I am having issues with the pivot constraint I am making a steam shovel model and trying to do a simple rig with no bones. I threw on a pivot constraint so that I could move the main arm and still have it rotate around an axis. It works great, but now when I try to move the whole machine (ie. turn it on the z-axis), it goes kind of crazy. Here are some pictures to show what I mean: 

Note that I have "Always" selected for the "Pivot When:" option. I tried changing this to just the Y-axis to no avail, so that is not the answer. For those curious, here is the .blend file
It is a Loop Dependency. You can't make a children object parent father of is father.
A constraint look like a parent in Blender, so you have to make some parents organization in your file.

Arm Rotation is Father of Shovel.
But, Arm Rotation is in the Pivot constraint of Shovel, so, it's child of Shovel.

This parenting are avoid, you have to use bones and armature to simplify.