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6x2 lorry tractor unit with lift axle
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This is my attempt at building a European-style artic lorry tractor unit, with a 6x2 axle layout with a liftable, non-driven second axle and driven third axle. It has coil spring suspension on all three axles (fully independent on first and second axles, live axle on third axle), steering, four-cylinder engine mounted under the seats and a three-speed gearbox. Most of the construction uses classic/vintage Lego Technic parts but with significant utilisation of the newer parts particularly in some of the moving parts.
About this creation


Driver's door open to show interior of cab. Both driver's and passenger's doors can open by cocking the lever outwards and then sliding the lever forwards. Each door can be locked in the closed position by cocking the lever inwards, which causes a hidden bar to chock against a plate on the inner side of the door.
The two wheels on side of driver's seat control the position of via rack and pinion mechanism, and the tilt of the backrest.
Gear stick is positioned to left of the steering wheel, and acts upon the grey rod that acts in turn on a rotor beneath and between the driver's seat and passenger seat. The rotor in turn transmits the vertical motion of the upper gear rod to thehorizontal motion of the lower final gear selector rod, which is situated alongside the engine cylinder block, and the gears themselves are situated behind and below the engine.
One effect of this arrangement of the gear selector is that the range of travel of the gear knob is very great - in this picture, with the gear knob at the top, first gear is selected; when third gear is selected, the gear knob is level with the driver's seat. Second gear is about two thirds of the way down between first and third.



Interior of cab from passenger's side. The "clutch" pedal is just visible behind the lower universal joint of the steering column, and is the only pedal which moves (it has a rubber band for return). The "clutch" pedal presses against a point wheel mounted on the pivot spindle of the gear stick, and whilst it does not prevent the gear stick from being manipulated without depressing the clutch pedal, it does prevent the gear stick from inadvertently descending under the weight of the upper rod and gear stick itself.



View of engine with passenger seats raised for access; cooling fan just visible at lower left of rear end of engine block. At this point I must explain something - this model was originally intended to be a Toyota Hiace-style van, with the engine accessed by tilting the passenger seats, but it didn't turn out the way i wanted it to so I ended up using much of the front chassis and transmission as the basis of a lorry instead. This is why i didn't attempt a more realistic tilting cab to enable access to the engine. This also explains why the model has independent double wishbone front suspension, as per 1980 Hiace vans, rather than the solid front axle suspension more likely on a tractor unit of this sort.
In this shot, the rack and pinion mechanism of the driver's seat is shown, along with the upper part of the gear selector mechanism. In the narrow gap between the right of the engine block and the left of the driver's seat, the lower part of the gear selector arm is just visible. The technique is very similar to that used on my "Yellow SUV" / "Pick up truck" / "Pneumatic tow truck" though the selector rod mechanism is of course much longer to reach from gear knob to gear box.



The front of truck is equipped with moving wiper blades linked by a rod so that they move in tandem.



The back panel of the cab can be raised to allow access by releasing the catches on the two bottom corners and lifting it manually, regulated by rod dampers.
The lorry is equipped with an adjustable roof spoiler to match the height of a trailer that may be towed. The spoiler is raised and lowered mechanically by the wheel at the top left hand side of the rear of the cab.
The rods and arms that form part of the axle lift mechanism are visible here. The gearing is obscured underneath the grey panel behind the white tank, and consists of a series of gears turned by the wheel on the lower left hand side of the back of the cab which turns a worm gear acting on the arms visible at the front of the rod assembly. The worm gear allows tension to be applied when the axle is lowered to keep these front arms rigidly pressed against the blocks in front of it to ensure that any vertical movement of the axle is absorbed by the suspension and not the lifting mechanism.
The "fifth wheel" for attachment of a trailer can tilt.



The front suspension is a double wishbone independent layout, as seen on the "Test Car" 8865. Each wheel requires three springs - two hard coils mounted in the "standard" fashion, and a softer spring mounted longitudinally at the front of the assembly. In order to operate smoothly and prevent inward "bulging", the lateral angle of the third spring is maintained by a bar linking the lower and upper wishbones.
The additional springs are necessary due to the weight of the front of the model, and even so some "preload" is observed and this is only remedied by substituting the softer third springs with the harder type, but this compromises the smooth operation of the suspension more than it is worth.
Part of the gearbox is visible to the right of the shot. The gear ratios are exactly the same as those used in the Test Car 8865, though back to front owing to the way the gear selector operates to ensure that first gear is engaged with the gear stick at its highest position.



Rear wheels removed to show suspension, with second axle lowered. The shock absorbers are mounted low and braced on the insides of the trailing arms, and as such are obscured by the wheels when in place. The driven third axle is equipped with a differential gear and an anti-roll bar.



Rear wheels removed, with second axle raised. The black rear arms of the lifting mechanism act directly on the shock absorbers to raise and lower the axle.




Underside of gearbox showing propellor shaft and sliding selector shaft (primary drive shaft obscured). The gear selector rod is visible behind and below the propellor shaft in this view. There is a slave shaft parallel to the selector shaft which slides in tandem with the selector shaft, which helps to keep the selector rod steady and perpendicular to avoid sticking.



Underside of rear axles. The construction of the trailing arms for the second axle, which are suspended completely independently from one another, are particularly massive to ensure that the wheels do not flop around when under compression and remain perpendicular.


Underside of front axle. Crown wheel and gear at bottom of steering column are visible in front of the axle (bottom of shot).



Roof spoiler raised and second axle deployed.



Spoiler retracted and second axle raised.



Comments

 I like it 
  November 5, 2015
I do love coming to your page... Technic at it's classic best, and some clever solutions here.
 I like it 
  November 1, 2015
Love it! I once tried a 8865 Technic truck, but nowhere as technical as this one.
 
By Daniel Wareing
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LEGO models my own creation MOCpages toys shop 6x2 lorry tractor unit with lift axleTechnic


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