10-08-2016 08:40

cad321

Re: pic: Its twins - prototype drives

Are both the 4in and 6in wheels powered? If so, how are you planning to account for the differences in circumference between the two wheel sizes?

10-08-2016 08:44

notmattlythgoe

Re: pic: Its twins - prototype drives

It looks like the 2 sets of wheels have different size sprockets, which I'm guessing is to account for this.

10-08-2016 09:21

ASD20

Re: pic: Its twins - prototype drives

How do you plan on connecting the two sides? We had a somewhat similar design this year and we needed a lot of reinforcement to keep the chassis from flexing.

10-08-2016 09:24

346CADmen

Re: pic: Its twins - prototype drives

Will you be sharing CAD?

10-08-2016 09:25

GarroH

Re: pic: Its twins - prototype drives

Just curious, what sort of efficiency loss do you get with using the right angle gears? Is the loss in efficiency/power worth the savings in space?

10-08-2016 10:47

MoistRobot

Re: pic: Its twins - prototype drives

Do you have a drop center on this? Just looking at the picture it appears all wheels make contact so I would say no. Doesn't seem it would turn well.

10-08-2016 13:12

aldaeron

Re: pic: Its twins - prototype drives

I am also interested to know this

10-08-2016 15:03

ThaddeusMaximus

Re: pic: Its twins - prototype drives

I know it's prototype but how thick are those side rails, and those are 1/2" bolts? How's the frame work here? Anything to try and stiffen things up in sideways flexure?

10-08-2016 15:31

D.Allred

Re: pic: Its twins - prototype drives

What is the weight of one pod as configured in the picture?

It hard to tell, but it appears you only have a few degrees of chain wrap powering the outside set of 4" wheels - definitely less than 90 degrees. You'll get bad wear on that sprocket.

As others have stated, your 6" wheels are touching the ground. Turning on carpet will give you trouble.

Keep working on the prototype and let us know how it performs.

David

10-08-2016 16:56

joeweber

Re: pic: Its twins - prototype drives

Between the bumpers and mechanisms installed this should stiffen up the frame. If I get a chance (it won't be soon) I will post the CAD drawings of the plates. We actually used the Andymark CAD to find the distances. Far as efficiency loss, I am not concerned, its small and our mechanisms are geared correctly with low power use so if we have loss at the match it will be over before it is a problem. We will just recharge the battery a little more. Between the front wheels, center and rear wheels their is a difference of .375 inch. The side plates are 3/16 thick. We do plan on cutting the bolts shorter or go with aluminum with C clips but we have had problems with the C clips coming off. The chain wrap is a problem, but we do not plan on using the center 4 inch wheels on future designs. We wanted to put double sprockets on that wheel but we only have 2 inches between the side plates and the chains barely clear now. One unit weighs 20 pounds.

10-08-2016 17:27

cad321

Re: pic: Its twins - prototype drives

that's an awful lot for just one side. You're probably looking at a minumum 45-50lbs chassis once you add in the rest of it. Is there anywhere where you think you could cut down on weight? One area I would suggest is not using such a heavy plate. 2386 has run chassis similar in build from around 2010-2014. We found that as long as you supported it correctly (standoffs strategically placed throughout the "pontoon") 1/8th plate will do perfectly fine and hold up to the forces seen in a typical frc game.

10-08-2016 17:45

Max Boord

Re: pic: Its twins - prototype drives

What are they geared to? low power use isnt just gearing. its efficiency and right angle gears are less efficient than spur gears like those found in conventional drivetrains. Size also has no meaningful effect on mechanical efficiency. Also are you saying overcharging the battery is a solution to pour mechanical design? i think what people are point out is that your drivetrain will push less hard, accelerate slower and reach a lower top speed as a result of the bevel gear stage.

.375 inch wheel drop is very aggressive even for a game like this years. .125 to .188 is pretty much standard.

So are you guys planning on running 6 inch wheels 4WD or some other configuration? 4WD is generally a bad idea in FRC and is inferior to a 6 or 8 wheel drive with 4 inch wheels for numerous reasons.

What exactly are you guys trying to get out of this drivetrain that could not be done with a standard 4 or 6 cim versa drive or even a kitbot? This is a 50lb drivetrain while people have made 25-35lb drivetrains out of 2x1 tube for years now. They are simpler, cheaper, have less custom parts, are stronger, easier to assemble and maintain, have more reliability can push harder, accelerate quicker, and weigh 20-50% less than this. the only advantage I see in this is that it gets the Cims out of the way and frees up some bellypan space. if thats your sole goal then i highly recomend checking out designs for gearboxes that put the cims over the top of the drive wheels. its a simpler and more proven concept than this and has many more upsides with none of the downsides i listed in this post.

10-08-2016 17:50

asid61

Re: pic: Its twins - prototype drives

Bevel gears are quite efficient. I'm sure that the OP is getting at least 90% efficiency out of them, unless the tolerances are way out of whack.
We used 3/8" drop due to the AM pneumatic wheels, and found that was a good number for us.For an 8WD, having a bit of extra drop doesn't hurt a lot as long as your CoG is near the center.
4WD is definitely not a good idea, agreed.
This drivetrain seems to save a ton of space compared to "normal" WCD setups, and doesn't take too many extra custom parts. They weigh less, but with a slight redesign this drivetrain too could be pretty light (maybe 40lbs for the whole deal). For this year, a 40-50lb drivetrain isn't usually a big problem because of the lightweight robots people ran in general, although the rest of the robot could be as overbuilt as this and cause you to hit 120lbs. Flipped CIM gearboxes don't save nearly as much space as this design.

10-08-2016 18:34

joeweber

Re: pic: Its twins - prototype drives

If I remove the four 4 inch wheels in the center of each quarter I will still have 8 wheel drive. Far as weight this is a PROTOTYPE. We need to test it and find out how we can improve it.

11-08-2016 19:45

Jonny_Jee

Re: pic: Its twins - prototype drives

I'm a little late to the party here but according to MEADinfo.org

1 Spur 1:1 to 6:1 94-98% Efficient
2 Straight Bevel 3:2 to 5:1 93-97% Efficient

reference:
"Comparison of Gear Efficiencies." MEADinfo.org . N.p., n.d. Web. 11 Aug. 2016. <http://www.meadinfo.org/2008/11/gear-efficiency-spur-helical-bevel-worm.html>.

11-08-2016 20:11

Cothron Theiss

Re: pic: Its twins - prototype drives

Are those figures assuming that exact center to center spacing is used? If so, spacing those gears out just a few thousandths can squeeze out some extra percentage points to make up for any loss. It's the first stage of gearing, so the loss in load capacity shouldn't be an issue.

Looking at that page, I realized the OP could probably switch to spiral bevel gears without much increase in price, depending on the supplier they're using, but since they already have the straight bevel gears they need, it probably doesn't matter.

11-08-2016 20:21

asid61

Re: pic: Its twins - prototype drives

Spiral gears tend to be 2-3x as expensive as straight tooth, and IIRC don't give an efficiency advantage as much as they do a quietness advantage.
The quoted numbers have no context, but I would assume most FRC bots will get less efficiency.

11-08-2016 21:09

GeeTwo

Re: pic: Its twins - prototype drives

I like the wide open interior space - it would have relived a number of constraints on manipulators this year.

With those long chain runs close to the carpet, I would be worried about snagging threads and other debris, especially as the chains get loose. Is there some reason you decided to route the idler offsets to pull the top chain down rather than lift the bottom chain up?

My initial understanding of the small wheels inboard was to have the ability for more contact points with the floor. If you reduced this from 6 wheels per module to 4, would you make the wheels of equal/closer size, or did you do this for some other reason?

13-08-2016 22:07

Seth Mallory

Re: pic: Its twins - prototype drives

I like what you are trying to do. Four of the last five years the GRT drive trains have been working on increasing the free space in the robot. With the three motor drivers the loss of a few percentage points of power is compensated by the increase in space for interesting mechanisms.

14-08-2016 16:33

Dave McLaughlin

Re: pic: Its twins - prototype drives

Nit-picking at best, but I can't help but notice a lack of fillets on both interior and exterior vertices. The sharp corners will introduce stress concentrations that could be avoided by including even small radius fillets (albeit larger than the inherent radius left but whatever cutting tool you are using). If you are going to continue using 3/16th material during competition I would urge you to reevaluate your truss pattern. I think that you can be much more aggressive on material removal.

I appreciate trying to free up space inside the frame, but I have to wonder what is the acceptable cost. Max hits the nail on the head with this question IMO.

I want to be clear that I am not trying to stifle creativity, which seems to be the current sentiment critical analysis brings about on CD.

15-08-2016 21:36

Nick Lawrence

Re: pic: Its twins - prototype drives

This is cool. I definitely like the idea of trying to save interior space in the robot for mechanisms. It sure can make moving multiple or large game pieces through a robot easier.

Only one thought - other than wheel bolts, are there any standoffs inside the frame connecting the inside and outside plates of the pods together? I would be a little bit worried about a significant impact to the plates - even 3/16" thick. Some carefully arranged standoffs could help mitigate this, especially near the corners.

Please keep working on this drivetrain, offseason projects are a great way to try new designs that push the limits of "should I really do this?"

-Nick

23-08-2016 17:56

ArtemusMaximus

Re: pic: Its twins - prototype drives

Looks like motorized roller blades LOL