pic: One Tough Train


2006 prototype drive train
Wt. 68 Lbs.
L 38" W 28"
Speed 6.5 FPS
5.5" Wheels
6 Wheel Drive
1/4" Alum base plate
3/8" Stainless Steel side and front bars

You don’t come around these boards too often but when you do I am certainly impressed with your work. You are becoming a great robot designer. Have any of your designs been built? If so, I’d like to see some pictures.

A few notes:
-It might be better to mount the CIMs by the 10-32 threaded holes in their face rather than claming them on the can.
-Why use a coupling on the CIM shafts rather than just putting a gear?
-The transmission directly drives the wheel, I LOVE IT!
-The transmission has three plates, make sure there are never 3 bearings/bushings on a single shaft.
-For the sprockets, what size are they? I would recommend going the smalles you can (which would be 13T for #25 with 1/2" bore)
-Finally, have you considered a welded box tubing frame, they can come out to be very lightweight.

Last, I would like to offer you a picture of our frame in case you wanted to take any design cues from it. http://www.chiefdelphi.com/forums/pictures.php?s=&action=single&picid=10020 The pillow blocks slide for chain tension. Big tubing is 1.5x2 (probably should have used 1x2 and the small tubing is 1x1. All 1.8" wall.


First of all … very nicely done (inventor).

Few comments.

Very simple design, I like it. Looks like most of your weights are in the back of your chassis, I think you might want to look into that and make sure the chassis has central gravity. I can’t really tell if your middle wheel of the chassis is lower than the other two according to this picture. If the middle wheel isnt lowered, you might want to do that for better turning. Did you think about making the chassis go a bit faster? You can always change the ratio (output transmission sprocket and the ones on the wheels). Looking at the top view… it looks like you sprockets are 1:1. What is the ratio of the transmission itself?

:: I heard them AndyMark Transmission is good:: :slight_smile:

I really like your design. I’d like to know what you plan to do with it, as mentioned before. The center of balance is towards one end, but it seems that if you put a manipulator closer to the empty end that wouldn’t be a problem?

Hopefully all would balance out well. I also noticed it’s riding pretty low. Are we anticipating another low necesity game?

And while I’m on it…how would the design, if used, be altered for a water game?

hey the 6 wheel drive looks awesome but you should put some chain in so that people can tell exactly how it works ar first glance. I like the wheels too and the idea is great.

…I take it there will be slight adjustments if the size limit changes next year…or if they have a “Platformish” thing again
but still…The weight is perfect for a strong base…trasmission is good!..and best of all…THOSE IS SOME TIGHT RIMS…hahah

Maybe some doesnt know how to make chains in inventor (I have realized that it was very hard to make). One who has worked on a robot before would be able to look at that chassis drawing and would be able to tell where the chain goes. Now if it was a mechanism, then it’s a totally different story. Team 103’s (your team) inventor submission looks great this year. I saw that they used chains too on their robot. Mind sharing? Please tell us how to make the chains in Inventor. Thanks…

Rod… I had another question for you…

I can’t see how you mounted the Cim Motor on the transmission. Are they secured? Is it just two plates sandwiching those two motors together? Please explain or share a closeup picture? Thanks (if I am not mistaking, Rod has posted a picture of this transmission before the season started, just went through the image gallery, couldn’t find it)

… The chassis looks great, would be nice to see it running around in reality. :slight_smile:

That looks interesting…

I mostly concur with sanddrag, except for that sprocket thing.

A couple of years ago, in 2003, Woburn used those same little sprockets, and promptly broke something like 13 #25 chains over the course of three events. Since there’s no such thing as too much horsepower, the chain simply wasn’t up to what we demanded of it. (It undergoes higher stresses as sprocket radius decreases.) You might consider looking at Tsubaki’s General Catalog, Section 1, pages A-6 and A-7, where it gives power vs. speed vs. teeth-on-small-sprocket charts for #25 and #35 chains. (I’m always recommending this, it seems.)

Some quick calculations say that for your 5.5 in wheels at 6.5 ft/s, you need a wheel speed of 271 rpm. Plug that into the chart, and you get a rated power of 0.17 HP for #25. Two CIMs at 341 W each are good for about 0.91 HP, or 5.3 times your chain’s rated capacity. (You’re screwed :(; that’s even worse than Blizzard 4, in power-to-rating terms–it was an already-horrifying 5.1, even after accounting for its extra power!)

Of course, there’s an answer: move up to #35, where the rated power for a 13 tooth sprocket (note that it’s bigger than the #25 version, in all dimensions) is 0.94 HP, which is perfect.

I don’t know how much of a safety factor chain manufacturers build into these ratings. I’ve got empirical evidence to demonstrate that it’s not 5, however. Maybe 2, if you like to live dangerously–but gambling that the safety factor exceeds 5.3 really won’t be pretty.

I’m not sure how or why you broke your chains but we ran a 6 motor 2 speed with #25 13T sprockets without a problem. While you are correct in saying you can easily exceed the working load, the breaking point is over 1000lbs. Also, it is not about the force the motors produce, it is about the force the wheels can hold on the floor. But yes, larger is safer.

I think a lot of it had to do with the style of play in 2003, versus this year. You mentioned elsewhere that your 2005 robot didn’t have much pushing and shoving to do; by contrast, there were plenty of abrupt starts and stops in the 2003 game, not to mention outright collisions, where closing speeds could top 20 ft/s. Those represent shock loads not accounted-for in the 1050 lbf static breaking load (actually, for cheap #25 chain, it can be as low as 780 lbf, per the ANSI spec.). Add to this the fact that the three-motor transmissions would accelerate the robot very quickly, and reverse direction just as abruptly, and it’s clear that the chains were doomed. But even with a much more relaxed driving style, and better chain tensioners than we had in 2003, 5.3 times the working load doesn’t seem too safe.

A few answers
The gears used are a 55 tooth 3/8 face 20 DP for output and a 5 tooth stem pinion on the end of the Cim motor. The stem pinion has a 3/8" Dia. so it can not be mounted on the Cim shaft. That is why the Cims use a coupling and are not face mounted. I have put 3 bearings on a shaft many times and never had a problem. Our robot this year uses the same gearboxes and works perfect. The sprockets are 14 tooth #35 with 5/8" Bore. This year we had to add weight to our robot (17 Lbs.) We have a 55 Lb. drive train this year, that does not count the arm. So next year if we have a 68 Lb. drive train it should be about right. I read these boards every day, just don’t post too much. Our designs have been built, they are posted here both the CAD drawings and this year’s Robot. CAD drawings are titled Light weight drive train, we built a slightly modified version of it.
See Ya

For 3 bearings on a shaft, it is a geometric overconstraint. Unless you can keep tolerances to under .001, you are losing efficiency.

We use plain sleeve flanged bearings. The center bearing is bored out .020" over and is more of a gear spacer than a bearing.

Ah, okay. So you really don’t have 3 (close fitting) bearings on the shaft. Also, now I can see the need for couplings. Looks very small/compact!

I’m not sure if this would be a good idea but it seems like those couplings are taking up a lot of space in your otherwise compact design. If you got a gear just big enough to bore out and key to fit over the CIM shaft to keep the tooth count small (for 20P I believe this would be a 12T) then you could increase the 55 tooth gear just a little bit (maybe to 60) or maybe even keep it that size for a faster robot then you could move the CIMs in closer and mount them by the face to that first red plate.

We are at 11:1 now. If we used 12T to a 60T we would be at 5:1> That would be way to fast for us with no low end.

How stiff is that chassis, anyways? It definitely looks good, but we tried a bent sheet chassis one year and we definitely had problems with it flexing. and especially with it twisting since it didn’t have any stiffness at the corners.

The bottom plate is 1/4" thick Alum. I hope it will not flex too much

I just realized; you’ve got an 11:1 reduction there, meaning that your quoted speed numbers are sort of conservative (I’d say you estimated 3000 rpm from the CIMs). With any luck, under normal loading (no pushing), your motors will tend to spin closer to their free speeds (5342 rpm) than their max. power speeds (2671 rpm).

Actually (using free speed), your top speed would be closer to 11.6 ft/s. I’ve found that free speeds of motors provide a better approximation of robot top speeds than anything else–with the extra voltage from a full battery roughly accounting for the load on the motors from normal driving, and allowing an estimate of free speed = operating speed. (This varies, depending on the drivetrain, and is clearly not the product of rigourous calculation! If anything, it overestimates the speed.) The 6.5 ft/s @ 3000 rpm figure seems more consistent with assuming that the robot will run most of the time at full power. That’s not a realistic number to work with, because you’ll never know what your power output actually is, when pushing something around–you certainly can’t hope to be at max. power in any given pushing match. If anything, when pushing another robot, you’ll be on the slow side of max. power (that is, the high-torque side).

And one more note: I was using the max power figure earlier in the chain calculations, while quoting free speeds. In reality, since max. power occurs at 50% free speed, you should (if you want to be rigourous about it) divide the speed I gave above by 2, and use the corresponding power figure on the chart. With some rough interpolation, and the #25 14-tooth instead of the 13-tooth sprocket, you’re at about 0.12 HP rated. But I think that these compounding safety factors are getting a little absurd. I wouldn’t bother with this last step, if only because this is a mere robot, and we don’t need to overbuild it to last forever without failing. As long as it lasts the season intact…

Lookin’ good, Rod.

Why not put the gearboxes on the center wheels and use two output sprockets (one to the front and one to the back)? This will eliminate the possibility of this drivetrain becoming a 4wd (3wd on one side, and 1wd on the other) with the loss of just one chain, or a 2wd with the loss of two.

Sorry if this was brought up earlier in the thread… it’s almost 3am, and I’ve been doing something that a lot of 21+ year olds tend to do :stuck_out_tongue:


Looks good, similar to the 254 - 60 drive. I was going to mention what Bill just said, it might be better to drive your bot from the center wheels. That way, if you throw a chain, you’ll still have at least 4 wheels that are powered by the CIMs. This is how we had our drive set up at first this year, but then decided to switch out our front and back wheels with the AndyMark Omniwheels.