Subject:
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BobCat's Big Brother
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Newsgroups:
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lugnet.technic
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Date:
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Fri, 26 Jul 2002 14:35:06 GMT
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Viewed:
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926 times
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While dismantling my interpretation of Jennifer Clark's BobCat
(http://www.brickshelf.com/cgi-bin/gallery.cgi?i=132218) and the Lego Front
End Loader (http://guide.lugnet.com/set/8464) I noticed that the wheels and
bucket from the BobCat were the same relative dimensions as those from the
FE-Loader - the concept of "BobCat's Big Brother" was born. That was in
Feburary! Five months later and the concept is reality -
http://www.brickshelf.com/cgi-bin/gallery.cgi?i=216961
The model has been rebuilt 4 times in that period (lives 2 and 3 were short
lived and after version 3 I'd had enough and the parts went back into stock
for a prolonged period and I took up castle building!) Some pictures of the
version 1 prototype (more of a working-concept than a model) can be seen
here (http://www.brickshelf.com/cgi-bin/gallery.cgi?f=13706), and more
pictures of the final version are here
(http://www.brickshelf.com/cgi-bin/gallery.cgi?f=12271)
For those interested some build notes follow.
The total reduction between the motors and the wheels is 36:1 which is
probably too much as the final model drives very slowly. In the initial
design it was 5:1 comprising a 24t gear on the motor shaft driving a 40t
gear on (effectively) the same axle as another 8 tooth pinion which drives
the 24t gear mounted on the rear half-axle. However, under load, this
arrangement coupled with the large diameter of the wheels is sufficient to
'back-drive' the non-powered motor when skid-steering. This has the
unexpected result that when applying power to only one motor the model
actually drives forward (albeit in a very large diameter turn due to the
drag caused by the non-powered motor).
The solution to this problem was to introduce a worm gear into the drive
chain to inhibit this 'back-drive' effect. With a worm gear in the drive
chain the problem now becomes one of how to transfer the drive in a
sufficiently small width (the chassis is 8 studs wide at the rear and only 6
at the front, and has to accomodate independant drive to each side) over a
significant height, as the motors have to be mounted above the rear axle in
order to achieve the correct balance and to avoid dragging on the ground
during a 'wheelie'. The final solution was for the worm to drive a new 12t
double bevel, which in turn drives an 8t idler pinion, which drives another
12t double bevel mounted on the same axle as the final 8t pinion which
drives the 24t gear mounted on the rear half-axle. These final two gears
are mounted outside the chassis frame and fit inside the wheel hub. The
12-8-12 gear combination provides no overall reduction and also neatly
mounts in three beams stacked on top of each other to provide the required
height change while only being four studs wide.
This gear train however, introduces the problem of insufficient space for
the bevel gear on the rear half-axle that transmits the drive to the front
wheel. The solution to this was to turn the motors around and mount the
gear train behind the rear axle, thus freeing up space for the bevel
take-off point. Unfortunately, this moves the centre of gravity of the
motors which affects the balance of the model while skid-steering! The fix
for this was simple (compared to the previous problems!) - a train weight is
built into the rear 'engine housing' of the model to compensate for the
change in position of the motors.
The universal couplings in the longitudinal axles (between the front and
rear wheel axles) are there primarily as I don't like placing a single axle
in more than three bearings - and these axles pass through 4 cross-bracers.
However, they also introduce a certain amount of 'play' on the front wheels
which helps to 'skip' the non-driven front wheel when skid-steering. For
extra chassis rigidity, a 6x16, 4x6 and 4x8 plates are fitted as an
'under-tray', but these were removed to expose the underside in the photograph.
The bucket arm and linkage arms are a combination of studless beams and
standard beams. The standard beams are finished with tiles to achieve the
same smooth finish as their studless siblings. Over such a length the
studless beams seem to flex more than a standard combination of 'beam and
plate' construction, being particularly noticable at the joints. To
overcome this, joints overlap by at least 3 holes, preferably 4, and every
hole is fitted with a friction pin or an axlepin (where the bent beams have
an axle hole at the end) to reduce the 'play' in the joint. This does make
aligning the beams during assembly very difficult and you certainly don't
want to have to dismantle and re-assemble them too often! [Note to Lego:
Can we have a black friction axle pin for this purpose please, as the grey
end in the joint offends my asethic eye.]
With such a large arm, the Technic pneumatic pistons do not have a
sufficient 'throw' to simply scale up the mechanism from the original
BobCat. The initial design extended the lower linkage arms past their
bottom pivot point and mounted the pistons horizontally along the chassis.
This had a number of real and 'practical' problems. Firstly, this
arrangement caused the front axle to be poorly supported and sub-optimal for
effective skid-steering. In addition, the pistons would quickly get covered
in 'mud' (aka carpet dust) and become a maintenance nightmare. Lastly, and
by no means least, to lift the bucket the piston arm has to 'pull'
(contract), and I have found the pistons to be less effective in this
direction than when 'pushing'. The final solution was to simply mount the
pistons to form a triangle between the lower linkage arm and the 'vertical'
portion of the bucket arm. The pivot between these two arms is a 3 long
axle with stud, the stud of which faces inwards and acts as a 'stop' to
ensure the piston doesn't go past the 'point of singularity'.
The top linkage arm front pivot (on top of the cab) took a remakably long
time to find a working solution. Due to stresses while lifting (probably
caused by differential expansion rates of the pistons), a gray pin tends to
pop-out, while a long gray pin tends to pull the cab apart. It was
therefore necessary to 'cross-bond' the two arms at this point - what was
needed was a 10U pin or a 10U axle with 'pin' ends. The solution required a
quick redesign of the cab frame and uses two 'pin long with stop bush'
(http://guide.lugnet.com/partsref/search.cgi?q=32054) and a 6U axle.
Cross-bonding the lift point of the pistons with a similiar arrangement
('padded' with pin joiners for an asethetically pleasing diameter - not
clear in any picture) has also alleviated the 'differential' lift of the
bucket arms.
The bucket tilt mechanism is a straight forward arrangement of liftarms,
doubled up where necessary to give a visually appealing 'bulkiness' rather
than for any mechanical requirement. The pneumatic hoses from the bucket
pistons run along the arms to the rear end of the top linkage pivot point at
which point they are joined so that only one hose form each side needs to
descend the lower linkage arm and enter the cab. The hoses for the larger
lift pistons are similarly joined, thereby reducing the number of hoses on
each lower linkage arm to two each side. The mounts for the flex-system
pipes on the lower linkage arms are a 1x1 tile with clip on a 1x1 plate with
horizontal clip on a Technic 1/2 pin pushed into one of the holes in the
beam. This was necessary as the 1x8 tiles on the top of the lower linkage
arm are 'structural' in that they join the main beam to the 1x1 Beam with
Hole that forms the end pivot point - a shorter tile 'pops' off under load.
The chassis/cab is a combination of old style beams and plates in a 'Model
Team' (fill in the holes) and new style studless beams in a 'Technic'
(outline the frame) approach - its worked better than I was expecting. [The
cab roof is blue as I'd run out of black double bent beams - they're used at
the rear end of the bucket arm, and I'd forgotten I had 4 yellow ones at the
bottom of the parts box - I may get around to swapping them over.] Once
again I have 'borrowed' the front grille from the Lego Digger
(http://guide.lugnet.com/set/8862) - why re-invent a wheel?
MLCad/LDraw plans will be made available at a later date.
William
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