All good stuff
The easiest way to visualise a spur gear arrangement is to look at the picture 7 minutes and 12 seconds into this Utube video MichaelG supplied
(one mouse click stops the movie, another mouse click starts it)
Then in your minds eye, turn every gear into a spur gear, the main drive gear arms will have a separate spur gear arrangement to each drive shaft, so you will have two seperate spur gears at the top and bottom of the arms
To generate the "differential effect" the two spur gears now required at the top and bottom of the arms will need to be offset and meshed with each other as well as being meshed with their own respective driveshaft
easy peasy lemon squeezy
Edited By Ady1 on 20/10/2012 10:21:32