AC120 Demag carrier is a 60 metricTon truck, with superstructure and boom. There is 5 axle. For scale reasons, I don't respect, in this version, the truck wheel configuration. Only 2 driving axles and 3 steering axles. The superstructure is centred between driving axles.
Driving axles are equiped with
double wheels. Chassis is symetrical around the turntable. Of
course, the front axle (not yet done on the picture) need to steer
stronger than the rear ones : all the steering wheels describe
circles with same center.
Direction rack are drive by 8 tooth pinion.
Steering axles aren't very well fixed to the chassis, due to its
limited width.
A big difficulty in this kind of crane is to assure a real rigidity between carrier and superstructure. Unfortunately, we don't have Lego big parts for such big models. Charge on top of boom generate big torque. You can't simply use Lego turntable, but you need to increase lever arm to reduce efforts apply on Lego parts. I use tile flagstones on top of carrier to support superstructure.
By doing this, the turntable
only support a vertical effort, instead of a torque. Despite of
this improvement I need to fix very strongly the turntable to
the two elements.
Turntable driving is also critical : all Lego gears are in
plastic and warp under effort before supersturcture accept to
turn. Then boom hardly shock during rotation. To manage the problem,
I drive the turntable with 2 eigth-tooth pinion.(see the picture)
All crane trucks use outriggers
during lifting. Usually the carrier take of the ground, only lift
on its outriggers. Hydraulic jacks adjust the crane position to
be perfectly horizontal. Unfortunately, Lego jacks are pneumatic
and not strong enough while a hosting. On my model you need to
install shoes handly.
Version 1
First version is a simple technic beam
with rack and pinion. The two sides are synchronized with same
drive.
But efforts will be very stong in a simple beam, and the rack need to be hide in the chassis. Then it limit a lot the stroke, and the removable part is big.
Version 2
Outrigger is now made of 4 technic beams. Stroke is bigger but rack and pinion disapear. Removable part is small and easy to carry on the truck.
DecorationColor choice is really important : you need a big amount of parts for such structures.
Internal chassis parts are grey, carrier is black and red, superstructure yellow, and booms black and yellow. The result is a bit clumsy, but not far from the original truck...
Design problems on this version
I've some design defects : first of all, chassis isn't rigid enough. It's due to several construction details. I've done a wrong choice while assembling beams, I didn't respect right stacking, and its hard to lock the beams. Some transmission axles (driving, steering...) go through the chassis and reduce rigidity.
Steering doesn't work properly : efforts are too important and position of front and rear wheels are different.
Outriggers are too short and bad fixed to chassis. There no room enough to install electric motors.
So I decided to dismantle this version to start a new one.
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Second carrier |