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Experimental modal analysis was also used to find the natural frequencies of the real transmission housing and to ensure that the model did not deviate too much from the real housing.Gears shafts and bearings were modeled as point masses and beams. The model was excited at the bearing positions by applying forces in the frequency range from 1000 to 3000 Hz. The force amplitude was chosen as 10% of the static load from the gears. This choice could be justified because only relative differences are of interest, not absolute values. The finite element analysis was performed by Torbj-rn Johansen at Volvo Technology. The author’s contribution was the evaluation of the results of different housing geometries.A number of measuring points were chosen in areas with high vibration velocities. At each measuring point the vibration response due to the excitation was evaluated as a power spectral density (PSD) graph. The goal of the housing redesign was to decrease the vibrations at all measuring points in the frequency range 1000 to 3000 Hz. 2.6 Results of the noise measurements The noise and vibration measurements described in section 2.3 were performed after optimizing the gears and transmission housing.The total sound power level decreased by 4 dB. 2.7 Discussion and conclusions It seems to be possible to decrease the gear noise from a transmission by decreasing the static loaded transmission error and/or optimizing the housing. In the present study, it is impossible to say how much of the decrease is due to the gear optimization and how much to the housing optimization. Answering this question would have required at least one more noise measurement, but time and cost issues precluded this. It would also have been interesting to perform the noise measurements on a number of transmissions, both before and after optimizing the gears and housing, in order to determine the scatter of the noise of the transmissions. Even though the goal of decreasing the gear noise by 10 dB was not reached, the goal of reducing the gear noise in the wheel loader cab to 15 dB below the overall noise was achieved. Thus the noise optimization was successful. 3 SUMMARY OF APPENDED PAPERS 3.1 Paper A: Gear Noise and Vibration – A Literature Survey This paper presents an overview of the literature on gear noise and vibration. It is pided into three sections dealing with transmission error, dynamic models, and noise and vibration measurement. Transmission error is an important excitation mechanism for gear noise and vibration. It is defined as “the difference between the actual position of the output gear and the position it would occupy if the gear drive were perfectly conjugate” [1]. The literature survey revealed that while most authors agree that transmission error is an important excitation mechanism for gear noise and vibration, it is not the only one. Other possible time-varying noise excitation mechanisms include friction and bending moment. Noise produced by these mechanisms may be of the same order of magnitude as that produced by transmission error, at least in the case of gears with low transmission error [4]. The second section of the paper deals with dynamic modeling of gearboxes. Dynamic models are often used to predict gear-induced vibrations and investigate the effect of changes to the gears, shafts, bearings, and housing. The literature survey revealed that dynamic models of a system consisting of gears, shafts, bearings, and gearbox casing can be useful in understanding and predicting the dynamic behavior of a gearbox. For relatively simple gear systems, lumped parameter dynamic models with springs, masses, and viscous damping can be used. For more complex models that include such elements as the gearbox housing, finite element modeling is often used. The third section of the paper deals with noise and vibration measurement and signal analysis, which are used when experimentally investigating gear noise. The survey shows that these are useful tools in experimental investigation of gear noise because gears create noise at specific frequencies related to the number of teeth and the rotational speed of the gear. 3.2 Paper B: Gear Test Rig for Noise and Vibration Testing of Cylindrical Gears Paper B describes a test rig for noise testing of gears. The rig is of the recirculating power type and consists of two identical gearboxes, connected to each other with two universal joint shafts. Torque is applied by tilting one of the gearboxes around one of its axles. This tilting is made possible by bearings between the gearbox and the supporting brackets. A hydraulic cylinder creates the tilting force. Finite element analysis was used to predict the natural frequencies and mode shapes for inpidual components and for the complete gearbox. Experimental modal analysis was carried out on the gearbox housing, and the results showed that the FE predictions agree with the measured frequencies (error less than 10%). The FE model of the complete gearbox was also used in a harmonic response analysis. A sinusoidal force was applied in the gear mesh and the corresponding vibration amplitude at a point on the gearbox housing was predicted. 3.3 Paper C: A Study of Gear Noise and Vibration Paper C reports on an experimental investigation of the influence of gear finishing methods and gear deviations on gearbox noise and vibration. Test gears were manufactured using three different finishing methods and with different gear tooth modifications and deviations. Table3.3.1 gives an overview of the test gear pairs. The surface finishes and geometries of the gear tooth flanks were measured. Transmission error was measured using a single flank gear tester. LDP software from Ohio State University was used for transmission error computations. The test rig described in Paper B was used to measure gearbox noise and vibration for the different test gear pairs. The measurements showed that disassembly and reassembly of the gearbox with the same gear pair might change the levels of measured noise and vibration. The rebuild variation was sometimes of the same order of magnitude as the differences between different tested gear pairs, indicating that other factors besides the gears affect gear noise. In a study of the influence of gear design on noise, Oswald et al. [5] reported rebuild variations of the same order of magnitude. Different gear finishing methods produce different surface finishes and structures, as well as different geometries and deviations of the gear tooth flanks, all of which influence the transmission error and thus the noise level from a gearbox. Most of the experimental results can be explained in terms of measured and computed transmission error. The relationship between predicted peak-to-peak transmission error and measured noise at a torque level of 500 Nm is shown in Figure 3.3.1. There appears to be a strong correlation between computed transmission error and noise for all cases except gear pair K. However, this correlation breaks down in Figure 3.3.2, which shows the relationship between predicted peak to peak transmission error and measured noise at a torque level of 140 Nm.