Phased array ultrasonic testing of gear tooth

Abstract

This paper proposes a new testing method, phased array ultrasonic testing, for the internal defects of gear teeth, which can be used to inspect the teeth of final products, breaking the limitations of incomplete volume coverage of conventional ultrasonic testing. This paper mainly discusses the inspection process of phased array ultrasonic testing, and verifies the feasibility of phased array ultrasonic testing for gear teeth defects of finished products by using physical and chemical inspection technology. The practical application results show that the phased array ultrasonic testing technology uses sector scanning mode to scan the area comprehensively between the pitch circle and the base circle, and the inspection image displayed by the equipment is intuitive, which can clearly identify defects. The number of defects inspected by phased array ultrasonic testing is consistent with the results of the physical and chemical inspection technology. The above summary shows that phased array ultrasonic testing is feasible, and the test results are reliable.

Keywords:

• phased array ultrasonic testing
• ultrasonic testing
• gear tooth
• wind power industry

PUBLIC INTEREST STATEMENT

Gear, as a vital part of mechanical transmission, needs strict control in terms of quality. Currently, the widely recommended technology, conventional ultrasonic testing, is not suitable for the internal defect inspection of finished parts. The principle of phased array ultrasonic testing is similar to that of conventional ultrasonic testing, which is based on the principle of pulse reflection method, namely collecting ultrasonic reflection wave and interference wave in different media and converting waves into electronic and digital signals on the instrument screen. The differences between the two technologies are that phased array testing adopts S-scanning, which means it has more angles of ultrasound, equivalent to having multiple angles of the probe working at the same time. Moreover, the angle is controllable, so the dead zone is smaller, and the inspection efficiency is higher to a certain extent. Phased array inspection has higher sensitivity and resolution because of the focusing function.

1. Introduction

The wind turbine gear box is located in the middle of the wind turbine and usually operates at high altitudes in harsh environments, so failure usually occur and the maintenance are difficult to carry out. According to statistics, the cost of fault repair is as high as 1/4 of the quotation of wind power equipment (Bin et al., 2009; Xuefeng et al., 2011). Therefore, it is very important to study the influence of the material defects on wind turbines. The gear box is mainly composed of ring gears, planet gears, gears, gear shafts, rolling bearings and other components, among which the gear is the main transmission component, and its failure has attracted much attention. Research data indicated that gear failures account for 60% of the gear boxes (Xiangdong, 2017), and the main failure modes of gears are broken tooth, pitting, wear and scratch, etc. In this regard, Zhigang et al. (2021). analyzed the impact of broken tooth on gear box and found that the broken tooth as the most serious failure form, which will directly affect production efficiency, and generate unplanned replacement and maintenance costs. Therefore, the suggestions were put forward to strengthen the material inspection of gear and the on-site inspection of gear box. Yanxin et al. (2019) used dynamic simulation software to study the influence of gear broken tooth fault on the position of high-speed gear, and the simulation results provided a basis for the serious consequences of gear failure. At present, most of the research (Huaiju et al., 2018) are still focus on the analysis of the gear box gear tooth defects with the method of physical and chemical testing, nondestructive testing methods have not been widely promoted and used. In other words, more effective and comprehensive nondestructive testing methods in gear have not been used. Meanwhile, effective non-destructive testing (NDT) technology needs to be developed in many applications, including oil and gas, medical imaging applications, railway, automotive, marine, and aviation industries (Hongyuan & Mahmoud, 2023). This paper shows the specific method of using phased array ultrasonic testing (PAUT) to inspect the gear teeth and formulates a PAUT process scheme through inspection and application results, providing a new idea for the internal defect inspection of the gear.

2. Theory and Methods

2.1. Ultrasonic testing standard

Forgings in the wind turbine gear box are inspected in accordance with “ISO 6336–5:2016 Calculation of load capacity of spur and helical gears—Part 5: Strength and quality of materials.” The standard stipulates that carburizing and quenching steel, in the rough working state, should be inspected in accordance with “ASTM A388/A388M–19 Standard Practice for Ultrasonic Examination of Steel Forgings” or “EN 10,228–3 Non-destructive testing of steel forgings—Part 3: Ultrasonic testing of ferritic or martensitic steel forgings” before heat treatment. For the MQ and ME grades of fatigue strength of gears, the reference reflector during the inspection is flat bottom holes (FBH), and the reflected signals with a size of φ2.0 mm or above need to be recorded.

2.2. Machining and inspection process of wind turbine gear box gear

Gears require multiple-processing processes from raw materials to finished products. Different processes may lead to different defects, which will increase the risk of tooth fracture. Therefore, various NDT methods have been carried out during the processing processes to control the quality of the gear. Figure 1 is the traditional processing and inspection processes of the gear. According to the “ASTM A388/A388M–19 Standard Practice for Ultrasonic Examination of Steel Forgings,” the forgings should be inspected by ultrasonic testing (UT) technology after the heat treatment. After shot peening and finish turning, removing the oxide scale on the surface of the products and perform the UT. Figure 2 is the current processing and inspection processes of gears.

Figure 1. Flow diagram of the forging traditional processing and inspection process in the wind turbine gear box.

Figure 2. Flow diagram of the forging current processing and inspection process in the wind turbine gear box.

2.3. Conventional UT of wind turbine gear box gear

There have been many experimental studies (Huaiju et al., 2018) showing that the cracks of tooth face are generally taken first at a certain depth below the tooth side and expands towards the middle of the tooth profile of the loaded tooth face and the tooth root of the unloaded tooth face, then forming the tooth face fracture. The area that is easy to crack is dangerous area, as shown in the shaded part of Figure 3, which should be paid attention to during inspection. At present, the working depth of tooth is up to 60 mm, and the thickness of addendum is up to 7 mm. Teeth in these range can be inspected by changing the probe (size and frequency).

Figure 3. Schematic diagram of the dangerous area prone to cracks of gear tooth.

The space between the two gear teeth is smaller than the size of the conventional UT probes, such as straight probes and angle beam probes, so the conventional UT probe cannot inspect from tooth side. Also, considering that the tooth face is curved, the coupling between the conventional UT probes and the tooth face is too poor to meet the requirements of inspection sensitivity; therefore, the conventional UT of gear tooth is mostly inspected from the top land. The status quo is that the straight probes from top land can only inspect the middle position of the gear tooth and the angle beam probes from top land cannot be moved from side to side due to its size, the detection areas of these two probes are limited. The diagram of inspection area of straight probes and angle beam probes are shown in Figure 4.

Figure 4. Diagram of inspection area of straight probes and angle beam probes.

2.4. PAUT of wind turbine gear box gear

Compared with conventional UT probes, some PAUT probes are smaller in size, so the PAUT probes can be moved from side to side at the top land for S-scanning, and the multi -angle scanning makes the sound beam cover the entire dangerous area, as shown in Figure 5.

Figure 5. Diagram of inspection area of phased array ultrasonic probe.

3. Field application

3.1. Tooling Research and development

A fixed tooling as shown in Figure 6 was made for placing the PAUT probe, which can ensure that the probe is always kept in the middle position of the gear tooth, reducing the inspection error of inspectors during manual scanning, increasing the reliability of the inspection results, and realizing the precise positioning of the probe.

Figure 6. Schematic diagram of fixed tooling for placing the phased array ultrasonic probe.

3.2. Calibration of instruments

After several tests and verifications, model 32/32 PAUT instrument is selected for inspecting and the probe parameters are shown in Table 1. The sound velocity of the wedge used is 2330 m s⁻¹, and the wedge angle is 319.55K.

Table 1. PAUT parameters

Frequency (Hz)	Element pitch (m)	Element number (PCS)	S-scan angle (K)	Stepping angle (K)
$10\times 10^6$	$0.5\times 10^{-3}$	16	308.15 ~ 338.15	274.15

3.2.1. Probe delay and sound speed measurement

The reference blocks shall be manufactured according to the processing requirements of “ASTM E127 Standard Practice for Fabrication and Control of Flat bottom hole Ultrasonic Standard Reference Blocks,” “ISO12715–2014 Ultrasonic non-destructive testing- Reference blocks and test procedures for the characterization of contact search unit beam profiles” and “ASTM A388 Standard Practice for Ultrasonic Examination of Steel Forgings.” Ultrasonic acceptance test shall be carried out on the reference blocks before inspection, and the reference blocks were used after passing the test. The material of the reference blocks should be the same or similar to the acoustic performance of the tested products.

Before testing, placing the PAUT probe on the reference block with radius of 0.05 m and 0.1 m, and then moving the probe to find the maximum signal echo, finally pressing the “Calibrate” key to indicate the completion of sound velocity calibration and delay calibration, respectively. The structure of the reference block is shown in Figure 7.

Figure 7. Structure diagram of delay and sound speed calibration reference block.

3.2.2. Sensitivity curve settings

According to the acceptance level, side drilled holes (SDH) reference blocks shall be performed to make the ultrasonic inspection sensitivity curves using at least three holes. It should be noted that the maximum buried depth of the hole must cover the inspect range. The structure of the reference block is shown in Figure 8.

Figure 8. Structure diagram of sensitivity curve setting reference block.

3.2.3. Sensitivity curve correction

The sensitivity curve should be corrected after set. Placing the probe on the reference block as shown in Figure 9, and correcting the gain value of the sensitivity to reach full screen height (FSH) 80%. The reference block showing only demonstrates a situation where the FBH is buried, the buried depth of the FBH shall be greater than or equal to the size from the addendum circle to the working depth circle during the actual inspection.

Figure 9. Structure diagram of sensitivity curve correction reference block.

3.2.4. Dynamic verification

The block shown in Figure 10 is a tooth shape simulation block with artificial defects. There are three SDH in the test block, all of which are $\phi 1\times 10^{-3}\times 20\times 10^{-3}\mathrm{m}$. The first hole is $5\times 10^{-3}\mathrm{m}$ from the top land, the second is on the pitch circle, and the third is on the working depth circle. The appearance of the simulation block is similar to the inspected product, which can verify whether the PAUT process can inspect the defects on the gear tooth.

Figure 10. Schematic diagram of simulation block with artificial defects.

3.2.5. Field inspection

It should be noted that the teeth surface shall be cleaned to remove obstacles that affect the inspection and the coupling agent used in verification shall be consistent with that used in field inspection. Using the calibration curve as the scanning sensitivity, and the scanning interface shall present the S-scan and A-scan images. According to “ISO 13,588 Non-destructive testing of welds—Ultrasonic testing—Use of automated phased array technology,” inspection should be carried out indoors, which is not affected by the external temperature, and the indoor ambient temperature should be within the range of 0 ~ 50℃. The gear inspection is carried out in a constant temperature room, so the temperature state can meet the inspection requirements.

3.2.6. Quality Levels

According to ISO 13,588, whether an indication in phased array images is defective should be comprehensively evaluated according to the signal amplitude relative to general noise level, and the acceptance quality levels shall be established between purchaser and manufacturer. In this paper, the sensitivity curve is made by reference reflectors, and the amplitude is corrected for FSH 80%. Therefore, the acceptance level is that when the indication amplitude is equal to or exceeding FSH80%, it will be judged as a defect.

4. Results and discussion

4.1. PAUT results

The inspection results found that there was an indication in the device image with more than 80% amplitude, which is defined as a defect according to the quality level, and the location of the defect is $21\times 10^{-3}\mathrm{m}$ from the top land. The test result is shown in Figure 11.

Figure 11. Inspect result representation of PAUT.

4.2. The physical and chemical inspection

Using physical and chemical inspection to analyze the PAUT inspection results and verify the correctness of the results.

4.2.1. Scanning electron microscopy and energy dispersive spectroscopy analysis

The gear destructive test is carried out according to the test result of PAUT, the diagram of scanning electron microscopy (SEM) was obtained as shown in Figure 12, which showed an obvious defect in the entire section of the broken tooth. The SEM surface shown that aluminum is the main element of the defect, and this idea was also well verified in the energy dispersive spectroscopy (EDS). Six rejected products from 2022 to date are randomly selected for destructive testing, and the results of the number of defects were shown in Table 2, which are consistent with the PAUT test results. Consequently, PAUT method could be judged accurate and reliable.

Figure 12. SEM and EDS of broken tooth section.

Table 2. Date statistical of the distance from top land and face of the defects

IDX	Distance from top land/m	Distance from face/m
1	$10.95\times 10^{-3}$	$5.8\times 10^{-3}$
2	$13.78\times 10^{-3}$	$4\times 10^{-3}$
	$7.5\times 10^{-3}$	$4.62\times 10^{-3}$
3	$10.74\times 10^{-3}$	$5.25\times 10^{-3}$
	$12.19\times 10^{-3}$	$7.4\times 10^{-3}$
4	$12.55\times 10^{-3}$	$6.1\times 10^{-3}$
5	$18.43\times 10^{-3}$	$6.8\times 10^{-3}$
	$15.19\times 10^{-3}$	$8.35\times 10^{-3}$
6	$20.2\times 10^{-3}$	$5.2\times 10^{-3}$

5. Conclusion

1. In this paper, PAUT technology was used to inspect the gear teeth, which has the advantages over conventional UT technology that PAUT can make up for the large dead zone and provide a new idea for gear defect inspection.

2. The PAUT technology adapts the S-scan method to conduct a comprehensive scan of the pitch circle and the working depth circle of gear teeth, and the inspection image represented by the PAUT device is intuitive. The number of defects inspected by PAUT was consistent with that observed by SEM, which verifies the feasibility and reliability of the PAUT.

3. The PAUT technology in this paper is suitable for the inspection after the bobbing and before the finished product. This process ensures the quality of the product before the factory, and it can also remove the defeats inspected by PAUT in the subsequent grinding process to avoid processing waste and reduce costs.

4. The PAUT technology provides a strong guarantee for the safe operation of the gear transmission system. It also provides a technical reference for similar testing of other gears such as rail transit, engineering machinery, and heavy machinery.

5. In fact, PAUT technology still has limitations in internal defect inspection. The greater challenge is that PAUT technology is affected by the instrument gain and product surface status, and the defect cannot be accurately positioned and qualitative. In terms of comparison, the total focusing method (TFM) imaging technology, a newly development technology, has a value in the analysis of defects quantification. It can make up the shortcoming of PAUT, but the specific application also needs to improve research and expansion (Yan,).

Abbreviations

PAUT	=	Phased array ultrasonic testing
NDT	=	Non-destructive testing
UT	=	Ultrasonic testing
SDH	=	Side drilled holes
SEM	=	Scanning electron microscopy
EDS	=	Energy dispersive spectroscopy
TFM	=	Total focusing method
FSH	=	Full screen height

Geolocation information

No. 30, Houjiao Road, High-tech Park, Jiangning District, Nanjing

Date availability

The data used to support the findings of this study are available from the corresponding author upon request.

Acknowledgments

The authors would first like to thank the master, whose expertise was invaluable in formulating the research questions and methodology. The insightful feedback pushed authors to sharpen the thinking and brought authors work to a higher level. In addition, the authors are grateful to thank the teammate for their wonderful collaboration and patient support.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The authors received no direct funding for this research.

Notes on contributors

Liang Wang

Liang wang received his bachelor degree in Materials in the school of materials science and engineering at Nanjing Institute of Technology, Nanjing, 2007. Since 2007, he joined Nanjing High Speed Gear Manufacturing Co., LTD., currently he is engaged in ultrasonic testing, phased array ultrasonic testing, magnetic particle testing, penetration testing, corrosion testing, visual testing, eddy current testing and other process development, responsible for the department’s non-destructive work.

Chenchen Ma

Chenchen Ma received her Bachelor degree in Materials from Xuzhou university of technology, Xuzhou, 2018; Master degree of engineering from Hohai university, Nanjing, 2020. Before 2022, she joined a material research institute and was mainly responsible for process development. After that, she joined Nanjing High Speed Gear Manufacturing Co., LTD., currently research interests include ultrasonic testing, phased array ultrasonic testing, magnetic particle testing and other non-destructive testing, and responsible for supplier audit and other related work