Introduction: In industrial production, when using eddy current displacement sensors for measurement, due to the short-term, temporary nature of the measurements, the correct installation of the eddy current displacement sensors is often overlooked, leading to significant testing errors and causing problems for vibration fault diagnosis and shaft balancing. To improve the accuracy and reliability of equipment vibration testing, the following article from Shensi Measurement & Control introduces the correct installation method and key points of eddy current displacement sensors.
The following points should be noted when installing eddy current displacement sensors:
1. To avoid resonance and loosening of the structural support, the sensor support must be positioned at a frequency higher than the frequency corresponding to the highest rotational speed of the equipment. Otherwise, resonance of the support will distort the measurement results. The American CTC manufacturer specifies that the natural frequency of the eddy current displacement sensor support in the vibration measurement direction should be 10 times higher than the machine's highest operating frequency. This is often difficult to achieve in practice. Generally, a natural frequency of the support in the vibration measurement direction that is 2-3 times higher than the machine's operating frequency is sufficient to meet the vibration measurement requirements.
To improve the natural frequency, the structural support is generally made of 6-8mm thick flat steel, and its cantilever length should not exceed 100mm. When the cantilever is longer, shaped steel, such as angle iron or I-beams, should be used to effectively improve the natural frequency of the support. During testing, to prevent the support or eddy current displacement sensor from loosening, the support must be securely fastened to a stable supporting component, preferably fixed to a bearing bush or bearing seat. The connection between the eddy current displacement sensor and the support should be achieved by tapping threads on the support and then tightening with nuts; do not use a method of drilling holes in the support and tightening with double nuts.
2. Avoid Cross-Induction and Lateral Clearance: When two eddy current displacement sensors mounted vertically or parallel to each other are close together, cross-induction occurs, reducing the sensor's output sensitivity. To avoid cross-induction, the two sensors should not be too close. The required distance between the two sensors varies depending on the type of eddy current displacement sensor. Insufficient lateral clearance, primarily due to conductors on both sides of the sensor head, significantly reduces the sensor's output sensitivity. The correct lateral clearance b should be greater than or equal to d (the diameter of the coil at the top of the sensor). The lateral clearance must consider not only the cold state but also the expansion changes of the cylinder and rotor after heating. The exposed height c of the sensor head generally has no specific specification, but based on field experience, too small a c will also significantly reduce the sensor's sensitivity.
3. Correct Initial Gap: Various types of eddy current displacement sensors require a specific gap voltage (the gap between the top of the sensor and the object being measured, typically indicated by voltage on the instrument) to achieve good linearity. Therefore, a suitable initial gap must be adjusted during installation. The maximum static range of an eddy current sensor should not exceed 2.5 mm. To obtain better linearity under dynamic conditions, the working gap should be within the range of 0.3~2.8 mm, meaning the gap voltage indicated by the instrument should be 2~16V.
After the rotor rotates and the equipment is under load, the rotor will displace relative to the sensor. If the eddy current displacement sensor is mounted on top of the bearing, its clearance will decrease; if mounted horizontally on the bearing, its clearance depends on the rotor's rotation direction; when the rotation direction is constant, its clearance depends on whether it is mounted on the right or left side. To obtain a suitable working clearance value, the rotor's speed from static to operating should be estimated during installation, with the journal raised approximately half the bearing top clearance; the horizontal displacement depends on the bearing type, the clearance on both sides of the bearing, and the unit's sliding pin by 0.20 mm. When the sensor is mounted on the right side in a horizontal position, the clearance c increases after the rotor rotates; when mounted on the left side, d decreases.
The displacement of the journal within the bearing bush is related not only to the rotational speed but also to the active load of the equipment. For low-mass steam turbine high-pressure rotors and shafts with reducers, the combined effect of partial steam intake and gear-transmitted torque can push the journal cone to one side of the bearing bush, with the displacement potentially approaching the diameter clearance of the bearing bush. When adjusting the initial sensor clearance, in addition to considering the above factors, the maximum vibration value and the original rotor wobble value must also be taken into account. The initial sensor clearance should be greater than half of the maximum amplitude of the shaft's possible vibration and the original shaft wobble value.
4. Axial Position Selection: From the perspective of measuring shaft vibration, the measuring point should be as close as possible to the center of the bearing bush. However, this is often limited by the installation location of the eddy current displacement sensor, sometimes necessitating a certain distance from the bearing bush. Field vibration tests show that the shaft vibration amplitude increases with the distance between the measuring point and the bearing bush. Other factors to consider regarding the axial position of the measuring point include the machining accuracy of the shaft at that point and the uniformity of the magnetic conductivity on the shaft surface. Generally, before formal installation, it is best to use a dial indicator to check the wobble value at that point.
If the sway value exceeds 50 μm, a different measuring point should be selected. Otherwise, excessive sway at low speeds will cause inaccurate readings at high speeds. Uneven magnetic conductivity on the shaft surface cannot be directly determined by the naked eye. Only after confirming that excessive sway is caused by excessive mechanical sway on the shaft's surface can it be determined that excessive low-speed shaft vibration is caused by uneven magnetic conductivity on the shaft surface. Eliminating this fault requires selecting a different measuring point.
5. Radial position selection: In accordance with the requirements of ISO DIS 7919/2, the installation of the shaft vibration sensor should ensure that the two bearing sensors are on the same axial plane and perpendicular to each other.
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