Ok, basically, I went for compact with this one. It’s a two stage gearbox, the second stage is a planetary built in to a custom wheel. The wheel is 10" in diameter, final ratio is 1:19.2. Comments?
Can we get a view without that outermost plate and bearings so we can see the gears inside?
Is a 10 inch wheel too big? (I don’t know, so I ask), and as big, I mean overly large for the size of a robot that would be made for a FIRST competition…
Also- would it be possible to make this a 2 speed?
Sanddrag, I’ll post an exploded view tomorrow showing the planetary gearset.
Craig, my team has used 10" wheels with much success since the 2002 season. Also, I suppose you could replace the first stage with a shifter but that would defeat the point somewhat, this is meant to be simple and compact.
I have been thinking about an in wheel transmission/gearbox like this, but the thing that worries me most is impacts and side loads.
I suppose if you can find a solution to that it would be great.
Keep the ideas flowing.
Matt,
Is the ring gear attached to the wheel? If so, what forces the wheel to turn? Most planetaries will allow the moons to spin around the sun when the outer ring is stationary.
I suppose this will be cleared up when you post the exploded view. I believe you want the moons attached to the wheel with the outer ring fixed to the frame.
As I understand it, having seen the exploded view, the sun gear is on the motor output, the planets are stationary, and the ring gear is locked to the wheel. If I’m not mistaken, this setup will cause the ring gear to spin in the opposite direction as the motor, with a reduction equal to the number of teeth on the ring, divided by the number of teeth on the sun gear. Is this correct?
Over all, I was pretty impressed with this design. Matt showed it to me a few weeks ago, and I was considering using it in a summer project, though I ended up going in a slightly different direction. It seemed to me like a pretty easy, compact way to do a direct-drive gear reduction.
It’s not shown in the picture (yet) but six bolts will hold the ring gear to wheel, it’s actually 1/8" thick, while the wheel has an 1/8" flange, total wheel thickness 1/4". The Sun and Moon gears are all idling on shafts (see those bearings in the plate? those are for the Sun and four Moon gears) while the ring will spin, and in turn the wheel. I will post the exploded later today, for now I need to clean my room before my parents yell ;).
I just posted the exploded views, two are already approved, the other two should be shortly, I hope this clears up any questions.
This is the same as I see it. I missed the planets being fixed. Do you know what the ratio would be if the outer ring is fixed and the wheel is attached to the planets?
A very interesting design. The first of the summer season!
are you sure about this gear thickness? are the planet and sun gears an equal thickness? and what DP are these gears? while i don’t have any calculations (i’m in the library “studying” for a final), 1/8" seems a little thin to me.
Very cool. That could be a very interesting design, not only for drive wheels, but for other devices.
Here’s my question - What keeps the ring gear from moving cross-vehicle? I think there are a couple of ways to solve this, but I was wondering what you had in mind.
what’s going to support the side load of the wheel? . looks like all the weight of the robot is going to be forced onto the bottom planet gear and In all my experience gears don’t like to be smashed together like that at all. , also if that wheel takes any side load it’s going to come off. both of those things can be solved with a big bearing. but Where Re you going to get one? those big bearings are hard to find, not to mention expensive. Thats a cool design but if i was planing on making a robot i would use something much simpler and less expensive.
The side load problem is the one next on my list, this is a completely new area for me, so I’m kind of winging it, I have a few ideas to prevent the carrier gear from slipping off, I’m not sure about the downward forces on the planets, I was thinking of rotating them 45 degrees to spread the load across the two bottom gears, this is all brand new territory for me, and as far as I know, FIRST in general, though I could be wrong about that. Does anyone know of any similar designs used in FIRST?
I’m not an expert on gears by any means, but I think just spreading the load across the gears won’t help that much. Gears are meant to operated at fixed distances on fixed shafts. 25 pounds of force pushing the gears into each other isn’t going to make them happy. Even if you could get it down to 5 pounds, you’d still lose a lot of efficiency. I think your best bet is to figure some way to get a bearing supporting the wheel so you’re not using the gears as a bearing sort of surface. This would help with the side loads too.
I’m thinking a big thin section ball bearing that goes around your array of bearings there and slips inside a rim on the wheel. At that diameter a brass bushing might work as well.
Hey nice design. One way to reduce the range of loading on the gears would be to find a way to support the wheel on it’s own bearing. It may have to be a needle roller bearing with a large inner diameter to keep the design compact but it should work out with a bit more structure. That way the wheel loads go to the bearing and the gears don’t have large time varying loads that will occur even without impact with the present design.
Also, you can get away with using fewer gears now that the loads don’t have to be distributed that much. That should save a little on weight.
Thanks for all suggestions, I’m taking them all under consideration, now I have a question for all you gearheads out there…
What the heck is a needlepoint bearing? 
I’m learning, one step at a time…
needle bearings use thin cylindrical rollers instead of a balls (in the case of ball bearings).
Indeed, and for larger diameter shafts, they are generally a whole lot smaller than ball bearings that will get really huge and heavy.
Andy Baker’s suggestion in the other thread was also a really good idea for taking the load off the gears and constraining the wheel motion, just depends on what you choose.
Jevawn
Matt -
Look here for some examples of needle bearings. You can get them straight from the manufacturers (sometimes - depending on how many you want to buy), or from standard sources like MSC, McMaster-Carr (see page 1023 of their catalog), Small Parts, etc.
-dave