​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. ​Gears and ​sprockets are also commonly mounted to rotating shafts and used to transmit force (via other gears or a ​chain). In all cases, the goal is to get the force generated by a motor to the mechanism that should be moving (wheels, arms, etc.).

For lighter loads, you can make 3D-printed couplers; you can find all sorts of useful pre-designed parts by searching for FRC on thingverse.com

• 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
• The Denso throttle motor can be coupled using:
  • a ​custom printed gearbox