Meshing Gears

[FizMan]

I'm pretty sure there's a thread aboot this somewhere, but me being on a break at work, don't have time to sift through the site.

Anyhow, lots of talk aboot dogs in transmissions letting you 'shift-on-the-fly'. What are the problems when it comes to meshing two gears directly?

______
 |  |
=== |
 | ===
 |  |
______
 
shift to:
______ 
 |  |
======
 |  |
 |  |
______

I hope you get the picture; it's a top-down shot of a simple system pushing one gear to align with a second. (the === are the gears, the | are the axles, and the _____ is the housing)

And is shifting like this not possible on the fly? Didn't that team with the 4 speed automatic transmission do it on the fly? (forgot the team name)

If there's already a thread like this, you can point me in the direction of it too if you'd rather (don't want to waste your time writing a post if the information is there)

Thanks for your help.

[Joshua May]

It's possible, many tranny's are shift-on-the-fly, and the team you're thinking of is Team 33.

I haven't built one of these gearboxes yet, so I don't know the problems, but I'm pretty sure that at least team 116 used a lathe to "round-out" the edges of the gears so that they would mesh better, you just have to make sure that everything is aligned correctly. If you don't get the image of the "rounded gears", they would look something like this from a view facing the gear.

 _____
/     \
|     |
|     |
|     |
|     |
|     |
\_____/

[Lisa Perez]

We TRIED to have a 2-speed transmission ('tried' being the key word ) but one of the main problems that we encountered was that when we shifted, sometimes the two gears would not mesh or "catch on to the other". Thus the shift did not work, and the shifted gear would come in conflict with the other.

There is always that possibility that one gear will not catch on to the other while being shifted (even if the two shafts rotate at the same speed). We ended up with an eaten up gear and we had to change back to a shift-less transmission. But hey, that one worked

[FizMan]

So would a system of meshing the gears like that (rounding them as well) work alright for a transmission where the robot has to stop first, then shift? Any problems with that? What if the gears happen to have their teeth aligned so when you try to push the gear in, one of the teeth hit each other directly on the side? That's where the rounded teeth come in handy?

[Joshua May]

Originally Posted by FizMan

That's where the rounded teeth come in handy?

I believe that's where the rounding comes in handy. Also, at least in shifting while moving, the gears are moving so in the time that it takes to shift, there will be multiple times where the teeth do ot hit each other and should slide together. Now I'm not an expert on shifting gearboxes, I haven't even used one, I'm just familiar with some of the theory in them.

[FizMan]

I suppose it'd be a good idea to fashion multiple spares of the shifting gears in this case, eh? I mean, I suppose it's possible to damage them.

[Lisa Perez]

Originally Posted by FizMan

I suppose it'd be a good idea to fashion multiple spares of the shifting gears in this case, eh? I mean, I suppose it's possible to damage them.

Yeah.. We didn't really think of rounding.. That'd might've helped.

At any rate, does anyone know to what extent the rounding will help? Because if teeth still manage to hit each other on their sides, eating of the gears can still happen. And if one rounds too much, the teeth won't get enough grip on each other. What teams out there did round their gears and what was your experience doing this?

As for the spares.. We welded the gears onto the shafts.. Didn't really think of that, either

[Joshua May]

Originally Posted by Lisa Perez

Yeah.. We didn't really think of rounding.. That'd might've helped.

At any rate, does anyone know to what extent the rounding will help? Because if teeth still manage to hit each other on their sides, eating of the gears can still happen. And if one rounds too much, the teeth won't get enough grip on each other. What teams out there did round their gears and what was your experience doing this?

As for the spares.. We welded the gears onto the shafts.. Didn't really think of that, either

I got this information from the White Paper on 116's tranny. The white paper can be found here, it gives the amounts to round and everything. You'd really have to ask them about how it worked.

[sanddrag]

I would think using a dog type shifter is much better.

[FizMan]

Originally Posted by sanddrag

I would think using a dog type shifter is much better.

What about in transmissions like say, 33's 4 speed one? Does it make that much of a difference? Though I suppose, it wouldn't be that hard to modify it to use a dog instead.


Also, you think there'd be any problems with meshing helical-gears in this fashion without dogs and all? Just moving it to lock up with another helical gear?

[sanddrag]

Originally Posted by FizMan

Also, you think there'd be any problems with meshing helical-gears in this fashion without dogs and all? Just moving it to lock up with another helical gear?

Any rotating part ramming into another rotating (or stationary) part has inherent problems. It's the nature of the business. The better transmission is the one that copes with these problems best.

[FizMan]

What I meant was, are there any UNIQUE problems with meshing helical gears as opposed to umm "normal" teethed gears? (don't remember their name) And/Or are the same issues between the two not as much (or more of) a problem with Helicals?

[Kevin Sevcik]

This is more or less the exact tranny that Team 57 used this year. We had an issue with gears disintegrating, but that was poor design and wasn't the shifting gears. We beefed up that pair of gears and things were fine. We also had an issue with the shifter getting stuck, but that was an alignment problem and was fixed with the judicious application of a hammer.

On to some points brought up in this thread...

Yes, there will be times with the gears won't mesh perfectly when you shift. If you think about it, Dog shifters have this problem as well. Anyways, we had no issue shifting on the fly. Occasionally one side would engage before the other, but it was never very bad. The best thing you can do is shift the trannies with pneumatics. They'll naturally act as springs and keep pushing the gears together till they mesh.

I will say that the proposed method of "rounding" the gears isn't really productive. Especially if you that to both of the meshing gears. That won't really make the gears mesh better because you still have the same basic cross-sections trying to mesh. What you really need to do is dremel the sides of the gears down to points. That makes it much easier for the gears to mesh. You can also make it easier by setting the gears a little farther apart than would be strictly advised by pitch diameters. This increases backlash and makes the gears slightly weaker, but it makes meshing even easier.

Also, you can reduce the chance of breaking the gears by moving the shift as far up the gear train as possible. If you can shift gears on the low torque end of the tranny they don't have to be as strong so you can reduce the size.

Finally, I think this kind of transmission is a bit easier and more forgiving. Dog shifting trannies almost mandate a CNC shop to get the dogs right. A reasonable gear meshing tranny will just need a good drill press with an X-Y table without much slop, and possibly a lathe.

[Lisa Perez]

Originally Posted by Kevin Sevcik

Yes, there will be times with the gears won't mesh perfectly when you shift. If you think about it, Dog shifters have this problem as well. Anyways, we had no issue shifting on the fly. Occasionally one side would engage before the other, but it was never very bad. The best thing you can do is shift the trannies with pneumatics. They'll naturally act as springs and keep pushing the gears together till they mesh.

We did use pneumatics.. but that's part of the reason that the gears ate each other up

[Kevin Sevcik]

I think this is probably the advantage that our method of rounding gave us. the gears more or less had to mesh, so it wasn't really a problem. I think a little time and care with a dremel can make this kind of tranny work rather well.