Mercedes-Benz / NAW SK 3550 8x4/4 with Modular Platform Trailer


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Mercedes SK 3550, picture 1

Features

Truck:

  • Rear axles driven by 9V motor
  • Front axles steerable by 9V micromotor
  • Cab tiltable to reveal engine
  • Openable doors and sun roof
  • Detachable ballast platform with fifth wheel underneath
Trailer:
  • Platform modules with steerable wheels
  • Drawbar module to access steering motion
  • Bumper module with flashlight to access steering motion


Introduction - Truck - Trailer - In Action
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Introduction

Although Mercedes-Benz is a German company, the original of the truck pictured here has partially been manufactured in Switzerland. Based on standard components from Mercedes-Benz, the heavy haulage tractor units used to be customized for their special duties at NAW. Located in the eastern part of Switzerland the company NAW (Nutzfahrzeuge Arbon und Wetzikon AG) was a well known name for heavy duty trucks.
The model designation SK 3550 8x4/4 reads as follows: SK stands for 'Schwere Klasse' meaning 'heavy class' and is the successor product line of the NG 'New Generation' Mercedes-Benz top of the line trucks. 35 stands for 35 tons maximum gross vehicle weight while 50 stands for the turbocharged Diesel engine OM 442 LA with 503 horsepower. 8x4/4 finally means that of the eight wheels four are driven while another four are steered. The maximum weight to be pulled is 250 tons.
The model is in scale 1 / 18.75 which at first glance seems to be a strange number. But scale was dictated by another creation, the Liebherr LTR 1800 crawler crane. And there, two studs on the model equal 30 centimeters in reality which resulted in an even number of studs for the three meter wide upper structure. The reason for the same scale on both creations is the fact that I wanted to build a heavy haulage truck being able to transport components of the crane. Another reason was to give an impression of the dimensions of the LTR 1800 in reality.

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Truck

Mercedes SK 3550, picture 2

The Mercedes-Benz SK trucks were available with three different sized cabs. My model features the largest one which offers enough space for a comfortable bed behind the drivers place. Furthermore one could also chose wheter there should be side windows on the rear half of the cab or not. I opted for the latter one. Nevertheless the places where the windows would have been are clearly visible as there has been manufactured only one pair of side panels with window openings which could be closed by additional blanks. Therefore rubber was used to seal these panels which resulted in black frames on the side walls of the cab which can be seen on my Mercedes as well.

Mercedes SK 3550, picture 3

The cab features openable doors and sun roof as well as a detailed interior. The latter one includes a bed with pillow and blanket and many smaller details like cup holders, sun visors and a reading lamp. Details on the outside of the cab include rear view mirrors, flashlights, horns, a Mercedes star consisting of a printed round tile and directional lights on the fenders. Furthermore there are self made stickers with the model designation number.
The cab can be tilted to reveal the engine, a turbocharged V-10. Although there would have been plenty of room under the big cab, the engine mockup itself is squeezed in a small space between the two frame rails. The reason for that is that there is an old style 9V motor linked with a three stage reduction gear with a total ratio of 1:27 to drive the tandem rear axle assembly. There are two worm gears on the propellor shaft that directly drive the 24 tooth gears on the differential cages. This is a space saving construction and the biggest torque is produced in the final stage of the gear train.

Mercedes SK 3550, picture 4

The two front axles feature rack and pinion steering. They are linked with a common shaft with two different sizes of pinions - 14 teeth on the front axle and eight on the second. A 9V micromotor placed between the first and second axle and between the two frame rails drives the steering shaft with a one stage belt drive. Steering motion is transfered to the steering wheel in the cab which revolves when the wheels turn. While the cab is tilted, connection between the steering column and the common shaft is interrupted.
The Mercedes can be used in two different configurations to pull heavy loads. At the rear end of the frame in a lowered position there is a towing attachment. Furthermore there is a fifth wheel on top of the frame which can be placed in different positions along the frame rails to configure the truck for different trailer settings. When pulling a trailer connected to the towing attachment ballast is needed to make use of the mechanical advantage of the two driven rear axles. Therefore a ballast platform can be connected to the frame. On my model it is equipped with a box containing some tin and representing a reinforced concrete block. The weight of 1.5 kg is supported by the frame at three points - two on the front end of the platform and one at the fifth wheel. The ballast platform features openable side walls and a towing bar used when pulling or pushing trailers without a drawbar or when coupling more than one truck to move a bigger payload than one truck can bear alone.

Mercedes SK 3550, picture 5

Heavy tow tractors usually are quite short in length compared to standard trucks and feature up to five axles. Therefore there is often no room on the frame for all the special equipment that can be found on heavy duty trucks. These components are then put together on a rack which is placed behind the cab. My model features such a rack as well. First of all there is a large fuel tank at the center place. A standard tank would not be sufficient for towing applications as the 500 horsepower V-10 is quite thirsty. On the left side there is an additional oil cooling unit. On the right side are the exhaust pipes covered in the same way as the cooler on the left. Finally there are several air tanks for the breaking circuit placed on top of the fuel tank. A small platform behind the rack gives access to the different components and the hoses that connect to the trailer. On the model these are represented by the two wires connecting to the drive and steering motor respectively.
Finally the truck is equipped with a heavy duty front bumper complete with brushguard, register coupling and additional headlights. To be able to tilt the cab the brushguard can be tilted away as well. Further details include an air filter unit on the right side of the frame between the first and second axle and a fire extinguisher at the corresponding place on the left side of the truck. Axles two to four feature single fenders. Stickers with a candy cane pattern for the front bumper, license plates and the Mercedes-Benz logo complete the model.

Mercedes SK 3550, picture 6Mercedes SK 3550, picture 7Mercedes SK 3550, picture 8
Mercedes SK 3550, picture 9Mercedes SK 3550, picture 10Mercedes SK 3550, picture11

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Trailer

Mercedes SK 3550, picture 12

First there was the trailer. Although the description of the truck comes first on this page, the idea to build a modular trailer was there before I decided for a truck to pull it. Inspiration for my model I got from the products of several well known European trailer manufacturers. Therefore the creation pictured here does not represent a certain make or model of a modular trailer system existing in reality.
The key element to the trailer are the wheels. I used the Modelteam wheels with white rims of which I have 26. Together with two of the wheels with grey rims I was able to build seven axle lines with four wheels each. There are two modules with wheels - a three and a four axle line unit. Each axle line consists of two pairs of wheels which can independently oscillate around an axis in front to back direction regarding the trailer frame. Every wheel pair is steered. In reality there would also be hydropneumatic suspension that allows to level the trailer and lift it to take up loads or lower it to pass low bridges with a high load. It was not possible to realize a working suspension on the trailer as even the smallest pneumatic cylinders were already too big. Nevertheless the boogies of the wheel pairs look more or less like the boogies on real trailers.
Steering works with rack and pinion. A common shaft holds three different sizes of pinions on the smaller module and the same three sizes of pinions on the four axle modules. The third and the fourth axle of the bigger module have the same steering angle. The common shafts are being connected with axle connectors when coupling the different modules. The Technic axle is also present on the high bed module without wheels. Therefore a steering shaft passes the trailer from front to back with possibilities to access steering motion from both ends. For the rear end there is a bumper module with a flashlight which acts as a steering wheel. A drawbar module can be coupled to the front end of the trailer and forces the wheels to steer when the drawbar is turned sideways by the pulling truck. Usually it is needed to steer with both devices at the same time because the long steering shaft (10 studs per axle line) twists too much over its length so that the wheels at the end of the trailer would only steer a little bit when the first wheels have fully turned.
In total there are five trailer modules. A three axle module, 30 studs long; a four axle module, measuring 40 studs, a high bed intermediate module, 30 studs long and the rear bumper and drawbar modules. This results in several different combinations. First of all the wheeled modules can be used alone. They can be coupled together to get a seven axle trailer and they can be combined with the high bed module to get a flat loading bed with a total length of 100 studs which equals 15 meters in reality. Payload then is about 60 to 70 metric tons as the load limit per axle line is 12 tons. Coupling of the modules works with six stud long Technic axles going through the holes of two Technic bricks per module so that there are four bricks side by side at a separation point. To prevent the modules from being twisted around the longitudinal axis at the separation points there is a Technic pin at each end of a module which plugs in the corresponding hole on the counterpart.
Details on the trailer include rear lights and indicators, warning signs and lights, fastening points and a control pad on the drawbar module. Width is 18 studs which equals 2.7 meters in reality.

Mercedes SK 3550, picture 13Mercedes SK 3550, picture 14Mercedes SK 3550, picture 15
Mercedes SK 3550, picture 16Mercedes SK 3550, picture 17Mercedes SK 3550, picture 18

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In Action

Mercedes SK 3550, picture 19

Two of the different trailer combinations can be seen on the following pictures. In both cases the load consists of crane components of the Liebherr LTR 1800.
The first configuration sees the truck in action without its ballast platform. To get enough weight on the tandem axles the load - consisting of the three stage telescopic boom of the Liebherr - is coupled to the truck using the fifth wheel. At the other end, the boom rests on the combination of the three and four axle modules forming a seven axle platform roller. This method of transporting the boom can be seen in reality as well. It has the advantage that there is no dead weight for an extra long trailer stretching over the full length of the boom. Instead the load is self supporting and is an integral part of the trailer. To be able to pull the empty trailer after the boom has been unloaded by the crane, the drawbar module stays connected to the platform roller. This way it can be coupled to the truck using its towing attachment.
The second configuration consists of the three and four axle modules together with the high bed intermediate part. Thus a long and even loading platform is created ideally suited to transport the quite long but not that high upper structure of the Liebherr LTR 1800. Despite the height of the trailer bed above the road of 1.2 meters the overall height of the trailer and the load stays below that of the truck. This time the truck is equipped with the ballast platform and the trailer connects to the hitch of the Mercedes. Another possibility would be to use a gooseneck on the trailer and the fifth wheel on the truck.
The second configuration is also suited to haul the crawler frames of the crane. The last two pictures show this transport.

Mercedes SK 3550, picture 20Mercedes SK 3550, picture 21Mercedes SK 3550, picture 22
Mercedes SK 3550, picture 23Mercedes SK 3550, picture 24Mercedes SK 3550, picture 25
Mercedes SK 3550, picture 26Mercedes SK 3550, picture 27Mercedes SK 3550, picture 28
Mercedes SK 3550, picture 29

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