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Unread 12-20-2002, 05:48 PM
Simon G Simon G is offline
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Question Thunder Chicken CCT

This is specifically directed towards Paul on team 217, but if anyone else can help, I'd appreciate it!

I'm very interested in understanding and perhaps building a version of the CCT (the one that's in the white papers) I've been doing quite a bit of analysis of the CCT setup, and I have some questions:

1. I calculate the last stage of the bosch gearbox to have a 4.31:1 ratio, with a sun of 13 teeth, and a ring of 43 teeth. However, I counted 14 teeth on the planets, which seems to be one tooth short... why do they allow such an amount of backlash?

2. Given the above gear ratio, and the ratio of the initial gear train between the chip motor and the sun, I end up with a total gear ratio of almost 61.6:1, with a 276 max RPM (if you run the chip motor at 30Amp max RPM of 4048) Correct?

3. I assume you don't exceed 276 RPM for the angular speed of the ring gear (the one driven by the worm). Thus your maximum speed ratio goes from 1:1 up to 4.31:1. Could you actually run the ring faster than the sun, to get a output of faster than the max RPM? That doesn't seem like a good idea...

4. OK, my main question:

Given the above ratio, and the torque that the system puts out at the max torque of the input motor (I get almost 36 NM @ 30A), your reaction torque (ring torque) is about 28.5 NM. Yes?

With a worm rato of 30:1 (60 teeth / 2 thread worm), you need 0.95 NM of torque @ up to 8280 RPM to drive the worm gear assembly. Yes?

At 30A each, the drill motor gives you 0.157 NM, and the FP motor 0.174 NM, which doesn't add up to enough torque to spin the worm. Furthermore, at that load, the FP will only spin at 7700 RPM. Correct?

How do you get this to work? It seems to me that you would be tripping the breakers on the drill and FP motors at high loads, as you're asking too much of them. Or do you have a further gear reduction outside of this box, such that you don't need as much reaction torque?


Sorry for the long message, but I'm truely enthralled by your design, and I'd like to understand it further. It's quite a neat idea.

Thanks!
Simon G.
Mentor, Cheshire HS
Sikorsky Aircraft

Last edited by Brandon Martus : 12-20-2002 at 09:27 PM.
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Unread 12-29-2002, 01:31 PM
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ThunderChickens CCT

Simon,

I apologize for taking so long to get back to you, but I have been on vacation. I will answer in the order you asked your questions.

1. The reason is interference. There are 3 different interference conditions to deal with when designing planetary gears. Bosch chose to allow more backlash to avoid the interference, because backlash is not that big of a deal for the kind of drill it was designed for.

2. Your gear ratio is slightly off, but here is the exact number:
(44/9) * (48/16) * (43/13 +1) = 63.18:1
That ratio is the speed ratio of the output if the ring gear is not spinning. At max speed of Chip that would be 88 RPM. The sun gear will spin at 375 RPM at max chip speed. The speed at 30 amps is important, because that is the speed you will be going at max pulling time. That speed is 64 RPM at output and, as you said, 276 at the sun gear.

3. Oh, but we do spin the ring gear faster than the sun. Our design speed for the ring gear is 600 RPM. That would make our total speed ratio range from 63.18:1 to ~8:1. You are right that there are some limitations when spinning the ring gear that fast.

4. I will try to address all your concerns here. All of the things you said in question 4 are true, however, we never spun the ring gear while at design load!! The worm gear setup I chose gave us pretty good backdriving resistance. What I mean is that the FP and Drill did not have to work when sitting still (most of the time). The point at which a worm gear set is backdriveable is highly dependent on 2 things: lead angle (number of threads) and Mu between the gear mesh. Every once in a while we would start backdriving the worm gear and it would look like we were in neutral. We weren't in neutral, but we were in a no move situation. You could stop us with just a pinky finger, but you could not move us back with 200 lbs pushing force. We did not like this condition, so we had a "pulsing" button on one of the joysticks that would pulse power to the FP and drill to stop backdriving. After a few trials (and burnt motors, as you eluded to), we got the pulsing routine to work. We did not have any further gear reduction, just used the backdriving resistance of worm gears.

Now, our design intent for the CCT had 2 main objectives: Max pushing force of 280 lbs and max speed of 10ft/sec. We knew you had to get to the goals fast and once you had them it did not matter. What we found is that with 1 goal, we could spin the ring gear at 600 rpm and move quite easily (drained the battery), but once we had 2 goals and transferred the weight ... no more ring gear spinning except for anti-backdrive. We had it designed for a specific purpose, but if I were to design it so I could use the spinning gears at all times no matter what, then I would spin the ring gear at about 75% of the sun gear speed at max speed (contrary to our 200% of sun gear speed). This only gives you a 3:1 jump in gear ratio as opposed to the 4.31:1 jump you talked about. The advantage of the 3:1 jump is that you get max torque available at all times, even at high speed; but you do not get the variation in gear ration that you require.

The one thing I will do differently this year is LOCK the ring gear in place when I am slow/torque mode, probably with pneumatics. You can do it with the CCT as designed, but need to add some holes. Another thing, do not directly couple the motors to the worm shaft as I did. Put the motors below the worm shaft and add another gear stage (1.5 or 2:1), it will make replacement easier.

I hope I helped.

-Paul
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Unread 01-03-2003, 10:59 AM
Simon G Simon G is offline
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Thanks & more

Thanks for the detailed reply. It was very helpful. We actually looked at a similar design here at Sikorsky a while back for large scale helicopter applications...!

One more question. Your current worm lead angle is very close to 10 degrees (9 deg, 24' to be exact), which would give you backdriving in extreme conditions, especially with your smooth, ground worm...

Why didn't you use a single thread worm and a 30 tooth gear instead? (thus cutting your lead angle)

Was it a geometry (packaging) issue, or am I missing something critical? Your worm shaft thrust would go up with a smaller worm gear, but the torque requirements would be the same, right? Does the efficiency drop with a smaller lead angle? Is there a limitation on RPM with a single thread worm too?

Thanks!
Simon
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Unread 01-03-2003, 11:12 AM
Simon G Simon G is offline
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one other thing

Sorry, but one more quick question...

It's unclear how you actually attach the drill ring gear to your housing that holds the worm gear. I saw there was a note ref. on your 111-000 assembly drawing, but I didn't see the actual note. It's all steel... are you welding it or brazing it together?

Thanks again
Simon

PS... did you actually spend ~ $130 per bearing for the Kaydon reali-slim part?! (we have a very slim budget) Or do you have a contact who might donate something like that?

Last edited by Simon G : 01-03-2003 at 11:16 AM.
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Unread 01-03-2003, 01:18 PM
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CCT Answers

Simon,

The worm lead angle has a huge impact on efficiency. I would really want to use a quad thread worm, but I needed some backdriving resistance, so I chose dual. A single lead was just too inefficient.

The ring gear is press fit into a steel part (TC-2002-111-005)
that the worm gear (TC-2002-111-001) is bolted to. We used liquid Nitrogen (just because it is fun to play with) to shrink the ring gear and then pressed it into the steel part.

FANUC Robotics (the company I work for) is the largest non-military customer for KAYDON. I know our sales representative personally and he gives me any standard KAYDON bearing for free. In return, KAYDON is placed as a sponsor to our team. So, to answer your question; no we do not pay one penny for the KAYDON bearings. If you decide to use the CCT and need a couple of those bearings, I can probably get you a couple.

-Paul
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