1 ultrasonic testing (UT)

Ultrasonic testing is a kind of nondestructive testing technology. When ultrasonic waves enter an object and encounter defects, some sound waves will be reflected, and the receiver will receive and analyze the reflected waves, so as to accurately measure the defects and determine the position and size of the defects. Ultrasonic testing is suitable for detecting regional defects inside the detected object. The advantages of ultrasonic detection are strong penetration, light equipment, low detection cost, high detection efficiency, instant detection results and automatic detection, and it is particularly sensitive to harmful cracks and defects in defect detection.

2 Coal Mine Machinery Operation Status

Most of the machinery and equipment used in coal mines operate in harsh environments such as dust, humidity and harmful gases, and are often subjected to huge impact loads and are in a state of high-intensity operation for a long time. The alternating load generated by high-speed operation and heavy-duty working environment is very easy to amplify the internal defects of the material or the defects caused by the processing technology when the spindle is processed, and form dangerous cracks. There are also defects caused by improper operation of the driver, improper design and installation, improper forging of the spindle, etc. Fatigue crack is caused by the change of material strength and stiffness of the spindle itself during working. If these dangerous cracks are not found in time, it may lead to the sudden fracture of the mechanical spindle, causing major safety accidents and bringing unnecessary losses to the mine.

3 Coal mine needs to detect the mechanical spindle

The spindles to be detected are: main ventilator spindle, lifting roller spindle, pulley spindle, conveyor roller spindle, cage or skip lifting spindle, overhead passenger device driving wheel and bypass axle, etc. The main shaft is constrained or covered by structural parts assembled on the shaft, and the parts on the shaft are areas with concentrated stress, which are prone to surface or internal cracks. If other nondestructive testing methods are used for testing, these assembled parts need to be disassembled from the spindle, which not only wastes a lot of manpower, material resources and financial resources, but also directly affects the normal production of coal mines. In order to solve this problem and provide better conditions for the operation of coal mine machinery and equipment, it will be better to use ultrasonic flaw detection to detect the spindle without disassembly.

Ultrasonic testing technology of mechanical spindle

4. 1 Prepare

4. 1. 1 Grasp the actual situation of the spindle of the inspected machine. After the inspectors arrived at the inspection site, they first communicated with the mine and asked about the basic information of the spindle. According to the information provided, they can master the material, heat treatment state, geometry, size, assembly structure and quantity, stress state, on-site inspection conditions and environment of the spindle, thus providing conditions for ultrasonic inspection. Secondly, according to the information we have, we will work out a test plan with the mine.

4. 1.2 Selection of ultrasonic testing parts According to the transmission structure, stress state and stress concentration degree of the spindle, the variable diameter parts of the spindle coupling, the connecting parts between the roller and the spindle, the variable diameter parts of the spindle, the fixed end of the motor and the root of the keyway are selected as the key ultrasonic testing parts.

4. 1.3 Cleaning of ultrasonic flaw detection surface At the selected flaw detection parts, clean the pollutants with cotton yarn, and smooth the corroded parts with sandpaper.

4. 1.4 When selecting probes and standard test blocks for ultrasonic testing, the probes should be selected according to the grain size of the spindle material to be tested, and generally, 2.5MHz probes are selected for ultrasonic testing. According to the shape and length of the spindle to be inspected, the standard test blocks CS-I, CS-2C, CSK-Ⅲ A, CSK-Ⅱ A and RB-2 are selected as the sensitivity inspection of ultrasonic testing.

4. 1.5 instrument sensitivity adjustment The sensitivity of the detection instrument is realized by adjusting the knobs such as Gain, Attenuator and Emission Intensity on the ultrasonic flaw detector. The radial detection adopts the straight probe detection method, and the sensitivity adjustment of the straight probe includes the workpiece bottom wave adjustment method and the reference block method. When the radial spindle length S≤3N (near-field area), the block comparison method is adopted, and the spindle length s > 3n (near-field area) is adjusted by the large flat bottom bottom bottom wave adjustment method. The adjustment of the detection sensitivity of the tilt probe is to adjust the sensitivity of the detection system to level 2 or 3 by using CSK-IIA or RB-2 test blocks.

4. 1.6 Selection of coupling agent Engine oil, transformer oil, glycerol, water and sodium silicate are often used as coupling agents in ultrasonic testing.

4.2 ultrasonic testing method for spindle

There are two kinds of probes for spindle ultrasonic testing: straight probe and inclined probe. The straight probe mainly detects the exposed part of the spindle, and the inclined probe mainly detects the variable diameter part of the spindle coupling, the connecting part between the roller and the spindle, the variable diameter part between the spindle and the fixed end of the motor, the connecting part between the spindle and the fan blades and the root of the keyway.

4.2. 1 straight probe scanning

1) radial scanning: let miners open the end cover of the spindle with a wrench, coat the coupling agent on the end of the spindle, and put the longitudinal wave straight probe on the end face of the spindle for scanning, with the pressure of 0.5 ~ 1 kg and the speed of 20 ~ 50 mm/s, 100%. In the scanning process, the probe is used for "W" overlapping scanning. While the probe is scanning, the waveform changes of the instrument screen should be observed at any time and the displayed information should be judged one by one.

2) Circumferential scanning: Apply coupling agent to the exposed part of the spindle, and perform 100% circumferential omni-directional scanning with a straight probe at the same pressure and speed. While scanning with a straight probe, observe the waveform changes of the instrument screen at any time and judge the displayed information one by one.

4.2.2 Scan the reduced diameter part of the spindle coupling, the connection part between the roller and the spindle, the reduced diameter part between the spindle and the fixed end of the motor, the root of the keyway and other unexposed parts with an oblique probe. The scanning should be carried out along the radial direction of the spindle, with a pressure of 0.5 ~ 1 kg and a speed of 20 ~ 50 mm/s, 100%.

5 defect location, quantification and evaluation

5. 1 defect location

Defect location is to locate defects according to the horizontal scale value and scanning speed of defect echo on the oscilloscope screen of flaw detector. When longitudinal wave is detected by a straight probe, the distance from the defect to the detection surface can be directly obtained from the position of the defect wave on the horizontal scale of the instrument after the baseline scanning line of the instrument is adjusted according to the ratio of1￠ n. For example, when the time baseline is adjusted according to the sound path ratio of1~ 2, the spindle bottom wave should appear at the 10 grid. When the defect wave appears in 6 grids, the distance between the defect and the detection surface is 2× 60 =120 mm. When the shear wave oblique probe detects the main shaft, the position of the defect can be determined by the refraction angle (β) and the acoustic path X (polar coordinate system), or by the horizontal distance L and depth of the defect (rectangular coordinate system).

5.2 "Quantification of Defects"

The quantification of defects refers to determining the size, quantity, length and area of defects in testing. The accuracy of defect quantification is directly related to the success or failure of testing. Only by accurately determining the size of defects in mines can measures such as replacement or maintenance be taken in time to avoid major accidents and eliminate hidden dangers in time. At present, the quantitative method, equivalent method and length measurement method of spindle defects. The transverse fatigue crack depth of spindle is measured by equivalent method, and the crack length is measured by length method. Equivalent methods are commonly used in spindle flaw detection, such as equivalent block comparison method and base wave height (dB) relative comparison method.

5.3 Defect Assessment

After the inspection is completed, according to the wavelength, quantity and waveform characteristics of the defects and the requirements of GB/T6402-2008 Ultrasonic Inspection Method for Steel Forgings JB/T1581-2014 Ultrasonic Inspection Method for Steam Turbine, Rotor and Spindle Forgings, the mine can make an accurate evaluation of the defects. In a word, ultrasonic testing technology can detect the internal defects of mechanical spindle structure without destroying parts, and can not only make qualitative evaluation, but also quantitatively evaluate the size and location of defects, and give the evaluation results, which provides a reliable guarantee for the normal operation of coal mine machinery and equipment, and also provides a reliable guarantee for the safe production of coal mine enterprises.

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