Most commonly used couplings have been standardized or normalized, in general, only need to correctly select the type of coupling, determine the type and size of the coupling. When necessary, it can be vulnerable to the weak link of the load capacity check calculation; When the speed is high, the centrifugal force on the outer edge and the deformation of the elastic element should be checked, and the balance check should be carried out. Coupling can be divided into rigid coupling and flexible coupling two categories. Rigid coupling does not have the ability of buffering and compensating the relative displacement of the two axes, which requires strict alignment of the two axes. However, this kind of coupling has simple structure, low manufacturing cost and assembly and disassembly. Easy to maintain, can ensure that the two shafts have a higher neutral, the transmission torque is larger, widely used. Commonly used are flange coupling, sleeve coupling and sandwich coupling, etc. Flexible coupling can be divided into inelastic element flexible coupling and flexible element flexible coupling, the former class only has the ability to compensate the relative displacement of two axes, but can not buffer vibration reduction, common slider coupling, toothed coupling, universal coupling and chain coupling; The latter type contains elastic elements, in addition to the ability to compensate the relative displacement of the two axes, but also has a buffer and damping, but the transmitted torque is limited by the strength of elastic elements, generally less than the inelastic elements flexible coupling, Common elastic sleeve pin coupling, elastic pin coupling, quentin coupling, tire coupling, snake spring coupling and spring coupling, etc.