Hi,
i am doing the ‘rigging mechanics’ tutorial from the documentation.
http://www.blender3d.org/documentation/htmlI/x7891.html
can anyone tell me what the ‘pivot axes’ are? are they an empty object? or how can i generate this kind of object?
thanks.
From that tutorial
“For the sake of simplicity here there are only the two axes for the piston, made with plain cylinders”
In other words, it’s those two cylinders highlighted in white in figure 57.
thanks. it was to easy. ‘manchmal sieht man den wald vor lauter bäumen nicht’ as we say it in german (means something like: sometimes you can not see the forest because of so much trees)
Don’t worry, I read it a few times and looked at the image then read it again before I was convinced of what it referred to.
Some people would say it was “hidden in plain view” 
The main thing to think about when doing steam-engine style mechanics, or “reciprocating anything,” is that it’s often easiest to build the system such that the moving-parts move on their own, and the pieces which in real-life would drive the machine … follow those moving parts. You will become very familiar with the “track to” constraints.
One complexity that was not discussed at all in the aforementioned demo is what to do if the driving-cylinders are not parallel to the center of whatever they are supposed to be moving. Rotations and such always occur in a straight line. To deal with this, you can use your other good friend, the Empty. It’s an object in every respect, except that it consists only of a single invisible point. But it can be parented to other objects, and objects can be parented to it or can be made to track it.