So I have a Toro Super Recycler mower, model 20057, year 2005 (early serial number).  It starts and runs fine, but the propulsion mechanism is not working.  I adjusted the linkage and still not working.  When running, I can squeeze the handle, which is supposed to make it move, but it goes nowhere.  If I lift the rear of the mower off the ground the wheels spin.  

I took the transmission apart, and all the gears mesh up fine.  When I try to turn the axle by hand, it turns, but there is some resistance.  

Here is the part that confuses me.  There is a gear on the end of the axle on each side.  That gear meshes with a gear on each driven wheel.  When I take the wheel off, the gear on the axle just turns freely.  I don't see how or what is supposed to engage that gear and make it turn with the axle.  It's the same on both sides.  Is this normal?  What's going on here???

Here is my best guess:  When the wheels are on the ground, there is too much resistance for the drive to overcome it and propel the mower, i.e. the belt slips around the pulleys.  There is something about those gears that meet with the wheels that just doesn't make sense.  If anyone can explain how they are supposed to engage, I would really appreciate it.

Here is the link to the parts diagram if anyone can help:

Toro Parts Page

seems the gear has a woodruf key or a rolepin from the pix.
blow the pix up and see

Sho 'nuff.  I'll be dipped.  I sure didn't see one when I took the gear off the end of the axle shaft.  I'll have to re-check tomorrow.  

Thanks for the keen eye.

does the gear turn freely both ways?  if so the key is wore out or it and the spring is jammed full of shit and stuck down.  Also check the wheels to make sure those arent worn out.