Rookie Team Redesign

[joelg236]

It's that time of year again... Two days left in build season and my rookie team is left in a precarious situation.

Our last 6 weeks were spent on a "rotational lifting" design. The idea was that this was much simpler to accomplish, easily diagnosable and a K.I.S.S. type way to do things (even though we could only stack 3 totes + a bin).

And until today, it worked well, especially considering our resources and manpower (very very tight on budget).

The design went like this: 80-20 framing on AM14U2, a minCIM geared 100:1 direct driving a ~30 inch arm that has lexan clips that fit under the lips on totes, and around the bin.

But over the last few weeks we've slowly been leading towards the issue that our versa planetary gearboxes are breaking - even in what could be considered "normal" game situation use. Lifting stationary and without other factors, there isn't much of an issue. But when driving and just generally being more "busy", we've managed to break the sunflower gears three times, as well as (apparently, no visible damage) the housing 3 times.

And we're out of supplies to replace the ones that broke today.

We had thought our issue was solved a few days ago when adding some thicker tubing to a "leverage bar" on the back of the arm (picture). Well today proved that wrong.

I'm wondering if anyone might have some advice or experience in this kind of situation. It's mostly out of the cards to "buy a solution", and our machining abilities are limited but not impossible. Advice regarding design, the team, strategy, or just in general would be appreciated. I would love nothing more than for us to "succeed", in the sense that we end with something we're proud of.

Ideas I've had:

• Similar to 148's "HP bot", we could act as a backstop for 2-stacks, to make things quicker for HP bots that are out putting stacks on platforms
• Bring our arms lower and closer to centre for less torque on the motor, limiting how high of stacks we can make (we don't have the gearbox for this, but maybe a PG188 or similar could do it)
• Bulldozer noodle removal

It's worth noting that yes, I am aware of the fact that the miniCIM with 100:1 is not advised on VEX's website. We made that decision, aware of this potential problem. It was necessary to get enough torque with the arm length we wanted. I thought long and hard about it, and obviously hindsight is 20/20.

Thanks!

[AdamHeard]

a 4 stage one of these should be able to take a minicim and handle the torque better.

http://www.andymark.com/product-p/am-2550.htm

Same bolt pattern.

-Adam

[joelg236]

Originally Posted by AdamHeard

a 4 stage one of these should be able to take a minicim and handle the torque better.

http://www.andymark.com/product-p/am-2550.htm

Same bolt pattern.

-Adam

So 181.4:1? That would be good (speed was quick anyway), but $200 isn't really in our budget without some favor somewhere.

[Christopher149]

How easily could you convert to a simple elevator with some box tube and some bearings to form a carriage for your U-hook? You could winch the elevator up and down with a fairly pedestrian gearbox, like a CIMple box probably (we're using two Toughbox Nanos powered by CIMs).

[joelg236]

Originally Posted by Christopher149

How easily could you convert to a simple elevator with some box tube and some bearings to form a carriage for your U-hook? You could winch the elevator up and down with a fairly pedestrian gearbox, like a CIMple box probably (we're using two Toughbox Nanos powered by CIMs).

That sounds ambitious for 2 days with mostly rookie members. I'm also not confident about the materials required, if we could get them / make them.

[Seth Mallory]

Contact the captain of 192 at gunnrobotics192@gmail.com. He can look to see if we have something in stock. If we do then we can bring to Salt Lake with us. Also he will have about 40 students who are our robot designers and builders with him to help you if you ask.

[IKE]

Shocks or impacts are likely what is killing your gearboxes. Every bump causes really high torque back into the arm. My recommendation is to remote the gearbox from the arm. A low pivot "hip joint" on the front side would reduce loads. Using sprockets to drive it. Or possibly setting it up as a winch off the back.

[mlantry]

http://m.grainger.com/mobile/product...ler-Body-4X176
you could find a way to integrate one of these. All you need are to sides and a rubber insert it will help to absorb shock from moving around on the gearbox my team used one made from aluminum last year on our catapult reset that we kept breaking gearboxes on due to shocks from shooting, made it seem likevthe problem was never even their.

[rich2202]

Not relevant to your question, but:

Last year, there was a lot of concern about stored energy. How much energy does your bungee cord store when fully lowered? If it is more than nominal, the Robot Inspector may make you cover the cord so, if it breaks, no one is inadvertently injured.

[joelg236]

Originally Posted by IKE

Shocks or impacts are likely what is killing your gearboxes. Every bump causes really high torque back into the arm. My recommendation is to remote the gearbox from the arm. A low pivot "hip joint" on the front side would reduce loads. Using sprockets to drive it. Or possibly setting it up as a winch off the back.

A winch would limit our range of motion (since the arm needs to go all the way back for bins). I'm intrigued by the idea of reducing shock, since we have lots of spare belts and some sprockets, so it would just be a question of mounting. I'll be trying this today.

[joelg236]

Originally Posted by rich2202

Not relevant to your question, but:

Last year, there was a lot of concern about stored energy. How much energy does your bungee cord store when fully lowered? If it is more than nominal, the Robot Inspector may make you cover the cord so, if it breaks, no one is inadvertently injured.

It's not as much energy as it looks. The competition bot will have a good securing method to make sure the tension is constant.

[DaveL]

Another hardware option is the BaneBots p80 transmission with a CIM motor.
http://banebots.com/pc/P80K-S4/P80K-443-0005

They come in a variety of ratios.
Like an earlier post, I would recommend relocating the motor/transmission from the pivot point. Sprockets and chain would be my approach.

If you have or can get a drive transmission, that might be another option with reduction gearing.

Dave
Mentor 2976

[Ether]

Originally Posted by joelg236

...a minCIM geared 100:1 direct driving a ~30 inch arm... our versa planetary gearboxes are breaking

On the bright side, what a wonderful teaching moment (no pun intended).

[seg9585]

Originally Posted by joelg236

A winch would limit our range of motion (since the arm needs to go all the way back for bins). I'm intrigued by the idea of reducing shock, since we have lots of spare belts and some sprockets, so it would just be a question of mounting. I'll be trying this today.

A winch would not have to limit your range of motion. From your picture, mount the winch above the drive CIMs and attach a tension cord to the extreme back of the mechanism (where the tubing is connected). That should give you full range of motion.

[IKE]

Originally Posted by joelg236

A winch would limit our range of motion (since the arm needs to go all the way back for bins). I'm intrigued by the idea of reducing shock, since we have lots of spare belts and some sprockets, so it would just be a question of mounting. I'll be trying this today.

Shock loading is a really tricky thing to improve, but awesome when you get it right. What you are looking for is something that can support your nominal load fully, but then flexes a little bit if you exceed that. Using a coupling like these spider couplings might work:
http://www.mcmaster.com/#spider-couplings/=vxqhqt

It looks like your arm has about a 3 foot reach for center of mass of a Tote. I think this would equate to about 25 ft*lbs or 300 in*lbs. I personally would size the coupling and buy 3 spiders (different material). They have urethane ones rated for 290 in*lbs, and a Buna Nitrile the same size rate for 140 in*lbs, and a Hytrel rated for 400 in*lbs. I think you might be happiest with the Buna Nitrile (I personally would order 2 of those to have a spare). I suspect that the torque rating is a Max Continuous at operating RPM, and for your application would add a nice cushion to the lifting and bumps. Should it be undersized, then the same geometry urethane would be a good alternate.

All this being said, you are outside of VEXPROs load rating for that gearbox. They do their rating chart based off of motors and ratios as the motors have the capability to apply enough torque to do damage to the gearbox in those scenarios.
http://content.vexrobotics.com/vexpr...s-20150106.pdf
IE, any of the above fixes, and you are still operating outside of the recommended limits of the gearbox.

For this, my overall recommendation would be get a new 5:1 ratio box, and rebuild it to be 50:1 ratio, and then use a chain loop for the arm. I personally like big sprockets on arms as it reduces the tension in the chain and thus compounded loading effects Something like a 36 tooth sprocket or larger attached to the arm, and then driven by a 12 to 15 tooth sprocket.

the same 300 inch*lbs needed by the arm will equate to about 150 lbs of chain tension (radius of a 36T #35 chain sprocekt is a bit over 2 inch).