Bogdan Valceanu

[MOC] [WIP] Crawler crane in 1:20 scale

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Hi guys,

I’ve been following the forum for some years not, but I’ve been mostly a “lurker in the dark”.

But, after 2.5 years designing and building when I had some spare time, I can finally share my 1:20 crawler crane MOC with you.

For now it has somewhere around 40.000-60.000 parts (don’t know exactly how many).

I’ve tried to build it as close as possible to a real crane in terms of assembly and functionality, with the usual constrains that you have with building out of Lego at this size and scale.

As a disclaimer (and as a direct apology to Lego purists), the slewing bearing is not lego, but a cross roller bearing. The drive of the bearing is Lego, using the ¼ gear racks from the 42055 BWE. Took me about 6 monts to find a bearing that fits in size with the internal gearing of the gear racks, so that the driving axles still fit trough. Also, if you look really close, there were some occasions where I got out the Dremel for some adjusting (mainly panels), as I didn’t want to sacrifice strength, or design. Again, sorry to the purists.

The crane is powere by 4 BuWizz, one in each central counterweight (between the crawlers), and one in each superstructure counterweights. I love the fact that you can still operate the BuWizz while it’s charging, so I have a power bank battery next to each BuWizz. Like this you get hours and hours of play time even with a large heavy model like this.


The drive is as follows:

- Each crawler is powered by 4 PF XL Motors (and one BuWizz per crawler) geared down 240:1. The motors don’t drive the ends of the crawler, but 8 sets of 2 gears underneath each of the crawler chassis. Due to weight reasons I used metal axles from Eezo’s Brick Machine Shop from the US.

-Slewing is done again by 4 PF XL Motors which sit in the base of the superstructure.

- Each winch is driven by a PF L motor. They were powered by 2 PF L motors, but because the winches are worm-driven I had some issues with them not running synchronous and overloading the motors. The winches can be individually taken out from the superstructure for maintenance and use 1mm wax rope.

Structurally the main building technique for the crawlers, undercarriage and superstructure is an array of 5x7 technic frames. I’ll come back on another post with some pics of the various building techniques and technical details. Maybe I’ll also do a more detailed video on this sometime soon.

The crane is not finished yet. I still need to build the superlift tray and telescope, which will be EV3 controlled so that it self-adjusts, and sadly I need to rebuild all the boom. Until a main boom length of 4.5m everything is ok, but with more than that it starts to bend too much. In the video below I’ve build 4m of boom, because it was pretty windy when we shot the video.

The goal would be 7m someday. I want to keep the boom in the main boom+short fixed jib configuration. A luffing jib would be easier to lift (most large Lego cranes that I saw are built in a luffing jib configurations), but as I work in wind turbine assembly, I want to build the boom configuration that we use mostly.

Here some pictures from the assembled crane:

And here a video about the crane (without wanting to advertise for the channel)


As mentioned before, I’ll follow-up with some more pics from the building phase and building techniques.

I hope you guys like the crane. 

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Indeed over the top for sure. Quite interesting to see it all go together, how much weight can it lift? Excellent build even with modified parts. 

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Wonderfully terrific build.  Thank you for sharing.  As a fellow lover of Lego cranes, I knew I was going to love this post from the title alone.  For me Lego cranes never get old, despite there being so many of them.  Couple of things:


- Modularity.  :thumbup::thumbup: more and more we are seeing this, and I think it is just wonderful.  Especially for crane builds, b/c that is how they are in real life as well. 

- Using the stainless steel axles.  I do not now whey they are not used more: especially for large builds like this.  I have used them for several applications and although they are unforgiving in terms of inserting them into axle holes for other Lego elements, they are also unforgiving in providing strength.  Another application I would like to see them more is for large, complicated vehicles, rock crawlers, etc. in drive trains and wheel axles.  They don't minimize backlash; they ELIMINATE it.  Not purist - but many purists don't even have a problem with them b/c they are not cutting, modifying, etc. elements.  They are just taking elements that are actually in the Lego family, and using much stronger versions. 

- Love the video.  The first shot with the mobile crane in the background in the crane yard gave the impression that all the cranes in the yard were Lego.  Brilliant! 

I am sure I will have more to say later.  Please post more photos, I know I for one would like to see more.  Again, wonderful build and thank you for sharing. 

Edited by nerdsforprez

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What a monster...

Very nice!
I know from experience that building big with LEGO can be quite a challenge structurally.
Where did you have the most structural design problems in this MOC?

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Opps... I’ve obviously come to through to instead of Eurobricks. My bad. :sweet:

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Absolutely amazing. That video must have taken you the better part of a day to shoot. But it is totally worth it. The segments look already quite optimized for stiffness & weight. I'll be interested to see what solution you come up with to improve on that.

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My jaw literally dropped while watching your video, i just saw a way beyond imaginable masterpiece. Woah! :laugh:

Your crane shows what ABS can perform when a trained engineer gets in touch with excessive amounts of Technic parts :vader:

I`ve never seen such a sleek and compact supermonster of a crawler, hats off

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Thanks for the wonderful feedback from all of you, this means a lot to me to get this kind words on my first MOC from the forum that I've been following for years. 

I want to start the series of technical and build details with the most important question of them all :excited:

" Why is the crane operator called Alfredo? "

When building the crane cab, I stumbled on a 5L liftarm with an "Alfredo" sticker on it. It came like this from a Bricklink order and went unnoticed until building the cab. 

So, I carefully pealed the sticker off and used it on the dashboard of the crane cab. 

When researching where the sticker came from, it turned out that it was on the 2009 8264 Hauler ( ).



Edited by Bogdan Valceanu

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Alfredo lives also in my two-Technic-fig collection! The other guy has sort of a red racing overall... At least the true identity of the blue-white dressed guy has now officially being revealed, thank you! :laugh: Nice detail for technic nuts!

(Btw, i wonder if any other technicfigs have become "Canocical" names along the years... )

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With a small delay due to us attending the Modelshow Europe in the Netherlands this weekend, I want to start posting some more technical info about the crane.

I will also post the LDD files, but these are partially drafts, I did deviate a little during the build, so please take them more as a “suggestion” and to see some of the building techniques.

First, the undercarriage.

As most of the structural part of the crane, this uses a loooot of 5x7 frames. These are set up an interlocking and alternating manner, so in the end the piece is almost “solid”. This gives excellent stiffness and strength in all directions, without too many “slack points”. It took 5 designs and 2 builds to get a satisfying stiffness.

As mentioned also in the first post, the slew bearing is not Lego, but I wanted to use the ¼ gear racks to drive the slewing motion. I searched for almost 6 months to find a bearing that fits over the gear racks and matches up with the hole pattern, and still be able to pass the axles trough to drive the gears (more detail in a later post).

The bearing plays a structural role, as it stiffens up the undercarriage, and it’s held to it using 5mm thread rods that are fixed on the underside of the undercarriage. Where they get fixed I used some of Brick Machine Shop’s aluminum liftarms as “load spreaders” so that the forces are not concentrated just where the nuts are located.  

Link for the LDD file: 















Edited by Bogdan Valceanu

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What I had just seen??? So gigantic! Absolutely incredible! 
What are you using as a counterweight? Metal powerbanks? :laugh:

And where are you from :)


My 2 meter Liebherr looks tiny now :sceptic:

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13 minutes ago, Aleh said:

Crane is here ^)

Yeah I know, already saw this. I was happy to meet Ian Webb in person, I've been following him for years. 

You can also watch my video about the Modelshow Europe. This is just the preview, the full episode will be online in the weekend:


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57 minutes ago, Bogdan Valceanu said:

I've been following him for years. 

The same as me. btw could you please answer a couple of my questions above?

I'll see this and wait the ful video for sure!

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2 minutes ago, Aleh said:

The same as me. btw could you please answer a couple of my questions above?

I'll see this and wait the ful video for sure!

Sure. I'm from Romania (I've update my profile now, I forgot to put it in) :laugh:

The counterweights have 0.5mm lead sheets cut into stacks, and also the BuWizz controllers with a powerbank battery for each controller. The total counterweight is around 15-16kg

I will give some more details when I post the details about that. 


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Continuing with the details, it’s now time for the superstructure and slew drive. As mentioned before, consider the LDD files as being a draft, as I deviated in some details during the build.

The superstructure is based, again, on the 5x7 frames in an alternating manner, similar to the building style of the undercarriage. The main structural modules are: the two side frames, the bottom part (which includes the slew drive system) and the paneling. The paneling is split again in left/right and the back and is also used structurally to stiffen the whole superstructure.

I had some issued with the slew drive, so I had to disassemble completely the superstructure and building in modules really eased the disassembly work.

All 5 winched can be removed for maintenance and improvements. At first they were powered by 2 motors each, bus due to small differences in the rpm’s between the motors and the fact that the winches are driven by worm gears, they ended up in different positions and overloaded the other motor, so for now each winch has a PF L motor in it. In the picture you can also see Mindstorms motors, I changed that later on.

As the slew bearing out of Lego would be impossible to build for my target height of the crane (hopefully someday 7m), and still to be rigid enough, I searched for 6 months for a suitable bearing to fit the hole pattern and size of the ¼ round gear racks from the BWE. Not only did the exterior have to match up with the outer diameter of the gear racks, but also the interior diameter was critical, so that the drive axles can fit trough to the internal gearing. If you look one of the pictures with the red circles, you can see how close the holes where the axles pass trough come to the interior wall of the bearing.

The bearing is overkill for the crane in terms of its thickness/height, but it was the only one that I could find size-wise, plus as a bonus, it’s heavy enough (around 4.5kg) to act also as a central ballast. The bearing-to-undercarriage connection (on the outer part of the bearing) fit the hole pattern perfectly, but the interior PCD didn’t fit with the superstructure connection, so I had to build an adapter plate for the connection (again, my apologies to purists).

The connection plate is a 2-piece aluminum construction, the top part connects to the superstructure using again 5mm thread rods, and the small circular part that fits inside the bearing. I had to build this as a stabilizing part to prevent the drive gears axles from bending and skipping some teeth.

I used stainless axles metal gears from Eezo’s Brick Machine Shop just for ease of mind, as reaching this part of the crane for maintenance requires a lot of disassembly. Finally, it turned out that the crane turns so easy and fast that I think plastic gears would have been strong enough for this.

Here the dropbox link for the LDD files (including cabin, A-frame and counterweight supports):


And pictures of course:


























Edited by Bogdan Valceanu

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