"Automatic Transmission"

[Tytus Gerrish]

Ahh Cool im getting mentioned now as an inventor

I Personaly Can,t stand automatics Its All Manual For me, But on an atonomus robot i see the advantages And i Didn't steel my idea from the thunderchickens I have only known about it for a month now when Brandon said a similar device already exists, ive been ctudying the CCT and it uses the same mechnical princapls as the "Gerrish Gearbox" But It Is Very Diffrent!. I went with a reverce diff thinking its how Dual-Engine Helicopters merge their power. Ive yet to draw the Srag chutches to prevent backdrive in my models, its gonna get in there tho

BTY: I NEED AUTOCAD!

[Jnadke]

Originally posted by Tytus Gerrish
Ahh Cool im getting mentioned now as an inventor

I Personaly Can,t stand automatics Its All Manual For me, But on an atonomus robot i see the advantages And i Didn't steel my idea from the thunderchickens I have only known about it for a month now when Brandon said a similar device already exists, ive been ctudying the CCT and it uses the same mechnical princapls as the "Gerrish Gearbox" But It Is Very Diffrent!. I went with a reverce diff thinking its how Dual-Engine Helicopters merge their power. Ive yet to draw the Srag chutches to prevent backdrive in my models, its gonna get in there tho

BTY: I NEED AUTOCAD!

Nobody ever really said you stole any ideas. We were just saying that the concept has been done before.

Both from a physics standpoint and from a design standpoint a bevel gearbox "differential" is the same as a planetary gearset, only re-arranged. In both cases, the stall torques need to be the same on both motors in order to prevent backdrive (unless a clutch mechanism is used), and in both cases the overall speed of the gearbox is the average of both motors.

More discussion has already been done here. The differential you have designed is merely a planetary gearset with the carrier fixed.

[P.J. Baker]

Not quite...

Tytus' tranny is not "merely a planetary gearset with the carrier fixed." Think about it. He has two inputs and one output. Holding the carrier fixed in a planetary gear set leaves you with only one input and one output. As I see it, the design is more like a special (and physically impossible) case of a planetary set where one input bevel is the sun and the other is the ring:

The output speed of Tytus' differential will ALWAYS be the average of the speeds of the two inputs. The output speed of a planetary gear set (i.e. the rotational velocity of the planet carrier) will only be equal to the average of the sun gear speed and the ring gear speed if the sun gear and the ring gear happen to be spinning at the same speed (when this happens the planet gears stop spinning and the sun, ring, and carrier all have the same speed).

Looking at the equations that relate the speeds of the sun, ring, planets, and carrier to each other, the only way that the carrier speed is the average of the ring speed and the sun speed is if the sun and ring gears have the same radii – which is not possible. So, like Paul C. has said before, the bevel differential is a close cousin of the planetary gear set, but it is not the same thing. As far as his claim that the CCT is not a differential, I’m not so sure, but I think that is more a matter of semantics than physics.

P.J.

[Paul Copioli]

PJ,

No. A differential and a planetary set (like the one is the CCT) are different. The differential, by definition, has 2 outputs (or inputs); for the car example it is the 2 side gears. The CCT, and planetary gears in general, only have one sun, one carrier, and one ring gear. The differential for a car (and Tytyus' implementation) has 2 suns, 1 carrier, and one ring gear. Unlike the differential, the planetary gearset with 2 motors has independent control of each motor's Torque/Speed profile (i.e. one will NOT override the other if they are both supplied with power.

-Paul

[P.J. Baker]

The analogy between the differential and the planetary set is as follows:

one of the input bevels is the sun

the other input bevel is the ring

the intermediate bevels are the planets

the ring gear is the carrier.

There are not two suns, it's just that the sun and the ring are the same and are actually interchangeable.

In a standard planetary set, if the planet gears are made infinitely small, the sun gear and the ring gear approach eachother in size and the behavior is almost identical to the differential design proposed by Tytus.

P.J.

[Paul Copioli]

P.J. is right. The differential idea proposed by Tytus is exactly like the CCT with a 1:1 ratio between the sun and ring gear. Hey, even us unsung FIRST heroes are wrong from time to time.

That being said, only one motor contributes to the output torque of the system. The other motor acts like a speed increaser at the same output torque (if balanced correctly). All the equations derived in my whitepaper for the CCT apply to Tytus' design.

-Paul

[Jnadke]

Originally posted by Paul Copioli
P.J. is right. The differential idea proposed by Tytus is exactly like the CCT with a 1:1 ratio between the sun and ring gear. Hey, even us unsung FIRST heroes are wrong from time to time.

Yes, I was going to mention that, but ran out of time. The bevel/differential gearset can can do the one thing that the planetary gearset can't: achieve a 1:1 gear ratio between the sun and ring gears. In order for this to happen, the sun gear would have to be large enough such that it approaches the size of the ring gear. Of course, this can't happen, but calculus says the limit exists. As the sun gear size increases (and therefore the planet gear size decreases), the gear ratio decreases (assuming that the ring gear is the input and the sun gear is the output and the planet carrier is fixed).

Of course, luckily for us, the stall torques of FIRST motors are never the same. So a more compact dual-motor gearbox can be made by simply using a planetary gearset, taking advantage of the difference in gear ratios.

That being said, only one motor contributes to the output torque of the system. The other motor acts like a speed increaser at the same output torque (if balanced correctly). All the equations derived in my whitepaper for the CCT apply to Tytus' design.

-Paul

Actually, both motors will contribute to the output torque of the system. However, in order to avoid backdrive, both motors must be configured to have the same stall torque. So basically, you would end up with twice the torque of a single motor. However, the overall top speed will be the average of the two individual motors.

This is complete opposite to the standard spur gear method of combining motors, where the free speed must be configured the same on both motors. The overall torque will be the sum of the two motors, but they will share the load unequally.

[Andy A.]

I relize this thread is getting off topic, but I just have to ask.

Why not just use 2 identical motors on a differential type gearbox? As in, use the two drill motors on the left side, and the two Atwood motors on the right and then just gear them to give you the same speed. With two motors of the same stall torque you avoid the main detraction of a diff gear box, which is the lower torque motor being over powered when stalled.

I'm still not completely clear on what happens next, when the motors actually start to show their torque differences. Would the side powered by the more powerful motor 'overpower' the other side in a pushing match, and pull the 'bot to one side? Am I correct to think that the drills would be geared higher, and the Atwood's geared lower? Or does the Diff mix this all up??>

Like I said, I don't really understand how it all works out- I've yet to receive any formal mechanical education. I know that 121 has used two drills on one side and two Atwoods on the other (this past year I believe?). From what I remember, they used a fairly traditional transmission, except the two were not identical. My team has never used multi motor gearboxes, so I'm a little in the fog here.

Am I just missing something painfully obvious here? I get the feeling I am. Again, sorry for pulling this thread further off topic, but my curiosity is killing me.

-Andy A.

[Jnadke]

You wouldn't notice any difference, except that the drill motors are slightly more powerful than the CIM motors, so one side will accelerate faster than the other. Although you won't really notice it in pushing matches, or when cruising at top speed, you will notice it everywhere in between. So, when accelerating from top to full speed your robot would travel in a slight arc.

Also, it's extremely hard to match free speeds exactly. Although, in general, the numbers will be so close it won't make a whole lot of a difference. Especially if you have a 4-wheel drive robot. If you have a robot that's 2 wheel drive with casters you will definately notice it, other than that.

Next year, the situation could change though. This year you could have gotten away with it, because the power of both motors are approximately the same (the drill is 50W more powerful, I think, which is 8-10%).

[Tom Schindler]

Originally posted by Andy A.

Why not just use 2 identical motors on a differential type gearbox?

I'm not sure what type of gearbox they used, but i'm pretty sure that 121 used both drills on hte left, and both chips on the right last year.... someone from their team might be able to give more detail

Tom

[Solace]

that is definitely what 121 has done for the past couple years. I have talked to members of that team, and they say it works great.

[ahecht]

Originally posted by Jnadke
Also, you have the issue of torque transfer between two non-interlocking metal surfaces. How did you guys solve this problem? I suppose you could use a high COF material and use acid-etching to make the metal surface porous. Or am I making this problem larger than it is?

Yes, you are. The toroids were made of machined cast iron (only because FIRST didn't allow steel thicker than 1/4" that year), and were very smooth. We ran CVT at fairly high speed, and did further gear reduction afterwards, so that there was never that much torque going through it. We never had any trouble with the it slipping.

[ChrisH]

After seeing 190's CVT I was a little research on something else and found to my surprise that the coefficient of friction for iron on steel was actually fairly high. BUT the rolling resisitance is very low and the contact area is small.

Which explains why locomotive wheels are still cast iron. Actually it might have been in a railroad book that I found that little gem. Unfortunately I don't remember the reference.

[Madison]

Originally posted by ChrisH
Which explains why locomotive wheels are still cast iron. Actually it might have been in a railroad book that I found that little gem. Unfortunately I don't remember the reference.

Locomotives also drop sand on the rails at times to increase their traction.

[Jnadke]

Originally posted by ChrisH
Which explains why locomotive wheels are still cast iron. Actually it might have been in a railroad book that I found that little gem. Unfortunately I don't remember the reference.

Actually the coefficient of friction of Cast Iron on Steel is 0.4. This is compared to the coeficient of friction of Steel on Steel which is 0.8. By comparison, some rubbers (neoprene, gum) can achieve a COF of 3.0 or more on most surfaces.

The real reason cast iron is used is that it won't deform (compared to steel). Steel can deform as much as 35% before it fractures, whereas cast iron will only deform less than 1%. This is a good thing when you want the wheel to stay round under all that weight. Also, cast iron as better vibration dampening than steel. Keeps everything from rattling as it goes over the many joints of the railroad tracks.