[patrickrd]

A possibly slightly relevent anecdote, and then my recommendation...

Last week for one of my classes (a fluids dynamics class) we did a pinewood derby truck race at the end of this semester. I am sure lots of you did these in boy scouts. Anyway, it turns out friction between the axle and wheel is a major thing that slows down the pinewood derby car. The default setup is basically a bushing. Since we were required to use the provided wheels and the provided axle (a nail), but there were no rules against wheel modification or using other parts, I decided to install ball bearings After about 6 hours in the machine shop I modded the wheels so that I could install these tiny 1/8" ball bearings. Even though the axle (nail) was smaller than the bearing inner diameter (such that the axle could wobble a bit inside the bearing), the truck was flying down the track. We ended up cruising into 2nd place of about 50 teams... With a very non-aerodynamic truck

So, the moral of the story... If you can use bearings, use them... They have less friction than bushings. On the other hand, if some friction is acceptable (e.g. you are not looking for near 100% efficiency), then bushings might be OK. There are also types of bearings that are smaller than ball bearings... Roller bearings. I have no experience with them, but something to look into if you're interested.

One more thing... Also consider how much force the shaft will have on the mounting surface. If there will be little stress, a bushing will have little friction. However, if you are supporting the connection between the wheel and robot, for example, there will be tons of friction (F = mu * N). Even though you can get the friction really really low with a good machine finish, it will still be proportional to the load. A ball bearing, on the other hand, is not significantly affected by the load as long as you conform to the loads that it is rated to handle.

- Patrick