Attaching wheels to wiper motors

You will find all kinds of connectors and U-joints through McMaster Carr
http://www.mcmaster.com/

I've done this in the past....

Plastic wheels (lawn mower sort of thing)- Press a nut into the bore of
the wheel, then screw the nut and wheel onto the shaft. Works OK with
light loads, but use thread lock or similar or the wheel & nut unscrews
itself very easily.

With metal wheels - (caravan jockey wheels) I've welded a nut onto a
large steel washer (carefully so as not to damage the threads) then weld
the washer to the center of the wheel. This is very strong and reliable
but the wheel can unscrew itself under heavy load.
Another option is to not weld the nut to the washer but to use it to
bolt the washer (and wheel) onto the shaft, this is only feasible if the
  wheel shaft is of sufficient diameter to get a long reach socket
spanner in there.

For heavy loads, fasten a flat steel sprocket onto the shaft (you might
want to taper the hole to suit the taper on the shaft) with a nut. and
drive the wheel via a chain. That way you can carry a lot more weight on
the wheel.
bicycle chain can deliver a lot of power and is easy to get hold of,
along with tools for splitting and joining it.
I've used this technique in the past to drive wheels varying from 6inch
jockey type wheels to 15 inch bicycle wheels.

If you have the facilities, it's relatively easy to replace the shaft
with a longer one on a lot of wiper motors (well at least the ones Ive
used here in the UK). If you remove the plate from the rear of the gear
housing. The shaft and the nylon gear wheel are often held in place with
a 'C' clip on the shaft under where the nut goes. Once the clip is off,
a slight twist to disengage the worm gear should bring the shaft and
gear out.
Clamp the shaft in a vice and use a hacksaw to cut through the Nylon of
the lump on the back of the gear wheel and the end of the shaft. Try not
to damage the gears.
With care you should be able to push out the shaft (it's splined so
don't try to twist it. you will need to use your vice as a press)
the resulting hole may need to be carefully reamed out to suit a piece
of 'bright' bar the same size as the original shaft.
Drill 2 or 3 3mm holes through the side of a  large 'repair' washer then
weld it onto the shaft.
Slide the gear onto the shaft and drill through the holes in the washer
and fit some nuts and bolts through to clamp the gear to the washer.
I usually leave some shaft out through the back of the gear and drill a
hole in the cover for it to come through (handy if you want an encoder
on the back or two wheels driven from one motor. Put a strong(ish)
compression spring onto the shaft before fitting the modified cover and
it will serve to keep the gear in place without trying to fit a 'C' clip
again.
I've done this mod with a wheel at each end to replace the pedals and
chain in a childs ride on toy.
It gave about 3 years reliable service, being ridden by my child over
uneven ground and often with a friend being pulled along in a traailer
behind...

I don't claim to be an expert on this just my experiences to date. I
have never had access to lathes and milling machines for this sort of
thing, so a vice, hacksaw, small welder etc are all I've ever needed.

Hope this rambling helps
:-)

Jon Sutton.