Couplings can be divided into two categories: rigid couplings and flexible couplings.
Rigid couplings can not buffer and compensate for the relative displacement of the two axes, and require strict alignment of the two axes. However, this type of coupling has simple structure,can ensure large transmission torque. Commonly used are flange couplings, sleeve couplings and clamp couplings.
Flexible couplings can be divided into flexible couplings without elastic elements and flexible couplings with elastic elements. The former type only has the ability to compensate the relative displacement of the two axes, that is, varying degrees of axial, radial and angular compensation, but it cannot buffer and reduce vibration. The common ones are slipper couplings, gear couplings, universal couplings, chain couplings, etc.The latter type with elastic elements,in addition to has the ability to compensate for the relative displacement of the two axes, also has the buffer and damping effect, including flexible couplings with nonmetallic elastic elements and metal elastic elements, However, the transfer torque is limited by the strength of the elastic element,inferior to the  flexible couplings of inelastic elements. Commonly used are elastic sleeve pin couplings, elastic pin couplings, plum coupling, coupling withe rubber type element, falk flexible coupling, flat spring coupling,etc.