42009 Mobile Crane with new gearbox




All the honors for this MOC go to Mowlwurf. I found his 42009 Mod - new gear boxes on rebrickable.

I like this kind of operating a gearbox: Technic like reality! The motors are always on. You use one switch per function to switch forward / idle / backward. It is the same principle as in Red Excavator.

With this new gearbox you now can operate all 6 motorized functions much more easier, independent and in parallel.
- Outriggers in / out
- Outriggers up / down
- Turn crane right / left (this is an additional motorization, in stock 42009 this function is not motorized!)
- Boom up / down
- Boom in / out
- Main winch up / down (geared up 3:1, but be careful: there is no more clutch gear involved. If the winch is rolled up, it continues to pull further and may stress your construction)

I think this is a big improvement to the wonderful 42009 Mobile Crane. This was my first LEGO set starting my AFOL activities after my dark ages. It will always be a special one for me.

I made some minor modifications for a smooth flow in the gearboxes, but I kept the original idea and design from Mowlwurf.
And I geared up the main winch by 3:1, because it was too slow for me.
And I used the old technic metal hook . I like it, but it is rare and expensive. So you may also build it with the hook in stock 42009 to save money.

You need 235 additional parts compared to 42009-1. Use rebrickable's compare feature to get your missing parts. 

Includes 380 pages step-by-step instructions as PDF made with LPub3D.

Change Log:


Swapped 1 gear to lift the boom as suggested by TechnicBRICKs.


Major change in the main gearbox of the crane:

When playing with my first version I detected that the motors often get too weak to lift the boom and stopped moving. You needed to switch off/on batteries to get them back to work, but only for a few seconds and they stopped again. I analysed the problem and found two main causes:
- The heavy load of the boom (especially when fully extracted and in low height) put a very strong force to the gearbox and squeezed all gears to the beams leading to high friction.
- The torque of each motor is limited, if two motors are connected in parallel to one battery box.

So I redesigned the complete gear train using only one motor for all functions and skipped out 6 gears by using a direct and short link from motor to gear switches on the lower level of the gearbox.

I removed the 3l axle pin connector between the 2 linear actuators, because this part pushed the force to the gear behind when the boom is under load and caused a lot of friction.

Now it runs very smooth with only 1 motor and has enough power to lift "heavy" loads.
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Name 42009 Mobile Crane with new gearbox
Designer jb70
Designed 2019
Inventory 2730 parts
Theme Technic
Modification of 42009-1 Mobile Crane MK II
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