pic: 3247 Robopack Frame Design

Here is our teams first non-kit frame. We just got done building it and about to have it sand blasted to matte finish and anodized blue. For the left and right sides we are using milled 1.5" x 3" x 1/8" rectangular tubing. For the front and back it is water jetted .25" plate. The frame size is 30" long 26" wide.

I like your design! Looks like it will be a nice bot. Nice color too!

It definitely looks good.

Are those CIMple or toughboxMINIs, and are they being direct driven?

They are CIMple Boxes. We also have some toughboxMINIs ready and configured to replace them if we find the CIMples inadequate.

If it’s not yet too late, please replace the flat plate on each end with something more rigid.

The only thing preventing your frame from becoming a wonky rhombus instead of a nice rectangle, as it’s shown here, is a solid off-center collision.

It is too late, but there will and is more bracing with in the frame to resist that action. And how we are mounting that front plate to the frame is very resistant to moving in that way.

A Cimplebox is not suitable for direct driving a six inch wheel. Did you “do the math” and figure out what your gear ratio would be?

Yeah, the 1/4 plate you have in the front and back are securely attached to the robot, but Madison’s concerns are valid. It’s not so much the plates physically moving as much as them parallelograming. (Is that a word?)

Easiest way to fix both these issues would be to drop a Toughbox Mini in (with an appropriate ratio) and to add the AndyMark standoffs between the gearboxes, in addition to any “additional support” you might have planned (e.g. replacing the front and back pieces with C-channel or tubing). A rigid belly pan would also help a little.

We have the AndyMark standoffs on there I just haven’t had the chance to put them in CAD

Could you explain to me why we should you the Toughbox vs. the CIMple?

The CIMple Box’s reduction is only 4.67:1. This is fine in the Kit of Parts drives of 2011 / 2012, because they came with a chain reduction to further reduce the CIM output, but when you are direct driving you are locked into that gear ratio.

If we plug your drivetrain into JVN’s Mechanical Design Calculator*, your drivetrain has a free speed of about 30 feet per second (approximately 24 feet per second after friction and other losses). That sounds great, except that you’ll never, ever get going that fast. Either you would spend the entire length of the field accelerating, or in this situation, you’re so overgeared that you will probably trip circuit breakers just trying to move or turn. Basically, you don’t have enough torque to move yourself.

The Toughbox Mini is a 2-stage reduction, so you have a lot more flexibility with your final ratio. I don’t know if your team has the 2013 KoP Toughbox Minis (10.71:1 ratio) or the off-the-shelf ones direct from AndyMark (12.75:1), but both of those ratios are much better for 6 inch wheels. The former will give you a speed of about 10.5 feet per second after losses, which is a pretty good, fast speed for a robot that only expects to play a little defense, while the latter will give you a speed of 8.8 feet per second after losses, which is better for inexperienced drivers and more defensive play.

*The “right” answer to your questions would be explaining all of the math of drivetrain gearing to you, but it’s near the end of Week 4 so I’d rather just “give you the answer” and have you look into the physics behind it when you have free time.

The problem with direct-driving Minis is that the gearbox is taller than the 6" wheel.

I think the flat plate mount might be, but I am pretty sure the actual “box” isn’t. The output shaft is under 2 inches away from the bottom of the bent plate gearbox. Mounting this might be tricky.

Thank you very much, I will address these issues at today’s meeting.

The MINI works perfectly, I origanly designed the frame to fit it but we wanted more speed. Thus we tried using the CIMPles.

What is your opinion on changing the gears inside the MINI to a ratio of 8.45:1? Will this give us enough power? We are wanting the higher speed.

What you’ve shown doesn’t help you avoid parallelograming, to borrow Chris’ word, in the least. The plate isn’t very resistant to bending along its thickness, so it’s as likely to bend in the middle irrespective of how it’s attached to the side rails. The giant pockets you’ve put into it closest to where it meets the side rails exacerbate the problem.

It may be that the additional structure you plan to add is enough to keep everything square and that end plates serve a light-duty or decorative function, but I can’t make that call without seeing the rest of the robot – and I’d be happy to take a look at it in confidence if you wanted to PM me a picture. As things are, though, I’d strongly suggest you do something to those flat plates to increase their resistance to bending. At this stage, riveting or bolting some angle along the top and bottom edges would be the simplest solution. You might also be able to get away with very securely attaching your bumpers several places along the length of those plates. Thick, high-quality plywood will provide a lot of stiffness.

If this happened on my team, I’d insist on a change. A bent frame can cause all kinds of other problems with the machine and can be nearly impossible to correct while at an event. At best, it’ll make driving behavior less predictable; at worst, you’ll no longer meet frame perimeter or bumper requirements and be unable to compete.

was finishing out the frame and anodizing it expensive? We’ve not gone that route yet and usually build with the c-bars. Just curious…we’re not funded by our school but we’re quite fortunate to have great sponsors/patrons:D


For our matte finish and anodizing it will be sand blasted and anodized by team
2341 who are in the same town as our team and working out of Gordon cooper technology center. I could ask them how much it cost them for the anodizing supply’s and setup.

There are other components not shown that will beef it up more then enough. For example where our shooter is mounting, is 2 of the 1.5"x3"x1/8wall tubing laying on the side going across and firmly fastened to the frame, and then there is also pieces of the AndyMark hex shaft connecting the gearboxes. We have already addressed the stability of the frame flexing and have incorporated it into the rest of our design.

I nor my mentors are worried in the slightest of the frame doing as you described. Thank you for the concern though. :slight_smile:

Okay :slight_smile: Good luck!