Tow Truck: 12 Pounds of Lego Technic

Cyrus Tabrizi, 7/10/12
Tow Truck (1)

      The Tow Truck is my largest and most complex creation up to date. It features remotely-controlled all-wheel drive, all-wheel steering, adjustable suspension, pneumatic stabilizers, four-function rotating crane, motorized tow lift, and a working tow arm. All in all, it has 14 RC functions (even though the Power Functions system only handles 8) and weighs a total of around 12.2 pounds – roughly 80% of all my parts combined!! Physical construction began during the last week of 2010 (planning and thought was put in the three months leading up to winter break, and was finished sometime in June of 2011 (Although it took half a year from start to finish, most of the technical portions were finished in the first two months of its construction). Upon its completion, it was entered into the Lego Technic Challenge "Build a Cool and Tough Crane" (Theme for June, 2011). It was one of the ten entries that were picked by Lego's judges to enter the public round of voting but it did not make it past this round.

      In total, the model used 4 XL motors, 6 M motors, and 1 RC motor. All this power-hungry machinery was powered by three battery boxes and controlled by 5 PF receivers -“5 receivers?” you might ask. Yes, when a model is more than two feet in length, one must use extension cords and when, in my case, more than two extension cords are needed, then it’s time to get creative. I determined logically that if a second receiver were to be connected to the output of another receiver, then it could serve the same purpose as an extension cord under the following parameters:

Conditions for Using a Second Power Functions Receiver as an Extension Cord Replacement:

      1. The first and second receivers must be set to the same channel as the remote and the second receiver must be connected to one of the first receiver’s outputs (for the purpose of this explanation, the second receiver will be attached to the red output of the first receiver). Additionally, the first receiver must be connected to an appropriate power supply, and there may not exist interference or disruption that would cause the two receivers to receive different IR signals from each other.

      2. The red output of the first receiver (or whichever of the first receiver’s outputs the second receiver is attached to) must be turned on. If this and Condition 1 are met, then the second receiver will be turned on and the red output of the second receiver will respond in place of the first receiver’s red output – the second receiver will have served the same purpose as having an extension cord be connected to the first receiver’s red output.

      3. The red output of the second receiver must be on (Conditions 1 and 2 have been met) for the  blue output of the second receiver to correspond with that of the first receiver. Neither of the second receiver’s outputs will work (the second receiver will be off) if the first receiver’s signal is blocked or if its red output is not on, but, conversely, the first receiver’s blue output (or whichever output is not being used by the second receiver) will work regardless of whether the second receiver’s signal is blocked or not.

      It is important to note that using this fifth receiver as an extension for another does not mean that all the outputs of these two receivers can or should be used to power a motor –each receiver is still limited in the number of motors it can power at one time. Subsequently, only one output on the first receiver was used to power a motor - the second output was used to power the extended receiver- and only one output of this extended receiver was used to power a motor (as explained previously). Additionally, this arrangement (that of using a fifth receiver) does not increase the number of channels or functions the system can handle (a total of eight) – it only means that two receivers are running on the same channel and receiving the same signal. To exceed eight RC functions in this model, I decided to use three gearboxes -two in the truck and one in the crane (this will be discussed in depth later on).

8Wheeler 8258

      While the platform is loosely based on the platform of my Modular 8 Wheeler (shown above on the left), my inspiration for this model came from my purchasing the 8258 Crane Truck (shown modified above on the right). At the point in this hobby at which I bought this set, I had not built anything nearly as complex as that truck. It was interesting to have so many functions controlled by a single M Motor and, despite the, in my opinion, relatively poor performance of some of these functions, the set was somewhat fun to play with and roll around –this, in fact, was more fun for me than the rest of the motorized functions simply because the decently large truck moved so smoothly. I immediately thought to motorize the steering and the drive train with an M and an XL motor, respectively, and I succeeded to some extent in my quest. I didn’t want to tear the model apart so I replaced the engine block with the XL and put an M motor at the rear of the cabin (my efforts are documented on Brickshelf) – it worked but I wasn’t content with the result. After playing with the crane truck for a few days, I destroyed it -the reason I bought it in the first place was because I thought it would give me the best bang for the buck considering the number of linear actuators, 11L gear racks, and specialized gear train pieces it had. After building and taking it apart, I had this picture in my head of the crane truck rolling on these enormous Power Puller tires with huge travel in the suspension, all-wheel steering, large stabilizers, and some sort of legit manipulator – i.e. tow arm or crane – I couldn’t get it out. One of the first things I did was to draw a picture that gave a crude representation of the truck after making these modifications.

      That is what I had planned for the Tow Truck. Unfortunately, Power Puller tires were (and still are) out of the question for their weight and cost – the latter more importantly, but their weight and their size becomes an issue when trying to motorize a huge truck with 8 of them – it may be possible now with the new portal hubs, but this piece wasn’t even in the drawing board – I would think- when I was working on this model. So instead of those tires, I went with the newer 94.8 x 44 balloon tires from the 8297 Off Roader. I had wanted to build an RC 8x8 vehicle since before this project and the first step towards doing so was building the Modular 8 Wheeler that I mentioned earlier – I had ordered four more tires and the pieces required to build the suspension and steering assemblies (universal joints, steering links, CV joints, steering arms etc) from Lego Direct to complement the four assemblies and tires that I had from the Off Roader. With this, I built the Modular 8 Wheeler as a test bed, destroyed it, and began construction on the Tow Truck.

In the Works (25)

      The base of the truck has four axles, each consisting of its own steering and suspension assemblies. In this, nothing really varied between the axles. One difference is that the rear two axles were flipped around, putting each of their steering racks behind their respective axles whereas the front two axles have their steering racks in front of their suspension assemblies. All eight wheels were steered via one axle that ran along the very bottom of the undercarriage up until the front of the cabin so the rear two axles were flipped to have them steer in the opposite direction as the front axles, improving the truck’s steering radius. Originally, I was planning to use the only four XL motors that I have for the sole purpose of powering the truck’s drivetrain – this would mean giving each axle its own motor. I knew that I would have to power the steering with something that had a lot of torque so I tried gearing down an RC motor with a worm gear and some 24t gears but I found that the setup would not be enough to steer the entire truck through that one axle. Instead, I decided to drive the rear two axles with a single XL motor and use the fourth XL motor as the steering motor. This remained at the front of the truck below the opening cabin and fake V8 engine (behind the V8 engine, there is a small V2 that could almost be thought of as a fanciful turbocharger of some sort).

Tow Truck (18)

      In addition to having motorized drive and steering for all eight wheels, all eight wheels have adjustable suspension. This is done in nearly the same fashion as is done in the 8297 Offroader where two 2L beams (hole and axlehole) are attached to each shock absorber. At the hole-end, they are connected to the shock absorber by a 3L pin, and, at the axlehole-end, to a motorized axle. There are four such axles, two in the front and two in the rear. Each axle handles two wheels and each pair of axles is rotated via knob gears and a worm gear. The worm gear in the front and the worm gear in the rear are, in turn, driven by a single axle, connecting the front suspension to the rear.  This axle is driven by one of the truck’s three gearboxes. When this function is selected, the output rotates, slowly adjusting the angle at which the shock absorbers are mounted to their respective steering arms and raising or lowering the steering arms as well. This actually does two things: it adjusts the riding height of the truck and it adjusts the stiffness and travel of the vehicle’s suspension – this is all the result of changing the geometry of the shock’s mount. Although each of these characteristics cannot be adjusted independently of each other, the vehicle’s ride height can be adjusted despite the weight that the suspension is supporting. The only condition here is that the ride height can only be raised when the vehicle is off the ground – the mechanism responsible for adjusting the suspension does not have the power by itself to lift twelve pounds of truck. While it cannot accomplish this feat alone, it can, however, do so when done in conjunction with the extension of the truck’s four pneumatic stabilizers. When these two operations are run simultaneously, it is possible to remove the wheels from the ground and rest the entire weight of the truck on these four stabilizers. Doing this is no easy task and it requires a significant amount of time for the truck to lift, but, if conditions are right, it normally manages to do so.

Tow Truck (15)

      The pneumatic stabilizers consist of four pneumatic cylinders connected to a pneumatic switch in the compartment behind the cabin. This switch is controlled remotely via one of the outputs from the main gearbox (will be discussed again later). The gearbox outputs a rotational force which is transferred to a worm gear which is connected to the switch via some lift arms. To power the four cylinders, an RC buggy motor drives a worm gear which then spins a belt wheel connected by a pin to a pneumatic pump. This arrangement is not ideal and barely provides enough pressure to lift the truck but was picked because I had no pneumatic T piece to use a second pump (all of the ones I had were used to connect the cylinders in the first place). If I had had another one, I would have used two RC motors and two pumps – I actually tried this setup and it ran much faster and much smoother – the pumps were connected oppositely to lower the tension caused by the pumps’ coils – with only one pump, such an arrangement is not possible. To turn the RC motor and pump the cylinders, yet another output from the main gearbox was used. This output was connected to a PF switch which was connected to the RC motor via an extension cord – this cord was not needed as an extension but simply as an adapter for the old connection type present on the RC motor. All of this was housed in the compartment behind the cabin.

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.

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.