Model of a pushback tug. Features a powerful propulsion system, elevated cabin and lights.
Completion date: 07/08/2011
Power: electric (Power Functions)
Dimensions: length 40 studs / width 16 studs / height 35 studs
Weight: 2.07 kg
Propulsion: 2 x PF XL geared 24:1
Motors: 2 x PF XL, 1 x PF Medium
Pushback tugs, also called pushback tractors, are low-profile vehicles used to push and tow aircraft around the airport. They are in common use, as using a pushback tug is cheaper than having an aircraft move under its own power, and safer than having an aircraft move using jet engines. Pushback tugs come in two variants: the older one, with tractor connected to the nose gear by a tow bar, and the newer one which scoop up the nose wheel and lift it off the ground, effectively attaching themselves to it. I have chosen to model the newer variant, which is much more complex and more interesting. It was not modelled after any particular make of a pushback tug, just after the typical look of a vehicle of the newer variant, as I wanted to focus on performance rather than on accurate look.
My goal was to make a model capable of towing 20 kg of load on a wheeled trailer. The trailer was essential to reduce the friction – according to my tests, the model was able to tow roughly 6 kg without a trailer, with the load being simply moved on the floor. As I was unable to determine whether the real tugs have suspension systems of any kind, I decided not to use any suspension in the model. There are two reasons for it: first, the suspension of any kind would make the drivetrain more likely to fail under tension, and second, a suspension system seems needless in a vehicle designed to drive solely on the perfectly flat areas of the airports.
The front axle of the model was steered and the rear axle was driven. The model steered poorly, as its center of gravity was close to the rear axle and as the rear axle had no differential. The propulsion system consisted of two PF XL motors geared down 24:1 each, generating a total torque of 696 N.cm, that is 48 times the torque of a single non-geared PF XL motor. The rear axle was reinforced to handle this kind of torque: it consisted of a number of short axles rather than a single long one to limit twisting, and the gear wheels on it were doubled and connected together with four pins each in order to lower the risk of physical damage.
Tests ran on the model have shown that its construction can handle the required tension (it was built around a massive frame, and the area around the rear axle was heavily reinforced) and that it has plenty of torque (I was thinking of adding another two PF XL motors if needed). Thus the limit of its towing capacity depended only on the traction of the rear axle’s four wheels. I’ve equipped the model with massive body and installed six weighted bricks around the rear axle to make the model heavier and thus improve the traction. The weighted bricks added extra 0.33 kg while taking relatively little space.
The body of the model was simple and had little functions (I decided to make the nose gear attachment mechanism non-functional, and one of several reasons was that it was very difficult to observe how it works in real vehicle). The cabin was elevated, just like in many real tugs, using a motorized scissor lift mechanism and a single small linear actuator.
The model was very simple, but turned out able to achieve its goal: it towed the trailer with 20 kg of load on it. At this point the model was already having problems with traction, depending on the area of the floor, and the trailer was so heavily burdened that its wheels kept slipping off axles every couple of minutes. I was afraid to physically damage the trailer, even though it was built around two layers of massive 16x8x1 bricks, and hence I did not test heavier loads. It would be interesting to see how much such a vehicle could handle if it was carrying the load directly on it, as this would ensure sufficient traction, but such a solution involves plenty of problems resulting from a huge load being placed on the model’s structure. I suppose it is possible to make a Lego vehicle that can carry 20 kg of load on it, but it should be probably significantly different, e.g. with tracks or with wheels with no tires on them.