pic: Gerrish Gearbox MK-III #2

[Bill Beatty]

Tristan

I missed the 4:1 gearing of the drill motor. I agree, the differential is unnecessary.

I am wondering how the team (I don't know their number) that built the transmission I saw in Chicago is doing. I am not sure I agree with the design, but the workmanship was superb. Anyone out there who can bring us up to date with some info?

[ChrisH]

Originally posted by Bill Beatty
I am wondering how the team (I don't know their number) that built the transmission I saw in Chicago is doing. I am not sure I agree with the design, but the workmanship was superb. Anyone out there who can bring us up to date with some info?

From your earlier post, that sounds like Team 217, The Thunderchickens. Mentored by the current UFH Paul Copioli. They called it the CCT (Crazy Chicken Transmission)

[Gadget470]

By the by, Paul made a white paper for 217's 2002 version.

[Paul Copioli]

I sent a private message to Tytus, if he wishes to share it with the group I have no objections. Basically, I agree will Mr. Beatty regarding the differential. It is unnecessary in this design. Remember that the differential is meant to allow torque transfer to 2 axles from one input and also allowing the axles to travel at different rotating speeds. Its major design limitation is that it will transfer torque to the path of least resistance (one tire on pavement and one tire on snow.. snow wins and car goes nowhere).

There are other uses for a differential (redundancy in helicoptors, etc.), but speed matching is not one of them.

Regarding the CCT , it uses a planetary gearset with one motor coupled to the input (sun) gear and one to the ring gear. When selected correctly, you can get a very wide speed range with the SAME output torque. We had a worm gear because we cheated on our gear selection to get more speed. The result was backdriving of the ring gear. If we had used a different gear ratio, the back driving would not have occured (we have experiments to prove it), but we decided we needed the speed.

It took a sacrifice of 2 regionals (Chicago included) to get it right, but we all thought it was worth it. It was working by the 2002 Nationals (ask team 27 ... sorry Kyle, I had to add that). We have our patent application in and our pending approval from the U.S. patent office.

If you need further details regarding the CCT, please look in the whitepapers section.

-Paul

[Adam Y.]

Its major design limitation is that it will transfer torque to the path of least resistance (one tire on pavement and one tire on snow.. snow wins and car goes nowhere).

Why not use another differnential design that goes around that limitation? That may work backwards insteand of a regular differntial.

[Madison]

Originally posted by Adam Y.
Why not use another differnential design that goes around that limitation? That may work backwards insteand of a regular differntial.

The only way to overcome that weakness is to build a torsen or limited slip differential -- both are quite a bit more complicated.

[Jnadke]

This gearbox still doesn't address the issue of two motors having dissimilar torque characteristics.

Contrary to what Bill Beatty said, such a gearbox is excellent if you have two motors having identical torque but different free speeds.
To extend on what Bill said, the team was the Thunderchickens, 217.

When you think of gearboxes and motors, you can't think of movement. You have to think of the potential to do movement. In other words, you have to do think about forces acting within a gearbox. If you have a 300lb man, and a 100lb man, each leaning against eachother on a 45 degree angle, which will prevail? The obvious response is the 300lb man, because he has more force, or weight.

It's similar with motors. If you have a motor that outputs 500Nm of torque, and another motor that outputs 600Nm of torque. When such a gearbox encounters a position of stall (fighting against a stronger robot, for example), the extra 100Nm of torque will be wasted back-driving the opposing motor.

I'm not saying such a gearbox is bad. I'm just saying it's highly inefficient.

However, such a gearbox can be made to be extremely compact, if you think along the correct lines. You're focusing on a completely wrong topic here. The issue of two motors with dissimilar torque characteristics has already been solved by teams such as the Technokats by merely gearing down one motor.



EDIT: I didn't see/read the 2nd page.

I see you've added the gear-down of the drill motor since your initial design, that's a good start, but still you're focused on the wrong topic.

As for the people saying the differential is redundant. They way I see it, the differential is serving as a torque transfer mechanism. Most shifting gearboxes I've seen are large and bulky with a 3-stage gear mechanism. Using bevel gears as a way to combine the first and 2nd stages together, as this gearbox does, is a good start at creating a smaller and more efficient shifting gearbox.


Design Theory
One thing you could do is use different size bevel gears in the differential to combine the 4:1 geardown into the gearbox itself. I would mate the Chiaphua motor to the 1st stage using a shaft coupling. I would then, on the other side, mate the mate the Drill to the 1st stage using a 2:1 reduction. For the differential, I would use a 15 tooth and a 24 tooth (16 pitch) bevel gears to create a 1.6:1 difference between the two shafts. This would serve as the torque-transfer mechanism. I don't know if it's possible to match up different size bevel gears of the same pitch, but I'd assume it would be safe. The bad side is that you'd only be able to use 1 intermediate bevel gear instead of 2 as standard in a differential, so you'd have to support the shafts at the bevel gears so they maintain contact.

Basically, the gear closest to the chiaphua would be spinning at 5,500 RPM free-speed, and the gear closest to the drill motor would spin at 10,000 RPM free-speed. Match these gears up to the 2nd stage shaft normally as illustrated (using a 2:1 or more reduction I'd assume).


However, I'd need to note one major flaw in the gearbox design. In both cases, the intermediate bevel gear of the differential would have to be fixed (the gear that combines the two shafts), or no torque would be transferred. In which case the 1st stage gears would need to be spinning in opposite directions...
If you don't see where I'm going with this, your gears in your shifter would be spinning in opposite directions. Whenever you wanted to shift, your robot would suddenly stop, and move in the opposite direction...

[Paul Copioli]

Jnadke,

What you describe in your design theory section is exactly what the CCT is with using bevel gears. The differential without the second gear, as you describe, is not a differential at all. It is a planetary gear set. As you describe it, it would work EXACTLY like the CCT from 217, but it is not a differential. By definition, when you remove the second inside bevel, it is no longer a differential.

I will put together a little paper showing the physics (with numbers and equations) behind Tytus' gearbox design, but I am fairly confident it will behave as Bill Beatty and I have described.

-Paul

[Jnadke]

Yes, this is true. It should be noted that none of these designs are differentials. A differential has 1 input and two independent outputs. These are merely bevel gearbox designs.

Indeed, the gearbox will behave as you described. I was referring to Bill Beatty's original message, which when I read it now, I must have misunderstood what he was getting at. Somewhere in my long message I may have forgotten what my original point was, so just chuck it into the garbage.


In either case, both traditional spur gearbox design and a planetary design each have distinct advantages when it comes to multiple motors. With the former, you have to match free speeds, and with the latter, you have to match stall torques.

[Tytus Gerrish]

a diffrental is basicly a planetary gearset with Bevels, it may take some staring at it to see, but its true

[Bill Beatty]

Tytus

An interesting observation concerning differentials. I applaud your imagination and your thinking outside the box. Keep it up.

Your analogy is a bit backwards, but in the ballpark. Some comments.

Differentials do not need to be built with bevel gears. Although bevel gear differentials are the most common, there are other configurations, the simplest of which can be built using only spur gears.

Differentials are most commonly used with one input and two outputs, however they are used with two inputs and one output, just as with you proposed gearbox.

A planetary gear arrangement can be used as a differential and I agree with you, the Thunder Chickens are using it as a differential.

Keep at it. Keep stirring the pot. You just never know what might come out.

[Paul Copioli]

O.K. Mr Beatty (and Tytus),

Let's break down a differential in planetery gearset lingo:

1. The input from the engine (transmission) is the equivalent of a ring gear in a traditional planetary (team 217 drives this gear also.

2. The ring gear in a differential (the big gear that houses the differential gears) is really the carrier in a planetary set and it is being driven ONLY by #1 and is not the output. The 217 CCT carrier is the output and is driven by the sun gear (see below) and the ring gear.

3. The little differential gears are the planets in a planetary set and they are free to spin. Same as 217 CCT.

4. (The biggest difference) The side gears in a differential that drive the wheels are free to spin and are both outputs for the system. They would equate to the sun gear in a planetary set. Equal torque is applied to each wheel no matter what.
The 217 CCT uses this as another input (the main driver), so it is a true planetary, not a differential.

The main difference between the differential and the planetary, in general) is the differential has 2 sun gears (the output to the wheels) that are being used as outputs. The planetary gearset has only one output. The CCT, in addition to the differences above, also uses the carrier as the output and the ring gear and sun gear are inputs.

While I agree that a differential is a relative of the planetary gearset, the 217 CCT is NOT a differential.

-Paul

BTW - I agree with Mr. Beatty that a differential can easily be used with spur gears instead of bevels.