[MOC] Linde P250 Tow Tractor and luggage carts build log

[johncarpenter]

Hello everyone, I want to introduce my new project: The Linde P250 Tow Tractor:

 

This is only my third project after returning to Lego Technic after 20 years, so I consider myself a beginner as I have little experience with studless building. After my last project (RC skid steer) where I had a lot of headaches fitting everything in, I wanted to build something simpler for my next project. I picked the Linde P250 as the reference for this build. The starting goal was not to have an accurate scale model, but to have a simple remote controlled model that could drive around. The real one has two versions, a short and long wheelbase. I picked the long wheelbase version to have more room for internals. According to the scale I chose, the model was going to end up 13 studs wide, which looked tight but doable. As I only planned to have driving and steering, I thought that this should be fine. After picking some suitable wheels and printing out the blueprint, I started off with the exterior design, sides and front first.

I was studying pictures and videos of the real thing and this is where I started to add some things to the original plan. There is a lever in the cabin that locks and unlocks the trailer coupling. Using this, the operator can grab or release trailers without getting out of the cabin and walking around to the back. Also, I saw that this particular model is available with normal or sliding doors, so I wanted to try the sliding of course. Finally, the tractor can be driven (very slowly) forwards or backwards from outside using a pair of buttons, again to make working with trailers easier. I knew that this function was going to be challenging, due to the limited space available at the back of the vehicle. There goes the idea of keeping it simple this time...

I started to redesign the sides to have sliding doors. There was a tricky point with the doors: On the real thing, the door has a piece which closes the gap between the bottom of the sliding door, and the cabin floor. If this piece were to be fixed to the door, it would interfere with the front wheel when the door is slid back. Therefore, it swings into the door and disappears when the door is slid open. I had to either make something similar or leave a gap there.

My first version of the sliding doors worked, but had two issues: The door slid back only 5 studs, which seemed too little and the swing up piece did not work great.

I iterated on this to fix both problems: I replaced the flap thing, and played around with the geometry of how it slides up. Also, I changed the vertical door post and the rail support at the top of the door to be half width linkarms. Now, the door was able to slide 6 studs, and the swing up piece looked good and worked reliably. I did play around with making the door open even wider, but it looked like anything more than 6 studs would mean a lot of other compromises, so I settled on that.

I also worked on the front: The first iteration looked way too narrow, so I moved the headlights outward:

In parallel, I was looking at how to package the internals. I wanted to use the normal AA battery box, but at 11 studs wide, it was challenging to fit. The most efficient way seemed to be to have it in the middle of the chassis, at the bottom. That way, it fit exactly between the side panels, and even protruded half a stud below the chassis, saving space by cheating slightly.

In researching the pieces I would need to build this in red, like the original, I realized that some of the thin liftarms I used on the door were not available in red. The issue if I used normal liftarms was that the door would slide just 5 studs. I redesigned the mechanism, to use a normal liftarm for the vertical part of the door, and used a short liftarm (yellow in the picture below) which was able to slide 1 stud under the roof. This way, the door was able to slide 6 studs, and did not need the thin liftarms. I also redesigned the sides, using 5x11 panels at the bottom, which improved both the looks and the structural stability, especially of the battery box mounting.

With the exterior starting to come together nicely, next up was to see if I could fit all the functions inside. I wanted to have an L motor for driving and a servo for the steering. For the "driving slowly using buttons on the side of the vehicle" function, I needed an M motor, a clutch to switch the drive to this motor, some gearing down, and a PF switch. I had selected the scale before deciding to add this feature, and it was looking quite unlikely that everything would fit inside.

This is where I am at right now. At this point, I'm thinking that I either need to go from a width of 13 up to 15 studs, or give up on the "button driving" function. I do think that feature adds a lot to this relatively simply model, so I would really like to keep it. Actually, widening by 2 studs has a lot of advantages at this point: Currently, the space available above the battery is exactly 9 studs wide. If I widen to 15, that would give me 11 studs of room in the top half, which means I can move the battery to the top. This would then free up 13 studs of width below, for the motors etc... As I still don't have a front axle, I think I will build that first with the servo. Based on that, I can see exactly how much space I have left and then decide if I will widen the model.

Edited March 8, 2021 by johncarpenter
updated title

[doug72]

Pictures cannot be seen, read FAQ on how to embed them.

[Mechbuilds]

Those image links look like deep web sites 

[johncarpenter]

Sorry, pictures should be fixed now...

[johncarpenter]

Here is the next update on this build:

I decided to widen the model to 15 studs, to have any chance of fitting everything in. With that change, I could move the battery up, and the mechanicals down.

The deciding factor for the layout was the clutch mechanism to switch between the main motor and the "button drive" motor. I started out with an L motor for the main drive. Due to the space available, I was not able to fit in any gear reduction, and it ended up far too fast and jumpy. As there was no available space for any other gearing, I decided to switch out the L motor with an XL, whose speed is about half as much as the L. This made the driving much steadier. The obvious place for the servo was in the middle, and the right side of the model had the M motor for the "button drive" motor. I started out trying to hook up the M motor using normal gears. However, it was difficult to get enough gear reduction in the restricted volume while still having enough gear reduction. Finally, I tried out a worm gear, which worked out much simpler and easier.

The way this function works is as follows: The clutch just forward of the differential takes power from either the L motor or the M motor. The L motor for normal driving is connected to the IR receiver. The M motor for the button drive is connected to a PF switch. To switch to the "button drive" the yellow liftarm sticking out of the left side towards the back is pulled out. This switches the clutch to the M motor.

Then, the liftarm can be tilted back and forth, for driving backwards and forwards. The red sliding 8t gear engages a 24t gear below, which is connected to the PF switch below (barely visible in the photos). The two small 2l red liftarms prevent the tilting when the liftarm is not yet pulled out to enable the function. I was really happy with how this feature came out.

The last feature is the trailer coupling at the back. I made a little handle which can be used to open or lock the coupling. Also, the function is forwarded all the way into the cockpit at the front. I will add a level there so that it is also possible to actuate the function from the cockpit.

After breathing a sigh of relief that all the functions fit inside, I completed some of the exterior details. I started on the load area, and added a swing up part, which will be used to access the battery box. This is pretty close to how the real thing is.

This is how the whole model looks at the moment.

Next, I will add some seats and other details to the cabin, and finish the shallow load area. Many of the details still need the pieces in the correct colors, most of which are on the way. The one headache is that the curved 4 long piece I used at the back corner is super expensive in dark bluish grey. I checked the similar but 2 long piece, but that one is not available in dark bluish grey at all. I looked around for other pieces that have the same curvature, but was not able to find any part that would replace it. Does anyone have ideas on this?

[Jundis]

I really like the style of this moc with all the clean lines but I don't see the problems for integrating a 2 speed gearbox (like this for example). A second drive motors makes the vehicle heavier and runs without use half of the time...

[johncarpenter]

2 hours ago, Jundis said:

I really like the style of this moc with all the clean lines but I don't see the problems for integrating a 2 speed gearbox (like this for example). A second drive motors makes the vehicle heavier and runs without use half of the time...

Thanks for your comment. I had the same idea about using only one motor for both normal driving and "button driving". The 2 speed gearbox could definitely be done. However, the part which I could not figure out is how to hook up the same motor to be driven by both the IR receiver and also the onboard pf switch. I asked about that in this thread, but it looked like this was creating more issues than it solved (2 switches etc...). I still might try that when I get an extension cable. If it works, I will rebuild it with a single drive motor as it is the more elegant solution.

[brickless_kiwi]

I really like the clean and tidy look of this moc. It's very well done. I spent a lot of time in my youth on Harlam tugs more for aircraft than baggage trolleys. Brings back some very cool memories. 

[HUWI]

just awesome!

It wouldn't have occurred to me to build something as banal as an airport baggage tractor.

And then also with the drive button ... Cool thing!

 

The airport topic is also on my agenda for later, then at least I'll know where to look for the tractor ...

[trekman]

That is a very neat looking tug. Very clean lines.  If this is your third attempt after a 20 year gap, then you are doing very well.  There is a lot of catch up in 20 years.  All that I would suggest is to build plenty of official models also, as you will pickup ideas and that you would take a long time to discover yourself.  Well done.

[johncarpenter]

Thanks for the encouraging feedback. I've gotten all the pieces in the correct colors, so the exterior is now finished.

 

I have done some work on the interior, but will probably refine it a bit more. The lever in the cabin to lock or unlock the trailer coupling is now working, you can see it between the seats.

Also, I want to do a matching luggage cart. Unfortunately, these two will have to wait a bit as I will be moving in the next months, and my legos are now packed up.

[Jundis]

Again, I love the clean looks of your build, the cab is so nicely done!

One suggestion to make the loading are smother: Maybe use such a bracket underneath the tiles?

Another question: Can the doors only slide open, when the wheels are not angled?

[HUWI]

Der ist echt toll geworden!

 

@Jundis die Türen gehen doch über das DBG, was meinst du, das Orginal?

Im Orginal ist das Loch vorm Radhaus ein Fenster, ander hintern Spitze schwingend gelagert ist. Schiebt man die Tür zurück, drückt das Radraus es hoch in die Tür.

P.S.: Die Orginalversion im Startpost hat die Tür mit Scharnieren an der A Säule angeschlagen und hat unten kein Fenster.

------------------------------------

It turned out really great!

 

@Jundis the doors go over the DBG, what do you think the original?

In the original, the hole in front of the wheelhouse is a window, the tip of which is supported swinging. If you slide the door back, the wheel pushes it up into the door.

P.S.: the original version in the start post has hinged the door on the a pillar and has no window there.

Edited August 31, 2020 by HUWI
P.S.:

[johncarpenter]

Yes, there are two versions of this vehicle, one with sliding doors and one with standard doors. As @HUWI mentioned, the first two pictures of the original show the standard, swing out doors, sorry for the confusion on that. I could not find a clean picture of the one with sliding doors. The third picture in my first post shows the detail of the sliding door version.

I tried to capture how the sliding door mechanism works, by taking out the seats: The grey part is hinged and offset 1 stud to the inside of the red door frame. When the door slides back, the grey part rides up on the wheelwell and pivots up:

This is how the grey "swing up" part of the door looks from the outside.

 

@Jundis, yes, I tried those brackets as well, but somehow it did not work out so well. I cannot remember exactly why, maybe the bottom edge did not look good. Anyway, after the move, I will try that, it could be better with those brackets.

Edited August 31, 2020 by johncarpenter
fixed broken gif

[HUWI]

I haven't seen the third picture, it's great! You've done a very good job with this tractor!

[johncarpenter]

After a long long pause, I was finally able to finish this project.

Since the last update, I built two luggage carts for this guy to pull.

I was not so sure about making luggage pieces. I tried out some of the existing Lego ones, but none of it really seemed to suit this project. Then, with a nudge from my significant other, I started to make a couple of suitcases. She then joined me, and we made many, many suitcases.

At this point, I wanted to take some pictures, make a video and call this one done. However... I saw the new Lego Technic Jeep Wrangler, and a new part: the 3x7 panel, in yellow. This triggered an idea I had had at the beginning of the project: The Linde P250 has a short wheelbase or long wheelbase option. At the beginning of the project, I had chosen to go with the long wheelbase option, as it would mean more space for internals. Another reason for that choice was that the 3x11 panel fit very well as the bodywork on the side. Now, with the 3x7 panel, doing a good looking short wheelbase version would also be possible. Knowing how tightly packed the long wheelbase version was, I knew that I could not fit all the same features into a shorter package. The obvious thing to drop was the button driving feature, which would save a lot of space and complexity. Also, I decided to do the lower half of the body in black instead of dark bluish grey, due to better part availability.

Here are some final pictures:

Finally, here is a video of them in action, hope you like it :)

I'm really happy with how this project came out. The tow tractors look good, work well, and the bigger one has the extra button driving feature, which is quite original. To be honest though, the most fun part of this was building the suitcases. There was something addictive about making one suitcase after another, adding this or that detail to each new one. It was so much fun, I'm thinking about what other projects I could do, that would utilize such a small repetitive element. Perhaps some other airport equipment might be in my future.

Thanks for reading, hope you enjoyed it!

Edited March 8, 2021 by johncarpenter
fixed youtube link

[Brickend]

Very, very nice MOC - it's great to see an under-appreciated vehicle get the Technic treatment.

Also like the iterative suitcase design process; smaller projects certainly 'evolve' quicker.

[Jundis]

Just love this moc! The looks are stunning, it has perfect functionality for this size and it works great. Bravo!

One small thing about your video though at 0:12: Have you never paid attention at the airport? The correct way to put the bags in the trailer is to throw them as unpleasently as possible!

[johncarpenter]

Thank you both for the feedback.

Yeah, I forgot about that when I was making the video. If I build something else related to luggage handling, I'll definitely keep that in mind :)

[MajklSpajkl]

Excellent and very fresh! Cool features and great looks. The beutton drive function seems really convenient for hooking up the trailers. I really like the slidding doors. With two trailers, it must be really fun to drive around. Excelent job on the baggage as well.

Great job overall!

[johncarpenter]

Thanks! Yes, driving around with two trailers was fun :) It required an enormous amount of space, though, especially for driving completely out of the frame on some shots. I had a couple of stressful moments where the whole ensemble almost fell completely off the surface. Luckily though, no bricks were harmed in the making of this video :)