|Articulated Hauler 6x6 (2011)|
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For tough off-road hauling conditions, Volvo (or Caterpillar, Terex) developed articulated haulers who are able to deal with these circumstances. Because these construction vehicles have a lot of techniques, I took the challenge to see if it was possible to design such a model in Technic LEGO®. The ambition was to have a full operated 6x6 drive-train, suspension and PF remote controlled driving & steering. I concluded from the start that the tipping of the dump bed must be achieved by using the pneumatic system, based on my experiences with the 10x4 Dump Truck (2009). It would be nice to add a pneumatic pump to automate the tipping as well, but it was clear that no space would be left within the model without a negative impact on the model's appearance.
The model was designed in 4 months, whereof the most important challenges were:
The 3-point suspension for both rear bogies, together with a 3-point bogie beam at both sides of the trailer chassis.
The front design of the tractor to have it's characteristic 'looks'.
Although I did succeeded in my opinion, the lack of center differentials come into account when the model makes sharp curves. Not a huge issue, but noticeable on flat surfaces. Unfortunately, applying center differentials was not possible due to the lack of space. Besides that, I was not sure if a center differential would be able to deal with the XL-motor force (especially the 12 tooth bevel gears). As well, I think the limits of the Technic LEGO® system are reached for models of this size, which I will explain later on.
The technic specifications of the
* 6x6 drive-train, whereof each wheel-unit has it's own differential unit
* Remote controlled driving (PF XL-motor) and steering (PF M-motor)
* 3-point suspension on both rear bogies, interconnected with a
3-point bogie beam at both sides of the trailer chassis
* Independent Tractor - Trailer movement by rotating hitch + frame steering
* Steering based on double Linear Actuator mechanism
* Working steering wheel within the drivers cabin
* Working V6 engine, including fan
* Manual pneumatic tipping of dump bed by 2 twin-cylinder units
* Headlights equipped with Power Functions LED's
Non-technic functions are:
* Swing-down front grill
* Engine hood open/close
* Maintenance 'doors' at both sides of the Tractor
* Dashboard with coffee machine and coffee cup
* Battery box appearance as air filter unit, including exhaust pipe
* Spotlights, flashlight, horns on cabin roof, steps for cabin access
The dimensions of the 'Articulated
* Length: 58 cm
* Width: 17 cm
* Height: 22 cm
* Tractor - Trailer steering: 33 degrees
* Rotating Hitch: 360 degrees
* Tipping angle: 43 degrees
* Ground clearance: 4 studs
* Weight: 2,4 kg (including batteries)
* Number of parts: 2.148
* Development time: ~125 hours
* MLcad time: ~50 hours
The dimensions of this model are based on the original Volvo A40D Hauler.
The common 'uneven' building technique for stud-less
constructions is used. The balloon tire size is 94.8 x 44 R (part 54120).
Unfortunately the wheel 56mm D. x 34mm Technic Racing Medium (part 44772) is not
available in yellow; as such the black version is used in combination with a
yellow wedge belt wheel (pulley) 'on top'.
The mudguards of the tractor needed to be circular, which was done by using the flexible tubes of Technic LEGO® set 8285 Tow Truck.
The interior of the tractor comes with a dashboard and again... a coffee machine.
The tractor comes with details as horns, headlights, spotlights, spotlight, rear mirrors, grills at both sides, a disguised battery box as air filter unit and exhaust pipe. At both sides, maintenance doors provide access to fake operation functions.
The left side of the tractor has steps to access the driver cabin.
Photo-shoot: Reconstruction of the wildlife habitat highway A2, nearby exit Vinkeveen (The Netherlands)
Drive train and V6 engine:
A single power functions XL-motor is used to drive the hauler. Due to the circumference of the 94.8 x 44 R tires, the PF XL-motor output needed to geared down in three stages, each one with a reduction of 5:3 (12 tooth double bevel - 20 tooth double bevel). This results in a total reduction of approximately 4,63:1. Due to the force of the PF XL-motor in relationship with the weight of the model, the drive train in the tractor consists of twin gears for each reduction step. The output of the 1st reduction step in the tractor is used as input for the trailer. Within the trailer, a single gear combination is used for the 2nd and 3rd reduction step per wheel unit.
Each wheel unit has of course a differential unit.
A V6 engine is located under the hood
together with a fan. Access to the engine is possible by opening the hood and
swing-down front grill.
The trailer has a replica of the original Volvo hauler 3-point suspension based on:
3-point bogie beam (a 1x16 Technic brick) at both sides of the trailer chassis, which can move independent from each other. The middle of this bogie beam is connected at the trailer chassis and is able to rotate (see red circle). Both outsides of the bogie beam are connected to both rear bogies by a rubber 'Technic axle joiner double flexible' (part 45590). See blue circles.
3-point rear bogies. Each rear bogie is connected at the 'front side' with a 'Technic Steering Arm 6.5 x 2 with Towball Socket Rounded' to the trailer chassis's 'Technic Steering knuckle arm with ball joint'. See green circle.
The combination of the bogie beam and the towball combination with the rubber axle joiner results in a large wheel movement while the body of the trailer stays leveled.
Due to the wheel movements of each rear
bogie, the drive train axle to the wheel unit must be able to change its
effective length. This is done by using a 'sliding joint' in real life vehicles.
The first attempt for the hauler model was based on the 'Steering CV joint axle'
(part 92906 which was released this year, 2011), together with the 'Steering CV
joint (part bb75). Although this works fine from a technical point of view, it
was not able to deal with the XL-motor's force. The joints just disconnect from
each other under heavy conditions.
The work-around I made is based on the 3L universal joints. The sliding of the bogie drive train is achieved by using a tan gear 20 tooth double bevel on the trailer chassis; I found out that this gear permits axles relatively easy to slide (see purple arrow).
The tractor itself as no suspension for
the wheel unit; the suspension is realized by the rotating hitch of the trailer.
The Volvo haulers do have a 3-point suspension as well for the wheel unit of the
tractor; however applying this to the model would lead to serious drive train
issues, regardless the fact if there would be space enough left.
Volvo uses are hydro-mechanical steering system. Within the model, this is realized by using a twin linear actuator mechanism which is powered by a single PF M-motor. A double clutch gear is used to safe the M-motor when the steering mechanism is blocked.
The twin linear actuators operates contrary, which means that one actuator 'pushes' whereof the other actuator 'pulls' during steering.
The design of the steering mechanism was more or less straight-forward, although it took some redesigns to have a sturdy chassis structure for the tractor.
The steering wheel within the drivers
cabin is connected to the steering mechanism and therefore operates as well.
Regardless the nice detail of the operating steering wheel, it became very handy
when the model is controlled by the remote PF unit: i.e. the 'operator' can see
if steering is 'going on' or 'stand-still'.
PF LED Headlights:
As you can see on the picture at the right side, the model has PF LED headlights.
For those from the Netherlands, at the background highway A2 with 5 lanes for both ways.
Switch to manual driving & steering:
It is possible to decouple the PF XL-motor from the drive train by shifting two gears '12 tooth double bevel' (same as on the Truck 8x4).
Steering decoupling is done by removing each 3L axle of both linear actuators at the trailer side. For proper steering, both linear actuators need to be retracted manually. Unfortunately there is no 'steering wheel' for manual operation; you just have to move the tractor unit into the right direction.
Remark: in manual operation, the gear reductions just work opposite, which means that driving needs to be done with some force, especially when curves are taken (due to the lack of 'center differentials'). Better manual operation is possible when the 3L universal joint between the tractor and trailer is removed.
Technical inside (cut-away render):
The articulated hauler fits precisely on the 'Truck 8x4 with detachable lowloader', which I designed in 2010.
Reaching the limits of the Technic LEGO®
First let me state that the articulated hauler model works fine in-house, especially on flat surfaces as a wooden floor.
And, of course the Technic LEGO® systems was never meant to be used in heavy outdoor circumstances. You can imagine that after dumping sand, the trailer drive train had to deal with a lot of resistance because the sand 'got in everywhere'. The only solution was a 'hot bath' to cleanup.
However, I ran into a couple of operating issues which I will share. Keep in mind that I design models according the original technical design. Of course some issues can be overcome by (for instance) applying a PF XL-motor output directly on the wheels to avoid a drive train.
Building models of this size requires
multiple reductions of the drive train to have a realistic speed of the
model. It's obvious that a PF XL-motor should be used given the necessary
force to drive the model.
Due to these reductions and the many axles involved, axle distortion comes into account which affects the driving in a negative way. This could be limited by a significant increase of the drive train rotations speed; but needs a 'portal axle' system to gear down at the wheel side. This was not option for this model without loosing the appearance.
I thought that the weakest point of the gear systems would be the new differentials due to it's single 28 tooth bevel. However, it turned out that this was not the point. The weakest point is still the 12 tooth bevel gear, used within the differentials. Yes, some of them did broke during the video-shoot.
Models of this size do weight
significant, the hauler weight is ~ 2,4 kg. It appeared that some of
5.5 L axles - used for the wheels - did bend significantly; especially the
axles for the wheels on the tractor. This gave the model the looks of the rear
wheels of a 'loaded Simca 1000 car
of the seventies'.
I think that bending is not related to the 5.5 L axles, but for all axles.
A lot of stress is taken by the Technic LEGO® parts which are used to connect the tractor - trailer and provides horizontal rotation (triangles, cross blocks 1 x 3). Actually, the turntable takes a lot of stress either but not in a negative way I noticed.
As well, a lot of stress is taken by the Technic LEGO® angle connector #1, used to connect the pneumatic cylinders at both sides of the trailer chassis, by dumping a full load of the dump bed. It was not able to make this more sturdy without loosing appearance.
Maybe an option is to have axles and gears of steal? No, for purists it would not be LEGO® of course... but Meccano®. :-)
Btw: the 3L universal joints are very
strong; even when the whole drive train stalled, no damage at all !
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