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Pushback Tug

August 8th, 2011 Leave a comment Go to comments

Model of a pushback tug. Features a powerful propulsion system, elevated cabin and lights.  

Datasheet:

Completion date: 07/08/2011
Power: electric (Power Functions)
Dimensions: length 40 studs / width 16 studs / height 35 studs
Weight: 2.07 kg
Suspension: none
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.

Photos:

1.jpg 2.jpg 3.jpg 4.jpg 5.jpg dsc04860.jpg dsc04865.jpg inspiration.jpg

Video:

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  1. qwertyuiop
    January 1st, 2012 at 00:25 | #1

    im planning on using the same wheels as you have here, as they are the biggest ones i have. there will be a clamp on the back that will secure the load to pull it.

  2. Sariel
    December 31st, 2011 at 09:50 | #2

    @qwertyuiop
    It should pull quite a lot. Much depends on what wheels you use and how much weight will be on these wheels.

  3. qwertyuiop
    December 31st, 2011 at 05:50 | #3

    im going to use a modified version of your 2 speed heavy duty gearbox to make a tug with 24:1 and 40:1 ratios. how much weight do you think 40:1 will be able to pull, with 2 xl motors as propulsion? (the model will only be around 1-1.5 kg)

  4. Gavin
    November 9th, 2011 at 22:58 | #4

    yeah the design of the pushback tug uses the weight of the aircraft for the traction. that’s why it’s so effective

  5. qwertyuiop
    November 7th, 2011 at 22:11 | #5

    thanks heaps

  6. Sariel
  7. qwertyuiop
    November 7th, 2011 at 09:12 | #7

    what type of wheels did you use?

  8. Sariel
    August 26th, 2011 at 22:44 | #8

    @mindstorm addict
    I just wanted to make sure the torque will be sufficient. Speed was of much lesser importance.

  9. mindstorm addict
    August 26th, 2011 at 22:44 | #9

    @Sariel
    I know, I’m asking this beacus you could have geared down it less

  10. Sariel
    August 26th, 2011 at 22:22 | #10

    @mindstorm addict
    I don’t have NXT motors.

  11. mindstorm addict
    August 26th, 2011 at 21:52 | #11

    I geared down two nxt motors at 3:1 ratio and It capacities was limited only by the wheels traction so I think you could have geared up a bit, or did you wanted him to be very slow?

  12. shawn
    August 16th, 2011 at 02:00 | #12

    I have 6 of them, but none of the bigger ones.

  13. Sariel
    August 15th, 2011 at 22:04 | #13

    @shawn
    No.

  14. shawn
    August 15th, 2011 at 22:01 | #14

    Have you used these wheels much? http://www.bricklink.com/catalogItem.asp?P=41896c03

  15. Mike
    August 10th, 2011 at 19:15 | #15

    http://www.youtube.com/watch?v=JpxNlPwMdRM&feature=related
    watching this, you can see the tug hopping while driving. This suggests that it doesn’t have any suspension whatsoever.
    This hooking-up mechanism is a bit different from what Sander’s video link shows.

  16. Sariel
    August 10th, 2011 at 00:06 | #16

    @blakbird
    Thank you Eric. I’m pleasantly surprised that someone connected with the aircraft industry speaks well of a model which is not particularly accurate or functional. Anyway, I hope my next step is finally a model of a plane.

  17. August 9th, 2011 at 19:10 | #17

    Very cool build! I’ve never seen someone do one of these before. Being in the aircraft industry, I know a few things about these vehicles. They are not used only for safety and fuel economy, but mostly due to the fact that airplanes cannot back up! The only way to move an aircraft back from the gate is to push it. This is usually performed via a “towbar”. A towbar is about a 3 meter steel link which connects to the steerable nose wheel of the aircraft and to the pushback vehicle. The nose wheel steering is disconnected and put in caster mode while it is being driven externally. The type of vehicle you have built is only recently invented and is called a “towbarless tow vehicle” because it does not require a towbar. Instead, it actually lifts the entire nose wheel off the ground and treats the airplane as a trailer. This allows faster airplane movement, but also means the two vehicle needs to be much heavier. The big ones, when loaded, can weigh over 100,000 pounds (50.000 kg).

  18. Sander
    August 9th, 2011 at 13:36 | #18

    Hi Paul,
    Considering you built this for torque reasons, it’s a great performance. Might there ever come a push tug 2.0, some hints and tips.
    This summer I’ve made many flights with KLM and watched the tugs as boarded the plane (737-300/700’s). Please note that in the real tug, your white bricks are in fact empty space, but I recon that’s why the bricks are white.
    Now the driven axle is steered as well, luckily because of the aft push tug wheels needing to lift the entire (front side of the) plane and being attached to flimsy arms, there’s simply no space for steering. So your driven axle, should have been steered as well (although your torque experiment wouldn’t allow this I guess).
    Concerning the mechanism, the photo of the blue KLM-push tug is pretty clear. It’s in three steps:
    1. tug drives up to wheels and drives the small black rubber blocks against the wheels (small cylinders).
    2. The aft yellow clamps fold around the wheels and restract so the nose wheels are held snuggly and tight.
    3. With both sides held firmly, the nose wheel can be lifted as a whole; the hoisting mechanism and hinge can be seen just in front of the first (step 1) holding mechanism.

    You can find some movies on it on youtube that show the abovementioned. For instance: http://www.youtube.com/watch?v=CKCd-mHC31Q&feature=related
    Building this reallistically would be a new challenge for your ventures, but you’d probably have to skip the power/torque goals you’d set yourself now. Besides the lifting mechanism you’d have to build a Batman Tumbler-like or ‘chain drive’ like chassis.

    But for this project? Great Work Paul, Also impressed on how you’ve combined the impressive technique with great esthetics.

  19. technikfreak
    August 9th, 2011 at 11:49 | #19

    nice work. now you could buildt an boeing 747 for pull ;)

  20. Sariel
    August 9th, 2011 at 00:29 | #20

    @Mehley
    Yes.

  21. Mehley
    August 9th, 2011 at 00:08 | #21

    Did you used worm gears to gearing 24:1?

  22. Emil
    August 8th, 2011 at 21:53 | #22

    Really nice Sariel. To tow 20 kg is unbelievable.
    Keep up the good work..

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