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 Drum or beater

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Mr. P
MikeNCR
Don Doerfler
Brandon Roberts
abrown
Cody
rjw
Frisco
Josh Raichel
Dan Curhan
Jeff L
Andy
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Cody
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Cody


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Drum or beater - Page 2 Empty
PostSubject: Re: Drum or beater   Drum or beater - Page 2 I_icon_minitimeMon Jan 10, 2011 12:58 am

Idk if I would say that drums really spin up any noticeably faster then a beater
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Don Doerfler
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PostSubject: Re: Drum or beater   Drum or beater - Page 2 I_icon_minitimeMon Jan 10, 2011 8:01 pm

Cody wrote:
Idk if I would say that drums really spin up any noticeably faster then a beater
yeah I agree I just think that you would get more momentum with a beater
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MikeNCR
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PostSubject: Re: Drum or beater   Drum or beater - Page 2 I_icon_minitimeMon Jan 10, 2011 8:59 pm

I'm fairly sure with some careful design choices you can get matching drum and beater MOI's in the same width and diameter.
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Jeff L
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Jeff L


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PostSubject: Re: Drum or beater   Drum or beater - Page 2 I_icon_minitimeMon Jan 10, 2011 9:37 pm

I'm going to correct you all, because I made a gigantic spreadsheet to answer this question a while back. A beater and a drum with the same diameter (tooth to tooth distance) will almost always result in a drum with a higher MOI. However, when the mass of both is held to the same number, you will be able to almost always make a beater with a higher MOI because you can make its tooth to tooth distance a fair deal larger without sacrificing strength.
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rjw
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rjw


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PostSubject: Re: Drum or beater   Drum or beater - Page 2 I_icon_minitimeTue Jan 11, 2011 10:36 am

well ...duh Rolling Eyes

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Jeff L
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PostSubject: Re: Drum or beater   Drum or beater - Page 2 I_icon_minitimeTue Jan 11, 2011 4:44 pm

rjw wrote:
well ...duh Rolling Eyes


Well, I'll state the obvious when people are clearly not aware of it.
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rjw
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PostSubject: Re: Drum or beater   Drum or beater - Page 2 I_icon_minitimeTue Jan 11, 2011 6:03 pm

To me, the obvious seems to be that a larger diameter weapon has a better chance of carrying more momentum...

To paraphrase your observation, it could be said that a 10" diameter beater will have more MOI than a 2" diameter drum....

Or did I miss something?
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Jeff L
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PostSubject: Re: Drum or beater   Drum or beater - Page 2 I_icon_minitimeTue Jan 11, 2011 6:44 pm

What I was saying was:

IF outer diameter is held equal THEN a Drum's MOI > a Beater's MOI.

IF mass is held equal THEN a Beater's MOI > a Drum's MOI.


EDIT: basically, if you're tight on weight but not on space for your weapon, go for a Beater. If you're tight on space but not on weight, then go for a Drum.
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rjw
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PostSubject: Re: Drum or beater   Drum or beater - Page 2 I_icon_minitimeTue Jan 11, 2011 6:57 pm

Again, I have to disagree

For any given diameter and length you can build a drum that will hold up as well or better than a beater.

Your thinking might only hold true, in cases where the beater will probably not hold up all that well.

Build a beater if you don't have the $$$ or capability to build a good drum

jmo...from my real world experience.
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Jeff L
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PostSubject: Re: Drum or beater   Drum or beater - Page 2 I_icon_minitimeTue Jan 11, 2011 8:33 pm

scratch rjw, stop jumping to conclusions. You haven't done the math.

The question I was aiming to answer was MOI maximization. Based on dimension or mass constraints, those are the correct outcomes; I didn't make four excel spreadsheets calculating the MOI and maximum possible kinetic energy for a range of weapon sizes and materials for disks, drums, beaters, and bars just for fun. While it is true that a cylinder is a far better shape for making high MOI spinning objects, under the circumstances I stated a beater does indeed come out ahead. I'll agree though, a beater will be somewhat inferior strength-wise, which is not as difficult to control for as it would seem (as I am sure the Plum crew can confirm).

EDIT: I've said before drums are the easiest way to get a high MOI. I'm just listing the cases in which a drum may be limited where a beater might not be.
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Mr. P

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PostSubject: Re: Drum or beater   Drum or beater - Page 2 I_icon_minitimeTue Jan 11, 2011 9:52 pm

rjw wrote:
Again, I have to disagree

For any given diameter and length you can build a drum that will hold up as well or better than a beater.

Your thinking might only hold true, in cases where the beater will probably not hold up all that well.

Build a beater if you don't have the $$$ or capability to build a good drum

jmo...from my real world experience.


why does a drum cost more?

I see aluminum pipe, aluminum side plates and steel teeth on a drum. beater has one piece of metal. the needed thickness of the metal makes the costs more than aluminum pipe. what am I missing? I know a titanium, s7, chromolly, beater will cost more than an aluminum drum.
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jeeves_m_d
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PostSubject: Re: Drum or beater   Drum or beater - Page 2 I_icon_minitimeTue Jan 11, 2011 10:03 pm

RJW machines his drums out of a solid piece of s7 teeth and body are all one piece

I agree that drums can be relativley cheap, the 7'' diameter drum I am putting on lolcano is only going to cost about $50 in materials and machining. Which is much less than what the slab of steel used to cut the beater out of had cost.

But RJW's is probably worth the price difference
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rjw
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PostSubject: Re: Drum or beater   Drum or beater - Page 2 I_icon_minitimeWed Jan 12, 2011 10:22 am

Jeff

Perhaps your math needs a second look?

Mr. P

Machining time on some solid steel drums with integrated teeth can be very pricey.

Most use separate endcaps, while some are true 1 piece units, with bearing surfaces, etc. all made from 1 single piece of material.

IMO, the best bang for the buck would be an alloy steel drum with welded teeth. On a 15 pound robot, wall thickness only needs to be around 1/8"

Aluminum body drums can work very well, but possibly NOT if facing hard steel weapons, especially some of the higher speed ones.

However, all this is kind of relative to how much action the robot will see...MLEC has several drumbots that have competed at numerous events and will hopefully continue to do so for a few more years.

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turtle
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PostSubject: Re: Drum or beater   Drum or beater - Page 2 I_icon_minitimeThu Jan 13, 2011 3:39 pm

a beater doesn't need to be made out of one piece of metal. our weapon has aluminum sides suspending steel bars. In my opinion a beater is better because it allows more of a contact area plus a larger force hit. our weapon is devastating (when we can keep it together) Embarassed
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Cody
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PostSubject: Re: Drum or beater   Drum or beater - Page 2 I_icon_minitimeThu Jan 13, 2011 4:24 pm

turtle wrote:
a beater doesn't need to be made out of one piece of metal. our weapon has aluminum sides suspending steel bars. In my opinion a beater is better because it allows more of a contact area plus a larger force hit. our weapon is devastating (when we can keep it together) Embarassed

This is only cause of the size not because its a beater
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turtle
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PostSubject: Re: Drum or beater   Drum or beater - Page 2 I_icon_minitimeThu Jan 13, 2011 5:32 pm

actually i think we make good contact because of the middle being open it allows the bot to get more into the weapon radius thus more hitting area. i realize that were talking about a fraction of a second difference but even this small amount has helped.
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Jeff L
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PostSubject: Re: Drum or beater   Drum or beater - Page 2 I_icon_minitimeThu Jan 13, 2011 9:18 pm

Hokay, after doing a good bit of rechecking numbers, yes: a beater of appropriate dimensions can beat a drum of the same weight's MOI - within a very specific set of sizes. Outside of that, a drum will win out for a big MOI.

One of those sets of dimensions is as follows:

12" long, an outer diameter of approximately 12", and a cross section of the bar equal to a 2"x1.5" rectangle (the thicker dimension is perpendicular to the striking surface of the bar; in other words, when the beater is laid flat on a table it will be 2" tall).

For comparison, the drum in question that is closest in weight is one of the same length with a 7.2" outer diameter and half-inch thick walls.

There is certainly a range of beaters that will outdo a drum of the same weight across any weight class, however as I mentioned, they WILL take up more space - A LOT MORE. With that concluding this debate, you can safely pick either a drum or a beater depending on what makes the most sense for your robot. For example, a robot that fits entirely within its weapon (such as LOLCANO) might be better suited with a gigantic beater than with a smaller drum. Other robots (like rjw's ultra-high-speed masterpieces) are more likely to benefit from a smaller drum with thicker walls.
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Cody
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PostSubject: Re: Drum or beater   Drum or beater - Page 2 I_icon_minitimeThu Jan 13, 2011 9:28 pm

I wouldnt want a 7 in drum let alone a 12 in beater.....
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Jeff L
Minister of Silly Walks
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Jeff L


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PostSubject: Re: Drum or beater   Drum or beater - Page 2 I_icon_minitimeThu Jan 13, 2011 9:30 pm

The numbers I was working off of were originally for 120 lb robots - scaling them to different weight classes should be pretty easy.
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rdubard
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PostSubject: Re: Drum or beater   Drum or beater - Page 2 I_icon_minitimeSat Jan 15, 2011 10:16 pm

You know, most of us (even me) would have given up on the math after running into conflict with rjw (we're not worthy--seriously have you seen the stuff he makes?), but I gotta hand it to Jeff cuz he kept hacking those numbers and found the 'eggbeater valley" where the precise combination of variables gives a different 'best' answer.

It's an unusual thing when someone admits they're wrong, but it's even more unusual when the go out of their way to investigate and prove that they were wrong. Good work, Jeffrey!

Now, would you care to share that spreadsheet with me (so I can use it)?
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Cody
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PostSubject: Re: Drum or beater   Drum or beater - Page 2 I_icon_minitimeSun Jan 16, 2011 12:29 pm

let some one else do the math....
http://www.teamcosmos.com/ke/ke.shtml
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Jeff L
Minister of Silly Walks
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Jeff L


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PostSubject: Re: Drum or beater   Drum or beater - Page 2 I_icon_minitimeSun Jan 16, 2011 3:03 pm

Looks like the link isn't working for me at the moment, Cody.
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Jeff L
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Jeff L


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PostSubject: Re: Drum or beater   Drum or beater - Page 2 I_icon_minitimeSun Jan 16, 2011 4:25 pm

It's working now, but it looks like the calculator itself is broken...
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Cody
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PostSubject: Re: Drum or beater   Drum or beater - Page 2 I_icon_minitimeSun Jan 16, 2011 5:23 pm

just did everything for a drum and it worked fine for me
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Stephen
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PostSubject: Re: Drum or beater   Drum or beater - Page 2 I_icon_minitimeMon Jan 17, 2011 12:00 am

I would go with a drum they are a lot harder to break for one and it's ur first time with a beater /drum u don't want to break it
Oh and there not that hard to build the hardest thing to build i think is the teeth But we just sent them to a shop to build them

But Im unexperienced and have no idea how to even start building a beater so it's ur decision
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