Heavy lift Crane boom Pulley design?? Will pay $$$ for IP

[abently]

Hi guys,

 

Have a mod'd 42042 (Catweasal MOC) and then added more boom, larger tracks + extra counterweight.

Decided to add a few more sheaves to assist the motor boom up with a load on the hook but am having trouble getting the sheaves to distribute the weight evenly across all 12 / 14 sheaves etc.

I've made sure all the sheaves have free play in them and are not binding up..... also made sure to reduce the angle of the rope from the motor to the first sheave to 0 degrees so that there is no side pull on the sheaves.   

I was thinking maybe I need to grease the shaft and side of sheaves to reduce the friction further but now that I think of it, could it possibly be due to flex in the shaft / frame at both ends that loads up the sheaves unevenly? On the other hand when I had the sheave designed 4 wide but 2 rows on both sides (so 8 sheaves wide in theory), I still had the problem with the last few sheaves / ropes having no tension on them...

Any ideas????

 

 

 

[allanp]

That sounds like an excessive amount of sheaves. With so many the weigh of the boom may not be enough to pull out so much string so fast. Can you post a link to a picture?

[nerdsforprez]

Agreed.  12-14 sheaves, unless lifting twenty plus pounds, is likely too much.  Also second the request for pics. Technic cranes used to be all the rage, very large ones using many sheaves. So it is possible

[bonox]

You may find some real life info useful. You're after a reeving method that puts the hoist line in the middle of the block. Have a look at this link for a 4 sheave block

https://www.thecrosbygroup.com/wp-content/uploads/2014/02/375_382.pdf

Then there are general reaving instructions from the crane manufacturers. You'll probably need to make a sheave block with a fair bit of weight to it as well use lighter string than the lego braided stuff. 

https://www.google.com.au/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwiYu-L_m9_TAhXBJJQKHSn1CZQQFgghMAA&url=https%3A%2F%2Fwww.manitowoccranes.com%2F~%2Fmedia%2FFiles%2FCrane%20Care%2FReeving_Information_v2.pdf&usg=AFQjCNHP6qdZ2OB_h3TlJ2sKc1G21lfb8w

and another

http://www.manitex.com/manuals/3500-000/adlib library/7600005-036/7600005-036_publish/.\7600005-036_Data\Operator_Manual_-_7600001-047\7600001-047-2_07-2003.PDF

[bonox]

And while i think of it, I think what you're after is termed Double Reeving, where the orientation of the lines are strung such that the hook will not move laterally when under load. With a high enough hook weight (which you can calculate using the formulae in the first crosby link if interested) to keep the lines straight, this means the hoist line is reacted by a support line at a point equidistant from the load line of the block.

[bonox]

btw, what are you using for sheeves/pulleys? If they are axle centres and all mounted together on one axle that will explain why only the first few lines are loaded and the rest go slack

[abently]

6 hours ago, bonox said:

btw, what are you using for sheeves/pulleys? If they are axle centres and all mounted together on one axle that will explain why only the first few lines are loaded and the rest go slack

Can you explain this???

Pics attached....

Don't pick me on the improvised axles, was just testing out my design before putting in a Bricklink order.

Edited May 8, 2017 by abently

[Aventador2004]

1 hour ago, abently said:

Can you explain this???

Pics attached....

I am not sure, where is the arm?

Quote

Don't pick me on the improvised axles, was just testing out my design before putting in a Bricklink order.

 I improvise axles a lot too, so it is fine.

Edited May 8, 2017 by Aventador2004

[doug72]

2 hours ago, abently said:

Can you explain this???

Pics attached....

Each pulley wheel should be free to rotate on the axle independent of the others.

i.e, they should have a round hole in the cente NOT a cross hole.

[abently]

Thanks Doug.

Yes, the sheaves/pulleys do have round holes and are fee to rotate independent of each other.

Edited May 8, 2017 by abently

[bonox]

yes - as doug noted, the movement of each pulley is different so they can't be joined together on one axle. You have hole centres so you're ok there. You may improve things by making sure they're free to spin and lubricate them as well.

The string looks like the single direction wound stuff you find in a lot of kitchens for tying chicken legs together. A collection of those under load will tend to introduce a moment that will twist blocks. You might try some thin braided stuff if you can find it. I think for that application the lego stuff might be ok - I was expecting you to be trying to reeve the load block not the luffing assembly.

 

The basic reason it happens is outlined in calculations in the first document I linked - the load in each line is not equal, as much as fairy fiction analysis land would have you believe. A big reason for this is internal friction in the (wire) rope as it goes around each sheeve. You can get away from it by controlling the twist of the rope, the choice of bearings and lubrication of the sheeves (not really applicable at lego scale), the size of the rope and the diameter of the sheeves. You may also get some value from a narrower total sheeve block width (perhaps the 1/2 stud  wide larger diameter pulleys, but these are on axle centres. If you can prove the concept though, you can machine out the axle centres or buy a custom set from efferman.

[nerdsforprez]

I think this is exactly the issue.  The "sheaves" are not real pulleys in the first place.  Lego designed them for wheels.  They are one stud wide, and when designed for other than their original purpose (pulley) the inside of the wheel has one stud width of surface area (friction) against the axle whereas the strings surface on the pulley (outside of piece), and therefore the friction it has on  it is much less than even 1/2 stud - so it easily can get bound up.  As mentioned above, much better to use a different element.  The pulley pieces would be perfect if they did not have axle holes.  But this can be easily rectified if you are willing to drill a hole through them.  That is what I did for this project (look at about minute 2:50 - you will see what I mean)

 

Now..... if you get a little skweemish because this is not a purist method I find it helpful to notice that many others have also recommended this trick.  A piece such as my home-made version is also available on Efferman's Shapeways site.   And honestly I believe it is only a matter of time before this piece DOES become an official piece.  So, might as well get a head of the curve!   As you can see, the little trick above works perfectly.  It is because the friction of the pulley on the axle (surface area) is more or less the same as the friction (surface area) of the string on the pulley.  More information is here on this mobile crane I built about a year ago as well as info on the piece offered on shapeways

http://www.moc-pages.com/moc.php/420370

 

 

 

 

 

[abently]

Thanks guys. Sounds like you have pin pointed the issue... =)

The string I was using is actually braided string but cotton... I plan to pick up maybe 0.5 - 0.75mm black fishing line instead.

I tried contacting some of the Bricklink Sellers in Perth where I am to see if I could find a local enthusiast nearby to save me the hassle of ordering and waiting for shipping but no one's responded since last Friday... =(

[MangaNOID]

not sure i'll add anything here but curious as to what everyone is on about with the small wheels being too wide with to much friction on the axel etc and want to understand.

the way I see it (which may be what your all saying anyway) is that there are too many small 'pulley' wheels in relation to the torque/energy required to go through (around) them. the larger 'pulley' wheel has a larger diameter and will thus drain less torque/energy creating smoother string action.

 

Edited May 9, 2017 by MangaNOID

[Burko-uk]

The sting is at a bit of an angle between wheel, so tries to twist the centerline of the wheel from the centerline of the axle, so it binds on the axle. Using a pulley (with the cross drilled out) reduces this issue as the angle of the string is much smaller as there is less offset between the string paths, and the pulley can cope with larger angles of twist before it binds on the axle because it's narrower, so a double win.