Pumped!

 This morning, I completed a couple of hours of animation training using a Udemy course. It's very good! It's easy to follow and pitched at the right level for me. Sadly, it's already run through the elements that I needed training on. The next six hours are about character rigging rather than rigid bodies. I'll complete the course, but not as a matter of urgency.

I did get a better understanding of some of the foundational animation, amature, parenting, and bone systems, which was helpful to build upon. So, what to do about my rigging woes? I went back to the YouTube videos and rewatched them, and it really helped. I got stuck, and things went wrong, but I went back to the very first video I watched—just one minute long—and I managed to replicate it. It's proof that I'll get what I need eventually.

A pump-type thing with a rigged piston.

A couple of flaws:

The piston rod and leave are not perfectly aligned. You can't see it here, but on the other side of the pump you can see them becoming superimposed. You must use the Shift+S tools that move an object's origin or 3D cursor.

The bone constraint "Limit rotation" wasn't quite working correctly. Pulling or pushing the pump too far separates the rod from the sleeve or pushes the rod impossibly deep so that it goes through the sleeve's pivot. Using the Limit Rotation constraint stops the bone from moving through an impossible axis, such as sideways instead of along the lever's arc. It should also let you set a maximum and minimum allowed angle of rotation to stop things from breaking. It just wasn't working for me. I must not have been reading the angle correctly.

Things I learned today:

When creating bones to use in an amature, duplicate them from existing bones. If you use Add>Armature>Single Bone, the resulting bone is part of a new amature, and you won't be able to parent it with an offset. I saw several commons on various rigging videos where viewers had experienced the same confusion.

Flipping between the modes can be a source of confusion if you slip into the wrong mode:

Object Mode: Move objects, no interaction with a bone's elements.

Edit Mode: Move bones and bone elements. For example, change the size of the bone. In fact, all you can do is add more bones, which will be added at the 3D cursor.

Pose Mode: Move bones within an armature, such as when posing a rig. 

To modify a bone or Parent a mesh to a bone, ensure you're in the edit mode.
To move a bone's position in a rig, ensure you're in pose mode.