DESIGNING THE MOBILE CRANE
LEGO® Senior Designer Markus Kossmann is a real expert at designing working model functions using LEGO Technic elements. That’s why when the time came to create a huge and feature-filled new Mobile Crane model for 2013, Markus was a natural pick for the design team!
“Every year, LEGO Technic releases a new flagship model, which is the biggest and most challenging model to build and develop. The flagship model has to be special, so choosing the right model is difficult. We had done mobile cranes before, like the Mobile Crane from 2005, which was a cool model and popular among fans. The new one couldn’t just be a copy of the old one; it needed to be better, bigger, and more modern-looking, with improved functions.”
- LEGO Senior Designer Markus Kossmann
The first step was to investigate which functions are essential for a mobile crane, and how they should be designed and controlled in the new model. Analyzing the 2005 version, there were a few things that the Technic team thought could be improved:
- Simplifying the steering by removing the gearing for different axle turning angles
- Creating motorized, more realistic supports, with extending outriggers and stabilizers
- Changing the pneumatics for lifting the crane boom to a motorized linear actuator
- Separating the extension of the boom and the operation of the winch
- Designing a more modern look using the latest LEGO® Technic elements
Generating ideas for the new functions and building a first rough sketch model takes around two weeks for a model of this size. The team also needs to design prototypes for any new elements needed to make the functions work. The elements are produced with a rapid-prototyping machine and can be built into the sketch model right away.
The first sketch model gave a good indication of the final size and proportions, how the functions would work, and where the critical areas were. Compared to the 2005 crane, the new version looked more modern, especially its cabin and the big boom arm on top. The new functions were working all right, but the support outriggers on the sides still needed to be motorized. The Technic team also wanted to add a fifth axle in front of the front outriggers to create a bigger difference from its predecessor.
To make the design process easier, Markus split the construction into three major modules: the chassis, with built-in steering, piston engine, and outriggers; the crane housing, with the motor, gearbox, and battery box as a counterweight; and the boom, with a two-stage extension system. All three modules would have to fit and work together in the completed model, so it was important to design them to interface with each other. All of the model’s functions would be powered by one battery-powered motor, split up into four different functions in a gearbox in the crane housing.
“In this development phase, I have to build all of the functions until they work perfectly, which involves a lot of testing, rebuilding, and testing again; you have to be very critical with your work and seek input from the other designers. While doing that, I also have to think about the building process, so that it can be properly shown in the building instructions.”
The chassis was very long and had to be solid and rugged so that the heavy crane could be placed on top. It couldn’t bend or break when lifting the model. A number of functions were built into the chassis, including a steering system, the V8 piston engine, the outriggers, and the gearbox for controlling them.
“I designed a strong bottom layer with the axles attached, where I could then add the outriggers and gearbox on top. All of these modules are then locked together by a top frame to create a strong structure. In the front of the chassis are the driver’s cabin and the motor compartment. These had to look very powerful and modern, with real details like rear-view mirrors and doors that can open.”
“The most challenging parts of the chassis were the outriggers, which had to be compact and work together in a synchronized fashion. We modified an 8-tooth Technic gear so that it could slide on a cross-axle and drive the linear actuators for lowering the support legs. It’s a really cool new function. The hard part was fitting everything into one module and finding an easy way to synchronize all four motorized outriggers. It took me quite a while to crack this one!”
The steering was less of a hard task. By changing the lengths of the wheel steering arms, the Technic team was able to give the axles different angles of steering to let the crane turn smoothly. If all of the axles had the exact same steering angle, the model would rattle and shake when it drove around curves!
The crane house is the heart of the crane, where the motor drives all of the crane’s various functions. Each function is selected by a gear switch on the side of the housing, making it simple to control the crane. The motor can power the outriggers, hoist and lower the boom, or extend the boom and the winch.
“The function for the outriggers is transferred through a turntable in the chassis, where it is split into extending each outrigger in and out, or lowering or raising the stabilizers. Another option was to build a gearbox to split the movement into five functions, and have both functions for the outriggers transferred to a turntable. It was quickly apparent that that wouldn’t work, though, because I wanted to build the chassis and the crane housing as two separate modules, and it would impossible to join them.”
It took a lot of time to figure out a gearbox layout that was buildable and stable under all conditions. It also had to be embedded in a solid structure, because the boom, which is attached to the housing, creates a lot of physical stress and can affect the functionality of the gear box.
“Therefore, I built a triangular structure to strengthen the front, while a 5x7 Technic frame on each side prevents any parallel movement of the beams on the side. Another 5x7 Technic frame is placed in the back, also to prevent twisting. In the back, the battery box acts as a counterweight just like on real cranes.”
“The boom also took more work than was expected. It is designed that it can extend in two stages; the principle of extending is the same as on the earlier Mobile Crane from 2005. So, easy job, I thought! I wanted it to look more modern like in real mobile cranes, where the boom is round on the bottom. I tried to create the round look by using 3x2x11 panels, but I realized that it was impossible to create a solid structure. I had to improve the element! The new panel has more Technic holes to attach beams for a strong build-up.”
Markus wanted to build the crane boom’s two extending stages with a light but very strong structure, using Technic beams that could extend a long distance. The first build-ups were very promising, but while testing the model, it was discovered that the mechanism didn’t work entirely smoothly when the boom was extended with a load of cargo hanging off the hook.
“Investigating together with my colleague Anders, who works in our test department and designed the RC Excavator from 2010, we managed to find the reason. The force of the load was bending the beams so much that they were pressed together and creating friction. I redesigned the structure of those parts, using Technic bricks for the second stage of the boom and running them on wheels. I also made sure there was a lot of room around the parts so they had space to flex and didn’t press against each other. All of those changes solved the problem, with the extra benefit that we reduced the power consumption of the motor to a third!”
“It was a long design process, stretching for almost one year, to complete the crane. The final model has 2600 elements and is the biggest LEGO® Technic model ever released. I enjoyed designing the new 42009 Mobile Crane Mk. II, and in the end I was very happy to have finished this model. Adding new functions and making it buildable for kids around 11 years old was worth all the effort. I hope you will enjoy building and playing with the new crane!”
- LEGO Senior Designer Markus Kossmann