In the field of mining management and extraction, drilling rigs are among the most efficient and direct drilling equipment. In complex and variable working conditions involving sedimentary rocks, volcanic rocks, and igneous rocks, the drilling rig operates by rotating under the power system to drill through hard ores, fully utilizing the hard and wear-resistant characteristics of the drill bit. The drilling rig employs wear-resistant and impact-resistant drill bits for rotary drilling and mechanical rock breaking, causing minimal environmental and geological damage. It is particularly suitable for small-diameter deep-hole mechanical rock breaking and exploration in complex geological conditions. Drilling and exploration with drilling rigs represent the most effective, direct, economical, least destructive, and most easily repairable process for ore extraction.
The top drive reducer directly provides power output for the top drive drilling rig. The driving methods of the top drive reducer mainly include electric motor drive and hydraulic drive. The drive system (power head device) is fixed on the support derrick, and the top drive system is connected to the winch and trailer system. The trailer’s ropes are used to lift and lower the power head device of the top drive drilling rig, achieving drilling and lifting operations. The power head device of the top drive drilling rig is suspended and installed directly above the derrick space, sliding up and down on the derrick guide rails. The top drive reducer drives the power cutting head to rotate and cut through rocks for drilling.
The top drive reducer drives the power head device for drilling and rock fragmentation. The top drive reducer is designed with gear transmission, primarily consisting of the power gear shaft group, the speed-increasing gear shaft group, the rotating main shaft, and the gearbox housing. The electric motor/hydraulic motor is connected to the power gear shaft group and the speed-increasing gear shaft group of the top drive reducer via splines. The rotation of the motor/motor drives the power gear shaft and the speed-increasing gear shaft to engage with the main shaft gear. The rotating main shaft is connected to the floating sleeve via splines. The power gear shaft drives the power cutting head to rotate and cut rocks.
The gearbox housing is typically a cast structure, but cast gearbox housings often have casting defects and poor appearance. During precision machining, casting sand holes are prone to appear, with a repair rate usually exceeding 30%. To reduce costs and shorten manufacturing time while meeting the strength requirements of the housing, a welded structure is adopted for the gearbox housing.
The power gear shaft and the speed-increasing gear shaft are connected to external input sources and operate at high speeds. Under the action of external alternating loads, they bear torsion, bending moments, and torque. The input shaft of the top drive reducer must resist fatigue fracture caused by long-term cyclic loading. To ensure smooth transmission, the material should have high toughness, and no cracks or torsional fractures should occur during operation. Under high-frequency vibration, repeated loads, and alternating loads, the material structure must maintain integrity at all times. Operating in a circulating liquid environment at around 80°C, it must have thermal expansion resistance and not be prone to fracture or deformation. Based on these working characteristics, the shaft material requires high surface hardness and wear resistance, while the overall shaft needs high strength and toughness, and the core must maintain high strength and toughness. Therefore, the high-speed heavy-duty gear material 18CrNiMo7-6 is selected, and carburizing heat treatment is applied.
Gear Shaft Group Design
The electric motor/hydraulic motor provides the power source for the top drive reducer. The external power source has a power of 2×75 kW and a speed of 1,500 r/min. The top drive reducer adopts a vertical cylindrical structure. The power gear shaft group and the speed-increasing gear shaft group engage with the rotating main shaft gear to provide power for the rotating cutting action of the power cutting head.
Gear Strength Verification
The power gear shaft group and the speed-increasing gear shaft group mainly consist of the gear shaft and bearings. The gear portion of the speed-increasing gear shaft is the same as that of the power gear shaft, so the design of the speed-increasing gear shaft can refer to the design of the power gear shaft.
(1) Minimum diameter of the power gear shaft:
$$d_{\text{min}} \geq A_0 \sqrt[3]{\frac{P}{n}}$$
where \(P\) is the input shaft power, \(P = 75 \text{kW}\); \(n\) is the rated speed, \(n = 1500 \text{r/min}\); \(A_0\) is the coefficient selected based on the allowable torsional shear stress \([\tau_p]\) of the material.
According to the material properties of 18CrNiMo7-6 and the \([\tau_p]\) for several commonly used shaft materials, the range of \(A_0\) is 97 to 112. When \(A_0 = 97\), \(d_{\text{min}} \geq \phi 35.74 \text{mm}\); when \(A_0 = 112\), \(d_{\text{min}} \geq \phi 41.26 \text{mm}\). The minimum diameter of the power gear shaft is designed as \(\phi 75 \text{mm}\), and since \(\phi 75 \text{mm} > \phi 41.26 \text{mm}\), it meets the requirement.
(2) Structural design of the power gear shaft:
The power gear shaft has a gear module \(m_1 = 8 \text{mm}\), number of teeth \(Z_1 = 17\), profile angle \(\alpha_1 = 20^\circ\), helix angle \(\beta = 10^\circ\), and is right-handed. The external spline has a module \(m_2 = 2.1 \text{mm}\), number of teeth \(Z_2 = 32\), and pressure angle \(\alpha_2 = 30^\circ\).
(3) Force analysis of the power gear shaft:
During operation, the power gear shaft is subjected to bearing reaction forces, centrifugal forces, torque, and the forces and torque from the meshing gear.
(4) Calculation of bearing reaction forces:
The torque acting on the gear spline from the motor/hydraulic motor (input torque) is:
$$T = 9550 \frac{P}{n} = 477.5 \text{N·m}$$
The driving force on the gear tooth surface is:
$$F_t = \frac{2T}{d_1} = \frac{2T}{m_1 Z_1} = 7022.06 \text{N}$$
where \(d_1\) is the gear pitch diameter.
The radial force acting on the gear is:
$$F_r = F_t \frac{\tan \alpha_1}{\cos \beta} = 2595.25 \text{N}$$
Calculation of reaction forces in the horizontal plane:
$$F_{NV1} = F_r \frac{L_1}{L} = 1272.96 \text{N}$$
$$F_{NV2} = F_r \frac{L_2}{L} = 1322.29 \text{N}$$
where \(L_1 = 129 \text{mm}\), \(L_2 = 134 \text{mm}\), \(L = L_1 + L_2 = 263 \text{mm}\).
Calculation of reaction forces in the vertical plane:
$$F_{NH1} = F_{NH2} = \frac{F_t}{2} = 3511.03 \text{N}$$
(5) Calculation of bending moments:
Calculation of bending moments in the horizontal plane:
$$M_{V1} = F_{NV1} L_1 = 164.21 \text{N·m}$$
$$M_{V2} = F_{NV2} L_2 = 177.19 \text{N·m}$$
Calculation of bending moments in the vertical plane:
$$M_{H1} = F_{NH1} L_1 = 452.92 \text{N·m}$$
$$M_{H2} = F_{NH2} L_2 = 470.48 \text{N·m}$$
Calculation of resultant bending moments:
$$M_1 = \sqrt{M_{V1}^2 + M_{H1}^2} = 481.77 \text{N·m}$$
$$M_2 = \sqrt{M_{V2}^2 + M_{H2}^2} = 502.74 \text{N·m}$$
Calculation of the equivalent bending moment at the section using the third strength theory:
$$M_{\text{ca}} = \sqrt{M_2^2 + (a T)^2} = 576.3 \text{N·m}$$
where \(a\) is the stress correction coefficient, taken as \(a = 0.59\) based on the cyclic characteristics of the torsional shear stress.
The load analysis diagram of the input shaft is as follows:
(6) Critical section:
Based on the operating state and nature, the power gear shaft rotates unidirectionally when driving the main shaft, which belongs to pulsating cyclic alternating stress. Taking \(a = 0.59\), the stress on the power gear shaft is:
$$\sigma = \frac{\sqrt{M^2 + (a T)^2}}{W} = \frac{M_{\text{ca}}}{W} = 2.29 \text{MPa}$$
where \(M\) is the bending moment on the shaft; \(W\) is the section modulus in bending. For a circular cross-section at the critical part of the gear shaft, \(W = 0.1 d_1^3\).
The gear shaft material 18CrNiMo7-6 steel undergoes quenching and tempering and carburizing heat treatment. The quenching and tempering parameters are: quenching at 850–880°C, tempering at 550–650°C. Referring to the table, the allowable bending stress of the shaft is \([\sigma_{-1}] = 60 \text{MPa}\). Since \(\sigma < [\sigma_{-1}]\), it is safe.
Power Gear Shaft Spline Strength Verification
The maximum output power of the motor/hydraulic motor is \(P = 75 \text{kW}\), the input torque on the power gear shaft is \(T = 477.5 \text{N·m}\). The input spline of the power gear shaft uses a German standard spline with number of teeth \(Z_2 = 32\), module \(m_2 = 2.1 \text{mm}\), pressure angle \(\alpha_2 = 30^\circ\). The effective engagement length (spline effective length) is \(L_3 = 56 \text{mm}\), and the material is 18CrNiMo7-6.
The compressive stress is:
$$\sigma_p = \frac{2T}{\phi Z_2 m_2 L_3 d_2} = 5.04 \text{N/mm}^2$$
where \(d_2\) is the spline pitch diameter, \(d_2 = m_2 Z_2 = \phi 67.2 \text{mm}\); \(\phi\) is the load factor, taken as \(\phi = 0.75\).
The compressive stress \(\sigma_p = 5.04 \text{MPa}\), and the allowable compressive stress for 18CrNiMo7-6 is \([\sigma] = 45 \text{MPa}\). Since \(\sigma_p < [\sigma]\), the spline strength meets the usage requirements.
Heat Treatment Research on 18CrNiMo7-6 Power Gear Shaft
The power gear shaft carries alternating loads during rotation and meshing. Under high loads, the minimum diameter area is prone to fracture. The core technology of the 18CrNiMo7-6 material lies in the carburizing and quenching process. Gears and gear shafts are prone to uneven thermal expansion and cold contraction during carburizing and quenching, resulting in residual stresses. Internal organizational transformations produce structural stresses, causing distortion and shape畸变, and local cracks may occur. By controlling the quenching time, temperature, and carbon potential reasonably, a superior渗层组织 and hardened layer depth can be obtained. The power gear shaft requires a carburized and quenched case depth of 1.4–1.8 mm, tooth surface hardness of 58–62 HRC, and core hardness of 36–43 HRC. To achieve its excellent performance, the process case depth is set at 1.7–2.1 mm. The carburizing and quenching temperatures for the 18CrNiMo7-6 power gear shaft are set at 820°C and 840°C for corresponding carburizing and quenching processes.
Carburizing and Quenching of Power Gear Shaft
(1) Experiment 1:
Quenching temperatures of 820°C/840°C, with pre-cooling for 3 minutes before quenching, and reduced cooling intensity. The process parameters and the metallographic structure of the carburized and quenched samples are shown in Table 1.
| Process | Martensite and Retained Austenite | Carbides | Core Structure |
|---|---|---|---|
| 1st Heat: 820°C equalizing 30 min, pre-cool 3 min quenching, slow cool 30 min, oil temperature 50°C | Grade 4 | Grade 1 | Grade 2 |
| 2nd Heat: 840°C equalizing 30 min, pre-cool 3 min quenching, slow cool 40 min, oil temperature 60°C | Grade 1 | Grade 2 | Grade 2 |
| 3rd Heat: 840°C equalizing 60 min, pre-cool 3 min quenching, slow cool 20 min, oil temperature 70°C | Grade 4 | Grade 1 | Grade 2 |
| 4th Heat: 840°C equalizing 30 min, slow cool 25 min, oil temperature 60°C | Grade 4 | None | Grade 2 |
Result analysis: Although the martensite and retained austenite in the 1st heat were qualified, the grade was 4, which did not fully utilize its advantages, possibly due to too slow cooling speed. For the 2nd heat, the quenching temperature was increased from 820°C to 840°C, and the slow cooling time was extended from 30 min to 40 min. By changing the cooling speed through slow cooling, a relatively better carburized metallographic structure was obtained. Based on the experience of the 2nd heat, for the 3rd heat, the oil temperature was increased from 60°C to 70°C, which can effectively reduce the core temperature before quenching and reduce quenching stress. The equalizing time was extended from 30 min to 60 min, and the slow cooling time was shortened to 20 min. The metallographic structure showed martensite and retained austenite structure with a grain size of grade 4. Pre-cooling treatment reduced the quenching temperature of the tooth part but did not have a positive effect on the shaft part and the carburized and quenched structure. For the 4th heat, the pre-cooling process was canceled, and the oil temperature was lowered. The martensite and retained austenite were grade 4. Next steps:
① Reduce the quenching temperature to 820°C to lower the quenching hardness of the shaft part.
② Cancel pre-cooling, add short-term strong cooling quenching, shorten the slow cooling quenching time, control the final cooling temperature of the tooth part and shaft part to 80–150°C, and use self-tempering to reduce quenching stress.
(2) Experiment 2:
Quenching temperature 820°C, cancel pre-cooling, add short-term strong cooling for 3 minutes, shorten slow cooling time to 20 minutes, oil temperature 60°C. The process parameters and results are shown in Table 2.
| Process | Martensite and Retained Austenite | Carbides | Core Structure |
|---|---|---|---|
| 5th Heat: 820°C holding 4 h, strong cool 3 min, slow cool 20 min, oil temperature 60°C | Grade 1 | Grade 2 | Grade 1 |
| 6th Heat: 820°C equalizing 60 min, strong cool 3 min, slow cool 20 min, oil temperature 60°C | Grade 2 | Grade 1 | Grade 1 |
Result analysis: From the metallographic analysis of the 5th and 6th heats, the metallographic structure has significantly improved compared to Experiment 1. Although the structure of the 6th heat is good, there is significant internal stress, and cracking occurred during subsequent keyway machining. By measuring the quenching return temperature and slice hardness, it was found that the cooling time of the 5th heat is relatively reasonable but can still be shortened. For the carburized and quenched slice with diameter φ120 mm, there is incomplete quenching, and the core hardness is uneven. After quenching at 820°C, the hardness decreases, and there is a certain hardness gradient. Next steps:
① Quenching temperature 820°C, retain the strong cooling process, determine the time based on the module size or the required hardness area of the shaft surface through experimentation, extend the holding time to further reduce quenching thermal stress.
② Based on the tooth hardness after quenching, discuss the feasibility of canceling the cold treatment to reduce stress. Initially, determine to control the strong cooling time within 3 minutes.
(3) Experiment 3:
Quenching temperature 820°C, extend equalizing time, control strong cooling time within 3 minutes, oil temperature 60°C, determine slow cooling time based on the oil outlet temperature. The process parameters and the metallographic structure of the carburized and quenched accompanying samples are shown in Table 3.
| Process | Martensite and Retained Austenite | Carbides | Core Structure | Remarks |
|---|---|---|---|---|
| 7th Heat: 820°C equalizing 3 h, strong cool 3 min, slow cool 15 min, oil temperature 60°C (carbon potential set 1.05%, measured 1.04%) | Grade 4 | Grade 4 | Grade 1 | Without cold treatment |
| 7th Heat: 820°C equalizing 3 h, strong cool 3 min, slow cool 15 min, oil temperature 60°C (carbon potential set 1.05%, measured 1.04%) | Grade 3 | Grade 2 | Grade 1 | With cold treatment |
| 8th Heat: 820°C equalizing 4 h, strong cool 3 min, slow cool 22 min, oil temperature 60°C | Grade 4 | Grade 4 | Low carbon martensite + indistinct free ferrite, Grade 2 | Without cold treatment |
| 8th Heat: 820°C equalizing 4 h, strong cool 3 min, slow cool 22 min, oil temperature 60°C | Grade 3 | Grade 2 | Low carbon martensite + small amount of free ferrite, Grade 3 | With cold treatment |
Result analysis: The carburizing equalizing time was extended to 3–4 hours, allowing the shaft core to cool down to the equalizing temperature. The holding time is longer, increasing production costs. After quenching, the final cooling temperature of the shaft was measured. For the 7th heat, the tooth temperature was 120–140°C, and the shaft temperature was 100–120°C. For the 8th heat, the tooth temperature was 100–120°C, and the shaft temperature was 110–150°C. The calculation method for cooling time is reasonable. From the slice hardness, the hardness within 5 mm of the surface of the φ120 mm test slice is 1–2 HRC lower than in Experiment 2, and the core hardness is 1–3 HRC lower than in Experiment 2, with the core hardness not exceeding 35 HRC. The cooling parameters of Experiment 2 and Experiment 3 are similar, but there are differences in hardness. The sample blocks without cold treatment have martensite + retained austenite at grade 4. If the process fluctuates slightly, the组织级别 may exceed the standard. Therefore, according to the cooling parameters (strong cooling time controlled within 3 minutes), cold treatment is necessary regardless of the control perspective of hardness and organization.
Solidification of Carburizing and Quenching Parameters for Power Gear Shaft
Verification of solidified carburizing and quenching process parameters at a quenching temperature of 820°C: equalizing 30 minutes, strong cooling 30 minutes, slow cooling 30 minutes, oil temperature 50°C. The test parameters and the metallographic structure of the solidified test process carburized and quenched samples are shown in Table 4.
| Process Parameters | Martensite and Retained Austenite | Carbides | Core Structure |
|---|---|---|---|
| 820°C equalizing 30 min, strong cool 30 min, slow cool 30 min, oil temperature 50°C | Grade 2 | Grade 1 | Grade 1 |
Under the conditions of 820°C equalizing 30 minutes, strong cooling 30 minutes, slow cooling 30 minutes, oil temperature 50°C, the metallographic structure of the test blocks shows martensite and retained austenite, carbides, and core structure all at grade 1, with good performance and no cracks generated.
Using the process procedure of quenching temperature 820°C, equalizing 30 minutes, strong cooling 30 minutes, slow cooling 30 minutes, oil temperature 50°C, with relevant equipment and process program numbers 36#+37# for batch testing. The process parameters and the metallographic structure of the carburized and quenched samples are shown in Table 5.
| Process Parameters | Hardness | Macrostructure | Microstructure |
|---|---|---|---|
| Carburizing + air cooling + high temperature tempering + quenching (strong cooling 30 min, slow cooling 30 min during quenching) + cold treatment + secondary tempering | 38–41 HRC | No significant macrostructure defects | Surface carburized layer structure: fine martensite + carbides + retained austenite; Core structure: lath martensite + small amount of undissolved ferrite; Inner layer structure (core): lath martensite + undissolved ferrite + small amount of bainite |
Using the process of carburizing + air cooling + high temperature tempering + quenching (with strong cooling 30 minutes and slow cooling 30 minutes during quenching) + cold treatment + secondary tempering (36#+37# process), the carburizing curve fluctuates normally, and the structure also meets the requirements. The surface layer was turned to check for cracks, turning 1 mm each time until 5 mm deep, observing after each turn. No cracks were found. After several months of storage, no cracks were observed. The 36#+37# process meets the carburizing and quenching requirements for the power gear shaft.
Conclusion
The design and development of the top drive reducer are mainly used in coal mine roof water hazard management, mine rescue, and other mining safety aspects. In the field of coal mining, the top drive reducer provides power output guarantee for safer mining of deeper coal seams. The top drive reducer designed in this study has greater torque and, equipped with a power cutting head, can achieve vertical drilling of over 1,000 meters. It has broad application prospects in oil and natural gas extraction, geological exploration, mine management, and other fields. The gear shaft design and carburizing quenching process ensure the reliability and durability of the power transmission system, making the gear shaft a critical component in these applications.