Basic Drive Train Drawings

[Allison K]

Does anyone have any drawings of a basic drive train? something that would be good for a rookie team.

The team decided last meeting that they would like to start doing more technical stuff to get ready for the build, and i figured drive trains was a good place to start. I looked in the white papers but thats mostly fancy stuff that I dont think would be good for our team at this point.

A couple of different ones would be good, as long as they're all fairly simple. A two wheel drive w/casters and maybe a crab drive and well i guess anything else you can think of too.

Thanks,
Allison

[ZACH P.]

check the FIRST website, there are a few exploded veiws of drivetrains.

[Cory]

Crab drive is not something I would call "Fairly simple" I would not attempt it on a rookie team unless it was comprised of many people who had formerly been on teams, and you have good machine shop support.

here is what I would do. A simple 4WD system using the supplied gearboxes would be decent. even better, imho would be a simple sprocket and chain setup with the drill motors. You can use the supplied mounts, and with just a little work mate the motor with another shaft driving a sprocket and chain reduction to the wheels. This is fairly simple, and from what I have seen, seems to be more reliable than the FIRST gearboxes, which dont allow you to chose what gear ratio you think would work best.

Cory

[Allison K]

Originally posted by Cory
Crab drive is not something I would call "Fairly simple" I would not attempt it on a rookie team unless it was comprised of many people who had formerly been on teams, and you have good machine shop support.

Cory

Yeah. I know. We wouldnt use it but the kids expressed intrest in learning what it is and how it works. I figured i could explain it better with a drawing.

[Sachiel7]

If you want, I can post some 3D renders of the "basic" 4wd system I've designed for next year (well, it's one of the designs) anyway, I wouldn't mind posting them up, if you'd like. They use the CIMs on a chain and sprocket system with sprocket reduction. It's a pretty basic design, I guess. The frame is built out of 1" extruded aluminum tubing, and just has some angle cutouts bolted to it at the corners. There are 12 flanged bearings (with set screws ::whew:: ) that support the axles, and bolt the frame together. NOTE: just so you don't get too confused, cause I had a few people flip over the design thinking it wouldn't drive, the wheels aren't locked to the axles. They come with internal ball bearings so they can coast on the axles. This system proved to work great last year, and since we've built it once (sort of) we think we can build it...in a day! Well, we hope. This newer system I designed focuses little on modifying parts, but more on connecting the proper parts. It's also designed to require basic tools to build. I guess I'll post a Whitepaper containing pics and an explanation of it for ya.

I've posted a hoard of pics and information on my coaxal multi-drive design. Just do a search for MultiDrive and it should pop up with some stuff.

[Allison K]

Originally posted by Sachiel7
If you want, I can post some 3D renders of the "basic" 4wd system I've designed for next year (well, it's one of the designs) anyway, I wouldn't mind posting them up, if you'd like.

That would be great. Thanks

Allison

[Mr. Ivey]

You really don't even need 4 wheel drive for a productive robot. Last year my team had Sparky running a rear wheel verticle mounted drive system, using the drill motors, that could easly push around another bot. A crab drive in my opinion isn't as productive overall as a old fashond fixed wheel drive system, just too much stuff to go wrong. If you have something that's innovative yet simple, you are likely to do well with your dive system.
Ivey

[Sachiel7]

Yeah, that's true. We just stuck with 4 wheel because it gave us a bit more traction, and we didn't have to deal with casters and the whatnot. 2 wheel bots can be built right out of the kit using the drill transmissions, I think there's some pics of it here:

http://www2.usfirst.org/2003comp/drive_train-exp.jpg
http://www2.usfirst.org/2003comp/drive_train-expII.jpg

That shows 1/2 of the drive
and a ZIP of my renders is up here:

http://www.chiefdelphi.com/forums/pa...le&paperid=247

Anyway, cheers!

[Mr. Ivey]

Sachiel7, those pictures you gave the links to, you don't even have to get that complicated with drive train design. You can just mount the motors somewhere on the frame, and have the motors in line with the wheels. Just couble the shaft that the wheels are on to the motor, and you have a basic drive system. My suggestion when doing this style drive system is using the drill motors, and remember that one motor is going in reverse. The motors spinning in opposite directions causes one wheel to accellerate faster than the other, but this can be fixed in programming.
Ivey

[Sachiel7]

I know, but I was giving an example of what we used last year, and what we plan to use again next year. We don't use the drills because we don't prefer them for drive (I'm not going to turn this into a Drill vs CIM thread, I'm just saying we prefer CIMs for our drive)
But yes, we had no problem with this system last year. If you wanted something basic, your idea would work fine. The motors didn't have much of a problem with the speed gap, as you said, programming fixed it fine.
Remember also that many rookies fall into the 2 wheel trap and put casters on the front, making turning very difficult.
For rookies reading this thread, make sure your casters follow behind your driving force. They weren't designed to lead.
2 wheel drive can be a very effective style too. It's also a good way to have a quick and easy drive right out of the kit.
But we're home schoolers, we don't live by the rules...

[Mr. Ivey]

Ohh, trust me I don't want to start a debate over drive motors. You perfer the CIM, I like the drill, both have their plusses and minuses. But I would like to make a point on where you put your casters, if you look under the bot my team built last year, you will see some pretty meaty casters. Here is a view under Sparky. While we have not always used casters (Sparky v3.0 was 4wd), we like them because the feel easier to drive with, if you have the proper casters. But still go with whatever your driver is comfortable with.
Ivey

[Sachiel7]

Yeah, those casters should work pretty good. They don't look like your typical drag casters. The further away from the pivot point the wheel is, the more likely you are to have turning problems. Those look pretty central.
My question is this though:
If you went down to 1 caster in the center of your front and made your back the front(the drive wheels), do you think It'd turn less anonymously? I personally didn't get to see sparky too much, but it did seem to be swerving around alot during the time I watched it. I'm not sure if that was intentional though.
Anyway, If it worked good, then great, there should be no problem. I also like what 422 did this year for front casters. They look two large (6x6x6"?) blocks of the HDPE material(or something like it) and lathed it down to half a sphere. They threaded through the center of the flat side and bolted them at the front corners of their bot. It worked great.
When it comes down to it, the casters function is simply to support the frame. If it's causing your bot to shift around, it's not serving it's purpose correctly, and needs fixing.
In the summer of 02 we modded up our bot for the MD state fair. When I got up there I found that they mounted 2 casters in the front with 2 rear drive wheels. It was a mess. It took me 2 days (and 4 hours of fixing programming after a new member 'tinkered' with it) to convince them to flip the bot so the back was the front and they had a center caster in the rear. All this was done on one joystick. It worked great after they finally let me try it.
What would be a really good steal is if you can get the casters where the wheel is a sphere, and the pivot point is directly over the wheel. This way, you don't need to worry about odd shifting.
Anyway, with the casters you have there I can see why you wouldn't have as much trouble, but a typical rookie team wouln't necessarily think of that. Just keep that in mind...

[Mr. Ivey]

Both Sparky 1, and Sparky v2.0 have casters, Sparky v3.0 was 4wd, and Sparky (Sparky v4.0) had casters. I felt that the drive system on 3.0 was cumbersom when I tried it almost 2 years ago. While 4*4 works on trucks and is great when I go rutin, I feel it isn't the best way to go on ALL bots. Some bots are very productive with 4wd. Back to the point, on every version of Sparky (not counting v3.0) we have used front casters, that were very strong. Also to hit on the swaying motion you asked about. I will say that unless used properly your bot will sway all over the place with casters. You need casters that are made to take on a heavy load, and are made to take the power from the back. About the sway that Sparky had last year, we meant for our year to be focused on having fun, and we made it so Sparky could "dance" around the field. So what you probably saw was our driver having some fun at the end of a round, like doing 360s or poping a wheelie, or just running around the field. But your idea of having a caster in the back is intriguing, I had never considered that. But isn't there a problem with stability, if you have a square frame, and a single caster in the back, it just seems like there would be a fair amount of teetering??? I would like to hear more on this design.
Ivey

[Sachiel7]

Well, If you think I was saying that every bot should be 4 wheel from the name, that's not what I meant.We just call this standard drive as opposed to our multi drive. Anyway, by no means do I think every team should be 4 wheel. I'm just giving an example of our system.
I mustv'e seen sparky dancin
Once you get used to building 4wd systems, It's not any different from 2 wheel, really. But I guess that really depends on your preference.
The wheel in the back eliminates all possible turning anomalies that might occur. In that design, we took the 2 CIMs, ran them through a gearbox to the wheels. They were 8" pneumatics, I think...
Anyway, the reason for using 1 caster instead of 2 is the way the drive force is executed. When turning, if you have 2 casters, they each will want to point a separate way, if they're behind the drive wheel on the opposite side of the frame. This can cause problems because you'r introducing opposing forces in your drive direction.
Here's an experiment: Find one of those office chairs that have small casters at the bottom. If you find one with an even number of wheels, let me know. The chair I'm sitting in right now at my computer has 5. And, IF it does have and even number of wheels, are they layed out in a perfect square like pattern? Chances are that you will have a very hard time finding this type of office chair. If you do, try sitting on the front edge of it, pushing yourself forward with your feet. Now turn, and try to go straight. You probably won't be able to do it without a major arch in your straighting out. The Reason:Triangular force.
My Chair's casters are set up as thus:
\_/
/|\
Now the angling in that little drawing is way off, but to the point, there are no 2 casters directly across from each other. Theres always 1 across from a pair of 2, forming a triangle.
Basically, the force of both wheels (the drive wheels) gets put on the one caster, and it finds a sort of median in the force and pivots to suit that position. It works pretty good.
You could try this with the edubot. Make 2 little casters (make sure the wheels isn't centered around the pivot point) and try some different configurations. You'll find that single centered casters tend to work more. They absorb both forces of pull.
Anyway, as far as frame stability is concerned, there were no problems at all. Since the caster could pivot around, it usually held up the side against the opposing drive force. It also helps to have your weight more centered on the bot with that design, so if it does tip, it should re-level itself up again.
I really liked Sparky 4 this year. Your self-righting device has inspired several of our teammates (and me too ) to brainstorm ideas for devices that aren't always game-related (self righting devices included)
I'll see if I can dig up some of the pictures from MD so you can see the bot.