What should I pay attention to when installing an eddy current displacement sensor?
“When using eddy current displacement sensor measurement equipment in industrial production, because it is a short-term temporary measurement, the correct installation of the eddy current displacement sensor is often not paid attention to, which causes significant test errors and brings trouble to vibration fault diagnosis and shafting balance. . In order to improve the accuracy and reliability of equipment vibration testing. The following is a brief introduction to the correct installation method and key points of the eddy current displacement sensor.
When using eddy current displacement sensor measurement equipment in industrial production, because it is a short-term temporary measurement, the correct installation of the eddy current displacement sensor is often not paid attention to, which causes significant test errors and brings trouble to vibration fault diagnosis and shafting balance. . In order to improve the accuracy and reliability of equipment vibration testing. The following is a brief introduction to the correct installation method and key points of the eddy current displacement sensor.
The installation of eddy current displacement sensor should pay attention to the following points:
1. Avoid resonance and loosening of structural supports
The vibration measurement frequency of the bracket of the sensor must be higher than the frequency corresponding to the maximum speed of the device, otherwise the measurement result will be distorted due to the resonance of the bracket. The American CTC factory stipulates that the natural vibration frequency of the eddy current displacement sensor bracket in the vibration measurement direction should be 10 times higher than the highest operating frequency of the machine, which is often difficult to achieve in actual operation. Generally, the natural vibration frequency of the bracket vibration measurement direction is higher than 2~ 3 times the working frequency of the rotational speed can basically meet the vibration measurement requirements.
In order to increase the natural vibration frequency, the structural support is generally made of flat steel with a thickness of 6~8mm, and the length of its cantilever should not exceed 100mm; when the cantilever is long, section steel, such as angle iron, I-beam, etc., should be used to effectively increase the natural vibration frequency of the bracket. In order to prevent the bracket or the eddy current displacement sensor from loosening during the test, the bracket must be fastened on the supporting part with good stability, preferably fixed on the bearing bush or bearing seat. The connection between the eddy current displacement sensor and the bracket should be tapping on the bracket. , and then tighten the nut, do not use the double nut to tighten the hole on the bracket.
2. Avoid cross-sensing and side clearance
When two eddy current displacement sensors installed vertically or in parallel are close to each other, cross-induction occurs between them, which will reduce the sensitivity of the sensor output. To avoid cross-sensing, the two sensors cannot be placed too close together. For different types of eddy-current displacement sensors, the required distance between the two sensors is different. Too small side clearance is mainly due to the presence of conductors on both sides of the sensor head, which significantly reduces the output sensitivity of the sensor. The correct side clearance b should be greater than or equal to d (sensor top coil diameter). The direction finding gap should not only consider the cold state, but also the expansion changes of the cylinder and rotor after heating. The exposed height c of the sensor head is generally not specified, but according to the field use, if c is too small, the sensitivity of the sensor will be significantly reduced.
3. Correct initial clearance
Various types of eddy current displacement sensors should have a certain gap voltage (the gap between the top of the sensor and the object to be measured, the general voltage is indicated on the meter), and the readings have good linearity, so the eddy current displacement sensor is installed. When displacing the sensor, an appropriate initial gap must be adjusted. The static maximum range of the eddy current sensor cannot be greater than 2.5mm. In order to obtain better linearity under dynamic conditions, its working gap should be within the range of 0.3~2.8mm, that is, the gap voltage indicated by the instrument is 2~16V.
After the rotor rotates and the equipment is loaded, the rotor will be displaced relative to the sensor. If the eddy current displacement sensor is installed on the top of the bearing, the gap will be reduced; if installed in the horizontal direction of the bearing, the gap depends on the direction of rotation of the rotor; when the direction of rotation is constant, the gap depends on whether it is installed on the right or left side . In order to obtain a suitable working clearance value, the rotor should be estimated from static to working speed during installation, and the journal lift should be about one-half of the top clearance of the bearing; 0.20mm. When the sensor is installed in the horizontal position on the right side, after the rotor rotates, the gap c increases; when it is installed on the left side, d decreases.
The displacement of the journal in the bearing bush is not only related to the speed, but also related to the active load of the equipment. For the low-mass turbine high-pressure rotor and the shaft with reducer, under the action of part of the steam inlet and gear transmission torque, the journal vertebra will be pushed to one side of the bearing bush, and the displacement value may be close to the diameter clearance of the bearing bush. When adjusting the initial gap of the sensor, in addition to the above factors, the maximum vibration value and the original sway value of the rotor should also be considered. The initial gap of the sensor should be greater than half of the maximum amplitude that the rotating shaft can produce and the original swing value of the rotating shaft.
4. Axial position selection
From the requirements of measuring shaft vibration, the shaft vibration measuring point should be as close to the center of the bearing bush as possible, but it is often limited by the installation position of the eddy current displacement sensor, and sometimes it has to maintain a certain distance from the bearing bush. It is obtained from the field vibration test that with the increase of the distance between the axonometric vibration point and the bearing bush, the shaft amplitude value will increase. Another consideration for the axial position of the axonometric vibration point is the machining accuracy of the rotating shaft and whether the magnetic permeability of the rotating shaft surface is uniform. Generally, before the formal installation, it is best to use a dial indicator to check the swing value of this point.
If the sway value is greater than 50u;m, another measuring point should be selected, otherwise the shaft sway value at low speed will be too large, and the displayed value of shaft vibration at high speed will appear unrealistic. The magnetic conductivity of the surface of the rotating shaft is not uniform, and it cannot be directly judged with the naked eye. Only after the excessive swing value of the rotating shaft is sent to be caused by the excessive mechanical swing of its surface can it be determined that the excessive vibration of the rotating shaft at low speed is caused by the surface of the rotating shaft. Caused by uneven magnetic conductivity, to eliminate this fault, only another measuring point location is selected.
5. Radial position selection
According to the requirements of the ISO DIS7919/2 specification, the installation of the shaft vibration sensor should satisfy that the two bearing sensors are on an axial plane and are perpendicular to each other.
Shenzhen Shensi Measurement and Control Technology Co., Ltd. is a high-tech enterprise specializing in sensor technology research and development, design, manufacturing and sales. The company’s main products are LVDT displacement sensor, eddy current displacement sensor, non-contact displacement sensor, contact displacement sensor, pull-rope displacement sensor, linear displacement sensor, high-precision displacement sensor, differential displacement sensor, resistive displacement sensor , Magnetostrictive liquid level sensor, capacitive liquid level sensor, inductive probe, etc. Products are widely used in machinery manufacturing, automobile, metallurgy, petroleum, chemical industry, scientific research institutes, aerospace, transportation construction and other fields.