Offseason 2012 chassis

[Aren_Hill]

Originally Posted by akoscielski3

Im not saying that CIMple boxes are bad, i'm just saying that they weren't good enough for maccanum drive. They need more torque for what we were using them for (it was just our fault for picking the transmissions)

If you properly designed it they would be part of a geartrain that had plenty of torque for a Mecanum drive, your wording of "weren't good enough" blames the gearboxes, not the person that improperly used them. I apologize if I sound harsh but you need to go some research.

I suggest you go look at JVN's mechanical design calculator and learn how many options there truly are ratio wise between a motor and wheel.

[akoscielski3]

Originally Posted by Andrew Schreiber

Did you have an additional reduction after the Cimple Box? Were you using 6" or 8" Mecanums?

unfortunatly we were direct driving them. this was the first year we tried maccanums, we never even tested with them before. And we were using 6". Another team in Pittsburgh was using 8" Maccanums and they could do everythign fine :$

[Andrew Schreiber]

Originally Posted by akoscielski3

unfortunatly we were direct driving them. this was the first year we tried maccanums, we never even tested with them before. And we were using 6". Another team in Pittsburgh was using 8" Maccanums and they could do everythign fine :$

This... is why we do the math.

[Thad House]

Does anyone know a good gear ratio for speed and 6 inch wheels. i would like to know what kind of reduction i would need in the chain, or if i should just move to toughbox mini's and reduce the ration inside of them to get more speed.

[thefro526]

Originally Posted by sst.thad

Does anyone know a good gear ratio for speed and 6 inch wheels. i would like to know what kind of reduction i would need in the chain, or if i should just move to toughbox mini's and reduce the ration inside of them to get more speed.

What's speed are you aiming for?

816 ran TB's (Tough Boxes) with 6" wheels from '08-'10 with a 25:22 reduction (25 tooth sprocket on trans, 22 tooth on wheel) and it worked well for us. Floor speed was somewhere around 11/12 fps.

[BrendanB]

Originally Posted by thefro526

What's speed are you aiming for?

816 ran TB's with 6" wheels from '08-'10 with a 25:22 reduction (25 tooth sprocket on trans, 22 tooth on wheel) and it worked well for us. Floor speed was somewhere around 11/12 fps.

What type of trans were you using in these years?

[Brandon Holley]

When you are reflecting on a failed design it is important to not get caught up in incorrect interpretations of that design.

For instance, the fact that one team was using 8" mechanums successfully vs. your 6" mechanums is really irrelevant. I think you understand this already but I just want to drive the point home. The size of your wheels is irrelevant to other teams designs. Your wheels must fit with your design like everything else does.

It appears you are looking to use off the shelf components for much of the geartrain of your drive system. That is a great start, many teams find success using tried and true off the shelf components. It does limit some of your choices however. Make sure you are analyzing what kind of speeds and torques your drivetrain will put out when you design your system. Instead of thinking in terms of "I want 6" wheels, directly driven off of toughboxes", think in terms of "I want a DT that can move at XX feet/second". Pick a starting point and start to design towards that.

For instance, assume your target is 10 feet/second. You can start your feasibility analysis at a very broad level and begin to focus it down to what fits for your team. I suggest doing this:

-The first would be to forget what exists already and determine what theoretical gear/sprocket/belt ratios can get you to your speed. A quick rule of thumb is not exceeding a 4:1 reduction in a gear set (this is due to the torque increase over that reduction and the yielding of gear teeth in that application). Don't worry about part numbers or specifics just yet, just determine what combinations would work to get you there.
-Next, start to look at if using an off the shelf gearbox can help save you design and manufacturing resources (if this is important to your team, which it should be.) See if with that rule of thumb an off the shelf gearbox can get you to where you need to be. It's important to note that you need to constantly be evaluating your design criteria. In your findings you may discover that using an off the shelf gearbox you can get to 11 ft/s very easily. You have to ask yourself in that situation is 10 ft/s an absolute must have? What if you purchase/make different wheels? What if you add an intermediate reduction? There are so many variables you can play with, you just need to keep working towards what works best for your team.
An example: Our team is fortunate enough to have a great deal of manufacturing resources at our disposal. We’ve made many custom gearboxes and fully custom drive trains so we are comfortable doing so. However, one Achilles heel for us is wheels. We’ve made our own wheels many times and they were successful but they ate up a great deal of our resources. A lesson we have learned is to design around off the shelf wheels. We can tailor the gearbox and other components to make certain wheels work, so we use that as a design constraint to start nailing down the overall drive design.

I think you are off to a great start. Just thinking about your future designs is more than most teams do so you are doing a good job! If you have any other issues or questions feel free to ask them here. Just about everyone provides some form of constructive criticism so keep firing away!


-Brando
(sorry for the long winded response, I guess I'm just passionate about the design process)

[Madison]

Originally Posted by Brandon Holley

-Brando
(sorry for the long winded response, I guess I'm just passionate about the design process)

Half a page is hardly long-winded and entirely what these forums are missing. Nearly everything else in this thread is pointless.

What we have here -- and what we often see on CD -- are pretty pictures with nothing substantive to offer. This thread is, so far, much of the same.

We see a bit of the criteria used in driving this design -- a desire to minimize machining time and to use COTS components. That's a great start. What's missing, though, are other goals and constraints -- speed, weight, cost, etc. Without first establishing those constraints, there's very little "process" going on and so, in my mind, there's not too much to discuss.

The next step is to fill in some of the broadly sketched strokes that take you from ideation to design. The wheels use purchased pillow blocks -- from what supplier? What do they look like? How are they fixed to the chassis?
How is the chain tensioned? How do you run chain to the outer wheelsets?

These things don't happen magically. This is where real design happens most of the time, it's the most interesting thing to discuss and it's the thing we see least here on CD these days.

Sorry to be Ms. Cranky Pants, but I've been waiting for awhile to see someone take the next step and offer real information describing how their pretty pictures came to be.

[Hawiian Cadder]

this drive-train looks pretty good. one thing that i might look at is using a timing belt from the cimple box to the middle wheel. belts are generally more efficient than chain. a second thing to look into would moving the gearboxes to the middle of 2 wheels, then you only have 2 chain runs, and are less likely to break a chain because you don't have all of the drive-train going through one chain.

[Vikesrock]

Originally Posted by Hawiian Cadder

belts are generally more efficient than chain.

Really? By how much?

[Tom Ore]

Originally Posted by Madison

...Sorry to be Ms. Cranky Pants...

<--- too funny!!

An example: our thought process for Logomotion. Basically we decided that we didn't need a lot of speed to zip around the field. We guessed that the HP would be able to toss the games pieces to our end fairly well. We decided we wanted the manuverability of Mecanums and wheel slip limit pushing at the highest speed we could get it. A bit of testing led us to 12.75:1 toughbox nanos direct driving 6" Mecanums.

This is probably quite a bit slower that most teams geared their bots - but it worked out well for us.

[Chris is me]

Originally Posted by Vikesrock

Really? By how much?

I've been told that belts are thought to be about 3% more efficient than an identical chain setup. I wouldn't use such a small margin as the primary motivation for using belts instead of chain, as drawbacks to belt designs require serious consideration (they require more planning and design since belts do not break and reclose)

[Hawiian Cadder]

Originally Posted by Vikesrock

Really? By how much?

i read somewhere than in frc #35 chain is like 87 percent efficient, #25 is somewhere in the low 90's and belts, appropriately sized are better than 95 percent.

[Thermal]

Belts are marginally more efficient than chain (and I say marginally because #25 chain is some pretty good stuff). As noted above, it comes down to a few percentage points. However, belt requires 75 times more tension for proper tooth interactions than equivalent chain. But don't take my word on it, take a gander over at http://chain-guide.com/

As for personal experience, I can say that if your chassis isn't bone solid rigid, you'll have issues with maintaining belt tension during use. Our tower caused our C-Channel chassis to twist (because open sided tubes like C-channel carry significantly less torque than closed square tubing) and maintaining tension from our gearbox to our wheels was an effort in futility.