Tow Truck: 12 Pounds of Lego Technic

Cyrus Tabrizi, 7/10/12
In the Works (15)

      On the matter of gearboxes, the Tow Truck had three: a “main” one in the center of the truck, a second in the rear, and a third in the crane. The first of the three was squeezed between the second and third axles. This gearbox used four M motors to control six outputs. The manner in which it did this was simple and could be likened to the way it was done in the 8043 Excavator – where three of the motors each have their own red driving ring and the fourth M motor shifts these three driving rings between two sets of 16t clutch gears – this allowed each of the three motors to control two separate functions (a total of six for the entire gearbox), but this also meant that only three functions could be run at a time. The only difference between this gearbox and that of the Excavator was the unique way in which it was fit in the tiny and awkward space in the center of the truck. In between the two XL motors that drove the second and third axles, there was a distance of exactly 11 studs between their backsides in which the gearbox could be fit. To do so, a 2 by 2 configuration was used where two M motors were mounted on opposite sides of both XL motors. This allowed space for the necessary gearing to be put in the space between these motors. To take the six outputs of the gearbox and connect them to their respective modules on the truck, nearly a dozen universal joints were used. Three pairs of universal joints went to the front of the truck and two went to the rear. The sixth output was used for rotating the crane and, since it was located just above the main gearbox, gears – instead of universal joints- were used to transfer the output of the gearbox to the geartrain responsible for rotating the turntable. The front-left output was connected to the pneumatic switch. The front-middle output was used for the adjustable steering. The front-right was used for turning the RC motor on and off. Since this gearbox only allowed three of the six outputs (one of two sets of three) to run simultaneously, one could choose between controlling the adjustable suspension and pneumatic lift, or between using the other functions instead – either way, they could simply toggle between these functions by a simple flick of the switch.

Tow Truck (2)

      The second gearbox was located above the third and fourth axles below the tow arm. While the first gearbox used four inputs and had six outputs, the second gearbox used only two inputs to control four outputs. These two inputs are, in fact, the rear-left and rear-right outputs of the first gearbox. The rear-right input was used to select any one of the second gearbox’s four outputs. The rear-left input was used to power whichever output was selected. To switch between the four outputs, two driving rings were used. The rear-right input went to a worm gear and a 24t gear which rotated two sets of belt wheels. One bent liftarm was connected to the left wheel and another was connected to the right wheel – the 24t gear and four technic triangles were sandwiched between these wheels. Each liftarm was then connected to a changeover catch and these were in contact with their respective driving rings. Thus, when the rear-right input was driven, these wheels would turn and the liftarms would move in such a way that the driving rings could be manipulated. If done carefully, one output could be driven at a time, but it was also possible to have more than one output selected, thereby driving two functions at the same time. This could be considered an inconvenience at times, but is worth the ability to have additional functions. In the case of the second gearbox, these four functions were related to towing in one way or another. The front-left output was used to lift and lower the tow arm by driving two linear actuators in the rear of the truck. The front-right output controlled the extension of the tow arm by driving a worm gear, a white 24t clutch gear, and the corresponding gear racks – the clutch gear was not used for the typical reasons – it was used because, for one reason or another, its teeth did not catch on the axle joiners that held the worm gear in place. The rear-left output went to a third linear actuator which lifted and lowered the tow lift – its range of motion is extremely small. The rear-right output drove a winch which ran through the tow arm and served as a functional tow cable – it was driven by yet another worm/24t gear combo to ensure that the winch had torque and that it wouldn’t slip during use.