I can show you example, but you must read HELP before asking so simple questions!
BTW, I work in SW2015, so the following images will help you.
This video show you what we'll have https://youtu.be/kEXS4TJeSi8
What we have?
Inner cylindrical face of the gear
Outer cylindrical face of the shaft
Two similar planes, e.g. RIGHT plane of GEAR & RIGHT plane of SHAFT
The distance between the 2 faces
Now you can move gear horizontally in the shaft (changing the distance) and then rotate the gear by rotating the shaft
Gear mates make yourself.