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The final conclusion is that it may not be possible to identify a single parameter, such as peak-to-peak transmission error, that can be directly related to measured noise and vibration. 3.4 Paper D: Gearbox Noise and Vibration -Influence of Bearing Preload The influence of bearing endplay or preload on gearbox noise and vibrations is investigated in Paper D. Measurements were carried out on a test gearbox consisting of a helical gear pair, shafts, tapered roller bearings, and a housing. Vibration measurements were carried out at torque levels of 140 Nm and 400 Nm with 0.15 mm and 0 mm bearing endplay and with 0.15 mm bearing preload. The results shows that the bearing endplay or preload influence gearbox vibrations. Compared with bearings with endplay, preloaded bearings show an increase in vibrations at speeds over 2000 rpm and a decrease at speeds below 2000 rpm. Figure 3.4.1 is a typical result showing the influence of bearing preload on gearbox housing vibration. After the first measurement, the gearbox was not disassembled or removed from the test rig. Only the bearing preload/endplay was changed from 0 mm endplay/preload to 0.15 mm preload. Therefore the differences between the two measurements are solely due to different bearing preload. FE simulations performed by Sellgren and -kerblom [6] show the same trend as the measurements here. For the test gearbox, it seems that bearing preload, compared with endplay, decreased the vibrations at speeds below 2000 rpm and increased vibrations at speeds over 2000 rpm, at least at a torque level of 140 Nm. 3.5 Paper E: Gear Geometry for Reduced and Robust Transmission Error and Gearbox Noise In Paper E, gearbox noise is reduced by optimization of gear geometry for decreased transmission error.
The optimization was not performed strictly mathematically. It was done by calculating the transmission error for different geometries and then choosing a geometry that seemed to be a good compromise considering not only the transmission error, but also other important characteristics. Robustness with respect to gear deviations and varying torque was considered in order to find gear geometry with low transmission error in the appropriate torque range despite deviations from the nominal geometry due to manufacturing tolerances. Static and dynamic transmission error as well as noise and housing vibrations were measured. The correlation between dynamic transmission error, housing vibrations, and noise was investigated in a speed sweep from 500 to 2500 rpm at constant torque. No correlation was found between dynamic transmission error and noise. 4 DISCUSSION AND CONCLUSIONS Static loaded transmission error seems to be strongly correlated to gearbox noise. Dynamic transmission error does not seem to be correlated to gearbox noise in speed sweeps in these investigations. Henriksson [7] found a correlation between dynamic transmission error and gearbox noise when testing a truck gearbox at constant speed and different torque levels. The different test conditions, speed sweep versus constant speed, and the different complexity (a simple test gearbox versus a complete truck gearbox) may explain the different results regarding correlation between dynamic transmission error and gearbox noise. Bearing preload influences gearbox noise, but it is not possible to make any general statement as to whether preload is better than endplay. The answer depends on the frequency and other components in the complex dynamic system of gears, shafts, bearings, and housing. To minimize noise, the gearbox housing should be as rigid as possible.
This was proposed by Rook [8], and his views are supported by the results relating to the optimization of a transmission housing described in section 2.5. Finite element analysis is a useful tool for optimizing gearbox housings. 5 FUTURE RESEARCH It would be interesting to investigate the correlation between dynamic transmission error and gearbox noise for a complete wheel loader transmission. One challenge would be to measure transmission error as close as possible to the gears and to avoid resonances in the connection between gear and encoder. The dropbox gears in a typical wheel loader transmission are probably the gears that are most easily accessible for measurement using optical encoders. See Figure 5.1.1 for possible encoder positions. Modeling the transmission in more detail could be another challenge for future work. One approach could be to use a model of gears, shafts, and bearings using the transmission error as the excitation. This could be a finite element model or a multibody system model. The output from this model would be the forces at the bearing positions. The forces could be used to excite a finite element model of the housing. The housing model could be used to predict noise radiation, and/or vibration at the attachment points for the gearbox. This approach would give absolute values, not just relative levels. REFERENCES [1] Welbourn D. B., “Fundamental Knowledge of Gear Noise --A Survey”, Proc. Noise & Vib. of Eng. and Trans., I Mech E., Cranfield, UK, July 1979, pp 9–14. [2] MackAldener M., “Tooth Interior Fatigue Fracture & Robustness of Gears”, Royal Institute of Technology, Doctoral Thesis, ISSN 1400-1179, Stockholm, 2001. [3] Ohio State University, LDP Load Distribution Program, Version 2.2.0, http://www.gearlab.org/ , 2007. [4] Borner J., and Houser D. R., “Friction and Bending Moments as Gear Noise Excitations”,SAE Technical Paper 961816. [5] Oswald F. B. et al., “Influence of Gear Design on Gearbox Radiated Noise”, Gear Technology, pp 10–15, 1998. [6] Sellgren U., and -kerblom M., “A Model-Based Design Study of Gearbox Induced Noise”, International Design Conference – Design 2004, May 18-21, Dubrovnik, 2004. [7] Henriksson M., “Analysis of Dynamic Transmission Error and Noise from a Two-stage Gearbox”, Licentiate Thesis, TRITA-AVE-2005:34 / ISSN-1651-7660, Stockholm, 2005. [8] Rook T., “Vibratory Power Flow Through Joints and Bearings with Application to Structural Elements and Gearboxes”, Doctoral Thesis, Ohio State University, 1995.