78
votes
Why are two wires used in railway overhead equipment?
Overhead line worker here.
It's about speed.
To answer your question, the purpose of the catenary wire is to get the contact wire to lay flat, so it isn't catenary itself. In your second photo, there ...
18
votes
Accepted
Why does a jumping rope have no bending stiffness and compression stiffness?
TL;DR: The bending and compression (buckling) stiffness is so small because the second moment of area of the fibres is small.
Bending stiffness
It does have a bending stiffness however it is really ...
- 24.3k
12
votes
Why are two wires used in railway overhead equipment?
It isn't to do with current-carrying capacity; it is to maintain good contact between the current-carrying wire and the pantograph pickup on the locomotive, as it allows the contact wire to be ...
- 451
6
votes
Why are two wires used in railway overhead equipment?
Why two wires:
A wire hanged at two points sags (it forms a "catenary curve"). The pantograph can only handle a certain sag before losing contact and arcing.
The acceptable sag depends on ...
- 499
5
votes
Accepted
What is the significance of hinge in the internal structure of the beam?
A hinge is a point where there is no restriction on rotation. For other points on a beam, the rotation to the left of a point must be equal to the rotation to the right of that same point; that is, ...
- 13.1k
5
votes
Why does concentrated shear force on a beam influence the whole beam?
So, we apply a point load to the end of the cantilever beam and can see that for external equilibrium, there must be shear reaction at the support. (Leaving aside for now the moment reaction that also ...
- 2,760
5
votes
Why do we always need to calculate the 'Moment of Inertia' about neutral axis for bending?
The derivation for bending stress is depended on the assumption that the strain distribution across the thickness is linear. i.e.
$$\epsilon(z) = a_1 z + a_0$$
where: $a_0$ $a_1$ are coefficients of ...
- 24.3k
5
votes
torque for a machine shaft
$P$ is power.
$P = \tau \omega$
$\tau = Torque$
$\omega = Angular Velocity (rad/s)$
https://byjus.com/physics/relation-between-torque-and-power/
- 8,109
5
votes
Accepted
Does lead screw's thread have to be trapezoidal?
It's not exactly clear to me what you mean by trapezoidal thread rod. Like you noticed Metric threads are also trapezoidal (just like ACME with slightly difference face angels).
Figure : Common types ...
- 24.3k
4
votes
Does same stress always produce same strain?
Since the bending moment is constant across any section, by the bending equation, the beam will deform as a pure circular arc, i.e. with constant radius of curvature across any small element length, ...
- 41
4
votes
Accepted
Moments of Fixed-Fixed Beam
When trying to figure out whether or not a given reaction will exist at a given support, it's worth remembering what a reaction actually is.
A reaction is the means by which the support resists the ...
- 13.1k
4
votes
Accepted
Solving for stiffness matrix numerically by a set of measurements
This is feasible and can be used to modify a theoretical stiffness matrix calculated by the Finite Element method to match experimental results more accurately. The FE model can then be used to ...
- 12.5k
4
votes
Accepted
Stretch in Infinitesimal strain theory
Taylor is straightforward:
$$
\sqrt{1+2x}
=\left.\sqrt{1+2x}\right|_0
+\left.{d \over dx}\sqrt{1+2x}\right|_0x
+O(x^2) \\
=1
+\left.{d \over dx}{1 \over \sqrt{1+2x}}\right|_0x
+O(x^2) \\
=1
+x
+...
- 238
4
votes
Accepted
Do all structures deform if put a load on them?
TL;DR: Yes, any structure deforms if you put a load on it.
Even adding an ant on top of a granite mountain will change (lower) the height of the mountain - imperceptibly so but it will still change it....
- 24.3k
4
votes
Why do we always need to calculate the 'Moment of Inertia' about neutral axis for bending?
Because we're not interested in the moment of inertia or the "c" ordinate of
a particular area element of the beam for their own sake - we're interested
in them as intermediate steps on the ...
- 851
4
votes
Does lead screw's thread have to be trapezoidal?
You can use regular threaded rod as a lead screw. The size and pitch (and grade) of the thread will determine your precision but 0.5 mm seems overkill for furniture positioning. The smaller the screw ...
- 2,347
4
votes
Why shouldn't we use Bernoulli beam theory if deformations and rotations are large?
Shear deformation causes the cross-section to twist, thus the plane no longer remains plane, which is one of the cornerstones of the Bernoulli Beam Theory.
Also, the large deflection invalidates the ...
- 8,109
4
votes
Accepted
Why shouldn't we use Bernoulli beam theory if deformations and rotations are large?
Apart from the other answers another reason why the Bernoulli-Euler beam theory falls apart at large deformation is due to the approximation about the radius of curvature.
The Bernoulli-Euler beam ...
- 24.3k
4
votes
Why does a jumping rope have no bending stiffness and compression stiffness?
Having low stiffness is part of the specification of a rope. It would be far cheaper to obtain the same tensile strength with a solid rod. Creating and manipulating multiple strands is expensive.
It ...
- 405
4
votes
Accepted
How many more degrees of tilt can the Millennium Tower in San Francisco withstand before it becomes structurally unsound?
Modern building failure due to tipping is rare but did occur, usually under strong earthquakes and loss of foundation (soil and structure), as the tilt will shift the gravity center of the building ...
- 8,109
4
votes
What is the significance of brittle materials and why do we use them?
Below is a list of the general advantages and disadvantages of cast irons. For applications where ductility is not a major concern, the reasons for the popularity of cast iron can be summarized by the ...
- 8,109
4
votes
Why do two simply supported beams stacked on top of each other deflect more than a single beam of twice the thickness?
Why do two simply supported beams stacked on top of each other deflect more than a single beam of twice the thickness?
Because the mating surfaces slide. The result is that you don't have the same ...
- 10.4k
3
votes
Accepted
Bending behavior of built-up C-Channel
Assuming your sketch has been drawn to scale, it won't be easy to make a hand calculation of the location of the shear center. The challenge is not the use of two different materials but that the ...
- 1,288
3
votes
Is a beam is subject to axial load, does it mean that deformation is the same in each point?
You seem to be mixing up strain and deformation. The strain, $\epsilon$ or change in length per length, is clearly uniform throughout the beam, and therefore the deformation is not. You get the ...
- 1,288
3
votes
Why does concentrated shear force on a beam influence the whole beam?
If we cut a section at any point of the beam, an equal and opposite force will be there (shear reaction) to maintain the equilibrium. So if we think it as a differential section, it will be easy to ...
- 31
3
votes
Accepted
Why is the thin-walled cylinder more likely to experience failure along its axis than along its hoop/circumference?
Actually, the pressure inside the vessel is uniform and constant everywhere. What happens is that geometry results in reduced stresses in one direction and not in the other.
There may be some ...
- 24.3k
3
votes
How to plot bending moment diagram from shear force diagram
First of all the bending moment, according to the usual convention (and the one you are presenting here) is positive.
However, the problem originates when you add the area of the shear force. (...
- 24.3k
3
votes
Accepted
What should be the relation between flange length and web thickness for an optimum I - beam?
The answer is "Yes", and the direct source to find such relationship is from the technical publishing, that offer steel design tables/charts showing the most optimum beam/column sections for ...
- 8,109
3
votes
Boundary condition at supported end of a rod
None of the three would occur if the material of the element holding the rod is "rigid", or is much stiffer than the rod. For such cases, my assessment is as shown below.
For the case that ...
- 8,109
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