Question concerning motors and axels

[corrado33]

So I'm new here....  :'( but anyway, I just got interested into building some robots and I had a quick question.  I was doing some modeling with sketchup and I was wondering something.  If my motor has a 6mm shaft, and my wheel has a 12mm keyed hole... how to I connect them?  The only thing I could think of was using sprockets and using a chain or timing belt to connect the two, but doesn't that lose efficiency? Not to mention it takes more room in my (already packed) robot.  Oh, and it'll mess with the gearing if the two sprockets aren't the same size.  I guess I could also use gears, but still, is there an easier way?  Also, how do I bring the axel through the wall of the bot.  With a bearing?  How exactly are bearings used in this case?  The axel just goes right through them and that's it?  

Aha found the FAQ, please ignore the next section.  
Next question.  I see all these bots on the internet using servos (usually modified) as drive motors.  Why not just use little motors? I know the only difference is that a servo is a motor that has some circuitry that allows it to move to a specific position(angle of rotation) which is very useful if you're making a robot arm or a automatic fish feeder, but not so much for a drive wheel... So is there any other advantage to using a servo rather than a motor?

I know this might seem advanced, but I'm no noobie when it comes to building projects or working with materials or even programming.  I have extensive experience in all three, I've just never built a robot before.  

Oh and I'm planning on using an old (or rather decently new) palm for my brain (following the CMU palm robot as an example).  I like the idea and it's a lot more powerful than just simple micro controllers.  

Thanks in advance!

EDIT:  Now that I think about it, I guess I could just make my own coupling.  I don't think it'd be that hard, just cut a piece of round cylindrical stock to size, then drill the appropriate size hole on each side.  It'd need to be perfectly centered though...  And drill in a hole for a set screw or somethin.  Anybody have any experience with this?  

EDIT #2:  Ok, with a little more research, it probably seems that making my own coupling isn't that great of an idea, cause it'll probably break something.  Here's the wheels I want to use... http://www.omniwheel.com/transwheel/transwheel-2000.htm  What wheel do you think would be the best?  I like the double row wheels, but I don't know if I'm going to be able to connect to them.

[waltr]

There are other wheels that have a 6mm ID.
http://www.lynxmotion.com/Category.aspx?CategoryID=39
http://www.superdroidrobots.com/shop/item.asp?itemid=383&catid=16
http://www.trossenrobotics.com/store/p/3126-VEX-Omni-Wheel.aspx?feed=Froogle

Article:
http://www.societyofrobots.com/robot_omni_wheel.shtml#omniwheel

[SmAsH]

The other option is to use a hub...

[corrado33]

The other option is to use a hub...

Wow... I feel dumb... I guess it's just not as elegant and I really didn't think of it lol. 

Ok now how do I get the axel through the wall?  Bearing?

[ArcMan]

Two common options are a ball bearing or a sintered bronze bushing (the cheaper option).
You can find stuff like this at Stock Drive:
http://www.sdp-si.com

Use a numbered drill about 0.005" smaller than the bearing O.D. and press fit it in.  You actually want the hole about 0.002" smaller, but the drill bit will wallow out about 0.003".  If you get the hole too big, a trick I use is to tap around the edge of the hole with a chisel to mar the metal.  The hole will then hold the bearing.

[corrado33]

Thanks for the replys guys.  I think I'm going to actually go with the wheel I have cause I found a (cheap) coupling that'll work with the different diameter axels.  Next question though.  So let's just say (this is just an idea) that I wanted to use gears to connect my motor to my wheel.  How would I stop the axel coming from the wheel from moving back and forth?  Does the bearing hold it in place?  Would I need a shaft collar?  If I used a shaft collar, wouldn't it rub against anything I put it next to (most likely the bearing?)? 

Again, I appreciate all of your help. 

[ArcMan]

Some larger bearings (like for a 1/2" shaft) have set screws to hold the axle in place, but most small bearings don't.
So, as you said, I use shaft collars (or hubs or gears, etc.) to hold the shaft in place.  I put nylon bushings between the shaft collars and the bearings/bushings to avoid wear.
Here is a picture of a little contraption I made for a robot contest.  Look closely for the white nylon bushings.



http://www.bennetwilliams.com/img/robots/DSCN0429.jpg

[corrado33]

I see, thanks!  However I've figured out a way for things to be much simpler, check out my cad design thread.

http://www.societyofrobots.com/robotforum/index.php?topic=9417.0
 

[redux]

Some larger bearings (like for a 1/2" shaft) have set screws to hold the axle in place, but most small bearings don't.
So, as you said, I use shaft collars (or hubs or gears, etc.) to hold the shaft in place.  I put nylon bushings between the shaft collars and the bearings/bushings to avoid wear.
Here is a picture of a little contraption I made for a robot contest.  Look closely for the white nylon bushings.



http://www.bennetwilliams.com/img/robots/DSCN0429.jpg

ahh, not to get sidetracked, but what does the mouse trap do?