simple mechanical linkage

Hi

I am struggling trying to figure out how to animate a mechanical linkage as a proper connected Blender object. I know how to do it by moving the 3 sub objects separately but I would like to know how to do it properly. This link explains in more detail and has a pic and a blend file. It would be great if anyone could explain how to continue this a a small tutorial.

http://blenderartists.org/forum/showthread.php?t=117779

Thanks for any help

Regards Geoff

See that thread, Geoff.