Beginners Guide: How to connect engine and gear

One of the most asked question is: How to connect engine and gear?
So, there are several ways to solve this problem.

Requirement for all ways (with fixed shaft)
It is recommended that the engine shaft fits perfectly into the gear. I used an engine with an 3,7 mm shaft. This helps to align the gear.
If your shaft is thicker: Make it thinner ;)
If your shaft is thinner: Than it is new for me.
But I have there an idea. Drill a hole into an axle and glue your shaft into the axle. When someone has tried it, please tell us about your experience.

Glue it!
At the beginning, I only glued it with some hot glue. With this construction I created my first video of the 42000 model. But it can't transmit high torques and have problems with heat.

Slit and glue it!
First cut a slit of 5 mm or more into your shaft. Now take some metal which will fit into your slit and cut an rectangle which will fit into your slit, and a gear over both. Glue it now with hot glue.

Mount an axle!
Some engines have no fixed shaft like the Multistar BabyBeast. The shaft is only fixed with 4 screws.

 

This gives us the possibility to print our own shaft, which will fit to any LEGO gear. I have constructed this for an unreleased project. After an successful test, I will publish this .STL file too.