paper: GUS Team 228's 2011 Robot CAD Assembly

[Jamie Kalb]

I can't wait to check out the CAD (I don't have Solidworks on this computer) but I have a few questions about the bot. First, did you guys do all the painting in house? If so, what exactly did you use to paint it? In the past, when I've used spray-paint, it gets scuffed up pretty quickly (and doesn't look nearly that nice to begin with, either).

Second, could you explain the chain routing on the elevator? My team used a two-stage elevator this year in VEX with one continuous loop, and we're looking for possible new methods to experiment with for next year.

Looks amazing, as usual. Thanks for posting the CAD!

EDIT: Took a look at the CAD. Outstanding work, and like Akash said, a great learning tool! Thanks again for posting it.

[artdutra04]

Originally Posted by Jamie Kalb

I can't wait to check out the CAD (I don't have Solidworks on this computer) but I have a few questions about the bot. First, did you guys do all the painting in house? If so, what exactly did you use to paint it? In the past, when I've used spray-paint, it gest scuffed up pretty quickly (and doesn't look nearly that nice to begin with, either).

Yup, we did all the painting in our shop (which is shared with the school's wood shop, which causes us to be extra careful because of the dust).

1. Remove all foreign contaminants (grease, machining oil, dirt, etc) from the parts, for which acetone does a great job. If you can't use acetone, denatured ethanol (ethyl alcohol) also does a good job.

2. Select the proper paint. We use enamel spray paint, as it forms a much more durable coating that's almost as strong as powder coating when fully cured. We experimented this build season, and used a "hammered finish" black paint and a plain orange paint, and liked both.

3. Carefully paint it. Not much to it, just don't make it run. Since we're in a snowy northern climate, we have to do this indoors during the build season, which means we usually do all painting at the end of meetings (with the dust ventilation system on, and after almost everyone else left) and let it cure overnight.



4. Let it cure. Even though the paint may claim to "cure" in a few hours, wait at least 24 hours before doing anything intense with it.

5. ???

6. PROFIT! Or at least trick people into thinking you have a powder coated robot. ;-)

Originally Posted by Jamie Kalb

Second, could you explain the chain routing on the elevator? My team used a two-stage elevator this year in VEX with one continuous loop, and we're looking for possible new methods to experiment with for next year.

There's two ways to power elevators, one with a single loop of chain (continuous), and one with one loop of chain per stage (cascade). If you want to learn more about the basic design of these elevator styles, Andy Baker has a great presentation about this here. While we utilized the "cascade" type of elevator, we modified the design to not need spools and only use continuous runs of roller chain (with springs to automatically tension it).

This is the view of the back of the robot. The two chains on the bottom are counter-rotating and each go to one side of the two parallel sets of roller chain used to lift the first stage. The vertical roller chain you see here is what's used to lift the second stage carriage.



So how does this work? Let's consider the outer (non moving) elevator member link 0, the first stage elevator link 1, and the tiny second stage link 2.

Link 1 is powered up and down by a loop of roller chain; the sprockets for these are fixed to the top and bottom of link 0. The chain/cord is bolted to the bottom of link 1, such that any rotation of the chain/cord turns into linear motion for the first stage. This loop of chain is powered directly from the gearbox. On our 2011 robot, these roller chains were on the front side of our elevator.

To power the second stage up simultaneously, this loop of chain is required to have two fixed points. One side must be anchored to the top of link 0. This then must run over a sprocket located on the top of link 1, then run down to a fixed point at the center of link 2. To have it powered down (not rely on gravity), you need to continue this with another section of chain starting from the fixed point on link 2, around a sprocket on the bottom of link 1, then back to the fixed point on link 0. When link 1 is moved up and down, link 2 will automagically do the same thing (and vice versa).

We created all our fixed chain locations using 25p roller chain and #4/40 bolts through the gaps in the roller chain. We also mounted springs in the elevator to both automatically tension the roller chain, but also to help absorb the impact (along with rubber pads on the hard stop) if we run into the hard stop at speed.

[Jamie Kalb]

Wow! Thanks for such a detailed response. I'm still amazed that the paint came out so well.

Yeah, I had seen that Andy Baker presentation before, but never fully understood quite how the cascade system worked. Your explanation makes a lot of sense, with very useful pictures, as usual. My team had some interesting situations with not being able to predict the postion of the middle stage midway through the travel, so the cascade routing is something we might experiment with.

One more question: do the springs in the chain also act as counterweights to offset the elevator weight, or are they just there for tensioning/shock absorbing?

[Jeffy]

Originally Posted by Jamie Kalb

One more question: do the springs in the chain also act as counterweights to offset the elevator weight, or are they just there for tensioning/shock absorbing?

A spring inline with the chain routing will provide chain tension. However, it provides a very equal force in each direction. The result being that it doesn't give any help to the elevator going up or down, just keeps the chain tight. (imagine if it were a string and you pulled on both ends with equal force)

[Jamie Kalb]

Originally Posted by Jeffy

A spring inline with the chain routing will provide chain tension. However, it provides a very equal force in each direction. The result being that it doesn't give any help to the elevator going up or down, just keeps the chain tight. (imagine if it were a string and you pulled on both ends with equal force)

Makes sense. Thanks!

[artdutra04]

Originally Posted by Jamie Kalb

One more question: do the springs in the chain also act as counterweights to offset the elevator weight, or are they just there for tensioning/shock absorbing?

We installed 2x ~8 lb constant force springs between the second stage carriage and the top of the fixed stage to prevent back driving.

[ttldomination]

Definitely Thank You for the excellent explanations.

I'll go ahead and throw my question into the mix. But, how are you powering the second stage, or the chain between link 1 and link 2? Is there a motor somewhere on link1/link2 or is that stage's chain slaved off another stage?

Thanks,
Sunny G.