If you are using your current method (without spinning the blade), weight paint it to "parent" it to the axle bone like this so that it turns correctly.
One (easy, almost cheating) way to animate it would be to make it a separate object. If the armature is already working correctly with the rest of the mesh, it is normally tough to go back and add more details. Even if I was building this from scratch, I would definitely make a part such as that a separate object.
The Blender Foundation had a problem similar to this when working on Cosmos Laundromat last year.
And guess what? Hjalti added a pocket! Since it’s too late to update
the rig (see the Alien Victor image above), Hjalti created a patch of
cloth with a rig and added it frame by frame to the animation. You’ll
find this animation on the Cloud as well.
Your problem will be much easier. Just parent the (separate) blade to a bone along the axle that it rotates on, and animate it as an object. I will not go into any more detail except to say that it can be done quite easily.
Add a "blade_control" bone that is parented to the axle control bone, add the needed vertex group, and then weight paint the blade onto the bone.
- Be sure to set the "center" of the blade control bone correctly (I set the cursor to the base of the axle control bone in the Gif)
- Parent the control bone with offset
- Make sure the weights for all of the other vertex groups are cleared for the blade
- Make sure the control bone only touches the blade (everything else is blue for it's weighted vertex group). To do this I used face select mode and hit L while hovering over the blade.
I had to split this into 2 Gifs:
As for animating at this point, you animate the rotation of the blade control bone with keyframes as you would with any other object.