Tow Truck: 12 Pounds of Lego Technic

Cyrus Tabrizi, 7/10/12
Tow Truck (5)

      Finally, there was the crane that sat in the middle of the truck. Normally, if I were to build a medium-sized crane like this one, it would have lots of flexibility and function. To do this, I would make use of 4+ motors and minimize the use of gears as much as possible, because, depending on the crane’s design, space is typically too limited to have large and complicated geartrains. In this case in particular, the main body, where any such gearing would go, had to be small enough to rotate in a relatively narrow space between the compartment behind the cabin and the back of the tow arm. Additionally, only 1 PF channel was left for me to use for the crane – this meant that where I had wanted to have four or more motors, I would only have two. With all this in mind, I decided there was only one way to go: build yet another gearbox. Doing so quickly made the crane my favorite part of the truck simply because it was the hardest part to build – the difficulty came mostly from fitting the gearbox and the two motors into such a tiny space and figuring out how to transmit the output of this gearbox to the rest of the crane – the main body measured a meager 11 x 11 x 12 studs (W x L x H)! By definition, this gearbox served the same purpose as the one used for the tow arm – two inputs, four outputs- but, in form, the two were completely different.


       It used two M motors as inputs – one to select the output and the other to drive said output. The output-selection motor shifted two driving rings in the same manner as the gearbox that was discussed previously – that is, it used yet another worm gear and 24t gear to rotate two liftarms and, consequently, shift each ring back and forth between two 16t clutch gears. While the gearbox for the tow arm used bent liftarms, the crane gearbox used two 6L half beams instead. What made this gearbox so unique was how I built the gearbox around the two M motors – they were literally the heart of the gearbox and were squished between all the arteries and muscles they were powering.

       This system ultimately allowed the crane to have four functions: a motorized winch, and an arm with three degrees of freedom. All of these were powered through a combination of gears and universal joints. The first degree of freedom, for example, was done through the use of a single linear actuator driven from the arm – a universal joint carried the output from the gearbox to a few 12t bevel gears which allowed the linear actuator to be used upside down. The second degree of freedom, however, was powered by two linear actuators side by side that were driven directly from the main body. To do this, bevels gears were used to translate the rotation of a vertical axle output to a horizontal one – this horizontal rotation is one that could be transmitted to both of the linear actuators via a single axle. The third degree of freedom was powered similarly to the first one in that it was done by a single linear actuator – it too used a universal joint to transmit the output past the first degree of rotation, and also used bevel gears to change the angle of the powered axis to a horizontal one. Next, a series of 16t gears transmitted this rotation to the top of the arm where it could drive the linear actuator responsible for giving the arm its third degree of freedom. Since all three of the degrees of freedom lie on the same axis, this flexibility could be considered redundant and unnecessary.


       The fourth and last function of the crane was that of the working winch. I put the winch in the second to last section of the arm to avoid all the chaos that was going on behind it – this left me with one problem: how to power it. There wasn’t enough space in the first section to send another set of 16t gears up to the winch in the second section and this would not be suitable for powering a winch – I try always to incorporate a worm gear into any winch mechanism I build because it prevents the winch from rotating when it is not being powered – a string of 16t gears can, if not braced properly, -such bracing was not possible with the nonexistent space that was mentioned before-slip under pressure. The question still remains: how do I power the winch? Other than using a chain, or a string of gears, my only option for powering a winch that sits a varying distance from its output source was to power it through a bendable axle – seeing how this was out of the question, I did the next best thing: I powered it through a sliding axle – an axle that, at one end, remains fixed at a single point, but, at the other end, is able to transmit its rotation along any part of the axle – this can be likened to being able to slide a gear along an axle and still being able to turn it. This, however, only complicates things further since sliding such a gear on an axle causes a significant amount of friction – enough to make it difficult for the arm to bend. To kill two birds with one stone, I put a worm gear at the end of the axle and housed it between two triangles along with an 8t gear – this way, I could solve the issue of the winch slipping under pressure and that of friction on the axle – a worm gear slides effortlessly. The only problem I had to work out afterwards was determining the length of axle and mounting position that would allow the crane to bend to the full extent of its three degrees of freedom while still maintaining sufficient contact with the worm gear. The end product worked flawlessly, and, in my opinion, looked pretty cool as well. To balance the forward-bias of the crane arm, the battery box was used as a counterweight and was mounted to the back of the second section of the arm. Lastly, there is a small cabin for the crane operator that is mounted to the side of the arm.

More pictures can be found on Flickr.