​Couplers transmit force; an example is connecting a motor to a gearbox or a gearbox to an axle or an axle to a wheel. Each device needs a way to connect to the other. The rotating part of a motor is the shaft and different motors have shafts of different shapes and sizes; to connect something to a motor, you'll need the appropriate coupler.

There are many ​off-the-shelf hubs that are designed to connect rotating things.

For lighter loads, you can make 3D-printed couplers. For example, the Denso window motor can be coupled using:

• to a 1/2" keyed shaft using ​this coupler
• to an off-the-shelf hub using ​this coupler