The noise of the piston vacuum pump is mainly caused by the impact, including the impact of the oil on the piston and the pump chamber wall, the impact between the exhaust valve plate and the valve seat (or pump body), the impact between the piston (ring) and the pump chamber wall. The impact between piston (rod) and guide rail is a new noise source recently discovered. The noise is not only related to the speed of pump, but also related to the transmission of sound.
Cavitation noise
Cavitation exists objectively in the piston vacuum pump. If there is cavitation, there will be noise. However, due to the low speed of the general piston vacuum pump, cavitation is not enough to damage the parts, so it has not been found. In the trial production of h150ab pump with speed up to 920 R / min, we found that the abnormal noise with high noise frequency and sound level was found, and the sound power was up to 100 DB, it was found that there were many shallow pinholes near the exhaust port on the piston (ring) after the disassembly and inspection. After it was removed and operated for several hours, the above phenomenon appeared again. Later, it was confirmed by many tests that this is cavitation. However, the operation time is short, and cavitation damage only appears on the surface.
Impact of exhaust valve
The exhaust valve should be closed quickly at the end of exhaust, otherwise it will increase the gas return flow and affect the vacuum degree and pumping speed, so when the exhaust valve is closed, the valve disc and valve seat (or pump body) will produce impact noise, which is related to the speed. In order to verify the noise, we used wool felt as valve plate to reduce the impact and noise. Of course, wool felt can’t be used as valve piece for a long time. After several hours of impact, its texture gradually becomes strong and its noise gradually rings.
Fluid impact
There are two kinds of impact of oil. First, the oil entering the pump cavity, driven by the piston, rushes to the exhaust port at a high speed and collides with the pump cavity wall and the exhaust passage. At the end of the exhaust, when the exhaust valve is closed, the compression chamber becomes a vacuum chamber. The oil at the exhaust port returns to the vacuum chamber at a high speed and collides with the piston and the passage. These two kinds of noises caused by the impact of oil are related to the vacuum degree, rotating speed, oil input, channel shape and exhaust speed of the pump.
The pump has high vacuum, high rotating speed, large oil intake, large flow resistance, high exhaust speed and high noise. The noise can be reduced obviously by properly controlling the oil quantity, improving the shape of the exhaust passage and reducing the exhaust speed. The above factors are studied comprehensively in Schindler vacuum equipment plant. The improved design of a pump has achieved a significant effect of noise reduction (8 ~ 10) dB.
Impact between piston and guide rail
The impact noise of piston (rod) and guide rail has not been found for a long time. First, it is relatively low in noise, and more importantly, it is difficult to separate from the total noise of the pump, so it is easy to neglect. We found that the impact noise of piston (rod) and guide rail does exist, and the impact noise of split guide rail is (2 ~ 3) dB higher than that of the whole guide rail. By analyzing the causes, we think that there are two times of impact between the split guide rail and the piston (rod) in each revolution, and the impact noise is larger because of the slightly larger gap between them, while the impact of the whole guide rail in each revolution is only once, and the gap between them is smaller, and the impact noise is also smaller. The above noise in Schindler vacuum equipment factory is objective and cannot be eliminated, but it can be reduced by controlling the gap.
Impact between piston and pump chamber wall
When the vacuum pump is running, with the rotation of the eccentric wheel, the piston (ring) rolls along the pump cavity wall. After passing through the exhaust port to the junction of the vacuum pump cylinder and the guide rail hole, there is a gap in the vacuum pump cylinder. At this time, each rotation of the piston (ring) has 0.01 s Left and right time from the pump cavity wall, as the eccentric continues to rotate, the piston (ring) contacts the pump cavity wall at the inlet again. Due to the high speed, the mechanical impact will be more serious, and the noise will increase (5-7) DB generally. It can be found that there is an obvious impact trace on the piston (ring) near the air inlet of the piston (rod). In order to eliminate this noise, the gap between the piston (ring) and the cavity wall of the vacuum pump must be strictly controlled to keep it within a small range. The above clearance is related to the clearance between the eccentric and the inner circle of the piston (ring), and must be controlled accordingly.
Conduction and shielding of noise
No matter the impact of pump oil or mechanical impact, the noise is related to the speed of vacuum pump. With the development of the speed of vacuum pump to high speed, this problem is more prominent. Therefore, it is necessary to reduce the noise from the aspects of sound conduction and shielding.
Oil tank is the main part of sound conduction. Compared with steel tank, the noise of cast iron tank can be reduced by (2 ~ 3) dB. The main reason is that the structure of cast iron is slightly looser than that of steel plate, so it has certain sound absorption effect. Moreover, the cast iron oil tank is thicker than steel plate oil tank, and its sound insulation effect is better. For example, if a film is sprayed on the inner wall of the oil tank, the effect will be better. The structural shape of the inner surface of the oil tank shall be designed to be asymmetrical to avoid resonance sound.
The exhaust valve cover is connected with the oil-gas separator, which has the function of sound insulation and noise elimination. If the exhaust valve cover is removed, the noise of the pump will be much higher. If the oil-gas separator is designed properly, it will also have certain noise reduction effect. For example, the “secondary cyclone oil-gas separator” designed by us has two collisions through the oil-gas separator during the transmission process, resulting in a certain loss of sound energy, which is conducive to noise reduction.
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