Mounting Gears on Shafts

[icius]

Just some basic questions about mounting gears on shafts.  I'm trying to stay with readily available stuff and nothing exotic.  So for the situation where both the shaft and gear rotate there seems to be these options:

1. Pressure fit (not sure how you would go about matching shaft and gear, doesn't sound like a 6mm bore gear would be a pressure fit on a 6mm shaft).
2. Gear with a set screw on a round shaft (seems like this would slip under high load?)
3. Gear with a round bore and set screw on a keyed shaft, like a "D" cut. (does this work?).
4. Gear with a keyed bore and set screw on a keyed shaft (seems like it would be the ultimate but how do you match them up from different suppliers?)

Then there is the option of the gear spinning freely on a fixed shaft(axle).  But how do you match these up?  It is the inverse problem of #1 above.  Does a 6mm bore gear spin freely on a 6mm shaft?  How much friction do you get with this setup?

Then there is the issue of 1/4" vs. 6mm.  Some sites seem to use this measurement interchangeably.  I could see maybe pressure fitting a 6mm bore gear on a 1/4" shaft.  But the other way around wouldn't the gear be too loose? 

In specific I like the selection at this site:

http://www.robotobjects.com/scripts/prodList.asp?idCategory=39

But I'm having trouble finding a nice selection of wheels and shafts that aren't 6mm.

Adam






   

[Militoy]

Your #3 choice has worked well for me - just use LocTite or Vibra-Tite on the setscrew threads. As far as using a 6mm gear or cam on a .250 shaft - that would be quite a press-fit. Why not drill out the center of the 6mm gear to .250 or even .252 (6.4mm)?

[Admin]

Stay away from pressure fit, its a hit or miss kind of thing.

I find keyed shafts are more reliable than set screws, cause set screws have loosened over time on me.

Another option is to drill and tap the shaft so that an extra long set screw can pass through the shaft and out the other end. This is my preferred method.

Why not drill out the center of the 6mm gear to .250 or even .252 (6.4mm)?

easier said than done. you know, every time i attempt to drill out a gear center, I end up unbalancing the gear by shifting the center . . . so hard to line up on a drill press . . . only use a precision mill if you do this!

[Militoy]

every time i attempt to drill out a gear center, I end up unbalancing the gear by shifting the center

When I drill any hole to enlarge it, I drill in stages, going up just a few sizes at a time. I have found that if the part is held down on a drill press just tight enough so it won't spin, but allowed to shift position a bit, the drill will self-center on the existing hole very accurately. It helps to use a sharp TiN-plated bit as well.

Your idea of pinning the gear to the shaft is a good one as well - I just usually don't take the time. I will drill a small indent on the flat part of the shaft, to accept the nose-piece of the setscrew. I've never had one that was treated with Vibra-Tite come loose.