According to its main functions, gear transmission can be divided into motion transmission, power transmission, or both. The main function of gear transmission is motion transmission. The research goal is to improve the transmission accuracy. The main function of gear transmission is power transmission. The research focuses on the following aspects: improving load-carrying capacity, reducing vibration and noise, improving transmission efficiency, saving manufacturing resources, improving processing efficiency and avoiding environmental pollution. In the papers of two international gear conferences held in Germany in 2010 and 2013, most of them focused on gear transmission with power transmission as the main function, and the main tasks of the research institute are shown in Figure 1. Based on the analysis of literature, this paper introduces the research status of gear transmission in terms of improving load capacity, reducing vibration and noise, improving transmission efficiency, saving manufacturing resources, improving processing efficiency and avoiding environmental pollution.
For the transmission of power based gear transmission, load-carrying capacity is a basic performance requirement, how to improve the load-carrying capacity of gear transmission has become the research focus of gear researchers all over the world. In order to improve the bearing capacity of gear transmission, it is necessary to understand the failure mechanism of gear transmission, and then put forward the methods to improve the bearing capacity of gear transmission. The main failure forms of gear transmission are gear tooth fracture, tooth surface pitting, tooth surface wear, tooth surface gluing, plastic deformation, etc. These failure modes are related to root bending stress, tooth contact stress, tooth sliding speed and direction, tooth dynamic load, surface processing quality, material heat treatment performance, lubrication state and other factors. For a specific form of failure, some factors play a major role, while the other plays a secondary role. Therefore, it is necessary to adopt different design and manufacturing methods for different failure modes to control the main influencing factors to improve the bearing capacity. At present, the tooth profile design is mainly used to reduce the bending stress of the tooth root, the contact stress of the tooth surface and the sliding of the tooth surface, the dynamic load is reduced by the dynamic design, the lubrication state of the tooth surface is improved by the lubrication design, and the surface processing quality and the performance after the heat treatment are improved by selecting the appropriate cold and hot processing manufacturing methods. In this paper, the latest research trends in gear design, dynamic design, lubrication design and gear surface processing are reviewed based on the literature analysis of the international gear conference in Germany.
1. Tooth design
In order to improve the load-carrying capacity of gear transmission, international gear researchers have made many innovations or improvements on the basis of the existing gear profile. The involute bevel gear can make the intersecting shaft transmission realize the line contact, which has the characteristics of high strength, good rigidity and stable transmission, so it is paid attention by the gear researchers. Fuentes et al.  studied the tooth surface modification method of involute bevel gear for the purpose of improving the bearing contact performance and reducing vibration and noise, and put forward a new modification method, which makes the tooth surface contact stress drop more than 50% compared with the traditional tooth surface contact stress. Traut et al. Also studied the tooth surface modification method of involute bevel gear, and verified that the method can be used in the interrupted working gear shaper or grinding machine by machining simulation. Traut et al. Studied the influence of manufacturing deviation on the bearing contact performance of involute bevel gear, and pointed out that compared with cylindrical gear, involute bevel gear is more sensitive to manufacturing error and tooth surface modification. Tsai et al. Studied the spiral bevel gear and put forward a kind of high load spiral bevel gear with approximate line contact. The spiral bevel gear has the characteristics of larger load capacity than the traditional bevel gear and insensitive to manufacturing error. As a kind of gear suitable for unidirectional transmission, people pay more and more attention to the asymmetric tooth shape. Asymmetric involute profile has the following advantages:
1) The bending strength and contact strength of gear teeth are improved;
2) The tangential contact force is reduced, and the micro pitting on the tooth surface is reduced;
3) It has good dynamic performance;
4) It is not sensitive to center distance error;
5) It is easy to process, and can be processed by rolling, shaving and grinding the same as ordinary involute gear.
Kruger et al. Discussed the basic problem of the design and calculation of the bearing capacity of the asymmetric involute profile, and put forward a calculation method of the bearing capacity of the asymmetric involute profile which can consider the plastic deformation. Dorofev et al.  optimized and calculated the stress of asymmetric involute tooth profile. Bercher et al. Carried out simulation and Experimental Research on the dynamic characteristics of asymmetric involute gear, and pointed out that the use of asymmetric tooth profile can balance the contradiction between noise reduction and load capacity improvement. Masuyama et al. Simulated and analyzed the strength of asymmetric involute gear. Alipiev studied the asymmetric involute gear with few teeth. Seidler et al. Studied the bevel gear with asymmetric tooth profile, and pointed out that the bevel gear with asymmetric tooth profile may be widely used in the situation with low requirements for reverse transmission.
2. Dynamic design
In the study of gear bearing capacity, dynamic design mainly focuses on dynamic load. Based on dynamics, Schlecht et al. Analyzed the tooth stress in the elastic range. Qin et al. Studied the dynamics of the wind power gearbox under variable working conditions and the dynamic load suppression of the gear teeth. Bai et al. Calculated and analyzed the dynamic meshing forces of gears under different working conditions. Schlecht et al. Analyzed the stress of the gear considering the dynamic characteristics of the gear transmission system. Weber et al. Simulated the dynamic tooth load and local strain history of involute gear. Beermann et al. Carried on the joint simulation to the dynamic and static load of the gearbox. Baumann et al. Calculated the load sharing coefficient and bearing capacity of planetary gears based on the dynamic analysis of planetary gears. Papies et al. Carried out multi-body dynamics simulation on elevator gearbox to obtain gear load spectrum, and then predicted gear life. Hu et al.  considered the dynamic characteristics of gear transmission system, established a stress-based fatigue calculation model of involute gear based on the mixed elastohydrodynamic lubrication theory, predicted the pitting fatigue life of involute gear, and pointed out that the surface roughness has significant influence on the critical stress distribution and fatigue life. More and more attention has been paid to the design of power split transmission based on dynamics. Liu et al. Established the translational and torsional dynamic model of composite planetary gear to study the influence of manufacturing error on dynamic load sharing, pointing out that the influence of manufacturing error of different parts on dynamic load sharing behavior is totally different. Wang et al. Established a simplified dynamic model of the gear transmission system with four paths of energy diversion to study the influence of the installation error on the load sharing, pointing out that the installation error of the low-speed stage has a greater impact on the load sharing than that of the high-speed stage, so it is necessary to study and control the installation error of the low-speed stage gear.
In order to improve the load-carrying capacity of gear tooth surface, the gear surface is usually treated, such as coating, changing the texture of gear tooth surface, shot peening, carburizing / nitriding, etc. These treatments can improve the Meshing Conditions of the tooth surface, reduce the failure of the tooth surface, and play an important role in improving the bearing capacity of the gear transmission. Stahl et al. studied the influence of surface texture and coating on friction coefficient and transmission efficiency by measuring friction coefficient and transmission efficiency on double disk tester and FZG efficiency tester. Fujii et al. Studied the effect of WC / C coating on the surface friction, wear and surface durability, and pointed out that WC / C coating can improve the life of gears, but when the coating peels off, its life is equivalent to or lower than that of uncoated gears. Krzan et al. Studied the scratch performance of WC Coating under the condition of reducing the immersion depth, and pointed out that WC coating can significantly reduce the scratch of gear, thus improving the bearing capacity, and the wear particles of coated gear are smaller than those of uncoated gear. Miyachika et al. Studied the influence of tooth surface carburization and tooth end Carburization on the residual stress and bending strength of gear teeth. Morikawa et al.  studied the effect of fine-grained shot peening on the tooth surface durability of vacuum carbonitriding gear, pointed out that vacuum carbonitriding can improve the bearing capacity of the tooth surface, but the shot peening after vacuum carbonitriding can reduce its bearing capacity, and the polishing after shot peening can make up for the shortcomings of shot peening and improve the strength of the tooth surface. Hohn et al. Studied the influence of grinding burn on the bearing capacity of gears. Klocke et al. Studied the influence of the residual stress caused by the finish machining of the hard tooth surface on the gear life.
4. Lubrication design
With the enhancement of people’s awareness of environmental protection, gear researchers also consider the impact of lubrication design on the environment when designing gear transmission lubrication. Brecher et al.  introduced the concept and design idea of environment-friendly gear lubrication system. Through the chemical transfer coating of biodegradable lubricant in the process of heavy-duty gear transmission lubrication, a high-performance gear surface lubrication film was formed, so as to realize an environment-friendly lubrication system. Ohshima et al. Conducted an experimental study on the influence of the flow characteristics of lubricating oil in and out of the gear meshing area on the lubricating performance. Li et al. Studied theoretically and experimentally the relationship between the micro pitting corrosion in the contact area and the lubrication conditions. Kubo et al. Used the contact bending fatigue testing machine to study the contact bending fatigue of the tooth surface caused by the crack expansion of the tooth surface caused by friction under the condition of mixed lubrication. Snidle et al.  studied the prediction mechanism of the mixed lubrication and the contact fatigue of the tooth surface. Jamali et al. Studied the influence of tooth surface modification on elastohydrodynamic lubrication of tooth surface, and pointed out that tooth top modification will produce high contact stress and thin lubricating oil film. Horl et al. Studied the low friction seal with texture sliding surface, and put forward a kind of surface texture which can obviously reduce the sealing loss of transmission system.