The RV (rotate vector) reducer has been widely used in the joints of agricultural robots, due to its compact structure, high torque, and small transmission errors. However, it is often subjected to high duty and frequent load, leading to surface damage (such as scuffing of internal components) and a significant decrease in the load-carrying capacity. The present study aims to improve the load-carrying capacity of the RV reducer in complex field work environments. The problem of contact surface damage was also solved during crop planting and harvesting. The surface coating strengthening was applied to the cycloidal gear tooth surface. Tetrahedral amorphous carbon (ta-C) film was coated on the cycloid gear tooth surface using plasma-enhanced chemical vapor deposition (PECVD). A series analysis was then carried out. Firstly, a high-precision virtual prototype was constructed to identify the dangerous working positions and the maximum contact stress during cyclic transmission. The theoretical basis was provided for the fatigue performance and damage behavior testing. Secondly, the numerical analysis was conducted on the coexistence of rolling and sliding during transmission. The entire cycle was summarized to identify and then avoid the severe points of the sliding wear. The finding also provided a strong reference to select the work positions in practical situations. Finally, a high torque acceleration degradation test was conducted under different experimental conditions on the RV reducer comprehensive performance test bench. The RV reducer was disassembled to characterize the macroscopic wear of the key components. The microstructure and elemental distribution of the wear area after the test were observed using material characterization. The results showed that the maximum meshing force was generated when the meshing phase angle was equal to 46.19º. The cycloid-pin pair showed that there was an outstanding linear contact and the contact stress, which increased from both sides to the middle to 346.13 MPa. The ta-C coating significantly reduced the frictional stress of the tooth surface pairs. The contact stress of the outer ring of the needle-roller pair was greater than that of the inner ring. Once the meshing phase angle was equal to 180º, the relative sliding speed reached the maximum of 0.18 m/s. Furthermore, the pendulum pair exhibited pure sliding, particularly when the meshing phase angle was equal to 46.65º. It implies that the present point was the position with the most severe sliding wear. The rolling speeds of the cycloid wheel and the needle wheel both approach 0 at the present position, indicating a trend of mutual restraint between rolling and sliding. The point around 46° was an extremely dangerous position in the transmission cycle. After coating modification, the efficiency reached 94.94% under rated conditions, with an increase of about 20 percental points. Also, the root mean square (RMS) of the vibration signal and the periodic spikes were reduced with the smoother transmission. Severe fatigue wear occurred on the surface of the key components in the uncoated reducer, with the localized flaky peeling and pitting. The reducer exhibited the localized conventional fatigue wear. The oxidative wear was significantly reduced after coating modification. According to the experimental observation, the ta-C coating shared the anti-friction, anti-impact, and anti-oxidation properties, which significantly improved the overall load-carrying capacity of the RV reducer. The findings can provide important references to optimize the load-carrying capacity of the RV reducer in agricultural robots.

Scuffing is one of the most typical damage forms in the gear transmission of agricultural machinery. The fast bursting speed and serious damage degree should be strictly avoided in the conditions of the low lubrication and extreme loading during the gear service. This study aims to improve the scuffing resistance load carrying performance in the transmission gears of agricultural machinery under severe lubrication, variable speed, and heavy-duty conditions. The surface coating strengthening was applied to the meshed tooth surface. A mathematical model was established for the relationship between the contact properties of coatings and gears, as well as the scuffing load capacity. The gear meshing, tribology, and thermodynamics were also utilized in this case. The finite element model of plane strain was selected to explore the influence of elastic modulus ratio between coating and substrate on the stress field distribution. Anti-scuffing gear coatings were achieved to clarify the effect of tooth surface frictional coefficient on oil film thickness and transient contact temperature, according to the calculation in ISO/TS 6336-22. Then, the carbon films were prepared by low-temperature plasma enhanced chemical vapor deposition technology. Surfaces of standard steel balls were coated with the tungsten containing ta-C and a-C:H coatings. Tribological properties of coating materials were evaluated using the four-ball method. A series of experiments of scuffing resistance were performed on the two coated gears in the FZG (Forschungsstelle für Zahnräder und Getriebesysteme) transmission test rig. The results showed that the carbon film friction pairs had the better running-in performance, compared with the uncoated components. The a-C:H coating presented the higher sp² C-C bond content than ta-C, resulting in a lower frictional coefficient. Also, the scuffing resistance capacity increased by 2 FZG loading stages in the ta-C coating, while at least 4 loading stages were found in the a-C:H coating one. In the process of engagement, the ta-C coating was peeled off, and then the peeled coating particles were pressed into, adhered to the contact surface, or discharged from the scratched surface, thus forming abrasive wear between the tooth surfaces, and finally the gear steel substrate was completely exposed. Furthermore, the failure mode was changed from abrasive wear to adhesive wear with the increase of frictional temperature. The surface material was subject to adhesive tearing. Moreover, both uncoated and ta-C coated tooth surfaces showed the significant competitive relationship between the thermal scuffing and micro pitting damage, which was mainly determined by the oil film thickness and contact temperature. In addition, the concave plastic deformation near the driving wheel pitch line was found on both surfaces. The a-C:H coating showed the conventional fatigue wear with the smooth and flat wear trace, where the tooth surface coating had the high integrity without outstanding damage. Theoretical analysis and experimental data were combined to enhance the gear scuffing resistance capacity. Although the frictional coefficient was low, the ta-C coating was more prone to the coating peeling and interface damage, due to the high elastic modulus ratio between coating and substrate. By contrast, a-C:H coating shared the relatively low elastic modulus ratio. The interface stress was smaller and more difficult to peeling, and the frictional coefficient was smaller, resulting in a relatively thicker oil film thickness and lower transient contact temperature on the tooth surface. The achievements demonstrated that the a-C:H coating exhibited the excellent scuffing resistance load carrying performance suitable for gear transmission. The finding can lay the foundation for the application of coating strengthening technology in the high-performance agricultural machinery gears, even the other transmission systems with harsh service conditions.