pic: DeWalt drive base: side module

[dbell]

[cdm-description=photo]28791[/cdm-description]

[Lil' Lavery]

Why not only have 1 DeWalt on each side, linked to both CIM motors? It would require a little more modification, but it seems like it would save size and weight.

[MrForbes]

this way when one of them fails, you still have drive on that side.....

clever design!

[Bill_Hancoc]

from my experience dewalts require further reduction from the output shaft before it gets down to a usable speed. We usually run a chain reduction.
Feel free to correct me if im wrong, but thats what we have always done.

[ducttapedude]

question, not sure if it's dumb, but why so much space between the plates, like, why so much space between wheels and chain routing ?

[Kyle Love]

question, not sure if it's dumb, but why so much space between the plates, like, why so much space between wheels and chain routing ?

There are two chains running to the middle wheel. One from the rear and one from the front, they sit side by side. The rear is offset to the outside, whereas the front is offset to the inside.

[AdamHeard]

There are two chains running to the middle wheel. One from the rear and one from the front, they sit side by side. The rear is offset to the outside, whereas the front is offset to the inside.

Those sprockets could be narrower though and probably save at least 1/2".

[Greg Needel]

from my experience dewalts require further reduction from the output shaft before it gets down to a usable speed. We usually run a chain reduction.
Feel free to correct me if im wrong, but thats what we have always done.

the reduction ratios of the NBD modification are 3:1, 4:1, 12:1. (12:1 being the same as the Banebots transmission this year) It would seem a bit fast to direct drive the wheels but wheel size plays a big factor in this (the smaller the wheel the more realistic this becomes), although I wouldn't do it as a cim motor at 3:1 reduction with 6 inch wheels would go ~46.5 ft/sec

[newton418]

from my experience dewalts require further reduction from the output shaft before it gets down to a usable speed. We usually run a chain reduction.
Feel free to correct me if im wrong, but thats what we have always done.

Definitely. Unless you want to have ~2" diameter wheels (in which case, the CIM would be the part touching the ground), you should really consider a reduction. My experience has been that an additional reduction in the ballpark of 4:1 (for a 7.5" diameter wheel) provides sufficient torque in 1st gear (for pushing/wheel slip), and plenty of speed in 2nd and 3rd.

Edit: Typos

[AdamHeard]

Use JVN's design calculator to choose your ratios, and model the results;


http://www.chiefdelphi.com/media/papers/1469


Don't forget that under robot weight, don't just put 120. Add in the weight of the battery and bumpers as well if you plan on using them.


This calculator removes all guesswork, speculation and "well, it worked for us....". It will give you the precise performance data along with helpful graphs.

[ducttapedude]

There are two chains running to the middle wheel. One from the rear and one from the front, they sit side by side. The rear is offset to the outside, whereas the front is offset to the inside.

oh, yeah, of course, lol, figured I was just missing something
thank you

[dbell]

Those sprockets could be narrower though and probably save at least 1/2".
Yes, i could make the sprockets narrower,(they are currently .75in wide) which would allow the plates to be closer together. Right now i don't have a real big reason to do that though.


Originally posted by Lil' Lavery
Why not only have 1 DeWalt on each side, linked to both CIM motors? It would require a little more modification, but it seems like it would save size and weight.

Several reasons: First of all i am using 4 dewalts because i know how to directly connect the CIM to the DeWalt, using the white paper. Making a gearbox to adapt 2 CIMs to one DeWalt would be something i have never done before. I think this would be possible though.
Another reason is because it would not really save that much weight. Each dewalt only weighs 1.5lbs and adding a 2-CIM adapter could add that much.
Also it wouldn't really save space because currently they only take a few inches at the ends. Which also distributes the weight well.
And finally because "this way when one of them fails, you still have drive on that side....." (posted by squirrel)


Originally posted by Bill_Hancoc
from my experience dewalts require further reduction from the output shaft before it gets down to a usable speed. We usually run a chain reduction.
Feel free to correct me if im wrong, but thats what we have always done.

Originally posted by Greg Needel
the reduction ratios of the NBD modification are 3:1, 4:1, 12:1. (12:1 being the same as the Banebots transmission this year) It would seem a bit fast to direct drive the wheels but wheel size plays a big factor in this (the smaller the wheel the more realistic this becomes), although I wouldn't do it as a cim motor at 3:1 reduction with 6 inch wheels would go ~46.5 ft/sec

Originally posted by newton418
Definitely. Unless you want to have ~2" diameter wheels (in which case, the CIM would be the part touching the ground), you should really consider a reduction. My experience has been that an additional reduction in the ballpark of 4:1 (for a 7.5" diameter wheel) provides sufficient torque in 1st gear (for pushing/wheel slip), and plenty of speed in 2nd and 3rd.


You guys are all correct.
I forgot to make this clear.
I am planning on adding the last stage to the DeWalt gearboxes. To my knowledge this stage is 4:1. This should be a good speed with the current 6 inch wheels i have. I have not yet tested adding this part of the dewalts so it may not be realistic. I'll see.

Thanks for your comments!
DB

[newton418]

Use JVN's design calculator to choose your ratios, and model the results;


http://www.chiefdelphi.com/media/papers/1469


Don't forget that under robot weight, don't just put 120. Add in the weight of the battery and bumpers as well if you plan on using them.


This calculator removes all guesswork, speculation and "well, it worked for us....". It will give you the precise performance data along with helpful graphs.

Gear inefficiencies, weight distribution, and coefficient of friction all must be estimated. For this reason, though the calculator is very nice and convenient, testing is still necessary to ensure you get the results you want.
Also, while the calculator is convenient, I would highly recommend that everyone create their own spreadsheet and run the calculations themselves. That way, if you don't get the results you want, you will have a better understanding of what you can change.

[fimmel]

also in team 885's experience the dewalts dont shift at exactly the same time.. this would cause binding as far as i know. out solution was to only use 1 dewalt/cim per side.
...just a thought
...forest

[team 1094]

thats very neat design you made. that would be cool to see in the 2007-2008 build season.

[Richard McClellan]

that's pretty cool....I'm curious though....what kind of interface shaft do you have to connect to the end of the dewalt for the direct drive setup?

[dbell]

that's pretty cool....I'm curious though....what kind of interface shaft do you have to connect to the end of the dewalt for the direct drive setup?

The shafts that go between the 2 red plates and hold the wheels are .75:" thick. In the end of those there is a .5" threaded hole. The DeWalt will screw into the end of the shaft. Then i will drill a hole through both shafts and put a pin through. Hopefully this will be solid and strong.

[newton418]

also in team 885's experience the dewalts dont shift at exactly the same time.. this would cause binding as far as i know. out solution was to only use 1 dewalt/cim per side.
...just a thought
...forest

Though I'm sure there are minor differences in the shifting times, we've never had a problem running 2 DeWalts on one side, connected by chain. Perhaps we weren't pushing them as much, it's definitely doable to have 2 DeWalts on one side of a drive train.

[fimmel]

Though I'm sure there are minor differences in the shifting times, we've never had a problem running 2 DeWalts on one side, connected by chain. Perhaps we weren't pushing them as much, it's definitely doable to have 2 DeWalts on one side of a drive train.
its cool to hear someone who has done this successfully.... we never did try linking 2 together but we were cautious. ill tell our head engineer about this since it makes my job as a part designer easier.

...forest

[AdamHeard]

The shafts that go between the 2 red plates and hold the wheels are .75:" thick. In the end of those there is a .5" threaded hole. The DeWalt will screw into the end of the shaft. Then i will drill a hole through both shafts and put a pin through. Hopefully this will be solid and strong.

3/4" diamater shafts will definately be strong enough; especially if they are steel. Depending on the size of the pin you're putting through, aluminum should be just fine. Either way, 3/4" is kind of heavy though. I guess you don't have much of a choice w/ the current interface with the dewalts though.

[Madison]

...

[Greg Needel]

3/4" diamater shafts will definately be strong enough; especially if they are steel. Depending on the size of the pin you're putting through, aluminum should be just fine. Either way, 3/4" is kind of heavy though. I guess you don't have much of a choice w/ the current interface with the dewalts though.

this is not picking on adam I am just using it as an example

There are alot of things that go in to the"strength" of a material in an application. Just saying that it will be strong enough shouldn't really cut it when contributing to a design.

As an example when you are talking about strength it is obvious that you are referring to not failing during application, but in what ways is the part strong enough. Are you concerned with the torsional strength to make sure the shaft wont twist off? What about the bending stresses of this kind of loading And don't forget to take into account the stress concentrations caused by the pin slot and the fatigue strength due to alternating loading...What about the shear strength of the pin itself?

I guess my point is there is alot more that should go into the design of a system like this, and just making wild guesses could cost you in the end. If you under design something you can get a failure and if you over design you have wasted weight and real estate for this mechanism.

Now I realize that 95% of the teams out there probably don't do/ know how to do any of this math and just throw stuff together (I know I have fallen to the same fate from time to time) but in the off season when we have plenty of time for this kind of top level design. I urge everyone to open a book or ask someone who knows for help on these problems rather then taking the "backyard engineering" approach and just winging it. Same goes for those offering suggestions on design to be able to backup your suggestions with some theory. Remember there are teams who live and die by the advice given on these boards and you want to make sure that they are given correct information.

[AdamHeard]

As an example when you are talking about strength it is obvious that you are referring to not failing during application, but in what ways is the part strong enough. Are you concerned with the torsional strength to make sure the shaft wont twist off? What about the bending stresses of this kind of loading And don't forget to take into account the stress concentrations caused by the pin slot and the fatigue strength due to alternating loading...What about the shear strength of the pin itself?


Looking at the amount of torsion put into the shaft (I just ran some similar calcs yesterday for a 1/2" shaft ) that shouldn't be the problem, What I was reffering to was that the point of failure will probably depend on the pin (either the pin itself, or the shaft where it engages the pin).


With so much time on your hands, and since you already modelled it anyway, you my as well learn to use the stress analysis built into inventor to check your it. I've been using this all season on critical parts and it has never failed me (remember to build in a safety margin).