I've not modelled this after any specific crane, but this model uses standard features that are found on tower cranes. There are a few deviations however, but the basic functions are the same.

Tower cranes are useful because they have a long reach and are tall. They have access to a wide area of a construction site and can move materials where they are needed. These cranes are light duty, typically lifting a only a few tons. As you can imagine, the lifting capacity decreases as the hook is moved further out towards the boom tip.

This model was no exception. I'm not sure what the lifting capacity was, but at the boom tip, it was no more than a fraction of a pound.

Total Height: 8.5 feet (2.6 meters)
Total Width: 8.9 feet (2.7 meters)
Boom Height: 6.5 feet (2.0 meters)

I started building this crane only three weeks before Brickfest 2003. I have quite a story to tell about my building activities in the summer of 2003. There was another crane model, one that I'd started building in the summer of 2002 and finally finished in June 2003. This crane met an unfortunate end and was destroyed late that month. It was a model of the MSG 50. I spent a lot of time in July 2003 developing my presentation for Brickfest 2003.

Only 3 weeks until the 'fest, I decided that I couldn't go empty-handed. I used a truss pattern from the MSG 50 and started working on this tower crane. I worked on it until the very last minute before I left for DC. In fact, I didn't have a chance to fully assemble and test the crane prior to leaving. It was a true first-draft prototype.

About half of my time was spent developing the turntable. There is a special section below to present this device.

Don't let the short construction time fool you. Except for the turntable, this tower crane was very simple and used many construction techniques that I've used before. It was easy and fast building. A lot of time was spent disassembling the MSG 50 model, which was a tangled mass of parts and string. My wife even helped me take apart and sort the old model!

Mechanical Details

At the bottom of this section on the left is an image labelled "Schematic". This simple diagram lays out the mechanical parts of the crane.

There were 6 "packages" that were inserted/attached to the crane structure. The motorized sheave and winch were inserted into the open truss structure of the back mast. The battery box was hung from the end of the back mast and served as ballast. The hoist trolly rode on straight plastic train tracks through the crane boom. The "dumb sheave" (see schematic) was located at the tip of the boom. This was just a simple large pulley mounted inside the truss structure. An finally the turntable was attached to the top of the column (see next section).

The crane was powered by a 12 volt battery and conrolled remotely by HiTechnic's 4-channel receiver (note: HiTechinic is currently not producing any products). There are a total of 8 motors: 4 for the winch, 2 for the sheave, and 2 for the turntable.

This little device was the most complicated and time consuming part of the crane. I knew I had to create a thrust roller bearing for the crane so that it would operate smoothly and would be able to slew (rotate) efficiently.

The turntable has a free-floating ring of 15 rollers. The rollers are 2x2 round bricks. The ring is created from several long lengths of flex tubing threaded through 1x2 liftarms. The ends of the tubing were a problem as they wanted to flex outwards. I inserted small pieces of wire into the ends of the flex tubing to help make the connection. You can see these wires where the flex tubing ends meet up.

This ring of rollers is centralized by four 2L bushings (or smooth rollers). The image "Core of Turntable" shows some of these centralizing bushings.

This sort of turntable is built around a large Technic turntable (the big gray thing with gear teeth) which has a problem of popping apart when too much moment is applied to the turntable. Instead of using a threaded axle through the center to help restrain it, I used 4 smooth beams and a set of counter rollers. In the image "Bottom View, Counter Rollers" you can see the ends of the smooth beams (white) and the yellow rollers, again 2L bushings.

The core turntable is engaged with two 8t gears, each connected to a separate motor. These motors are first reduced by means of a 24t and worm gear combination. This can be seen in "Turntable Mounted, 1st Drive with Worm Gears". This is how the model appeared at Brickfest and had a problem with slewing (rotating). After the top picked up speed in rotation, it would shock the gear trains when the motor power was cut off. This torqued the column and could be seen visibly. People reacted with surprize in seeing the column twist. However, the crane was structurally sound and could endure this load.

One solution to this was to "pulse" the motor so that rotation occured with a soft start and soft stop. However, this is not a permanent solution.

I rebuilt the turntable drive after Brickfest, and this can be seen in the bottom set of images. The worm gears were replaced with 3 sets of 8t and 24t gears to reduce the motor speed. The first set of 24t gears engaged by the motors were the white clutch gears. The long axles allow the motors to be placed higher up on the tower top so that the trolly and hoist lines could be passed through it.

This new turntable drive was a little better than the previous one that used worm gears. The gear reduction was a little more, so slower slewing speeds helped the torque problem. However, it would still shock when the motors were stopped. I believe the best solution to this problem is to have a proper way of starting and stopping the slewing motors softly. One method that springs to mind would be to use the LEGO train controller with its variable speed capability. However, this would require hard-wiring the model to a remote.

This video movie (a little out of focus, sorry) shows the turntable slew bearing in action. You can see the rollers and how they move with respect to the top and bottom pieces.

I didn't take any more video of the crane as it's really not that interesting. It spins, the hook moves up and down and back and forth. Pretty straightforward stuff!