Subject:
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Re: Articulation with "working" connecting rods
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Newsgroups:
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lugnet.trains
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Date:
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Fri, 15 Sep 2006 16:42:54 GMT
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Viewed:
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2282 times
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In lugnet.trains, Sebastian Dyson wrote:
> I'm trying to design an 8-wide 4-8-4 using Big Ben's wheels (CAD only, I'm but a
> poor student), and want the loco to be able to negotiate standard Lego curves.
> As the leading truck has an axle each side of the cylinders, I'm finding it hard
> to keep the cylinders fixed relative to the driving wheels without the cylinders
> and/or rods fouling the wheels of the pilot truck.
>
> I could raise the cylinders, as in Rosco's X class
> (http://www.br-eng.info/about/lego/mocs/x-class.htm), but I'd really prefer not
> to, as the prototype's cylinders are quite low (the bottom edge of the cylinder
> is about level with the leading truck's axles), and I can only move them so far
> outboard before they look ridiculous.
>
> Another option is to have no connection between the rods on the wheels and the
> cylinders and have the cylinders turn with the leading truck, which seems a
> little like cheating.
>
> So if anyone's got any tips or experience in this, I'd love to hear it.
An alternative solution, more complex than what I wrote earlier, uses a method
similar to Mike's, in that flex tubes allow the set of drivers to be split into
blocks, but different in the way the blocks are linked and kept synchronised.
The method is based on what I've used in my UK Class 14 diesel, which is an
0-6-0 with 6 BBB wheels, a jack-shaft and quite a long wheelbase - ordinarily
too long for the curves.
Underside view:
http://www.brickshelf.com/cgi-bin/gallery.cgi?i=1150094
Side/cut-away view
http://www.brickshelf.com/cgi-bin/gallery.cgi?i=1150093
(I'll be turning this mock-up into the proper 8mm scale body soon, with lots of
SNOT - it's handy having a scale drawing to follow)
Principles of BBB rod articulation:
1. Split the set of driving wheels and cylinder supporting bogies (in your case
4+8) into lengths short enough to go round curves, each set having 4 or 6 wheels
(use blind drivers in the middle of a set of 6).
2. All wheelsets having rods must be kept synchronised. This is achieved with
bevel gears in each set and inside the body of the loco. The pivot for a set
may be over one axle within the set but should be over the middle of a set if
there are sets both sides of it.
3. The sets of wheels are linked together with a 2L liftarm sideways (see
underside photo), which allows some extension between sets whilst keeping the
turning of adjacent sets coordinated.
4. The rods for each set use flex tubes and thin liftarms. A rod between sets
uses a flex rod that goes inside the tube on the next set, providing
extendability. Allow 2 studs length of bare flex rod outside the tube in the
straight position.
5. If there are more than 2 sets, or there are other bogies involved (such as
your trailing 4 wheels), the pivot(s) for the middle set(s) must be allowed to
float sideways (but not lengthways) within the loco body. Bevel gear
arrangements must allow the shafts between sets to turn independently about the
pivot(s) of the middle set(s). Also consider that too many bevel gears will
lead to prohibitive friction - try to have the pivot over an axle so there are
just 2 bevel gears in each set of wheels.
6. Provide enough motor power to the bevel gear shafts inside the body of the
loco to avoid the driving wheels being pushed by the train motor. I suggest a
20:12 gearing up from a 71427 gearmotor connected electrically to the train
motor. One gearmotor for every 6-8 wheels should be enough, double if you
intend to power the whole train from the BBB wheels (as can be done with the IR
battery system)
For your 4-8-4 I suggest 4-4-4-4 because 4-8-4 would leave the blind drivers at
the back of the 8 sticking out. Also 6-6-4 wouldn't work because the middle
wheels of the 6 at the front are smaller and therefore not blind, so the
wheelbase of 6 with cylinders attached would be too long.
Another possibility is 2-4-6-4, with the first 2 wheels free-running, attached
by a pivot to the next set of 4. The next set of 4 carries the cylinders, 2
small wheels and the first 2 drivers, with the important rod connection from
drivers to cylinders. A flex rod is attached to this first pair of drivers,
extending backwards to the flex tube on the set of 6 behind. The set of 6 has
blind drivers in the middle, with the pivot over the blind drivers. The pivot
can slide sideways within the loco body. The pivot for the set of 4 with
cylinders would be best over the drivers from a bevel point of view, but better
for the turning coordination to be the same distance in front of the drivers as
the distance between wheelsets on the following set of 6, assuming that the
pivot for the set of 6 is over the blind drivers in the middle. The last set of
4 wheels could be a train motor. A gearmotor should be somewhere inside the
loco body.
I guess I'll have to build this to illustrate it :-)
Mark
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Message is in Reply To:
 | | Articulation with "working" connecting rods
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| I'm trying to design an 8-wide 4-8-4 using Big Ben's wheels (CAD only, I'm but a poor student), and want the loco to be able to negotiate standard Lego curves. As the leading truck has an axle each side of the cylinders, I'm finding it hard to keep (...) (18 years ago, 4-Sep-06, to lugnet.trains)
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