Avoiding motor stalls

I broke another 9V motor. See my <http://news.lugnet.com/technic/?n=14319
original post> for the first story.

I'm building what amounts to a pitching machine for the Great Ball Contraption.
In most TECHNIC building you're building a gear train to produce lots of power
at the expense of speed. But in this case I need to build one that has lots of
speed and not much power.

I have a tire that spins {very} quickly, and as the soccer balls roll past it
they are accelerated and launched into the air. It's really cool, and I've been
working with a prototype structure to hold it all together, so it's flimsy.
Eventually I want to rebuild it to withstand day-long use, and that also means
fixing the gear train, because I've broken two motors with it so far.

In the prototype, I put a clutch gear right on the motor output, but even with
that precaution the motor has a hard time turning if something completely stops
the spinning tire. In both motors, something inside them eventually broke under
the strain, so now they are very difficult to turn. I definitely want to avoid
this in the future.

I have no camera available at the moment to show you what it looks like, so I'll
do my best to describe the gear setup.

The motor has a clutch gear on its shaft. This is attached to a 40 tooth gear
with a chain. On the other end of this gear's axle is another 40 tooth gear with
another chain going to a 16 tooth gear on a third axle. This spins another 40.
This 40 turns an 8 tooth gear on a fourth axle, which also turns a 24 tooth
crown gear, which turns an 8 tooth gear on a fifth axle. This last axle has the
tire on it.

If you can sort that out in your mind, this means the tire is spinning
{quickly}. Very, {very} quickly. It is fast enough that trying to stop it with
your finger is very easy, and that's when the motor stalls. Actually, it doesn't
technically stall; it still turns inside the clutch gear, but it has to work
very hard to do so.

What advice can you give me to avoid breaking any more motors? The clutch gear
works in normal gear trains because the motor doesn't have to work very hard to
begin with: it spins quickly and produces lots of torque farther down the line.
But here I'm doing the opposite. If I had an RC motor I would try using that,
since they are more powerful, but right now I'm stuck with normal 9V motors. Oh,
I also have one of those weird motors (no axle, but an axle hole) from the
monkey set, but I don't know how it compares to regular ones.