Change mechanism of vibration and noise characteristics before and after cavitation in hydraulic retarder

Abstract: In the hydraulic braking, the hydraulic reducer is often used in conjunction with other braking methods, which are used in the vehicle auxiliary brake. It is not transmission component, but the energy-saving component which works in the braking conditions. The use of hydraulic reducer in vehicles can effectively control the driving speed, ensure traffic safety, and increase the stability of the vehicle operation and comfort of driving. At braking of high speed device or large rotational inertia device, the inside of hydraulic reducer will cause cavitation, and the braking performance is reduced with temperature increment. The cavitation of hydraulic system can cause vibrating and produce lots of noise. In order to study the vibration and noise characteristics before and after cavitation in hydraulic retarder, a cavitation and vibration noise test bench was built based on the INV3020 data collection system and the high-speed photography system, which realized the synchronous acquisition for the performance parameters and vibration noise signals. The hydraulic reducer model was made of plexiglass material for the convenience of high speed photography. Firstly, by adjusting the opening of inlet and outlet valve of hydraulic retarder, the internal working pressure of the hydraulic reducer is controlled under several different conditions. The frequency converter is regulated to adjust the motor operating voltage, so that the pump speed of the impeller can be stable in several operation conditions. Combined with high-speed photography experiment, the accurate experiment condition was obtained for cavitation nascent stage, and the vibration and noise signals were measured before and after the cavitation in hydraulic retarder by using acceleration sensor and sound pressure sensor. The experimental results show that, the entire cavitation process can be divided into non-cavitation stage, nascent cavitation stage and serious cavitation stage based on the high-speed photography results. At nascent cavitation stage, the bubble first appeares at the back of the impeller close to the outer edge when the rotation speed of the impeller equals to 1 100 r/min and the inlet pressure equals to 0.04 MPa. As the pressure continues to decrease, the flow channel is gradually occupied by the bubbles, and it goes into serious cavitation stage. The vibration mainly occurs in radial direction of monitoring point, while there is no difference of vibration intensity between the 2 monitoring points. At nascent cavitation stage, the amplitude of vibration acceleration increases sharply, and with the degree of cavitation is further intensified, the amplitude of vibration acceleration is reduced. The change of the cavitation acceleration level of 2 000-3 000 Hz and 4 000-5 000 Hz bands before and after cavitation is obvious, which can be used as the characteristic frequency band of hydraulic reducer. For noise signal, the total sound pressure level in nascent cavitation stage is larger than that in severe cavitation stage, and the minimum value appears in the non-cavitation stage. The maximum value of noise signal is concentrated on blade passing frequency and its harmonic frequencies, which indicates that static and horizontal interference between the rotating pump wheel and the stationary shell in the low frequency band is still the main cause to the noise of the hydraulic reducer. In the 0-100 Hz low frequency band, there is no obvious relationship between sound pressure level and cavitation stages. In the 100-1 000 Hz middle frequency band, compared with serious cavitation stage and non-cavitation stage, the noise signal in nascent cavitation stage is slightly increased. Because the proportion of cavitation-induced vibration and noise components increases significantly at this stage, and the proportion of flow induced vibration noise is relatively reduced. In the high frequency range of 1 000-5 000 Hz, as the proportion of radiation noise signal of bubble burst increases, the serious cavitation stage shows the maximum amplitude of acoustic pressure value, the nascent cavitation stage takes the second place, and the non-cavitation stage shows the minimum value. The study could provide a reference for the study of cavitation vibration and noise of hydraulic retarder and the design method of it.