4x4 Crane

4x4 Telescopic Boom Crane
(modeled after Grove RT600)

This is a model of a typical all-terrain 4x4 truck-mounted telescopic boom crane. You will usually see this type of crane at large construction sites. They are easy to move, easy to set up, and can provide light and medium-duty lifting services just about anywhere on site.

I modeled this after a Grove RT600. I started building it in the fall of 2003 and planned to display it at Brickfest PDX in February 2004. Unfortunately, I did not completely finish the model.

Details of the Model:
Display Date: February 2004
Truck Chassis: 4 wheels with all wheel drive plus all wheel steering
Boom: 3-section telescopic box section
Maximum Lift: unknown
Scale: 1:15 (approximate)
Power: 16x AA 1.2v rechargeable batteries (8 for chassis & 8 for superstructure)
Chassis Controls: 4-channel R/C prototype from HiTechnic
Superstructure Controls: hard-wired remote with LEGO pole reversers
Total Motors: 11
Thrust Roller Bearing: 29, each with two 1/2 bushings

This model marked the end of a very busy and ambitious building period for me. From early 2001 to February 2004, I always had one or more projects ongoing, and each seemed to be larger or more complicated than the previous. I was active in attending various 'fests both in Texas (where I live) and out of state, and every project of mine had a target deadline being one of these events.

I'll get to the negative details and aftermath later on, but first I'd like to present the details of the model.

test of boom


Towards the end of 2003, I decided to build a smaller model. I wanted to focus on fine, mechanical details and build intricate working parts. Truck-mounted telescopic-boom cranes offer this sort of package - they're relatively compact and pack in a lot of details and moving parts.

I started by sketching and planning some of the difficult features. A lot of my inspiration came from Jennifer Clark's excellent model of a Demag AC 50-1. In fact, I copied several of her designs directly in my crane.

Sketches Made During Planning

Connection of Boom Sections - shows how the boom sections are connected together. A set of cables interconnects the three sections and allows simultaneous extension of all three by mechanically driving one section.

Boom Cross Section - cross section of boom. Shows the development of the geometry and the first 4 designs leading up to the final design used in the model.

Chassis - simplified sketch showing the layout of the main frame, pendular axle units, turntable, and outriggers.

Outrigger Details - development of the outriggers.

Steering - layout of steering geometry. I tried to get real Akerman steering geometry, but it didn't work out :-)

motorized axle unit

end view, no tire

unfinished chassis

wheel retention

outriggers extended

roller bearing

chassis with body panels

4 outriggers

boom w/ panels removed

Boom Inner Actuator

bottom of inner section

Boom Luffing Cylinder

boom - jib connection

secret pin

unfinished chassis

chassis - no axles

chassis done, no panels

turntable exploded

roller bearing



boom close-up

end of boom

3 boom sections

boom and jib

fixed jib


If you're a purist, turn away now :-) This model contained many chopped, hacked, and glued LEGO parts (and non-LEGO parts too :-)

Motorized Axle Unit: I started the model by building the pendular axle "units". I incorporated two 9V motors directly into each unit, and they also contained a differential plus two U-joints to allow for steering.

I'm not sure why I thought having BOTH axles rotate as pendular axles was a good idea, because it wasn't :-) Rather, it was poorly implemented.

I had to glue 6L axles into the U-joints for each wheel, and then drill small holes near the tip of the axles. After the wheels had been placed on the axles, I put a small length of wire through each hole to act as a cotter pin. This kept the tires on the axles, preventing them from sliding off. (When a wheeled model is extremely heavy, such as this, the axles will flex badly and the wheels will creep off the axles as the vehicle moves along.)

Each pendular axle unit was centralized by shock absorbers that were semi-fixed in one spot. As the axle unit rotated to one side, it would come in contact with the shock absorber. By having shocks on both sides of the axle, the axle stayed centralized. BUT, this design required that the shocks be very, very stiff and actually keep the axle centralized. As it turned out, the almost-finished model was way too heavy and the shocks too weak. I had to eventually fix one pendular axle to keep it from rotating and therefore keep the whole model upright (instead, it would list badly to one side).

Chassis: The base frame of the vehicle was made mostly of studless beams. This also turned out to be poorly implemented because, again, in the end, the near-finished model was so heavy that the chassis sagged badly - so badly that some of the mechanics had difficulty operating inside the greatly deflected network of studless beams.

The chassis housed the motors for steering, turntable (slewing), and outrigger extension/retraction. All 7 motors in the chassis were controlled by a 4-channel R/C prototype from HiTechnic.

Turntable: The design of the turntable was a little different than what I'd done before. Instead of using a typical large Technic turntable, I used a 40-toothed gear with a bunch of pulleys stacked on top. This created the main shaft, or pin, of the turntable. The whole stack was tied together with multiple threaded axles.

The turntable was centralized by the pulleys being surrounded by a 3x3 square shaft. This "shaft" was created by building studless beams around the 3x3 open square.

The rollers for the turntable were double 1/2 bushings. The ring (see photos to left) was created by use of long pieces of flex tubing. The ends of the tubing were spliced together with a heavy paperclip wire.

Outriggers: The outrigger boxes were fitted to the ends of the chassis and designed to be completely motorized. Each box contained two outrigger arms, and each outrigger arm had a rack gear running the length of it. The main structure of each outrigger arm was plates turned on their sides.

The tips of each outrigger had a hand-operated jack. This jack was basically a vertical arm with a rack gear driven by a worm gear. As you turned the worm gear by hand, the jack (vertical arm) would translate up or down. I had plans to incorporate a micro-motors to drive this arm, but 1) I didn't have time to implement it, and 2) they probably wouldn't have been powerful enough to lift the crane.

Superstructure: The base frame of the superstructure connected all the vital parts together. It connected to the turntable by several long axles and could be easily removed if needed. The superstructure had an electrical system independent of the chassis.

The three functions of the superstructure were:
1. boom luffing
2. boom extension/retraction
3. winch (only one was operational, the other was a dummy and non-motorized)

Luffing Cylinder: I built a special non-LEGO linear actuator for the luffing cylinder. It consisted of a short segment of aluminum tube, 3/16" threaded rod, a few nuts, and a special thrust roller bearing. See this photo to see a close-up view with notes.

I decided to make this special part from non-LEGO pieces because I saw something very similar in Jennifer Clark's Demag AC 50-1 model, and it looked very cool and like something I could do myself :-)

The end result was a very nice linear actuator that worked well.

Boom: The boom was the "key" element to the model. Before construction began, I planned out the cross section of the boom and built a test-section to prove the design.

The major challenge was to create a linear actuator to operate the telescoping boom. Again, I turned to non-LEGO parts to make this work, much like Jennifer Clark did in her model.

The boom turned out to be a very nice design. It worked fairly well, only hanging up now and then for a brief moment. It was slow, but then so are real telescopic boom cranes :-)

The boom was capped off with a fixed truss jib. This was attached to the side of the boom tip, and could be swung around 180-degrees to be stored against the main section of the boom. This is how it's really done in some cranes.

packing for the trip

on display

sagging chassis

on display

Bob & Ross' cranes with mine


I did NOT finish this model prior to Brickfest PDX (Feb. 2004). I mananged to complete all working parts, although most features were in first-draft status. I did not have time to test and re-design the things that needed it.

I slapped on a lot of yellow plate at the last minute to make the body panels. I did not finish completely covering the crane, however.

The first time the model was fully assembled and tested was in my hotel room the night before Brickfest PDX began. The first of many issues cropped up right away - the tires went flat. The near-final model was so heavy that it flattened the tires to the point where they were unusable.

My solution was to carefully pack wadded-up bubble wrap inside the tires. It took a long time to adjust and correct the wrap, but I eventually got the tires fairly close to being round. At least the crane would roll along and not look badly deflated.

The next problem was that the chassis sagged badly. The weight caused it to deflect too much (especially when raised up on the outriggers), which in turn caused some of the gear trains embedded inside to be racked. This affected performance - there were then issues with the steering and slewing of the crane.

Of course, as mentioned previously, the double pendular axle idea didn't work. I had to fix one of the axles to prevent rotation. I should have designed it this way from the start. That is how the real machines are designed after all :-)

My solution for motorizing the outriggers using Znap flex axles was a quick fix. The whole drive system was scabbed on at the last minute. I should have just left the outriggers unpowered, as the motor and flex axles could not provide enough torque to push or pull the stiff outriggers out or in.

Then the worst came. As I drove the model around on the floor at the convention center, I heard the dreadded click-clack sounds of gears slipping. Some of the 8-t gears in the gear train between the doubled 9V motors and the differential for the axles had slipped.

The steering was badly underpowered too. The wheels would absolutely not steer unless the model was rolling forward at max speed. And even then they steered very slowly. So, driving the model was a challenge as it took relatively long distances to steer the whole thing.

The top half of the model worked pretty good, thankfully. The boom luffed and extended. The winch worked as it should. The superstructure slewed okay, except for binding up now and then because of the bad chassis flex (the whole turntable drive system was down in the chassis, BTW).

I rushed the job and produced a poorly performing model. I did not give myself much time to test and redesign the things that needed it. And then there were very serious flaws like the chassis sagging that would have required massive redesign.

I decided that after Brickfest, the model would be scrapped.

But I have no regrets :-) This crane was a valuable experience for me. Plus, I went to Brickfest PDX '08 and got to meet Ross, Bob, and a young and very skilled builder - Lucas.

a real RT600

RT760 for comparison

RT760 for comparison


Here are some photos of the real thing. One shot is of a Grove RT600, and the others are a slightly different model just to give you some perspective. They are just typical 4x4 telescopic boom cranes. You can usually find one or more on large construction sites just about anywhere in the world.

BTW, over here (US), we call these things cherry pickers :-)

end of the crane

enough with the bricks!
on with photography :-)

When I returned home from Brickfest PDX in February of that year (2004), I stopped building with LEGO pieces. I stashed the crane away and turned to another interest of mine: photography.

I lost the desire to create MOCs, and my creativity just shut off. I was burned out.

I should have known better. I was doing too much and going too fast. Building this crane felt like real work, complete with deadlines and stress. This came at the end of several years of building at a hard pace and also marked another model that wasn't built well enough and had serious problems. During those years, I almost always had a project going and the project was usually for some big, upcoming 'fest or event.

I really needed a break, I didn't recognize that fact, and I just forced myself to keep building.

In retrospect, the crane wasn't a big deal. It didn't matter that I produced a poorly functioning model. But at the time I felt a strong desire to make complicated and fully functioning models and that I had to prove myself in some manner. In doing so, I pushed the fun out of this wonderful hobby.


Now, over four years later, I find myself finally catching up with my website and writing this page :-) Hard to believe...

I still built a few things during these past four "gray" or "semi-dark" years, but not at the pace like I used to. Photography occupied most of my free time and I've really enjoyed it. It's provided me a creative outlet and opened new opportunities. In fact, my photography is a huge part of my life now and has gone beyond just a simple hobby.

In May of this year (2008), the big LEGO switch flipped back on for me, and I started building again. My son (3-years old) really helped me get back into things. Now, he and I sit at my building desk and spend hours together. He doesn't actually work on the same stuff as I do, but he likes to sit nearby and parallel play. He builds his own little things and enjoys it. And he's always asking me if WE can go build :-)

I don't know what my future holds, but I'm really enjoying building right now. I'll just follow the thread and see where it goes. And I'll always remember to keep it fun and enjoyable.

I think I've learned a good lesson about getting burned out and experiencing way too much of a good thing. I'm certainly more wary of pushing my creative limits too far and putting myself into demanding, non-productive situations. In fact, part of the reason why I've started to build seriously again is to give my photography a little break :-)

best regards,
TJ Avery
(July 2008)

p.s. I invite you to look at my photography. I shoot mostly natural landscapes and wildlife. You can visit my photography site here:

Thomas J. Avery 2008