Eddy current displacement sensor installation method and precautions
In the industrial production, when the eddy current displacement sensor measuring device is used, since it is a short-time temporary measurement, it is often not paid attention to the correct installation of the eddy current displacement sensor, causing significant test error, causing trouble for vibration fault diagnosis and shafting balance. . In order to improve the correctness and reliability of the equipment vibration test. The following is the correct installation method and key points of the eddy current displacement sensor introduced by Shensi Measurement and Control.
The installation of the eddy current displacement sensor should pay attention to the following points:
1. Avoid resonance and looseness of the structural bracket
The sensor's bracket must be at a higher frequency than the highest speed of the device. Otherwise, the measurement result will be distorted due to the resonance of the bracket. The CTC factory in the United States stipulates that the self-vibration frequency of the eddy current displacement sensor bracket in the direction of vibration measurement should be higher than the maximum working frequency of 10 times of the machine. This is often difficult to achieve in actual operation. Generally, the vibration frequency of the bracket is higher than 2~. The 3 times speed working frequency can basically meet the vibration measurement requirements.
In order to increase the natural vibration frequency, the structural bracket is generally made of flat steel of 6~8mm thickness, and the length of the cantilever should not exceed 100mm; when the cantilever is long, steel should be used, such as angle iron, I-beam, etc., in order to be effective Improve the natural frequency of the bracket. In order to prevent the bracket or eddy current displacement sensor from loosening during the test, the bracket must be fastened to the stable support member, preferably fixed to the bearing shell or the bearing seat. The connection between the eddy current displacement sensor and the bracket should be tapped on the bracket. Then tighten with a nut. Do not use a double nut to tighten the hole on the bracket.
2. Avoid cross sensing and lateral clearance
When two vertical or parallel mounted eddy current displacement sensors are close to each other, cross-induction between them causes the sensor output sensitivity to decrease. In order to avoid cross sensing, the two sensors cannot be placed too close together. For different types of eddy current displacement sensors, the values of the distance between the two sensors are required to be different. The too small lateral gap is mainly due to the presence of conductors on both sides of the sensor head, which significantly reduces the sensor output sensitivity. The correct lateral gap b should be greater than or equal to d (sensor top coil diameter). The direction finding gap must consider not only the cold state, but also the expansion of the cylinder and rotor after heating. The exposed height of the sensor head is generally not specified, but depending on the field, if the c is too small, the sensitivity of the sensor will be significantly reduced.
3, the correct initial gap
Various types of eddy current displacement sensors should have a good linearity at a certain gap voltage (the gap between the top of the sensor and the measured object, indicating the general voltage on the meter), so the eddy current is installed. The appropriate initial clearance must be adjusted for the displacement sensor. The static maximum range of the electric eddy current sensor can't be greater than 2.5mm. In order to obtain better linearity under dynamic conditions, the working gap should be in the range of 0.3~2.8mm, that is, the gap voltage indicated by the meter is 2~16V.
After the rotor rotates and the equipment is loaded, the rotor will shift relative to the sensor. If the eddy current displacement sensor is mounted on the top of the bearing, the clearance will be reduced; if it is installed in the horizontal direction of the bearing, the clearance depends on the direction of rotation of the rotor; when the direction of rotation is constant, the clearance depends on whether it is mounted 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. The journal elevation is about one-half of the bearing head clearance; horizontal displacement and bearing form, both sides of the bearing bush and unit sliding pin 0.20mm. The sensor is mounted in the horizontal position on the right side. After the rotor rotates, the gap c increases. On the left side, d decreases.
The position of the journal in the bearing bush is related to the speed and is also related to the active load of the equipment. For the low-pressure turbine high-pressure rotor and the shaft with the reducer, under the action of part of the intake steam and the gear transmission torque, the journal will be pushed to one side of the bearing bush, and the displacement value may be close to the diameter gap of the bearing bush. When adjusting the initial gap of the sensor, it is necessary to consider the above factors, and also consider the maximum vibration value and the original rotor yaw value. The initial gap of the sensor should be greater than one-half of the maximum amplitude of the shaft and the original yaw of the shaft.
4, axial position selection
From the requirement of measuring the shaft vibration, the shaft vibration measuring point should be as close as possible to the center of the bearing bush, but it is often limited by the installation position of the eddy current eddy current displacement sensor, and sometimes has to maintain a certain distance from the bearing bush. From the field vibration test, the shaft amplitude value will increase as the distance between the shaft and the bearing pad increases. Another consideration of the axial position of the shaft vibration point is the machining accuracy of the reaming shaft and the uniformity of the magnetic field on the shaft surface. Before the official installation, it is best to use a dial indicator to check the sway value of the point.
If the swaying value is greater than 50u; m, the measuring point should be selected separately, otherwise the oscillating value of the lower shaft will be too large due to the low speed, so that the high-speed lower shaft vibration display value will not be realized. The surface of the shaft is not uniformly polarized, and it cannot be directly judged by the naked eye. Only after the swing value of the shaft is too large is caused by the excessive mechanical sway of the shaft, it can be determined that the vibration of the shaft under the low speed is too large. Caused by uneven magnetic permeability, to eliminate this kind of fault, only the location of the measuring point is selected.
5, radial position selection
According to the requirements of ISO DIS7919/2, the shaft vibration sensor should be installed so that the two bearing sensors are in an axial plane and perpendicular to each other.