If not please ask questions. I am trying to paraphrase
so ordinary mortals (non-engineers) can understand it :)
The University of Queensland School of Engineering. Quote:
"Mechanical failure is generally interpreted in terms of change,
some physical or chemical process having produced failure.
However the improper location and guidance of components
also constitutes an important mode of failure often ignored
in mechanical engineering practice."
Any object is said to have six degrees of freedom or movement.
Rotation about the X, Y, and Z axises.
Translation (movement) in the X, Y, Z directions.
In order to hold any object from moving in any way requires only
three connections. If a fourth, or more, connections are added
additional loads and stresses are introduced that cannot be determined
by simple means. A three legged stool is a classic example.
It is trivial to calculate the loads in the three legs and
the forces these loads apply to the floor. A four legged chair
on the other hand is a much more complicated situation and it
usually requires a finite element analysis computer program
to resolve the individual loads and stresses. The fourth leg
is therefore redundant and in fact considered undesirable.
Paul Lamar ...No rotor no motor.
A three legged round stool is more stable supporting a downward
force only when that force is applied on the area included in the
area defined by a triangle connecting the legs (since the legs are
not attached to the floor).
In this case, four legs have greater stability. I have crashed &
burned more than once on of those stools when trying to sit on the
edge. Also recall why the three wheel RTV's were replace by quads.
Fortunately for us, the three point mount IS constrained, hence a
better design.
Joel
Wondering why I get involved on subjects out of my Expertise.
Thanks Joel. Good thinking. Agreed.
You are absolutely right but for our purposes and audience IMHO it
is too much detailed. Do you think I need to go down that
slippery slope and draw a 3 legged stool where the force is applied
on the area included in the area defined by a triangle connecting
the legs ?
The constrained word is good but for the average person I think
the connected word is better.
-------------------------------------------------------
Ok here is another way of putting it. This is from Bruhn,
"Analysis and Design of Flight Vehicle Structures."
"A2. 5 Statically Determinate and Statically Indeterminate Structures.
A statically determinate structure is one in which all external reactions
and internal stresses for a given load system can be found by use of the
equations of static equilibrium, and a statically indeterminate structure
is one in which all reactions and internal stresses cannot be found by
using only the equations of equilibrium."
A 3 legged stool connected to the floor is statically determinate.
"If the structure has more external reactions or internal members than
is necessary for stability of the structure under a given load system
it is statically indeterminate."
A four legged stool connected to the floor.
"The additional equations that are needed to solve a statically
indeterminate structure are obtained by considering the distortion of
the structure."
In other words FEA which they did not have when this book was written
in 1965.
Paul Lamar ...No rotor no motor.
Most desk chairs used to be sold with 3 or 4 wheels,
but now they have five or six wheels.
If I'm on a three legged stool and lean forward onto two
of the legs the force on the third leg goes to zero.
If I'm on a four legged stool and lean forward onto two
legs then the force on the back two legs goes to zero.
As the force on the back two legs goes to zero it looks
more and more like a three legged stool.
Therefore my six wheeled desk chair is really just six
three legged stools.
-Jeff Martin
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