Building 12-sided Base

Finished Base

Bottom of Base

Actually a dodecagon (12-sided polygon), the base of the crane is made of many, many beams and plates. Duh! The bases of real offshore cranes are usually called crane tubs. They are circular structures made of stiffened plate.

I started construction of the model with the base, sometime in early June. I figured it would be the most difficult part, as it would be approximating a circular shape. It would also be very difficult because the base would accommodate the turntable of the crane.

After a little trial-and-error, I arrived at an arrangement of radial and circumferential beams that worked. It had 12 sides, and was about the right diameter I wanted.

Completion of the base set the tone for the rest of the model. I knew I could get it done before Brickfest in July. I constructed the base and finished the design of the turntable in one weekend. Basically, I got lucky. Almost everything I put together seemed to work in a minimum of revisions. As my friend put it, "the Technic Gods were smiling upon me" :-)

The strength of the base comes from the plates oriented radially (the black plates on the inside) and circumferentially (the gray plates on the outside- the shell). The plates are stiffened, or "backed", by Technic beams. This model makes use of plates in a structural way, and is very different from how I've used them before.

MODULE 1: The completed base is a module of the crane. As you will see below, each major part of the crane is a "module". The modular construction enabled me to easily disassemble and reassemble the model for easy transport. (relatively easy, that is- it was still difficult, but could have been worse!)

Close-up of Segments

Close-up of Bearing
(click to see detail)

Using 29 white 30.4 x 14 VR wheel hubs, I created a ring of rollers for the turntable bearing. This type of bearing, where the rollers travel on a circular path and are "sandwiched" by a top and bottom surface, is a thrust roller bearing.

Because the circumferential beams (see base above) are on their side, a flat surface is provided for the rollers. They do not ride on a rail or in a groove, so restraining the ring was a problem.

Fortunately, I quickly realized (again, the luck!) that the circumferential beams provided a crude circular channel for the ring of rollers. The bricks and plates connecting the rollers are deeper than the wheel hubs, so they are restrained from sliding off the base by the circumferential beams (see CAD images to left).

This ring of rollers can and does "float" around in the turntable as it rotates. Therefore it cannot restrain the crane from sliding off the base. The next part of the turntable is a kingping. It is a center pin that restrains lateral movement, and provides a center rotation point. The kingpin is made of a large Technic turntable (see next section).

MODULE 2: the ring of rollers can be separated easily, and can also be laid flat. If you make a break in the links of plates and bricks, the circle folds out flat.

Gear drive for turntable.
All 4 perpedicular axles
are connected to motors.

Top view of gear drive

Side view of complete model

The upper rotating frame (superstructure) is the heart of the machine. The large Technic turntable "keys" into the base. The motors driving the turntable are placed in the superstructure, allowing it to rotate 360° continuously (no wires pass through to the base, it is static).

• The Turntable:
I decided to go all-out on power, and set up 4 geared 9v motors to drive the base, with a 1:56 ratio. This turned out to be a little too fast! I should have used two motors and a higher ratio.

When controlling the turntable, I had to pulse the power on and off so that it wouldn't spin at full speed. If I held down the button, the thing would whip around very quickly (this is how I scared a few people at Brickfest ;-)

I was very impressed with the operation of the turntable. The crane rotated very smoothly and seemingly without much effort. Using a thrust roller bearing is definitely the way to go!

• The Frame:
The superstructure frame (directly on top of the turntable) is a pair of beams that run from the boom hinge pins to below the ballast. They are constructed of 6-wide plates that are turned on their side. The plates are stiffened with Technic beams and 1xn plates.

The back of the frame (under the ballast) extends down below the turntable. It grabs a "lip", or flange, at the top of the gray base. The purpose of this is upward restraint. When the crane is lifting a load, and starts to tip over, the back frame catches the lip so that the superstructure cannot flip over and off the base.

MODULE 3: this one is a little difficult to manage, as nearly all other modules connect to it. But it does come apart!

The white framework atop the superstructure consists of an a-frame and a back leg. They connect at a point above the crane, and provide the attachment point for the boom luffing block. Usually, the a-frame is in compression, and the back leg is in tension.

The a-frame also has small "protusions" in front. These are called boom stops. They prevent the boom from falling backwards on the crane if it is luffed too far back.

The main construction method of these frames is a sandwich of full-width liftarms and plates. Look closely and you'll see white liftarms in the middle, with 1xn and 2xn plates snapped over them.

MODULES 4 & 5: back leg and a-frame can be separated easily.

This large container is made simply of bricks. I designed it quite large so that I could use a lot of water as a ballast.

I planned to use water so that I didn't have to transport scrap steel, or other heavy material, to Brickfest. I placed plastic sacks in the boxes and then filled them under the faucet.

Unfortunately, the amount of water contained by the boxes wasn't a sufficient counterweight. I was barely able to lift my 9.5 pound R 996 model.

After the 'fest, I set the crane up at home and used steel for the ballast. I then was able to lift a substantial amount of weight. Read more...

The HiTechnic 4-channel R/C receiver is mounted atop the counterweight box. All electric connections are snapped off when the model is disassembled. The 2x2 connectors are color-coded with tiles so that I know what wire goes where. Of course I always leave behind my written guide!

MODULE 6: counterweights sit atop the superstructure using a minimum connection. Easy to remove! The large box actually splits in two, but that's usually not necessary.

There are three winches on the crane:
1) Main Hoist
2) Boom Luffing
3) Whip Hoist

The main hoist line winch and the boom luffing winch each have 4 geared 9v motors. The spool is connected to them with a 1:5 ratio. This arrangement provides a lot of power and a relatively high rotation speed. The result is a fast-acting model that's impressive to watch!

I should note here that this philosophy of "more motors, less gear reduction" results in a fast moving model that's pleasing to observe and operate. Most large models I've built in the past have been very slow and no fun to use. It was like watching snails crawl around.

The whip line (if you're wondering, the whip is the second hoist line- typically much, much smaller in capacity than the main hoist hook; it's used for light-duty jobs) is powered by two old-style non-geared 9v motors with a 1:9 reduction. It's very fast.

The winches sit in a cascading arrangement (one atop the other, but further behind the first). This arrangement was dictated by function, as the space between the a-frame and the counterweight box created a triangular compartment.

Almost everyone asks what the springs (Technic shock absorbers) are doing on the side of the winch. A fundamental problem with the given winch design is that if you pull hard enough on the line, the winches will unspool themselves. The springs, when flipped down, activate a locking mechanism- a piece engages with the 40t gears and prevents the spool from rotating and letting line out. However, the winch locks are manual, and if you forget to engage them after turning power to the R/C receiver off, you'll accidently boom down!

MODULE 7: you must spool in all string before removing!

Exploded view of joint
4L axle is threaded

Joint allows rotation
in two degrees

Plan view showing angled
chords of truss. Red mid-
section chords are rotated
inwards only (one degree) to
narrow the boom width

Hinge end of boom.

Close up of 16 part main
hoist block

Cantilevered boom (I'm
holding the end with my
hands. It's very strong!)

The boom is a large truss structure, obviously ;-) However, it is unique in that certain parts of it are angled. This presented a challenge, as making a 3D truss with LEGO parts is not easy.

One key element that I needed was a strong connection that would allow two beams to connect together end-to-end, but be able to rotate about two axes perpendicular to the beams. This is essentially the function of a U-joint, minus the torque part of it, but doing it with beams is not easy.

Not easy unless you find the right solution! And, of course, I did :-) The left set of pics (the CAD images) show the construction of the joint. The key element is a 4L threaded axle. It binds the joint together and makes it strong enough to handle significant loads.

I used these U-joints on the fixed jib tip of the boom. Look at the white triangular end of the boom. Notice how the main chords of the truss travel inwards and down as you move along to the tip.

I used simplified U-joints (not really a u-joint anymore) for the red mid-section. Notice how the main chords of the truss travel inwards (but not down). I connected the main chords very similarly to the U-joint, but instead of using a 1x1 Technic brick, I just connected the Technic beam of the main chord. This allowed one degree of rotation, and the clamping of the 4L threaded axle held the joint together.

I decided for this crane that I would have fixed, stiff cross bracing. The cranes I'd built in the past used string in an X pattern for the cross bracing of the truss, and did not perform well.

Using 1xn plates, and 1x5 and 1x6 1/2-width liftarms, I made the cross bracing. This construction performed very well, and I'm completely satisfied with it. However, it does take a bit of trial and error to figure out the proper bracing, especially for the non-rectangular parts of the boom.

MODULE 8: It's not really a managable module, and unfortunately, it doesn't split easily into sub-pieces. I had to nearly disassemble all of it when traveling to Brickfest, but you can read more on the shows page.