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Engine sizing (torque/power) question

  • #1

Homework Statement


There is a 1 kg (52x52x52mm) square block attached to a 200 mm long (d = 8 mm, weight = 0.03 kg) rod what is connected to an electric motor. The setup is vertical and needs to achieve about 90 rpm. So, the 1 kg square block will circle on the outer rim, connected to the motor by the rod.
The question is how strong motor has to be bought. It would be great to see the complete calculation logic because then I could use it with different sizes.

Homework Equations


Maybe angular velocity calculations, friction loss calc, gravity calc, not sure

The Attempt at a Solution


I am far from being able to do this calculation since coming from a non-physics are, it looks I am very underskilled.
 

Answers and Replies

  • #2
berkeman
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Welcome to the PF.
I am far from being able to do this calculation since coming from a non-physics are, it looks I am very underskilled.
What class are you taking? Have you been going to lectures and reading your textbook? What other classes have you taken so far?

EDIT/ ADD -- Also, your Profile page says that you have a Master's degree. What field is your Master's degree in? Just curious. Thanks.
About
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  • #3
CWatters
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The power required is hard to calculate because you need to know what resists the block spinning...air resistance? Friction in the bearings? Can you quantity these things?
 
  • #4
Welcome to the PF.

What class are you taking? Have you been going to lectures and reading your textbook? What other classes have you taken so far?

EDIT/ ADD -- Also, your Profile page says that you have a Master's degree. What field is your Master's degree in? Just curious. Thanks.
Business Administration....
 
  • #5
So, do you say there are no formulas to follow and I better go and buy multiple engines (stepper motors) and give those a trial and error test? That might work... No idea how to find friction, standard 6006 glass balls/nylon 66 bearing; air friction is at 200m above see, indoors.
 
  • #6
CWatters
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Now I think about it 90rpm isn't very fast so you might be able to ignore air resistance. Other things might well dominate..

How fast do you need it to go from stationary to 90rpm?

I'm not quite clear on the orientation but if the 1kg mass is being raised and it's not counter balanced it might also need a certain minimum torque to overcome gravity. Normally on a balanced rotor gravity cancels out so this isn't an issue.

Does it have to do anything else? Hammer in nails? Drive a load? Make electricity ? Just spin?
 
  • #7
CWatters
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So, do you say there are no formulas to follow
Not yet but there might be if you give us a bit more info as above.
 
  • #8
How fast do you need it to go from stationary to 90rpm?

I'm not quite clear on the orientation but if the 1kg mass is being raised and it's not counter balanced it might also need a certain minimum torque to overcome gravity. Normally on a balanced rotor gravity cancels out so this isn't an issue.

Does it have to do anything else? Hammer in nails? Drive a load? Make electricity ? Just spin?
There is no hurry, it reaches the speed whenever it can. Lower consumption is better. My biggest worry is that the motor would break because of the weight - as the weight is relatively heavy on the end of a rod. So, I am really after how I choose a motor what is strong enough. I would like to make one or two turns with a horse looking thingy driven by arduino and a stepper motor, then have it staying for a while, then making two turns, pretty much for fun. Well, I looked after how to choose a motor strong enough but could not find, or maybe could not understand. :)
 
  • #9
haruspex
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You may find it runs better if you balance it up with a second 1kg block mirroring the first, but maybe that interferes with the function you are after.
 

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