Subject:
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Re: New Civil Engineer letter
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Newsgroups:
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lugnet.technic
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Date:
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Thu, 5 Jul 2001 19:26:04 GMT
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Viewed:
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829 times
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In lugnet.loc.uk, George Haberberger writes:
> Some of the simplifications are used to avoid nondeterminate equations (ie.
> four equations and six variables). One example is that many joints are
> considered to be only constrained vertically and horizontally, not
> rotationally. If you were to model a truss with four elements meeting (which
> is most of the truss), you would have to use an axle in Techniq and end up
> with a four wide section, it's much easier to model in Meccano.
True, you do end up with a 4-wide eccentric connection. However, gusset
plates can be used instead of trying to line up 4 members on a common pin.
This can be accomplished by using extra beams or other members at the joint.
You may not end up with a perfectly non-eccentric joint, but you certainly
can reduce the width.
> Plus, if
> your axle went through a cross shaped hole, you have a resistance to any
> torque and your equations would be much harder.
Nothing can model a frictionless pin, despite the assumption made for a
truss. And besides, making trusses with real pinned connections is rarely
done anymore. Most of the members are "I" or box sections connected with
gusset plates. The connection is fixed, each member is rigidly fixed to
another. If tubes are used, they are all coped and then welded together.
> Another simplification is that only one end of structure is fixed, the other
> is free to move back and forth (again, this simplifies the equations
> immensely without much of a change in the answer). With bricks, you'd need
> tiles to properly model that.
And Lego offers tiles, many tiles.
> Another simplification is that materials are rigid and do not deform.
> Deformations change your structure and make your equations much more
> difficult.
Only if the deformations significantly alter the geometry of the structure.
Most elastic deformations are small and are neglected in any calculations.
If you want your structure to survive and not destroy the "building system"
you've invested in, you'd better not overload it.
> Again, Meccano should be better than ABS here.
Well, if you're going to build something and perform calculations to model
your models reactions, then steel or aluminum parts would be better. ABS is
a non-isotropic material. It has a different modulus of elasticity in
different directions. The modulus of elasticiy, E, of ABS in flexure is 2.07
GPa, and the E in tension is 190 GPa (Ref. Machinery's Handbook, 25th Edt.).
Your calcuations assume that the material is isotropic (i.e. it behaves the
same way in all directions- the modulus of elasticity and Poisson's ratio
are constant). Although steel and aluminum can sometimes be non-isotropic,
it is safe to assume they are isotropic (at least for simple calculations).
> Finally, Techniq is too limiting geometrically. There are configurations
> that are hard to pull off (ie. a seven cylinger rotary airplane engine) in
> TEchniq that can be more easily modeled in Meccano.
I agree. However, you really have to sit and think about this one. I'm sure
there are many things Meccano can do that Lego can't. However, I have an
idea that there are many more things that Lego can do that Meccano can't
(pneumatics, robotics, sophisticated electronics, etc.).
> Hope this clarifies things,
To ultimatly decide which system is best, you have to really consider the
purpose of having a "building system" in the first place:
1. Do you want something that is quick and easy to use, that will be able to
demonstrate a wide range of mechanical, structural, and electrical systems?
2. Do you want a realistic system capable of providing engineers with
materials identical to real-life situations and to perform calculations in
conjunction with experiment?
If you desire the first, then Lego is the choice. I think it has an
excellent selection of parts that can be used to model just about anything.
It's a convenient and easy to use system that requires no tools for assembly
and disassembly.
If the second is desired, I think you'd be better off with a home-build
system (i.e. stock up on plate, angles, tees, channels, tubes, etc. as well
as bolts and welding equipment; don't forget all the tools required for
assembly). In order to model more "mechanical" systems, you'll have to
purchase hydraulic and electronic components that can be very expensive
(relative to Lego).
To sum this up, the perfect building system would be just a scaled-down
version of the real thing. An ideal building system capable of demonstrating
real-life systems would be one that is easily obtainable, is relatively
inexpensive, provided a good inventory of parts, and could be used with
little effort. I think Lego fits the "ideal system".
T. J.
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Message is in Reply To:
 | | Re: New Civil Engineer letter
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| (...) I think he's right. I have a BS degree in Aerospace Engineering, and took many courses in engineering mechanics. The simplifications used in engineering mechanies are better modeled by Meccano (or Erector sets) than in Lego. Techniq is better (...) (23 years ago, 5-Jul-01, to lugnet.loc.uk, lugnet.mediawatch)
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