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Help with springs

  1. Jan 19, 2010 #1
    I think this is in the right section, sorry if it's not. First off, I'll apologize again for not knowing any terminology dealing with springs so I hope you understand what I'm wanting from you. Which is, say you have a circular compression spring that can hold 2 lbs without moving, what amount of weight would it take to compress the spring all the way. Another question I have is, hypothetically if a spring holds 2 lbs without moving and it takes say 50 lbs of weight to compress it all the way, that would mean the spring could provide 2 lbs of lifting power if it were sprung right? Isn't there a ratio to this, like for every 1 lb of support from a spring it takes a certain amount of poundage to compress it? Also what would the unit be for the stored power from a spring lb/ft, or lb/in?
    I'd really appreciate it if someone could assist me, thank you. Cory
     
  2. jcsd
  3. Jan 21, 2010 #2
    What you're looking for is the spring rate. In English units (like you're using above), it is generally listed as pounds per inch.

    For example, if I have a 4 inch long compression spring with a spring rate of 4 pounds per inch, and I put a 4 pound weight on top of it, the spring will compress 1 inch, because at 1 inch of compression, the spring will be exerting 4 pounds of force.

    Spring theory gets very complex, but that should answer your question.
     
  4. Jan 21, 2010 #3
    Thank you very much, I've been wondering about this.
     
  5. Jan 22, 2010 #4
    Bigaggie, in your example of a 4 inch long compression spring that has a spring rate of 4 lbs per inch you put 4 lbs on it and it compressed 1 inch. Say that the 4 inch spring was compressed 1 inch and the spring stayed compressed while the weight was lifted off, then the spring was released, how much weight by way of the stored energy would the spring be capable of lifting for that one inch? I'm assuming it's got to be less than the 4 lbs, right? thanks again cory
     
  6. Jan 23, 2010 #5
    To clarify, you're scenario is a spring with a rate of 4 lbs per inch compressed one inch, held in place at its compressed state, and then released, and you want to know the maximum load it could move back to its uncompressed state? I will assume that the spring is also acting against gravity.

    Theoretically, there is no load that would allow the spring to return to its uncompressed state. For example, if I used a weight of 2 lbs, the spring would have to support that 2 lbs. From Hooke's Law, the spring force is equal to the spring rate multiplied by the amount of deflection (in this case, compression). So with a force of 2 lbs, and a rate of 4 lbs, dividing the two will yield .5 inches. So in theory, any time you put a weight on that spring, it will deflect from its natural state.

    Now if you make the spring act in the horizontal, that's a totally different story altogether, because now you're no longer acting against the full weight of the object, simply its inertia and the friction against it. We'd have to be a lot more specific before we could really say what would happen there.
     
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