This LEGO Model is a model of the Mobile Crane Liebherr LTM 1200. I chose to build a mobile crane as it is one of the more complex construction vehicles, with 7 main functions all fit into a compact space.
About this creation
Liebherr LTM 1200 LEGO Technic model by sqiddster
This LEGO Model is a model of the Mobile Crane Liebherr LTM 1200. I chose to build a mobile crane as it is one of the more complex construction vehicles, with 7 main functions all fit into a compact space. The mobile crane has become sort of an ultimate build for technic builders... You’re not a real man until you’ve built one! (Or real woman, in the case of Jennifer Clark.) Anyway, I have sectioned this report into sections which relate to all the functions of the crane model. I hope you enjoy the read.
The carrier is the largest part of the crane (in the picture, the part with the wheels and the cabin) and is the most technically complex, at least in this model. It has 4 motors and 3 functions: The drivetrain, steering, and outriggers. The two receivers are in the engine complex behind the cab. The battery box was a pain to fit in, as I needed to use my rechargeable one for the superstructure, but I managed to find a spot in between axles 3 and 4. The steering mechanism is even redirected around it! The cabin was fun to build and I think it turned out looking pretty accurate to the original crane. It has a steering wheel, 2 articulated seats, and rear view mirror. The cab also has two opening doors.
The drivetrain is the part of the crane, located in the carrier, that gives the vehicle forward and backward movement. I used two XL motors for power. This is an immense amount of power... and geared down it was enough to move the crane. The two motors are positioned side by side in the engine block directly behind the cab. They are placed in a simple side-by-side orientation and are connected with sets of 24-tooth gears.
The power is transmitted to the driven axle through a driveshaft and gears transmit the power down to the pendular suspension module. The 3rd or centre axle is the one that is driven. I had originally planned (much more accurate to the original crane) to have an 8-wheel drive, and indeed, a pendular suspension module, based loosely off sariel’s: http://sariel.pl/2008/12/compact-pendular-steered-suspension, was designed to be both steerable and driven. Unfortunately, in a test prototype, the large amount of gears needed meant there was too many fault points and also an efficiency decrease meant that I decided it would be more practical to only have one axle driven and the rest steered. Since this was the central axle, it didn’t need to be steered, either. Anyway, I ran into a large problem with the last set of gears; due to the location of the differential, they were only secured from one side and due to the extremely high torque placed on them, they demeshed often. There was about one plate of distance between the differential and the gear set and I realised it would be possible to cut a ‘slice’, using a dremel, of a 3M beam and it was able to be kept in position by an axle 4M with stop. (Don’t tell anyone, but I didn’t have access to any of these, so I cut one of the 8M ones. Sue me.)
Steering a many-axled vehicle is a challenge. In your average car, the steering involves the front two wheels turning. However, when more axles come into account, if more than one of the axles is not steered, then significant tire skid would become a problem. A huge problem in real life, as it happens. In all honesty, it would not be such a problem at this scale, but it would still emerge some difficulties. Considering this and the fact that I was striving for realism in this model, it was really necessary to have an authentic steering setup.
Such a setup involves the front axle steering in a large range. The second axle must steer in a slightly smaller range, as it is closer to the drive wheel. Hopefully the pictures will make this clear. The centre axle doesn’t steer at all as it is the driven axle. Now the fourth axle must steer in the same range as the second axle, but in the opposite direction. The fifth follows suit, steering in a larger range than the fourth axle and in the opposite direction to the first and second. In the real crane, there are numerous steering setups, including crab steering, where all wheels are turned to the exact same direction, letting the crane move in a diagonal direction. I didn’t wish to grow gray hair at such a young age, so I didn’t even attempt this. Theoretically it would be possible at this scale, having a separate motor and receiver channel for every axle, but you would run out of channels almost as fast as your sanity. You can see this steering setup in the pictures.
Simple, right? Not really, especially as each steered axle utilises Ackermann Steering Correction. This system is used in all modern vehicles. What it does is steers the wheel on the inside of the turn ever so slightly more than the wheel on the outside of the turn, as it has to follow a shorter distance. Great fun.
The steering was achieved using a single medium motor, first using worm gear reduction to reduce speed and build torque. This was followed with an axle that ran the length of the crane, supplying steering to the entire crane. The different steering ranges were made possible by gearing differences. On paper, a (very) roughly 1:1.5 gear difference was needed between the less and more steered axles, but in real life, gear lag made it necessary to change this ratio to 1:3. The steering method used was rack and pinion steering.
The real Liebherr LTM 1200 uses independent suspension. However this is simply not possible at this scaleThe suspension is probably the most original part of the whole crane... The idea for it I came up with completely, although this is not to say it has not already been done, in one form or another.
The problem with LEGO shock absorbers is that they will only compress, and not extend. This is a problem when pendular suspension comes in play, so most LEGO pendular suspension modules are unsprung. I wanted to have a sprung suspension on this model, so I started thinking of a way to accomplish both. I ended up with what is shown as a simplified version (to facilitate understanding) in the diagrams below.
This setup enables pendular suspension to be sprung. The real crane has independent suspension, but this would be almost impossible at this scale. Perhaps the necessity of suspension at all in this model of the crane is minimal, especially as it only has two wheel drive and as such would have problems with rugged terrain, but I wanted to, and having no suspension in a model of this scale, for me, would be simply too easy and not to be condoned. Having said that, the suspension works very well.
The suspension uses 8 of the ‘hard’ 6.5M shock absorbers. I used these because I had four already and they are the most compact option, other than using rubber bands, but this is just not as realistic at this scale. It would be worth mentioning that the foremost axle is not sprung as the drive motors are directly above it, but rest assured
it is still suspended and has no effect on the performance of the model.
Outriggers are the ‘legs’ that come out of the side of the crane to support it when the boom is luffed (crane term for lifted). This is probably the most unrealistic part of the model, at least visually. In a real crane, the outriggers extend horizontally out of the sides to a large diameter then hydraulic piston deploy straight down, lifting the crane off of its wheels. With lego, hydraulics don’t exist, although pneumatics do become a substitute. However at this scale you have to choose:
• Great functionality, accurate imitation but 3 or 4 studs wide so off-scale and ugly
• Terrible functionality, accurate scale and imitation and reasonable appearance
• Fair functionality, accurate scale, reasonable appearance, compact, bad imitation
I opted for the option with the most positives. The design of the outriggers themselves are based off Jennifer Clark’s, but modified to give a longer reach. In the end of it all, they definitely don’t lift the crane off its wheels, but they at least stop the crane from wobbling on the suspension.
The superstructure is the part of the crane which holds the boom and rotates, mounted on the top of the carrier. It has 4 functions in total and 2 receivers. It is powered by the 8878 rechargeable battery box. Perhaps the most unique part of my superstructure (compared to other’s crane models) is that contrary to using the battery box as a counterweight, I placed it in the ‘arm’ which is on the other side of the cab. This meant I had the whole counterweight area to play with. I toyed with the idea of making the counterweight removable, however, the model became very complex very fast and I had to abandon this possibility. The superstructure is filled with the two receivers, weight balls, the rotation mechanism (powered by an XL motor) and the winch mechanism (powered by an M-Motor.) Another area of interest is the curved counterweight. The real crane has a curved counterweight, and as I was striving for realism, I wanted to have it, however, curves are not easily made in Lego. I ended up succeeding using lots of hinges. I had loads of fun building the operator’s cab, and I think it ended up looking pretty accurate.
The superstructure rotation went through a bit of refining. I always planned to have the motor on board the superstructure, as this would mean that the turntable would have no wires running through it, so that it could rotate forever without any wires getting tangled. It also let me work on the carrier and the superstructure as separate entities, which was a huge help when building.
The winch is the other built in part of the superstructure. It is directly connected to the M-motor through a chain drive system which hasn’t failed me yet. The winch is probably the most important part of the real crane although the simplest part of the model. The M-Motor doesn’t give much torque, although having the string reeved around the hook block and the end of the boom quadruples the torque. In tests, I achieved a lift of 2.1 kg. Needless to say, I was very happy with this result and it was above expectations for the model. The picture shows a lift of 1.5 kg.
It seems that I have unintentionally discovered a most interesting feature about the motor. When heavy lifts are made and the winch stops...er...winching, it doesn’t brake but instead coasts and the weight pulls the string down, until it is stopped by hand, and the motor then brakes. Curious.
Luffing is the technical term for raising the crane boom. One of the most interesting parts of this model is the setup used to accomplish that. Real cranes use huge hydraulic cylinders to accomplish this task. I decided to follow Jennifer Clark’s lead and use a custom-made linear actuator to do the job. Different from hers, however, I found a way to make it aesthetically correct using an ‘outer sleeve’ (coloured white) to both keep the proportions accurate and also to hide the turning screw. It works like this...
The XL motor is mounted on a pivot. Connected is an M8 screw with an axle shape cut into the end where it fits in the XL motor. This screw turns to luff the boom. Outside this screw is a piece of metal pipe (coloured silver, which can be seen when the actuator is extended). This has an M8 nut glued into it on one side to engage with the screw and on the other side, a hole drilled through its diameter and the edges rounded off to connect with the boom. I followed the real crane’s proportions throughout the making of this so it ended up being just long enough to do the job.
The motor to luff the boom had to be incredibly strong, as huge loads are placed on it as it approaches the horizontal, so it had to be an XL motor. Originally, I had a complex gearing system (again, much like Jennifer Clark’s) for the screw to be turned, but then I realised I had had a huge brain fart and I could just have put the XL motor straight in, with no gearing! In light of all this, the luffing ended up working up well, and the boom is able to be placed anywhere between 0 and 90 degrees on the vertical axis (It can actually go even further than 90 degrees! But then, what is the purpose of that?)
The cool thing about this mechanism is that it is directly connected to a mechanism that tilts the operator’s cab. So when the boom is vertical, the cab is lifted by about 45 degrees. This is the first time I have seen such a thing done in a mobile crane model and I think it works very well.
The boom extension was probably the part of the build that went through the most change. From the start, I had planned to have a 3-sectioned boom, and indeed there was enough space for it at this scale. I planned out a complex pulley system and everything, but things went pear shaped and I ended up reverting to a 2 section boom (which has much height anyway, as you can see in the pictures). It ended up being a choice between aesthetics and having 3 sections to the boom (i.e placing a gear rack and gears on the outside) but as you know I was not willing to compromise on aesthetics anywhere and having 3 sections to the boom is not imperative functionally... The real crane has 7 sections, so being choosy between 2 and 3 really wasn’t necessary.
The boom is extended by an M-Motor. Much like the mechanism for boom luffing, an M8 screw which runs the length of the core of the boom is connected to the motor. An M8 bolt is attached (attached may be a long shot, more like ‘trapped’ or ‘held in’) to the closest extremity of the second section to the motor when it is retracted. When the motor turns, the screw spins and the boom extends. The M-motor used was not the most desirable motor for this mechanism, in all honesty. I would have much preferred the meatier XL motor but this couldn’t fit in the back of the boom where the M-Motor is located.
I would like to thank Jennifer Clark: www.genuinemodels.com, Sariel: http://sariel.pl/, and M_Longer: http://www.flickr.com/photos/m_longer/ for ideas and inspiration, and most of all Dad for helping me, especially on the custom parts.
You can see a video of all of the functions here.