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NVH Measurement

<img src="/ueditor/php/upload/image/20260131/1769821142708174.png" title="1769821142708174.png" alt="1.png"/>The NVH testing setup is depicted in Figure 6. A Dytran 3055D1T accelerometer (Dytran Inc., USA) was affixed to the bearing housing adjacent to the drive gear to capture vibrational data, while a PCB Piezoelectronics 378B02 free-field microphone (PCB Piezoelectronics Inc., USA) was positioned 50 mm in front of the meshing gear pair to record airborne noise emissions. Data acquisition was carried out using the SIRIUSm data acquisition module from Dewesoft (Dewesoft d.o.o., Slovenia). NVH measurements for polymer gear pairs were conducted after 105 load cycles, a point at which the system was presumed to have reached thermal and mechanical steady-state conditions, with wear effects considered negligible in terms of influencing NVH characteristics. Acoustic and vibrational signals were recorded over a 10-second interval, employing a sampling frequency of 20 kHz. Representative signal traces are shown in Figure 7. For steel gear pairs, measurement was performed following a brief stabilization period, sufficient for torque and rotational speed to reach steady state, given the shorter operational duration of these tests. Three principal quantitative metrics are typically extracted from the acquired vibration and acoustic signals: the peak value (maximum instantaneous amplitude), the peak-to-peak value (the range between maximum and minimum amplitudes), and the root mean square (RMS) value. These parameters are illustrated in Figure 7. The peak and peak-to-peak values reflect singular extrema within the signal and are particularly sensitive to transient phenomena or high-amplitude anomalies. As such, these metrics may be disproportionately influenced by isolated impact events or momentary disturbances, which do not necessarily reflect the sustained dynamic behavior of the system. In contrast, the RMS (Root Mean Square) value provides a robust representation of the signal’s effective energy content over the entire sampling period.From an applicable standpoint, the RMS value serves as a quantitative measure of the system’s vibratory energy. In contrast to the peak and peak-to-peak metrics—which capture instantaneous amplitude extremes and are susceptible to transient events—the RMS value offers a time-averaged representation of the total energy contained within the signal, thereby providing a more comprehensive and stable indicator of vibrational intensity. The measured sound pressure was converted to sound pressure level (SPL), expressed in decibels (dB), which represents a logarithmic scale of the sound pressure relative to a standardized reference pressure of 20 μPa—commonly recognized as the threshold of human hearing. This threshold corresponds to the quietest sound perceptible to the average young, healthy individual under ideal conditions.Results and Discussion Effect of the Material Pair and Operating Conditions The mean RMS values of the sound pressure levels are presented in Figure 8, while the corresponding RMS values of the measured vibrational signals are shown in Figure 9. These results represent the arithmetic average of three independent test repetitions conducted under each operating condition, with error bands denoting one standard deviation to reflect variability in the measurements. For the benchmark steel gear pair, a relatively linear increase in sound pressure level was observed with rising torque and rotational speed. In contrast, the polymer gear pairs demonstrated a more complex and nonlinear acoustic response to changes in operating parameters. Notably, the steel gear pair consistently produced sound pressure levels approximately 10 dB higher than those of the noisiest polymer gear combination. Within the polymer gear group, a sound pressure level differential of approximately 10 dB was recorded between the highest and lowest performing material combinations at the lowest tested rotational speed. As rotational speed increased, a corresponding rise in sound pressure levels was observed. However, at the highest rotational speed, the variation in noise emissions across the polymer gear combinations diminished significantly. It is important to highlight that a sound pressure level difference of approximately 3 dB is typically considered the minimum perceptible threshold for the average human listener. Differences below this value, particularly at absolute levels exceeding 100 dB, are generally imperceptible under normal auditory conditions.It is important to underscore that all gear pairs were evaluated within an acoustically isolated chamber, ensuring that the recorded acoustic signals originated solely from the gear pair under test. In practical gearbox applications, however, the dominant source of radiated noise is typically the gearbox housing. This noise arises primarily from structural vibrations induced by the meshing gears, which are transmitted to the housing through the shafts and bearing interfaces. The RMS vibration values measured for the steel gear pairs were, in certain cases, more than double those observed for the polymer gear combinations. This substantial increase in vibratory energy suggests that gearboxes employing exclusively steel gears are inherently predisposed to higher acoustic emissions, due to more pronounced excitation of the housing structure.All material pairings were evaluated under identical operating conditions, including fixed torque, rotational speed, and actively controlled gear temperature. Nevertheless, the dynamic (modal) characteristics of the gear pairs varied due to differences in material density and stiffness. Consequently, the natural frequencies (eigenfrequencies) of each gear configuration were located at distinct positions within the frequency spectrum. For instance, the steel–steel gear pair exhibited eigenfrequencies that differed not only from those of the polymer–polymer pairs but also among the various polymer–polymer combinations themselves. When a mechanical structure is excited at or near one of its eigenfrequencies, it responds with significantly amplified vibration amplitudes—ideally exponentially approaching resonance. As a result, a specific material combination may exhibit elevated vibrational levels at one operating speed while demonstrating reduced response at another, depending on the excitation frequency relative to its modal properties. This phenomenon has practical relevance, as real-world gearboxes frequently encounter fluctuating loads and speeds. To verify that the observed NVH responses originated specifically from gear meshing and were not influenced by ancillary noise sources such as motor emissions, bearing noise, or environmental interference, fast Fourier transform (FFT) analysis was applied to both the vibration and acoustic signal datasets. It was critical to confirm that the dominant spectral peak in all analyzed signals corresponded to the gear meshing frequency. As illustrated in Figure 10, this criterion was met, thereby validating that the recorded NVH characteristics were attributable solely to the dynamic interaction of the meshing gear pair.The superior NVH performance of steel–polymer and polymer–polymer gear pairs arises from a combination of physical and material-specific properties. First, polymers have a lower elastic modulus than metals, which reduces contact stiffness during meshing. This softer interaction lessens impact-induced excitation and enables smoother load transmission, thereby lowering the intensity of impulsive noise. Second, polymers possess much higher material damping than metals, allowing vibrational energy to dissipate as heat rather than propagate through the system. Their lower mass and density further decrease inertial forces, which helps minimize excitation amplitudes and structure-borne noise. In addition, the conformability of polymer gear teeth enhances load distribution, reduces localized stress concentrations, and limits micromechanical impacts, all of which contribute to quieter operation. Finally, favorable tribological characteristics—such as reduced stick-slip tendency due to lower friction coefficients—help suppress high-frequency vibrations and tonal noise components. Collectively, these factors lead to a marked reduction in both sound pressure levels and structural vibrations.

29 Jan,2026

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