In the development of domestic coal machinery equipment, the localization of gear reducers has become a trend. However, domestic worm gears reducers often face severe issues such as excessive heating, low transmission efficiency, significant wear on worm gears, and short service life. The wear resistance of worm gears, in particular, has emerged as a critical technical challenge in the industry. As part of our research and development team, we have analyzed the faults in domestic worm gears reducers and investigated factors including materials, processing, assembly, and lubrication. This article presents our findings and improvements, supported by bench tests and underground industrial trials, which demonstrate that the enhanced reducer can operate without abnormalities for over six months in mining conditions.
Worm gears reducers are widely used in mining shuttle cars due to their compact size, high transmission ratio, and smooth operation. Typically, these reducers employ a two-stage transmission: the high-speed stage consists of spur gears, and the low-speed stage involves worm gears. The structure is designed to handle heavy loads and shocks in mining environments. However, the sliding friction inherent in worm gears transmission leads to substantial heat generation and wear, which are primary causes of failure. Our analysis focuses on a specific reducer with an input power of 24 kW, input speed of 1,420 rpm, input torque of 161 N·m, and a total transmission ratio of 34.6. The worm gears stage alone has a ratio of 32.73, which is critical for the reducer’s performance.
The main faults observed in domestic worm gears reducers include severe heating and oil leakage, as well as significant wear on the worm gears. Heating results from the high friction in worm gears engagement, causing thermal expansion that leads to seal failures and oil leaks. Wear, on the other hand, is influenced by material compatibility, surface quality, assembly precision, and lubrication conditions. For instance, after just one month of operation, domestic worm gears often show visible wear patterns, reducing efficiency and lifespan. To address these issues, we delved into the key factors affecting worm gears performance and implemented targeted improvements.
First, we examined the materials used for worm gears. The selection of appropriate materials is crucial for wear resistance and overall durability. In heavy-duty applications like mining shuttle cars, worm gears are typically made from cast tin bronze or cast aluminum bronze, while worms are often hardened steel. Based on our analysis of imported components and industry standards, we chose 38CrMoAl for the worm, subjected to tempering and nitriding, and ZCuSn12Ni2 for the worm gears. This combination offers good anti-galling properties and mechanical strength. The sliding velocity in our reducer is calculated as $$v_s = \frac{\pi \cdot d \cdot n}{60}$$ where \(d\) is the worm pitch diameter and \(n\) is the rotational speed. For our design, \(v_s = 3.48 \, \text{m/s}\), which falls within the range suitable for tin bronze materials. The material properties and their applications are summarized in Table 1.
| Component | Material | Characteristics and Applications |
|---|---|---|
| Worm | 20, 15Cr, 20Cr, 20CrMnTi, 20CrMnMo | Case-hardened (56–62 HRC), ground, for high-speed heavy-duty transmission. |
| Worm | 40Cr, 40CrNi, 35SiMn, 35CrMo, 38SiMnMo, 40CrNiMo | Hardened (45–55 HRC), ground, for high-speed heavy-duty transmission. |
| Worm Gears | ZCuSn10Pb1, ZCuSn5Pb5Zn5 | Excellent anti-galling, low mechanical strength, expensive, for sliding speeds of 5–15 m/s and continuous operation. |
| Worm Gears | ZCuAl10Fe3, ZCuAl10Fe3Mn2, ZCuZn38Mn2Pb2, ZCuSn12Ni2 | Moderate anti-galling, high mechanical strength, requires hardened worm, cost-effective, for sliding speeds ≤8 m/s. |
Next, we focused on the surface quality of worm gears. The micro-structure and physical properties of the surfaces play a vital role in reducing friction and wear. In our open R&D system, where manufacturing is outsourced to specialized enterprises, we emphasized strict quality control. We recommended using advanced machining equipment and implementing a traceable quality management system. For the worm, the final processing step involved precision grinding to achieve a surface roughness within design tolerances. The worm gears were also processed to ensure minimal deviations in tooth thickness and profile. The relationship between surface roughness and wear rate can be expressed as $$W = k \cdot R_a^m$$ where \(W\) is the wear rate, \(R_a\) is the arithmetic average roughness, and \(k\) and \(m\) are constants dependent on material and lubrication. By controlling \(R_a\) to below 0.8 μm, we aimed to minimize initial wear in worm gears.
Assembly precision is another critical factor for worm gears reducers. Misalignment or improper clearances can lead to increased friction and premature failure. We ensured that the housing was accurately measured using coordinate measuring machines to maintain the center distance between the worm and worm gears within specified limits. The backlash between the worm and worm gears was set to 0.38–0.45 mm by adjusting shims on the worm gears shaft. Additionally, we used the coloring method to check the contact pattern, aiming for a contact area of over 60% along the tooth length and 65% along the tooth height, biased towards the exit side to promote oil film formation. The alignment error \(\delta\) can be modeled as $$\delta = \sqrt{\Delta_x^2 + \Delta_y^2}$$ where \(\Delta_x\) and \(\Delta_y\) are deviations in the horizontal and vertical directions, respectively. By keeping \(\delta\) below 0.05 mm, we optimized the meshing of the worm gears.
Lubrication is paramount for the performance of worm gears. Inadequate lubrication accounts for a significant portion of failures in worm gears transmissions. We selected a dedicated worm gears oil with high anti-wear, anti-scuffing, and oxidation resistance properties. The reducer design features an upper worm arrangement, where the worm gears is immersed to one-third of its outer diameter in oil. The oil viscosity \(\mu\) was chosen based on the operating temperature and sliding speed, using the Stribeck curve to ensure hydrodynamic lubrication. The minimum film thickness \(h_{\text{min}}\) can be estimated as $$h_{\text{min}} = 2.65 \cdot R \cdot \left( \frac{\mu \cdot U}{E’ \cdot R} \right)^{0.7} \cdot \left( \frac{W}{E’ \cdot R^2} \right)^{-0.13}$$ where \(R\) is the equivalent radius, \(U\) is the rolling velocity, \(E’\) is the equivalent modulus, and \(W\) is the load. For our operating conditions, we maintained \(h_{\text{min}} > 1 \, \mu\text{m}\) to prevent metal-to-metal contact in the worm gears.
To validate our improvements, we conducted extensive bench tests and underground industrial trials. The bench test simulated the actual working conditions of the mining shuttle car, with a cycle of 40 seconds under full load followed by 5 minutes of no load, repeated over 4 days (8 hours per day). The test rig monitored input power, torque, speed, output parameters, housing temperature, oil temperature, and vibration. The transmission efficiency \(\eta\) was calculated as $$\eta = \frac{P_{\text{out}}}{P_{\text{in}}} \times 100\%$$ where \(P_{\text{in}}\) and \(P_{\text{out}}\) are the input and output power, respectively. Our tests showed that \(\eta\) ranged from 75% to 80%, indicating room for further optimization. After 0.5 hours of operation, the oil temperature was about 30°C higher than the housing temperature, reaching 100°C after 8 hours. This highlighted the need for high-temperature seals in the reducer design. Post-test inspection revealed minor wear on the worm gears teeth and copper particles in the oil, consistent with the run-in phase for worm gears.
| Parameter | Value | Notes |
|---|---|---|
| Input Power | 24 kW | Constant during load phase |
| Input Speed | 1,420 rpm | Constant |
| Output Torque | ~5,500 N·m | Calculated based on transmission ratio |
| Efficiency | 75–80% | Average over test duration |
| Oil Temperature | Up to 100°C | Peak after 8 hours |
| Wear Observation | Minor wear on worm gears | Copper particles in oil post-test |
Following the bench tests, we proceeded with underground industrial trials in a mining environment. The reducer was installed in a shuttle car and monitored over six months. After one week of operation, we replaced the oil and observed a small amount of copper particles, indicating the run-in phase for the worm gears. After one month, the oil change showed even fewer particles, suggesting that the worm gears had entered a stable wear stage. Throughout the six-month period, the reducer performed without any abnormalities, demonstrating the effectiveness of our improvements. This represents a significant advancement over previous domestic worm gears reducers, which typically lasted only 3–4 months. The success is attributed to the holistic approach addressing materials, surface quality, assembly, and lubrication for worm gears.
In conclusion, through systematic analysis and improvement of worm gears reducers, we have developed a solution that mitigates key faults such as heating and wear. The selection of compatible materials, precise control of surface quality, accurate assembly, and optimized lubrication have collectively enhanced the performance and lifespan of worm gears in mining applications. Our tests confirm that the improved reducer can reliably operate for over six months, marking a successful step in the localization of high-performance worm gears reducers for the coal mining industry. Future work will focus on further increasing the transmission efficiency and reducing the operating temperature of worm gears through advanced cooling techniques and material innovations.