Noise sources identification and noise reduction methods of hammer mill

Abstract: In order to solve the problem of big noise during the working process of the hammer mill, and search for the main noise source and noise reduction method of hammer mill, the noise signals were collected and analyzed by the virtual instrument software LabVIEW and bench test on the hammer mill. The sound pressure level of the hammer mill was tested and analyzed in no-load and load conditions, and the spectrum of the hammer mill was tested. The result of this test shows the no-load sound pressure level of the hammer mill is bigger than the load sound pressure level. The material loading or not only changes the size of the peak of the noise. Thus it can be seen the key of absorbing the noise is to reduce the no-load aeromechanic noise. The noise sources of hammer mill were superimposed, bringing about the multivariate source coupling problem. The distribution operation method was used to test the noise of the hammer, screen, feeding inlet and feeding outlet. Sound pressure level and frequency spectrum were analyzed to find the main noise sources of the hammer mill, according to the main factors of noise such as the hammers, screen, feeding inlet and feeding outlet. Noise pressure level was tested and spectrum was analyzed in the state of noise with a single factor and combination of multiple factors. The results are as follows: The noise signals contain 47, 180, 200, 361, 893 and 1263 Hz. The screen, feeding inlet and outlet have no effect on the main frequency component. The screen has the noise reduction function; the feeding inlet and outlet enhance the noise pressure in different degrees. The sound pressure level of the single rotor is very small, which reveals that the hammer has a great influence on the noise. The noise is strengthened when the hammer and the outlet work together; the noise of hammer mill is greatly reduced when it has no hammer piece. Based on the analysis of no-load noise spectrum of 2400-2800 r/min, when the speed increases, the noise amplitude increases dramatically, indicating that the effect of noise on high-speed grinder is evident. When the speed is 2400-2800 r/min, the main noise source is in good agreement with the noise base frequency and the second-harmonic frequency of the hammer mill; when the speed is lower than 2600 r/min, the broadband signal energy to noise frequency signal of grinder is larger, indicating mill noise of vortex at low speed is dominant; when the speed is higher than 2600 r/min, the discrete signal energy to noise frequency signal of hammer mill is larger, that is to say the rotating noise is dominant at high speed. In the working condition of no load and rated speed, the noise sound pressure level of the hammer mill was tested with no hammer, single row hammer, 2 row hammers, 3 row hammers and 4 row hammers respectively. It is known that the number of hammers only affects the amplitude of the noise and has little influence on the change of the main frequency by analyzing the spectrum diagram of the no-load noise with different numbers of hammer pieces. Besides, the structure of feeding inlet, outlet and screen was improved in design. Taking the outlet improvement design as an example, turbulent kinetic energy of original and improved separation device was simulated. The comparison results show that the kinetic energy of the improved outlet is less. It is proved that the improvement of the outlet is beneficial to the noise reduction in the experiment, the total sound pressure level was reduced by 3 dB(A), the sound pressure levels of test points were reduced by 1.9-3.6 dB(A), and the hammer mill production was not greatly affected. Some other methods of noise reduction were proposed. This research provides theoretical evidence for the noise reduction design of the hammer mill.