation possible production deviations have to be considered - the feedback from the production to the design department is essential.

To satisfy the new requirements the gear grinders manufactures had and have to improve their machines every year. Today, a serial production of gears in quality Q=1 according DIN 3962 (AGMA >15) is possible, if requested. This improvement was basically been possible with the substitution of the mechanical transmissions in the grinder with themodern CNC controls. By introducing the torque motor as the main table drive of a grinder the last gear transmission disappeared in the gear grinder field.

The accuracy of a gear grinder depends basically from the accuracy of the table drive. The modern torque motor shows together with the direct mounted encoder high advantages in comparison to the mechanical worm/worm gear drive. Problems like worm gear wear, backlash and deviations are not known in a torque motor anymore. Without excitation, it realizes an incredible constant movement of the table axis. Machine frequencies - the main reason for the almost disappearing of the generating grinders in the nineties - are not transmitted to the gear anymore. This and the possibility to realize topologicalmodifications could now lead to a Renaissance of the generating grinders. They can be built in such a way that also form grinding is possible on the same machine.