Research and Practice on Domestic Development of Key Transmission Gears

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Introduction
For a long time, the products of many manufacturing enterprises in China have been in the middle and low end of the global value chain, and the product design, key components, and process equipment mainly rely on imports. At present, in many 10-million-ton mines of domestic coal groups, as the main equipment for mining operations, such as continuous miners, bolter-miners, and comprehensive shearers, the overall level of localization is still relatively low. High-yield and high-efficiency mines still rely heavily on imported equipment, such as the JM series continuous miners of JOY Company, the MB series bolter-miners of Sandvik Company, and the SL series of EFK Company, and the 7LS series shearers of JOY Company. The materials selected for the key components of these equipment and the final heat treatment performance have a great impact on the reliability and stability of the whole machine, and gears, as the most common mechanical components, are widely used in the transmission devices of these coal mining machinery.

Although the imported mining equipment has outstanding performance, in actual use, it is found that problems such as difficult maintenance, high difficulty in troubleshooting, and difficulty in purchasing urgent and vulnerable parts are very common in the use of imported equipment. The long manufacturing cycle, high cost, untimely service, and numerous obstacles of imported accessories are prominent, which seriously affects the production continuity of enterprises.

This article takes the transmission idler gear of the cutting part reducer of the imported 6C drum shearer of M Company as the research object, conducts research and practice on domestic development, and explores a set of efficient and economical ways for the domestic development of key parts, helping enterprises resume production in the shortest time, reduce economic losses, break the technical barrier, and fundamentally improve the application range and reliability level of China’s mining equipment.

Characteristics Study of Rocker Arm Transmission Gears for Imported Shearers

  1. Working Condition Analysis of Transmission Idler Gear
    • The transmission system of the cutting part (hereinafter referred to as the rocker arm) of the imported 6C shearer of M Company is shown in Figure 1. The transmission idler gears are all spur gears, with an overall transmission ratio of 47.225, a motor power of 750 kW, and a rotation speed of 1,475 r/min. After the first and second-stage deceleration with different reduction ratios, the power is transmitted to the 2-stage planetary reduction mechanism by the idler gear set, and the rotation speed of the idler gear set is 777.36 r/min, which is relatively high. At the same time, the transmitted power of the rocker arm is continuously increasing, and it bears a large impact load during work, which requires higher reliability of the idler gear.
    • M Company adopts a full fillet design for the rocker arm gears, increases the pressure angle and the center distance of the gears, uses an ultra-deep tooth profile line to maximize the bending strength of the gear teeth; uses high-quality steel, and most gears select the “super steel” in the AGMA standard; adopts induction hardening and carburizing treatment processes; realizes the strength balance of the gears in the entire gearbox, optimizes the overall design, and uses the computer design program to evaluate the impact of load changes.
  2. Characteristics of Gear Raw Materials
    • Samples of the gear materials were taken, and their chemical compositions were analyzed by three testing institutions. The test composition indicators are shown in Table 1, and it is found that the steel grade is 3310 according to the domestic and foreign steel number manuals.
  3. Determination of Imported Parts Parameters
    • The new imported gears were measured in detail using a coordinate measuring machine and a gear measuring machine, and relevant technical data were queried. After comprehensive analysis of various parameters, the confirmed parameters and indicators are shown in Table 2.

Domestic Development Practice

  1. Selection of Raw Materials
    • Referring to the commonly used materials for gear processing in China, and querying the steel brand manual, the comparison results of the chemical composition and mechanical properties of the typical gear processing material 18Cr2Ni4WA and the American 3310 chemical material are shown in Table 3.
    • The impact energy of domestic 18Cr2Ni4WA is better than that of 3310, and relevant tests and research on heat treatment were carried out. The contents of retained austenite, carbide martensite, and ferrite were analyzed by metallographic analysis, and the surface hardness and core hardness were detected, all of which meet the requirements of heavy-duty gears. Finally, 18Cr2Ni4WA was selected as the raw material for the domestic rocker arm transmission gears.
    • Each batch of raw materials is strictly tested and inspected according to the internal inspection items and requirements of the steel, as well as relevant national and international standards, for the grain size, end hardenability, non-metallic inclusions, mechanical properties, etc. The relevant quality standards are: the size and allowable deviation of hot-rolled round steel shall comply with the provisions of Group 2 in GB/T 702, both ends shall be cut flat, and the cylindricity error shall not be greater than 75% of the tolerance of the nominal diameter, the bending degree ≤ 4 mm/m, and the total bending degree ≤ 0.4% of the length of the steel; the allowable deviation of the main chemical composition of the raw material finished product shall be implemented in accordance with the EN10084 standard; the chemical composition (melting analysis) shall comply with the requirements in Table 4.
  2. Calculation and Optimization Design of Parameters
    • Using professional gear simulation analysis software, combined with the actual measurement parameters and working condition parameters, the dynamic load simulation analysis of all gears is carried out. Based on the analysis results, an optimized design scheme that is closer to the actual working conditions is obtained, and the tooth profile is optimized.
    • State Simulation Analysis: The working condition parameters of the rocker arm are as follows: reduction ratio 47.225, motor power 750 kW, rotation speed 1,475 r/min, service coefficient 2.0, rated torque 229.32 kN, maximum torque 458.64 kN. The software simulation effects of the whole machine, rocker arm, and key transmission parts are shown in Figure 2.
    • Optimization Design and Modification: When the gear is subjected to the load, the shaft will produce different degrees of elastic deformation. At the same time, this load will also cause the deformation of the box. The superimposed result of the above multiple deformations will cause the gear to generate impact, vibration, and eccentric load during the meshing process. To this end, both ends of all teeth are modified to prevent the tip of the tooth surface from contacting.
    • When necessary, the tooth profile modification is used to compensate for the meshing deviation caused by the deformation of the shaft on the meshing surface, so that the tooth meshing surface is evenly distributed along the tooth width under normal load. At the same time, in order to avoid collision impact at the initial meshing stage and the final meshing stage, the tooth top and tooth root should be correspondingly modified to improve the power transmission capacity and reduce friction and noise.
  3. Selection of Manufacturing Process
    • The processing of gears starts after the test and inspection of each batch of raw materials are qualified. The main processing processes are forging the gear blank – heat treatment (normalizing) – rough machining – semi-finishing machining – finishing machining (gear hobbing processing) – heat treatment (carburizing + quenching + tempering) – strong shot peening – finishing machining (grinding the end face, grinding the teeth, etc.) – spline processing – comprehensive inspection (pairing research + flaw detection + parameter detection), etc. After passing the inspection, anti-corrosion and packaging are carried out.
    • Heat treatment requirements: carburizing: 930 °C, strong carburizing period 10 – 30 h, diffusion period 8 – 20 h; quenching: temperature 840 °C, insulation for 3 – 4 h, quenching oil temperature 70 °C; high-temperature tempering: 650 °C, insulation for 4 – 6 h; low-temperature tempering: determined according to the measured hardness value, the temperature is 160 °C – 200 °C, and the insulation is for 10 h.
  4. Quality Control
    • The quality of the whole process of gear processing is strictly controlled and tracked. After completion, it must be detected and inspected. The relevant standards are as follows:
    • The surface hardness value of the gear teeth reaches HRC 59 – 62 or HV 670 – 750;
    • The metallographic structure inspection shows that the carburized layer shall be composed of tempered martensite and residual austenite, the content of residual austenite ≤ 20%, and there is no network carbide;
    • The carbon content at the core is 0.28% – 0.35%;
    • There shall be no cracks, indentations, scratches, pits, burns, and other defects on the tooth surface;
    • Ultrasonic flaw detection and magnetic particle flaw detection shall be carried out in accordance with SEP 1921, and the workpiece needs to be demagnetized after grinding the teeth and magnetic particle flaw detection;
    • The dedicated gear pairing test bench is used for pairing research detection, and the contact area ≥ 80%;
    • The coordinate measuring machine and gear measuring machine are used to spot check the same products of each batch and measure various parameters to ensure they are correct.
    • After passing the inspection, the gears can be used for assembly.
  5. Application Effect
    • To verify the actual effect of the domestic research and manufacturing of gears, Shendong Coal Group selected the left rocker arm of the 6C/LWS636 shearer to install the corresponding domestic parts, and the right rocker arm used imported gears. Both were simultaneously put into an industrial comparative test underground. After the assembly was completed, a loading test was carried out on the 1,200 kW loading test bench. The highest temperature in the idler gear area was 90 °C, and the temperature of other transmission parts was lower than 95 °C. There was no abnormal noise or oil leakage, and all met the expectations. The LWS636 shearer was put into production in the 22107 fully mechanized mining face of a mine owned by the company on March 15, 2020. As of April 6, 2021, it has accumulated a progress of 4,500 m and produced 6.3 million tons of coal. From the usage situation, the performance indicators of the domestic rocker arm gear idler are good, the operation is reliable, and it meets the design requirements and can meet the needs of high-yield and high-efficiency coal mines.
    • At the same time, the processing cycle of a single gear is about 45 days, which is greatly reduced compared to the shortest duration of 180 days for imported parts. The average cost per piece is reduced from the import price of 40,000 yuan to about 18,000 yuan, with a cost reduction of nearly 55%. If calculated based on the 31 06 and 6C shearers used by the company, and an average of 10 gears are replaced for each machine, nearly 7 million yuan can be saved in accessory costs during each overhaul cycle, and the economic benefits are considerable.

Conclusion
By taking the rocker arm transmission idler gear of the imported shearer as the research object, combined with the actual working conditions, in-depth research on its stress situation, base material characteristics, design parameters, etc., combined with the existing production technology conditions, the domestic development research has been carried out, and a product that can be completely interchangeable with it, has highly matched performance indicators, the shortest cost and delivery time, and guaranteed supply has been manufactured. After the underground industrial test verification, it has been successful. At the same time, the domestic supply channel for accessories is stable and smooth, getting rid of the influence of the international environment on imported products, and the construction period and cost are controllable for enterprises, bringing good economic benefits to coal mining enterprises. Based on this, we can further summarize experience, improve the localization manufacturing level of coal mining machinery accessories, and promote the technical improvement and industrial upgrading of the relevant equipment manufacturing industry in the country.

In addition to the above research and practice, the following aspects can also be further explored and discussed:

  1. Further Optimization of Gear Design
    • Conduct more in-depth research on the load distribution and stress state of gears during operation to further optimize the design of the tooth profile and structure, improve the load-bearing capacity and service life of gears.
    • Explore the application of new materials and processing technologies to improve the performance and reliability of gears.
  2. Enhancement of Manufacturing Process Precision
    • Improve the precision of gear processing equipment and processes to ensure the consistency and stability of gear quality.
    • Strengthen the control and management of the manufacturing process to reduce the variation of gear performance.
  3. Long-Term Reliability Testing
    • Conduct long-term reliability testing and monitoring of domestic gears to accumulate more data and experience, and continuously improve the reliability and stability of products.
  4. Cost Analysis and Optimization
    • Conduct a detailed cost analysis of the entire process of gear localization, including raw material procurement, processing, testing, and logistics, to find ways to further optimize costs.
    • Consider the life cycle cost of gears, including maintenance and replacement costs, to provide more economical solutions for users.
  5. Technology Innovation and Intellectual Property Protection
    • Encourage enterprises to carry out technology innovation in the field of gear localization, and strengthen the protection of intellectual property rights to enhance the competitiveness of domestic products.
  6. Industry Collaboration and Standardization
    • Promote collaboration and communication among enterprises, research institutions, and universities in the industry to jointly solve technical problems and promote the standardization of gear localization.
  7. Training and Talent Cultivation
    • Strengthen the training of technical personnel to improve their skills and knowledge level, to meet the needs of the development of the gear localization industry.
  8. Market Expansion and Internationalization
    • After achieving success in the domestic market, actively explore the international market, and promote the internationalization of domestic gear products.

In summary, the localization development of key transmission gears for imported shearers is of great significance for improving the reliability and cost-effectiveness of mining equipment, and promoting the development of the domestic equipment manufacturing industry. Through continuous research and practice, we can continuously improve the performance and quality of domestic gears, and provide better support for the development of the coal mining industry.

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