Tower Crane

Me at Brickfest '03
Pic by Rob Hendrix


At TexLUG Fall 03

Unfinished Crane

Tower Crane

This is a model of a typical tower crane seen at construction sites just about everywhere. It's not modeled after any specific crane, but it has functions and features just like the real ones.

Tower cranes are useful because they can have a long reach, great height, and very small footprint. They are modular and can be easily transported once broken down.

Model Details:

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


Column Base


Top of Column Where
Turntable Attaches


Motorized Sheave Unit


Hoist Trolly


Winch Unit


Schematic


Close-Up


Counterweight/Battery Box


Sheave Unit Placed
in Truss


Trolly In Truss


Winch in Truss

Info

I started building this crane only three weeks before Brickfest in 2003. I had built a very large crane model (the MSG 50) but it had an unfortuante accident and met a hasty end.

I decided that I couldn't go empty-handed to Brickfest, so used the truss pattern from the MSG 50 model 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 about it.

The rest of the crane 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 parts from it.

Mechanical Details

At the bottom of the picture section on the left is an image labelled "Schematic". This simple diagram shows the cable setups for the hoist and trolly.

There were several small modules that were attached to the crane structure. The motorized sheave and winch modules 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 track rails (from the very old style train tracks) through the crane boom.

The crane was powered by a 12 volt battery and controlled remotely by HiTechnic's 4-channel receiver (note: HiTechinic no longer produces this). There are a total of 8 motors: 4 for the winch, 2 for the sheave, and 2 for the turntable.


Rollers


Roller Bearing Ring


"Core" of Turntable
Upside-down


Turntable Mounted
1st Drive with Worm Gears


2nd Drive With
Different Gears


Roller Ring


Rollers on Ring


Bottom View
Counter Rollers


2nd Drive
Close-Up

Turntable

The turntable module was the most complicated and time consuming part of the crane. I spent the time to make a proper thrust roller bearing so that the crane would slew (revolve) smoothly and efficiently.

The turntable had 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.

The turntable was built around a large gray Technic turntable. This turntable by itself will pop apart when too much moment is applied. So, I used 4 studless beams to act as a pin through the center. This "pin" had a set of counter rollers at the bottom (keeps the whole turntable assembly together under moment). 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 gray Technic turntable was driven by two 8t gears, each connected to a separate motor.

The initial turntable gearing was a little crude. When the power was cut after slewing, it had so much momentum that it would keep turning and twist the main column of the crane. The gear trains were shocked with a lot of torque while the previously spinning crane transferred energy into the base.

The quick solution at the time was to "pulse" the slewing motors so that rotation occured with a soft start and soft stop.

Later, after Brickfest, I rebuilt the turntable drive (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 clutch gears (the white ones). 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 greater, so slower slewing speeds helped reduced the torque-shock when power was cut. However, it would still shock a significant amount when the motors were stopped.

I believe the best solution to this problem (a problem common with all the large crane models I've built) 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.

Videos

This video (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:

Movie: 1.6 MB

This video is of the turntable upside down. This shows the bottom and the set of counter rollers:

Movie: 1.9 MB

Thomas J. Avery 2003