how can I position an object at a moving bonehead? I would like to draw a steam locomotive with rotating wheels, driven by a horizontal rod.

Cannot find an Object Constraint to couple an empty to the head of a rotating bone.

The base of a second bone should follow the head of the rotating bone.
The head of the second bone should move lineairly on one axis.
The base of a third bone (the horizontal rod) should be connected to the head of the second bone.

3        2/\
---------/  \1
You don't tell enough but if you just need a basic animation you can do it this way:


Create a 3 bone armature. Top horizontal bone is parented to right bone.





Switch to Pose mode, select the left and right bones, open the N panel (on the right of your 3D View) and in Transform > Rotation, choose the XYZ Euler mode instead of Quaternion. Create a first pose for those bones, then a second at 180° (on the Z axis for me), and in the Dopesheet click ShiftE > Linear Extrapolation. Now your bones rotate indefinitely.





Still in Pose mode, select the top bone, go in the Properties panel > Bone Constraint and give it a Track To constraint (might be a Copy Location as well) with the left bone as Target, a Head/Tail value of 1, To > Y, Space > Pose Space/Pose Space. This bone will follow the movement.