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Lifting vs pulling

  1. Nov 4, 2009 #1
    Ok so I was at the store today and I notised that the hoist they sell seam to be alot beefyer and stronger lookin them the come a longs ie hand winches and I was wondering why that was dose it take more force to lift 2 ton then it takes to pull the same, I was looking at a 1 ton chain hoist and it's all steel and thick chain looks real strong yet right next to it is a 4 ton puller made from thin steal and wire rope, now granted the puller dose use a pullie but that dosent change the fact that the force on the unit is still greater
    if any one can help please let me know. Would a chain hoist pull more that a similar rated hand winch?
     
  2. jcsd
  3. Nov 4, 2009 #2
    Force required to lift has to overcome gravity
    F = mass* gravity

    Force required to pull has to overcome friction.
    Friction Force = Force from gravity * Friction coefficient.

    Friction fore is much smaller then gravitational force, as coefficients of friction are much less then 1 usually.
     
  4. Nov 4, 2009 #3

    mgb_phys

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    Yes it's a question of force.
    If you lift a 1 ton object you need a force of 1000kg * 9.8m'/s^2 (ie g) = 9800N
    To pull the same object would depend on the friction between the object and the surface, for steel wheels on a steel rail (such as a railway car) the friction would be about 0.1% so you only have to pull with a force of 9.8N, equal to lifting only a 1kg.

    For very low friction like oil bearings it's possible to push huge masses, like a 100ton telescope, with almost a finger tip.
     
  5. Nov 6, 2009 #4

    stewartcs

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    If the chain hoist and the come-a-long have the same SWL (safe working load) of 2 tons (or whatever), then they can pull or lift the same amount. The position is irrelevant. I can take a come-a-long (puller) rated for 2 tons (SWL) and use it vertically to lift 2 tons.

    What you are probably noting is that most come-a-longs have typically lower SWL's than do chain hoist for the reasons stated above.

    CS
     
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