DrJB

Liebherr HS 855 Excavator

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I found out about this MOC while perusing through Sariels's book and am a bit surprised such machine was never discussed on EB (no results from Google).

In any event, on page 114 of the book, one can see the system of pulleys used to reduce speed (increase load). One requirement for the system to work is that all pulleys must be free rolling on a shaft as they do not have the same angular speed. I'm betting the MOC author had to modify/drill the pulleys to fulfill such function, otherwise not possible with stock Lego.

Anyone else sees the point or knows of a way to make this work?

Edit: Could a Moderator please update/correct the title to say Liebherr ... iOS auto-correct is not the smartest. Done, Thank You

Edited by DrJB

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No to the youtube video, but definitely yes to the brickshelf link in the thread you identified (Thank You). Here is a photo from that brickshelf folder, that shows exactly the set of pulleys I was referring to.

My presumption is that the stock pulleys could not be used as they have an axle hole. What one needs instead are smooth holes and most likely the author 'drilled' the pulleys.

liebherr-4913.jpg

Edited by DrJB

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In theory those pulleys need to free roll but I think those aren't, otherwise I don't understand the black pin that can be seen in the top right pulley.

Probably the pulleys turn and the rope slides slightly on them.

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I'm glad my book helps you to find good models, but...

My presumption is that the stock pulleys could not be used as they have an axle hole. What one needs instead are smooth holes and most likely the author 'drilled' the pulleys.

I don't know specific details behind Makorol's pulleys, but one thing I'm sure of is that he drilled nothing. The guy is a purist.

Also, you got the wrong video. The right one is below, it actually shows how these pulleys work.

Edited by Sariel

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You sould not need to drill. The string will "slide" in the grooves. I made a big crawler cane with my Technic beams and at 5 pounds on the arm the string stil slid in the grooves.

However the size of the string also important. Smaller size string can make a groove in the groove and bigger string doesn't hurt the pullies. The size in the pic is the right size for use under load, no wearing out of the pullies.

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... I don't understand the black pin that can be seen in the top right pulley ...

Good catch. that pin 'should' not be there. The plot only thickens.

... the rope slides slightly on them ...

The friction force must be very large as it grows like exponential (friction coefficient × angle). The only 'remote' possibility is that the cable is oiled/greased but then again, might not be enough.

... he drilled nothing. The guy is a purist.

Then I'm not sure this is working as it is supposed to be. The idea behind using multiple stacked pulleys like that is so that the motor needs only deliver a fraction of the torque/force needed to lift the load. In exchange, the travel of the rope gets multiplied by the same factor. Remember, in those systems, work/energy is conserved (almost, minus losses). The link below might prove useful.

http://www.explainthatstuff.com/pulleys.html

... The string will "slide" in the grooves ...

I can accept that the string will slide on ONE pulley only, but if it does so on 4 pulleys (×360 degrees), that is a lot of angle/friction, per my reply (above) that the force grows exponentially with wrapped angle.

I guess the simplest way is to build this and see how it performs, though I doubt it will, as the laws of physics have to hold.

Edited by DrJB

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The fact that the pulleys are tied together does not matter for a very simple reason, they are all the same size. It is a circumference/rotation thing, as every loop of string moves the same distance for a given shaft rotation.

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The fact that the pulleys are tied together does not matter for a very simple reason, they are all the same size. It is a circumference/rotation thing, as every loop of string moves the same distance for a given shaft rotation.

Not sure I agree, as the pulleys do NOT rotate by the same amount. If the first pulley rotates by an angle X, the neighboring pulleys will rotate by 2X, 3X, 4X ... etc, so that the delta/difference between each two neighboring pulleys is precisely X.

Thus, and because they do not rotate by the same absolute angle, the pulleys cannot all be connected to the same axle. Hence my observation, that those pulleys must be drilled so they can rotate freely on an axle.

@Sariel: any chance you can ask your friend/author about more details for the pulleys' contraption?

Edited by DrJB

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The pulleys most certainly DO rotate by the same amount, at least in this example. They must because they're all pinned together!

DrJB is right that if this were done in real life the pulleys would free-wheel on the axle. In this case my guess is that the pulley block will rotate at something like the average speed of the strings. The faster strings will suffer friction, but the slower strings will be pulled along by the faster-rotating pulley block and shouldn't (I don't think) contribute to the overall friction losses too much. If the axle that the pulleys are on was locked, then that's when you'd get your 4x360 degree exponential friction growth.

Owen.

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The string around each pully is traverling at the same speed and as each pully is the same diameter there will be no slippage as long as the string is wound on in same direction of rotation - does not matter if they are loose are fixed on the same rotating shaft.

Of more interest is the way the drag bucket is opererated when lifting a loaded loaded bucket. There must be some type of slip arrangement to keep tension on the drag rope to keep bucket in loaded position while the hoist rope raises it to the dump postion. The drag rope is then released and bucket tips down to empty it.

I once saw a working 4mm scale model of a drag line excavator which had a very complex arrangement to operate the hoist and drag ropes in the correct sequence.

The derricking sytem ( to move the boom) with its multiple pulleys is quite simple in comparison.

I have been contemplating building a drag line excavator using the boom parts from the 7632 Lego Crane but have to work out the operating rope drives first.

Edited by Doug72

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Hi guys! Here I am and I'll try to answer all your questions :)

In a real machine the pulleys can free rotate to minimize the friction force. But of course, we don't have LEGO pulleys with pin holes. So I was thinking about some kind of compromise.

I wanted the excavator to look and work like the real one, so I decided to use multiple pulleys (but I didn't want to drill them). I bought a string that was covered with some kind of slippery coat, so it could easily slide on the pulleys. It reduced the friction a little. Then I thought about something that captainowie said:

In this case my guess is that the pulley block will rotate at something like the average speed of the strings.

And it worked quite well. So I have 4 pulleys connected together and the whole "block" CAN rotate. Of course the string still have to slide on the pulleys, it produces a lot of friction and I'm loosing a lot of energy. But It's only a Lego model and for me, more important was to make it work and look just like the real machine rather than make it very strong and able to lift heavy loads :)

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@Makorol: Thank you for offering your perspective, and clarifying the design.

I did a web search and here is what I came up with. Apparently someone had already drilled through pulleys for the same reason I mentioned in my original post. I hope this clarifies the workings of such contraption for those still convinced that all pulleys CAN run at the same speed, when in fact they CANNOT (in a real setting of course).

http://www.wegmuller...nes/index2.html

Edited by DrJB

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<p>On my crawler I drilled out the pulleys for that reason. I found that they all rotate at different speeds depending where they are in the block. I'm running 12 pulleys in the main body and the same again on the A frame. Hope this image helps what I 10383513_703602849706988_4284550680951202269_n_zps42ddc6db.jpg

Edited by cheeze

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For the non purists, Efferman has pulleys with pinhole on his shapeways catalogue for this exact application.

Edited by OzShan

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At the cost they are. Seeing as you were going to use them, I think I'd rather just drill out original lego pulleys. But good idea

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