Infinite Recharge Hooded Shooter Design Questions

Hi everyone, my team is planning on revising our hooded shooter from last year so I’ve decided to see if anyone here has some advice to help us along.

I should probably include that our goal is to have a shooter that can shoot the balls into the upper port from maybe 30ft with good consistency.

So here’s what we assume to be optimal right now:

flat 4in rollers (like a stack of hard am stealth wheels)
about 1.5-2in compression
Smaller top rollers
Extra inertia somewhere in the system
as much contact time as possible
Slippery hood material
Higher RPM (main wheel) maybe 10k+ ? (to increase surface speed of 4in wheels w/o going bigger)
high rigidity is a goal
shots with less arc are more consistent (laser shots may be the term?)
reduce ball slippage on the wheel

Here are my questions:

How wide should the hood be ? 7.5in? 8in? bigger?
I’ve seem both, so are slippery hoods or grippyer hoods better? (slippery plastic vs aluminum backing) (maybe grippy are more consistent but don’t shoot as far?)
What release angle is a good starting point?
Are my above assumptions correct?

I know that’s a lot so thanks in advance to anyone who takes the time to help us out.

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Hood Material: I believe stickier hoods will give the ball more spin (assuming the ball slips on the hood) but our robot last year used a polycarbonate hood and the balls were so sticky we did not have problems though we did not try a more grippy material to compare.

Distance: 30ft distance is a little optimistic somewhere around 20ft is a good shooting distance. The closer distance should require less RPM so gear it differently to run the motors at a higher power. This should mean the wheel will spin up faster and maintain its speed between shots better. Its important to consider the further distance may seem like it will allow for faster cycle times but the above would mean you should spend less time shooting, and less time speeding up. On top of that the closer distance should increase the consistency.

Wheels: Stealth wheels are pretty good and cheap but don’t have much mass. Wheels like Fairlane wheels are grippy and heavy acting like their own flywheel but require custom made hubs. Colson wheels are also good options and easy to use but are not perfectly flat (use two to keep the ball in the center of the wheels). If you can try several kinds of wheels.

RPM: 10k RPM seems fast to me and if you are shooting from a closer distance then you need less speed also some wheels wont survive that RPM.

Checkout this thread Design Shooter - FIRST - Chief Delphi most of the advice is good. You will also see a post from me of our design last year. Use rough prototypes to eliminate ideas quickly (looking back we should have tested more variety rather then try to prefect our shooter design in the prototyping stage. Also if it may be nice to design your shooter to be able to manually adjust the hood angle after you have built it for better tuning (this could be some sort of adjustable rail like 254 2014 hood or a design where you just remanufacture a part or two with a slightly different dimension and changes the hood angle).

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All very helpful advice. Thank you

Not a fan of 10K RPM at all. If you want to hit the 30 ft shot, move up to 6" shooter wheels. In addition to the increase in safety, you can get NEOs/Falcons/miniCIMs into a controllable RPM range and get the exit velocity you need. I’d recommend 20-22 degrees for the laser-like shot. If you back off some from insane laser and get a little bit of arc on the shot you can increase the interval (short - long) where you can make shots without adjusting anything about the shot. There is certainly a place for adjusting shooter RPM and/or shooter angle based on vision or sensor feedback to hit the inner port from anywhere, but there’s a lot of value in having a really robust shot with everything fixed.

8" hood width should be fine. Not sure if going tighter helps any. I’d be concerned that the Power Cell might contact the sides if you are at 2" compression and 7.5" width. I like about 1.5" compression with these Power Cell balls but 2" might be OK as well.

I don’t think that most plastics that you would use for hood backing would be considered “slippery” with respect to the Power Cell. PC or PETG have pretty good friction with the Cell. I’ve heard of some people lining the hood with PTFE film to actually make it slippery. I think excellent results can be obtained with quite a range of friction with the hood backing. I’d consider many other factors to be more important.

If you do consider 6" wheels and also consider Colsons, note that you can lathe them to take off the slight crowning across the tread. You can also lathe down the side of the hub if you want to fit a couple wheels more tightly together than would be possible out of the box. Counterintuitively, a gap in the middle of two wheels or crowns on two wheels have a tendency to move a ball away from the middle of the hood rather than to hold it in the middle.

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My team built an adjustable shooter (with a moving hood) last year. In terms of distance, 30 ft is possible, but what we found is that the balls have so much inconsistency and damage that it was never worth our time to shoot from behind the color wheel. This is important to keep in mind in general—though it might not be as important for infinite recharge @home, there is massive ball inconsistency that undermines a lot of the theoretical advantages your shooter can have.

Either way, I’d definitely recommend moving up to 6" wheels. Beyond just getting higher RPMs, the heavier your wheels are—especially on the outer edges—the higher its moment of inertia, which basically means after the initial spin-up the wheel will maintain more speed after each ball it shoots. In other words, if you want to be able shoot pewpewpew rather than pew (re-spin-up) pew (re-spin-up) pew (re-spin-up) you probably want to be using bigger wheels.

We used about a 1.5" compression and that worked well. I believe during our initial prototyping we determined that that was the most consistent compression. I’ve seen teams pull off 2" pretty well though.

In theory, slippy hoods allow the balls to be more speedy, because the kinetic energy is used for linear motion rather than angular. However, again, the amount of slippiness depends heavily on the ball’s condition, and I’m not sure that you’ll find a material that slips super well on new balls with 1.5-2" compression, either. If you’re using big enough wheels and high enough rpms, grippy hood is easier and more consistent, but in general I wouldn’t stress this one too much. We used UHMWPE and it was fairly grippy.

We didn’t use top rollers and that worked well for us, as with many other teams, so for weight reasons I’d skip those, but I’m not the expert on that.

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If i wanted add weight to the flywheels with spare gears or metal disks or something after the fact how much extra weight do you think i should add for optimal results? 2lbs? 5lbs? more?

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Based solely on intuition here, 5 pounds seems quite excessive. I think it’d be a matter of trial and error after the rest of the robot is done and you know your remaining weight allowance. It also depends on the wheels you choose—what are you leaning towards? Keep in mind that the moment of inertia increases and the radius increases - in other words, for the same about of actual mass, you can create different results on the flywheel depending on how far out the weight is situated on your extra disks.

There’s also a certain compromise you might make here. What motors are you using? Depending on your motors and gear ratios, you might have to find a compromise between flyup time (higher moment of inertia = harder to flyup) and time between shots. You might find that for your team’s strategy one is more important than the other.

Hope that’s helpful! We used 4 AndyMark aluminum performance wheels as our flywheels with no extra weight and had super fast consecutive shots. (I really like those wheels for flywheels, as they are super weight efficient, with the mass distributed mostly on the outer edge.) For reference, those are about 0.7 lbs each. However, we were also using 2 direct drive falcons, which is pretty OP.

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It’s not a matter of weight, it’s a matter of weight distribution. You want the weight as far away from the shaft as possible to increase the moment of inertia.

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Balance it. You won’t regret it. It’s easy. Search “go kart wheel balancer”

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The Julia Design Calc has a section for plugging in the MOI/characteristics of a intertial wheel so that you can see how that effects the shooter system. Would recommend.

Anecdotally, my team used 4x 74t gears before switching to the SDS intertial wheel. They worked fine in a pinch.

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Balance it. You will regret not doing it.

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Yea ive definitely regretted not balancing my shooter wheels in the past - the 3 shooters I’ve made so far have all created various versions of a chainsaw sound which is certainly not ideal.

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If my team had a lathe i would definitely turn down some 4in colson wheels to be flat, and knock down the shoulders so they would sit flush with each other. However we do not have a lathe so i think I will just go with 4in stealth wheels and some extra weight. In terms of extra weight i will probably just make some steel disks with cutouts in the center to take advantage of the mass further from the rotational axis without adding unnecessary weight closer to the middle. I’m going to do something similar with a set of top rollers (stealth wheels + weight) and use 1 or maybe 2 falcons to run the main wheel and a falcon to run the top one. That way we can change the difference in surface speeds between the top and bottom (adding various degrees of spin) which I think may help make up for the lack of adjustability in our mechanism.