pic: Spectrum 3847 - 2017 CAD Model: Gamma Ray

What is your shooter drum made out of?

This is awesome, just wish my computer would load the step file so I could get into the model more.

The current version is 45A high-temperature silicone rubber tubing from McMaster, I believe 1" ID 1.25" OD, stretched around 1.25" aluminum tubing. We hope to Smith to some white 35A though to keep with our colour scheme (and maybe get some better grip on the ball).

We used that in our testing but it expanded too much. It looks like your’re using a belted 2:1 reduction from a 775pro, have you not seen this expansion as well?

What speeds were you testing at? We were testing primarily at 6000 rpm and didn’t notice any separation from the aluminum or excessive expansion.

Granted, we did used solid 4" conveyor wheels * to shoot in 2016, and those loved to expand so maybe we’re just used to shooter wheel expansion.*

Ah, that’s fair. 1.5/2 does just have a lot more material to expand, so I wouldn’t be surprised if our were also expanding but it’s subtle enough to miss. Our shooter wheels are also boxed up so uncontrolled expansion should be relatively contained.

How many balls can you guys hold? Doesn’t look like the traditional hopper to me, are you planning on intaking only?

Our current estimates sit around 50 balls or so. We will have a hopper, the lexan sheets that it is made of have yet to be fully modeled though. There will be one panel on either side and an angled panel out over the fuel intake that will spring out when the intake is down, but be forced back into frame when the intake is pulled back up.


Great bot as always! I really like the built-in handles.

Nice work!

Centrifugal acceleration* = w^2 * r. More RPMs and more radius = more force expanding your tube. Since this acceleration applies to all of the tube, thickness won’t matter, aside from the smaller force on the ID of the tube. The only material property that matters is some sort of ratio of density and elastic modulus, but that’d be the same since ou said it was the same stuff.

So all in all, not surprising your tube wanted to expand more than Spectrum’s. Are you guys going for a longer shot to need that much more surface speed?

*Yes centrifugal. Pedants are welcome to spend 15 minutes drawing a FBD with a tangential velocity, required change in velocity to create a circular path, implied acceleration, unbalanced force to create that acceleration, etc, etc. I’m going to pretend it’s a real force and get on with my life.

No just limiting the contact time and wheel size, as well as bad prototypes and the mentality that we will be able to spin slower with a properly built robot that has less friction deficiencies.

Nice job 3847! What are the servo motors on the front/back of the shooter for?

Thanks! We’ll only be using one of the two sets of three, but they’re intended to be lane blockers. If we line up to the boiler and one of our side streams isn’t on target, we want to be able to disable that stream instead of realigning our entire robot. We have one in the middle so that we can theoretically make a shot while our side is lined up with either the driver station or the front wall of the field.

This is so amazing!
My team and I just had a few questions:
What is your shooter compression and how did you settle upon this number?
What is your intake height and how did you find this number?

At the moment I believe it’s around 4 5/8" between the drums but the tower includes two plates that can be changed to allow us to increase compression if we need too.

What is your intake height and how did you find this number?

The height of the fixed intake roller is about 4.75" from the floor but that could also be changed by going to larger wheels. We currently have 1 5/8 compliant wheels from WCP but we could use 2" wheels if needed.

We wanted to design to the smallest amount of compression we could get to work and increase if needed. That way we start with the least load on the motors and add load as needed.

Great design! Ironically, just looking at your rather complex robot, I was inspired of a way to make our gear pickup MUCH simpler (replace two motors with one belt). Thanks!

Beautiful looking robot… and the CAD is nicely done.
I am looking forward to seeing how this plays on the field.

Well done!!

I’m equally worried about teams building drive base/bumper assemblies to the maximum allowed dimension, and then regretting this decision when they realize all the cool things they could have achieved with a smaller base that enabled over-the-bumper dropdowns. In past years, iterating in new systems was just a matter of finding the space and weight, this year there are going to be a lot of teams boxed out of improving their robots and adding features like Spectrum has because they assumed bigger=better early in the process.

I honestly expect the opposite to happen - teams built chasses that were too small so they could do over-the-bumper stuff, expecting a huge advantage, then realizing it was more trouble than its worth and are now sad about the substantial decrease in ball capacity they traded for that decision.

3847’s robot is a good example of leaving a conservative amount of space (just an inch or three) for OTB stuff, but I bet others left way more than they should have.