Purchase and block friction measurements and calculations

Discussion in 'Sailboats' started by Joakim, May 10, 2013.

  1. upchurchmr
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    upchurchmr Senior Member

    Joakim,

    In the late 70's I got a used Tornado that had blocks with stainless steel ball bearings.
    No friction measurements but compared to typical plastic ball bearings the sheaves spun very freely especially under load. I believed the balls retained their shape and therefore the reduced friction underload. The manufacturer was APM (American Precision Marine).
    To this day I don't understand why the SS ball bearings are not used.
     
  2. Joakim
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    Joakim Senior Member

    Here are more pictures of the system. Sorry about the backlight problem on some pictures. I also made more measurements and verified that the wire blocks are 38 mm.

    At about 0.5 m out of 8.5 m range taken in the force was 35 kg at 1:4 block, 15 kg at 1.8 block and 7 kg at 1:48 line. Obviously that is not possible since 1:8 is less than 1/2 of the 1:4. Probably had to pull too much from 1:4 to get slack in the remaining tackle.

    At 2.2 m I got 40 kg from 1:8 and 15 kg at 1:48 and the total force approximated from stretch was 230 kg. Thus 1:8 was actually about 1:6 and 1:48 was 1:15.

    At 4.0 m I got about 80 kg (my weight, not using a scale)) at 1:8 and 25 kg at 1:48 and the approximated total force was 310 kg. Thus 1:8 -> 1:4 and 1:48 -> 1:12.

    At 7.5 m 1:48 was 40 kg and the approximated force 460 kg. Thus about 1:11.5.

    The last 1:6 tackle doesn't seem to be less efficient than the 1:8 cascade.

    I measured all cases from both starboard and port side and couldn't find any difference between them.

    I did the same measurement for the main sheet which is 1:4 + 1:4. The first is 57 mm Carbo + 10 mm line and the second one is 29 mm carbo + 5 mm line. Both tackles seemed to work at about 1:3, but I only measured up to 40 kg after the first 1:4, thus having 13 kg at the second 1:4.
     

    Attached Files:

  3. John Perry
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    John Perry Senior Member

    I think I have seen published results of an experiment that showed that a large part of the friction in a pulley system is friction between the fibres of the rope, not friction at the sheave bearings. Unfortunately I can't remember where I saw this, perhaps someone else has an idea.

    If this is true the advantage of ball bearing blocks may be less than one might imagine.
     
  4. upchurchmr
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    upchurchmr Senior Member

    Very interesting comment about friction internal to the line.
    I would be interested in the report if anyone has it available.
    Does this lead us to some thoughts on how to reduce the total loss?
    Different sheave geometry or bigger sheaves (of course)?
     
  5. John Perry
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    John Perry Senior Member

    I found the paper on block friction, its by Martin Schoon, this is a link:

    http://hem.bredband.net/b262106/Boat/Blockfriction.pdf

    I quote the conclusions as follows:

    "The data presented here clearly show how the internal
    friction in the line dominates the total friction of a tackle.
    It follows that the design focus should be on line and
    block dimensions. Cascaded tackles should be considered
    as they have favourable friction properties. Choosing
    between plain bearing and ball bearing blocks is a
    distant third on the priority list."
     
  6. jehardiman
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    jehardiman Senior Member

    Figures 6 & 7 of that paper really show why a minimum 18:1 sheave:line is recommend for free running falls.
     
  7. daiquiri
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    daiquiri Engineering and Design

    Thank you for this paper. It is very interesting and eye-opening.

    I know the 18:1 (up to 20:1, depending on which register's rule one uses) sheave/rope diameter ratio from designing hoisting appliances, and also know how relatively small influence ball bearings have on block efficiency, but seeing graphically how much friction goes into just bending lines is very striking, though very logical too.

    Cheers
     
  8. Mikko Brummer
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    Mikko Brummer Senior Member

    Great thread indeed, I wasn't aware of all this. At first I was puzzled about the internal friction of a rope, but I think I get it now. As you bend the rope around the sheave, the layers of the rope (with thickness) need to move with respect to one another (there's shear). And when there is a lot of pressure in the rope, that slipping is not easy, hence the friction grows under load. Moving that bent rope around the sheave requires a force, which manifests as friction for the whole system.

    All this friction turns into heat and that's why the rope/sheave gets hot? I remember back in the IOR maxi days, with the wire sheets, Ludde Ingvall use to have this story about smoke rising from the winches when rounding marks... we used to smile at his exaggerating ways of telling stories, but maybe he was right? Maybe the inner friction could heat the wire so much that the grease inside the wire would evaporate in smoke??
     
  9. daiquiri
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    daiquiri Engineering and Design

    Your reasoning about how the friction is created is correct Mikko. And it also explains why the friction gets smaller as the sheave/rope diameter ratio increases. The bigger the sheave diameter, the smaller the difference in elongations between inner and outer fibers. At limit (infinite sheave diameter) a straight-line case is reached, where the relative movement between single strands and fibers is minimal. It never becomes zero, however, because single strands are braided together, so they are never straight and there is always a relative movement of the strands.

    Rather than grease, the smoke your friend has reported could have been due to water trapped in the rope. If the outside air temperature was low, one can easily imagine how the water vapor was created and released as apparent smoke. I'm guessing here, of course.

    Cheers
     
  10. Mikko Brummer
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    Mikko Brummer Senior Member

    Yes, of course, it could have been vapor. And all these years we have smiled a little behind Ludde's back. Mind you, he also told us how we should wear ceramic gloves on the foredeck when changing sails, as the friction heats the jibs being peeled on top of each other, in order not to burn our hands.

    Ludde skippered Union Bank of Finland in the 89-90 Whitbread race, and later become well known for his Nicorette Maxis, winning even Sydney-Hobart a couple of times. And once we finished the Fastnet Race 24 hours before the 2nd boat Sagamore, certainly still a record unbeaten. Wire sheets were replaced by rope right after the 89-90 race.
     
  11. jehardiman
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    jehardiman Senior Member

    Actually, one of the reasons the USN gave up on large nylon mooring lines is that internal friction would melt the cores, severely weaking the rope. Doesn't happen so much with the new kevlar/spectra/plasma, but you can still damage the cores by friction and improper tensioning (i.e. narrow V sheaves vice wide U sheaves).
     
  12. Joakim
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    Joakim Senior Member

    There must be clear differences between rope materials and constructions at the same diameter? Any measurements about that?

    What about low friction rings? They have typically very small radius for the rope compared to blocks at the same breaking load and yet they are used often in the tackles. Shouldn't they be very bad? German magazine Yacht has made a measurement (similar set up as the one linked earlier) with 5 mm Dyneema (core only) and 14-16 mm low friction rings e.g. Atanl 14x10, which is only 15 mm thick and should be worse than 19 mm block. Still the friction was "only" 15-20% of the load (300-700 daN ~kg, note order of magnitude higher than Martin Schöön had).

    Yacht has also tested several blocks and there have been clear differences between them. In their ball bearing block test the sheave diameters were 50-60 mm and 10 mm dyneema (FSE Admiral) was used in the same setup. Sprenger Gr 12 had the lowest friction despite the smallest sheave (50 mm). It had less than 8% friction at 250 daN load. The worst one was OH Marine Classic (57 mm) with almost 40% friction at 250 daN! The curves were not at all linear. Some blocks became better at higher loads and some clearly worse (even locking the sheave).

    Maybe the findings Martin Schöön did apply only to very light loads?
     
  13. daiquiri
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    daiquiri Engineering and Design

    Joakim, is there some trace of that article in internet?
     
  14. Joakim
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    Joakim Senior Member


  15. fcfc
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    fcfc Senior Member

    I am surprised nobody speaks of block technology.

    From my understanding, Ball Bearing blocks are not really Ball bearing blocks, as with typical mechanical ball bearings.

    In fact, the (plastic) balls only handle SIDE loads, and very LIGHTS loads. As soon the block has some tiny load, the balls deform, and the pulley works exactly as a plain bearing pulley, with friction.

    The only marine blocks that roll on something under load, not friction, are roller bearing (Torlon) blocks.

    One will immediately notice the wide price difference between ball bearing blocks and roller bearing blocks.
     
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