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Pulley friction in a overhead crane

  1. Sep 5, 2011 #1
    I need to adjust the safety trip at 110% of the nominal load ( 130 tons ).
    This means that I need to load the overhead crane with 143 Tons. With this load on the floor I start to rise the load with the crane, but I measure less than those 143 Tons ( say 135 Tons). This means that we can never achive the value os 143 Tons unless we stop the rising movement.
    I want to know what to do.
    In the atached file you can see the overall arrangement of the pulleys in the overhead crane and also values achived on the real test ( separate sheet). The load has 130 tons.
    In order to check the maths involved I developed several equations to show the different forces involved and the efficiency of the system ( note that f is the friction force in each pulley).
     
  2. jcsd
  3. Sep 5, 2011 #2
    I forgot the file
     

    Attached Files:

  4. Sep 6, 2011 #3

    Drakkith

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    Staff: Mentor

    I don't understand what you want. You want to load the crane with 143 tons, but you don't have 143 tons to load it with?
     
  5. Sep 7, 2011 #4
    Surely you need to suspend a calibrated load slightly smaller than your max just above the floor and gently add calibrated pigs until the trip operates?
    Adjust the trip setting as appropriate.
    Repeat this until the trip setting is acceptable.

    This will void the snatch effect you are referring to.
     
  6. Sep 7, 2011 #5

    A.T.

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    Gold Member

    To summarize your dilemma:

    - You want to protect all ropes with a single safety trip that measures just two of the eight rope forces.
    - Due to friction the forces on the not measured ropes can be higher or lower than on the measured ones, depending on the operation direction (rise/lower)

    Solutions:

    - Make the safety trip threshold dependent on operation direction (probably complicated)
    - Make the threshold so low that no rope can exceed the load it has at rest for 143ton, regardless of the operation direction (will limit the crane more)
     
    Last edited: Sep 7, 2011
  7. Sep 8, 2011 #6
    Thanks for your contributions!
    Could you tell me if the developed formulas are correct?
     
  8. Sep 8, 2011 #7
    Drakkith,
    I want to load the crane with 143 Tons in order to adjust the trip relay ( to avoid manipulation of the crane with that load or bigger).
    But due to friction (?) if i load the crane with 143 tons I start to measure 135 tons !!
    If for instance I can adjust the trip for 143 tons, later on the crane operator can lift 145 tons and the system allow it because it stats to measure ( say 137 tons) which is in the limit allowed. Only when he stops the movement the system measures the real load and than trips ( too late!!!)
     
  9. Sep 8, 2011 #8

    Drakkith

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    Ah ok I see now. Can you connect the cables to a device which will connect with the load and weigh it accurately, tripping a switch if the load is above the set amount? Or does this need to be built into the crane itself?
     
  10. Sep 9, 2011 #9
    The mesure device is a Load Pin Cell located in the shaft of pulley nº 5 which does not move at all. I cannot introduce any extra device on the rope because it will moves and can smahed against pulleys.
    If my formulas are right then it is a friction problem and because the friction is too high therefore I need to disassemble the pulleys system and check all the bearings of the pulleys. The main question is if I am wrong it will be too mutch cost for nothing!!!
     
  11. Sep 9, 2011 #10

    Drakkith

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    Staff: Mentor

    That's not what I meant, but I see your problem. Wish I could help!
     
  12. Sep 13, 2011 #11
    Any news?
     
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