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Belt/chain and relativity violation?

Question asked by 1-8WP1OO on Aug 27, 2010
Latest reply on Aug 28, 2010 by 1-8WP1OO

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Start with two pulleys and a belt/chain (Engage belt).

With one pulley twice the diameter of the other the smaller pulley should rotate twice for each revolution of the large pulley.

Now fix the large pulley or 'lock angle' and allow the axle of the small pulley to sweep a circle around the large pulley. The small pulley's angle appears to remain associated with the large pulley, not the belt, and only completes one revolution in violation of the theory of relativity. Is this a bug?

~Carl

still with SW2007

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Peter Biggert Aug 27, 2010 10:27 AM
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This sounds like the problem on some aptitude tests where you have two circles; one's diameter is two times the other's. If you roll the small circle around the large one how many times does it turn? If you just look at thier circumferences, you would come up with two as an answer, because one has a circumferece twice that of the other. Of course you would be wrong. The correct answer is the small circle turns three times because the path you traveled was a circle also.
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- 1-8WP1OO @ Peter Biggert on Aug 27, 2010 11:31 AM
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  Hi Peter,
  
  Absolutely. But relatively speaking a pointer on the small circle, aimed at the centre of the large circle, should be realigned twice during one full revolution. If the two pulleys are belt driven the net absolute revolutions would be one and, as you say, if the pulleys are 'gear' driven three. Still a bug?
  
  ~Carl
  
  still using SW2007
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  - Josh Brady @ 1-8WP1OO on Aug 27, 2010 12:50 PM
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    For the same reason a gear should go three revs (one extra for the circular movement), the belt should go just one. Gears change direction of revolution, so you add one. Belts keep same direction of revolution, so you subtract one.
    
    Just for fun, try it with identical diameter pulleys - you get 0.
    Try with diameter ratio of 3 - you get 2 turns.
    Diameter ratio of 4 - you get 3 turns.
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    - 1-8WP1OO @ Josh Brady on Aug 27, 2010 1:04 PM
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      Josh,
      
      Fun over. The belt/chain function does not work correctly in solidworks. Try it if you get a chance.
      
      ~Carl
      
      Still using SW2007
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      - Josh Brady @ 1-8WP1OO on Aug 27, 2010 3:33 PM
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        Works fine for me... just as expected.
        
        -Still using 2010
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        1-8WP1OO @ Josh Brady on Aug 27, 2010 5:48 PM
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        Josh,
        
        It works with gears but if your two pulleys are the same size it will just look like it is working with a belt. The planet pulley always remains aligned with the stationary sun pulley. Try it with different sized pulleys and see if you can get the planet to rotate.
        
        ~Carl
        
        SW2007
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        Josh Brady @ 1-8WP1OO on Aug 27, 2010 9:05 PM
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        I did. As I said, it works exactly as expected. What I posted above about diameter ratios happens.
        
        I don't know if they've fixed it since 2007. I don't have that installed.
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- 1-8WP1OO @ Peter Biggert on Aug 28, 2010 4:25 AM
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  Peter,
  
  I see that you are using SW2008. Does the belt/chain function work for you as described above?
  
  Can someone else try SW2009 and let us know. Thanks.
  
  ~Carl
  
  SW2007
  
  PS. As a work around can I refer to the measured value of 'arc length' in an equation or do I have to recalculate it?
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Belt/chain and relativity violation?

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