Created on:2021-06-11 10:27

Main parameters and common problems of hydraulic pump

Main parameters and common problems of hydraulic pump

1.6.7 vibration, noise and control

Vibration and noise are two common phenomena in the operation of hydraulic components including hydraulic pump. Vibration is the inherent characteristic of elastic material. Noise comes from vibration. The animal that causes noise is called sound source, so the control of noise comes down to the control of vibration.

With the high pressure, high speed and high power of hydraulic technology, vibration and noise have become prominent problems in the development of hydraulic technology. Because the vibration affects the working performance and service life of the main engine and the system, and the noise not only causes human hearing loss, but also distracts the operator's attention, and is more likely to submerge the alarm signal, causing personal and equipment accidents. Vibration and noise has become an important index to measure the performance of hydraulic pump.

(l) Theoretical analysis shows that the fundamental reason of the vibration is the size of the vibration element and the exciting force. The vibration can be measured by accelerometer instead of microphone in sound level meter. The main way to prevent, reduce and eliminate the vibration of hydraulic components and devices is to eliminate or reduce the excitation source (force), and reasonably design and match the inherent parameters of hydraulic components and devices.

(2) The generation, radiation and types of hydraulic noise can be seen from the following table. Hydraulic pump is the main noise source in all components of hydraulic system, which is called primary noise source. Other components, such as oil tank and pipeline, make little noise and are not independent noise sources. However, mechanical and liquid noise generated by pump and hydraulic valve will stimulate them to produce vibration, thus producing and radiating strong noise This kind of noise source is called secondary sound source. The noise of hydraulic system is the superposition of primary and secondary noise sources. Therefore, the vibration and noise control of hydraulic device should be considered from two aspects: component noise and device vibration noise. Obviously, reducing the noise of the hydraulic pump is the main way to control the noise of the whole hydraulic system.

 

The order of noise produced and radiated by hydraulic components

Component name

液压泵

液压阀

液压缸

过滤器

油箱

管路

溢流阀

节流阀

换向阀

产生噪声次序

1

2

3

4

5

6

7

5

传递辐射噪声次序

2

3

4

3

2

4

1

2

 

The noise produced by hydraulic components and systems mainly includes mechanical noise and fluid noise. For hydraulic pumps, mechanical The noise includes the noise caused by the bearing vibration, the mechanical collision noise caused by the collision between the parts in the pump, the noise caused by the poor lubrication and friction between the surfaces of the relative moving parts, the noise caused by the vibration of the whole hydraulic pump as a mass spring system, and so on; the fluid noise includes the cavitation noise caused by the oil suction cavity, the vortex separation noise caused by the change of the flow channel, and the load The sound of pressure shock caused by sudden change or trapped oil, the sound of pressure pulsation caused by flow pulsation, etc.

(3) The description and allowable standard of noise are used as the physical measurement of noise. The sound pressure level LP (DB) is one of the commonly used physical quantities in industry, which is used to describe the size or intensity of noise.

Lp=20lg(p/p0) (dB) (1-17)

Where P -- actual sound pressure, PA;

P0 -- the reference sound pressure (also called threshold sound pressure), P0 = 2 × 10-5pa.

Noise control is an important part of environmental protection and one of the quality evaluation indexes of hydraulic products. The noise health standard for industrial enterprises, which was published and implemented in China in 1980, is based on A-level (noise measured by A-weighted network in sound level meter). The allowable noise value of hydraulic pump in China is in JB / T 7041-2006_ JB / T 7039-2006 and JB / T 7042-2006 stipulate: for example, under the rated pressure and speed, the noise value of gear pump with rated pressure of 10-25mpa and displacement of more than 25-500ml / R shall be ≤ 85dB (a); the noise value of fixed vane pump with rated pressure of 16-25mpa and displacement of more than 50-63ml / R shall be ≤ 78dB (a). The noise value of swash plate axial piston pump with displacement > 25 ~ 63ml / R should be ≤ 85 dB (a).

(4) Noise measurement in order to analyze the noise of hydraulic pump, analyze the noise source and take appropriate control measures, it is necessary to measure the noise of hydraulic pump.

① The commonly used noise testing instruments include sound level meter, frequency analyzer and recording instrument. Sound level meter is a kind of noise measuring instrument which is widely used and suitable for the field. It can measure not only the sound pressure level and sound level of noise, but also the frequency analysis by filter, and use accelerometer instead of microphone to measure vibration. According to the measurement accuracy and application, the sound level meter can be divided into three types: ordinary type, precision type and pulse precision type. The noise measurement of hydraulic components and devices usually adopts the precise sound level meter. According to the display and reading methods, the sound level meter can be divided into pointer type and digital type (see Figure r for the appearance). The operation method and precautions of the sound level meter can refer to the product manual.

② It is ideal to measure the noise of testing environment and position in the anechoic room of free sound field built by human. The anechoic room should have good sound absorption conditions and no reflected sound. Except for the tested components, other devices should be set outside it to avoid influence. Therefore, the anechoic chamber for noise measurement is specially designed (Guangzhou Institute of mechanical science built the anechoic chamber in 1980s). However, in engineering practice, we often do not have the conditions of this kind of anechoic room, so we need to carry out the measurement in the general laboratory or workplace. At this time, in order to make the measurement results have enough accuracy, we should avoid the influence of other sound interference and sound reflection.

The distribution characteristics of sound field should be paid attention to when selecting the test location. In the shadow area of figure s, the sound pressure level will fluctuate with the change of measurement distance r, so it is not suitable to measure. Therefore, the measuring point should be selected in the far-field area of the free sound field (the sound field with negligible boundary effect) as far as possible. The characteristics of the measurement in this area are that the data is stable and reliable, and the noise will be reduced by 6dB (a) for every double of the distance R. therefore, the far-field area of the free sound field can be roughly found by the sound level meter.

1RS.jpg

The specific method of measuring point location selection is as follows.

a. It is 1.5m away from the surface of the tested device and 1.5m away from the ground. If the noise source size is small (e.g. less than 0.25m), the measuring point should be close to the surface of the device under test (e.g. 0.5m). It should be noted that the distance between the measuring point and the indoor reflecting surface should be more than 2-3m. Try to make the microphone face the geometric center of the device under test.

b. The measuring points should be evenly distributed around the measured surface, generally no less than 4 points. If the sound level difference between adjacent points is more than 5dB (a), additional measuring points shall be added between them. The arithmetic mean value of noise level of each measuring point shall be taken. The difference between the sound level calculated by this method and that calculated by energy average method shall not be greater than 7dB (a).

c. If the distance between two noise sources is close (such as hydraulic pump and its driving motor), the measuring point should be close to the measured noise source (0.2m or 0.1M).

d. If you need to know the harm of noise source to human body, you can choose the measuring point at the ear of the operator's position, or within the range of the operator's regular activities and work, and select several measuring points based on the height of the ear.

③ When testing, we should pay attention to eliminate and reduce the impact of the environment.

a. The influence of power supply, airflow, reflection, etc. If the power supply voltage of the instrument is unstable, the voltage regulator should be used; if the voltage is insufficient, the voltage regulator should be replaced. Outdoor measurement should be conducted in calm weather. When the wind speed exceeds level 4, the microphone can be covered with a wind shield or wrapped with a layer of silk cloth. The microphone should avoid air outlet and air flow. The reflectors in the measurement site should be excluded as far as possible. If not, the microphone should be placed at an appropriate position between the noise source and the reflectors, and try to keep away from the reflectors. For example, it is better to be more than 1m away from the wall and ground. When measuring noise, the microphone should keep the same incident direction at all measuring points.

b. Correction of background noise (background noise). Background noise refers to the ambient noise when the measured noise source stops making sound. The background noise should be more than 10dB (a) lower than the measured synthetic noise, otherwise it should be corrected, that is, the background noise value al should be deducted from the measured noise in the following table.

 

Correction value dB (a) with background noise

合成噪声和背景噪声级差

1

2

3

4

5

6

7

8

9

10

修正值△L

3.9

4.4

3.0

2.3

1.7

1.25

0.95

0.75

0.6

0.4

 

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