Faster or slower to crush cans?

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Discussion Overview

The discussion revolves around the effectiveness of different speeds for a can crusher that utilizes air-filled tires to flatten aluminum cans. Participants explore the implications of tire speed on crushing efficiency, considering factors such as momentum, air pressure, and strain rate sensitivity of aluminum.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants propose that spinning the tires faster may increase the crushing effectiveness due to the momentum of the heavy rubber tires.
  • Others argue that spinning the tires slower allows the air pressure in the tires more time to effectively crush the cans.
  • One participant notes that while aluminum has strain rate sensitivity, its effect is minimal in the context of crushing cans.
  • It is mentioned that the tire tread deflects during crushing, which may increase the crushing force at higher speeds.
  • A later reply suggests that air compression within the can at higher speeds could reduce the net crushing force.
  • Another participant shares their experience with hand-crushing cans and mentions techniques like pre-dimpling to facilitate easier crushing.
  • One participant describes their can crusher setup, including the use of a motor and a safety tube for dropping cans.

Areas of Agreement / Disagreement

Participants express differing views on the optimal speed for crushing cans, with no consensus reached on whether faster or slower speeds are more effective. Multiple competing theories regarding the mechanics of crushing remain unresolved.

Contextual Notes

Participants acknowledge the complexity of calculating the effects of speed, tire pressure, and air compression on crushing force, indicating that these factors may vary in significance.

John Archer
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I've built a can crusher that uses 2 air filled tires to flatten the cans.
My question:
Which theory is correct?
A) Spin the tires faster, The momentum of the heavy rubber tire will effectively crush the cans "flatter"
B) Spin the tire slower, giving the air pressure in the tires more time to flatten the cans

The tires are in constant contact with each other, and I normally run 20Psi of air in each tire.
1 tire rotates clockwise, the other, counter clockwise. When a can is dropped onto the tires, it is pulled in between the tires, and crushed.
 
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While aluminum has a strain rate sensitivity, the effect is small over the range of strain rates in crushing cans.

The tire tread deflects when crushing a can, so the inertia of the tire tread will tend to increase crushing force at higher speeds.

The air in the can gets compressed at higher speeds because it flows out through an orifice (the opening), which would tend to reduce the net crushing force at higher speeds.

All of these effects are difficult to calculate, but my best guess is that speed will have a small effect. I think that the largest single effect will be from tire pressure.
 
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Welcome to the PF, John. :smile:

Is this hand-powered? Or do you have 1-2 electric motors spinning the tires?

If hand-powered, how many cans to you crush at a time? Only one every hour or so, or do you do a whole trash bag full of cans at a time?

How do you orient the cans when you feed them in between the tires? On the same axis as the spin axis of the tires, or so that the top and bottom of the can contact the two tires at the same time?

And are you aware of the trick that makes crushing cans about 10x easier? It's pre-dimpling the sides of the can to weaken it...
 
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berkeman said:
And are you aware of the trick that makes crushing cans about 10x easier? It's pre-dimpling the sides of the can to weaken it...
Cool! I never heard of that.
 
Yeah, I crush my cans all the time by hand; it's really easy if you know the tricks. Pre-dimple the middle of the can with 4 dimples (two hands at the same time, thumbs and middle fingers). Then crush it end-to-end with a twisting motion. Be a little careful that the middle of the can doesn't tear during the crush, or you could nick your hand on the tear's sharp edge.

Easy peasy. :smile:
 
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berkeman said:
Welcome to the PF, John. :smile:

Is this hand-powered? Or do you have 1-2 electric motors spinning the tires?

If hand-powered, how many cans to you crush at a time? Only one every hour or so, or do you do a whole trash bag full of cans at a time?

How do you orient the cans when you feed them in between the tires? On the same axis as the spin axis of the tires, or so that the top and bottom of the can contact the two tires at the same time?

And are you aware of the trick that makes crushing cans about 10x easier? It's pre-dimpling the sides of the can to weaken it...
I am using (1) 1/2hp motor with a dual belt speed reduction system to slow it down to something less than frightening LOL!
1 tire drives the other.
Cans are dropped from above through a short tube (safety) and enter the tire crushing area vertically.
It can/will crush the cans as fast as I can drop them into the tube (they come out faster than they go in right now). As such, no need to dimple. Though that reminds me if stupid games we would play as kids. Stand on and empty can, try not to let it get crushed until you are only supported by the can, then, as quickly as you can, tap the side, but get your fingers out before the can gets crushed.
How did we survive ?

I'll try to post a video when I get home.
 
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Yeah, a video would definitely be fun! :smile:
 
John Archer said:
How did we survive ?
The most famous words in history!
 
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John Archer said:
B) Spin the tire slower, giving the air pressure in the tires more time to flatten the cans
Those small AL cans and those tires - I don't think there is much a difference regarding the tires itself, it is an overkill anyway.
The reason I would suggest to speed down a bit is due the cans instead: the air pressure building up within due the fast compression.
 

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