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  #16   Spotlight this post!  
Unread 09-13-2017, 10:23 PM
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Re: pic: What Could Carpedav Possibly B Working On?

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Originally Posted by carpedav000 View Post
My fear with going from single tooth to three-tooth is that with the intended weapon rpm (10,000-11,000), the opposing robot would ride up the wedge just fine, but the teeth would "grind" down on the other robot instead of hooking and throwing/breaking. I could lessen the weapon rpm, but I'm wanting to go with maximum impact. Unless someone knows this wouldn't be an issue.
That makes sense. If you only want one tooth, I would place a denser material where you would put two of three equally spaced teeth to get the center of mass back in the middle of the shaft. It will make balancing the drum easier than just putting weight directly opposite the one tooth.

Edit:
Or just provide mounting points on the outside of the drum to add mass for balancing, but I think with machined components you can have a designed drum that's balanced with out messing around with adding weights to it. The single tooth unbalanced will drive harmonics into the robot.

Last edited by Deke : 09-13-2017 at 10:36 PM.
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Unread 09-14-2017, 09:54 AM
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Re: pic: What Could Carpedav Possibly B Working On?

Quote:
Originally Posted by Deke View Post
That makes sense. If you only want one tooth, I would place a denser material where you would put two of three equally spaced teeth to get the center of mass back in the middle of the shaft. It will make balancing the drum easier than just putting weight directly opposite the one tooth.

Edit:
Or just provide mounting points on the outside of the drum to add mass for balancing, but I think with machined components you can have a designed drum that's balanced with out messing around with adding weights to it. The single tooth unbalanced will drive harmonics into the robot.
Tungsten Rods are available that have 2.5 x the density of iron. You could mill/drill one or two lengthwise holes inside opposite of the Key Feature and force fit them into the cylinder to fully balance it.

You would need calculate the mass moments of inertia, to find a solution that is balanced around the rotational axis. I could help with the calculation if you have the existing materials and full dimensions.
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Last edited by InFlight : 09-14-2017 at 10:10 AM.
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Unread 09-14-2017, 11:34 AM
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Re: pic: What Could Carpedav Possibly B Working On?

Another thought is to just machine out material 60 degrees on either side of the tooth. That would have the same effect as adding mass 120 degrees equally spaced around the drum.
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Unread 09-14-2017, 11:47 AM
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Re: pic: What Could Carpedav Possibly B Working On?

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Originally Posted by GeeTwo View Post
If you look closely, wind turbines, car wheels, and such tend to have prime numbers of spokes. This reduces the number of resonant frequencies to worry about providing damping. Likely not a significant effect for this application.
Not exactly. It doesn't have to be a prime number. The odd number disconnects the beam elements and essentially doubles the stiffness of those beams because they only cross half the diameter instead of all the way across. This would double the resonant frequency in those beams hopefully driving them above operating frequencies.

Stiffness is directly related to length, and natural frequency is the root of stiffness over mass.

The odd numbered spokes or mass placement creates a trigonometric geometry that is more forgiving to the center of mass allowing for more error in the manufacturing. It's slight, but slight helps at high rpms.

Another reference, see fidget spinners. I don't recommend an even numbered module spinner to help your focus...

So basically, because math.
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Unread 09-14-2017, 01:45 PM
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Re: pic: What Could Carpedav Possibly B Working On?

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Originally Posted by Deke View Post
Another thought is to just machine out material 60 degrees on either side of the tooth. That would have the same effect as adding mass 120 degrees equally spaced around the drum.
That's what I'm planning on doing, I just need some serious help with the math
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