Baseball/tennis ball launchers

[guss]

Why do baseball/tennis ball launchers use two spinning wheels to launch balls, and not one?

You know, these things:
http://static.electro-tech-online.com/imgcache/5399-BG_baseball_pitching_machine.gif

Couldn't they replace one of the wheels with a wall?

I suppose one reason is to eliminate spin, but are there any other reasons?

Thanks.

[Danger]

Spin reduction/control was my first thought, but there might be another stability issue. With only one wheel, the ball might feel obliged to bounce off of the stationary member (rail or chute or whatever). Two wheels keep it airborne.

[enigma]

It's spin.

The goal of a ball launcher is to have an artificial 'sparring partner'. Those wheels rotate so fast that they'd put a completely unrealistic spin on the ball. Even if it made it to the person trying to hit it, it would subsequently careen off in a random direction once it was hit. Not very good to practice against something that a real player could never produce.

[guss]

Thanks, that makes sense. I actually just came across some pictures of machines using only one wheel.

I am not designing a pitching machine, but something like it. Are there other ways besides reducing friction or adding a non-powered wheel to reduce spin on a one wheel machine?

[Danger]

The only thing that immediately comes to mind is that you could make a paddle wheel so it slaps the ball rather than rolls it. I'll apply some thought to it, but that's the first thing that leapt out at me.