How to make animation curves linear?

I want to make a seamless animation in Unity for a rotating obstacle.

However the curve of the animation refuses to become linear. I've tried changing the key frames from "clamped auto" to "auto" as most tutorials instruct me to do. When that didn't work, I tried changing the tangents to linear, but that didn't work either...

Am I missing something or is there a way to fix this?

Edit: The curve is linear near the last key frame, meaning there is no slow down at the end of the animation. The curve near the starting key frame refuses to become linear which makes the animation not seamless...

 

A better solution would be to script this, especially if you are using the animator instead of the animation component, since the animator has a lot of overhead.

Nevertheless selecting all keys and then doing this:


Should result in completely linear animation.