Hot answers tagged

42 votes

Why are screw holes in most of the door hinges in zigzag orientation?

More compact configuration that fully utilizes the area of the leaf. Avoid stresses concentrated on a single plane that is likely to cause the base material to progressively fail in shear or split. ...
  • 7,696
35 votes

Why are screw holes in most of the door hinges in zigzag orientation?

Because the screws go into wood and if the screws are in line then the wood will most often split between the screws in the grain direction and then the screws come loose.
  • 14.2k
25 votes
Accepted

How should I position two shelf supports for the best distribution of load?

As with all good things, it depends. If you can assume that your supports are totally stiff and that the loading on the shelf will be approximately uniform, then you basically have the following ...
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23 votes
Accepted

Why does a gear have extra holes?

Different gears have different drivers for having holes. One way you can categorize gears is by whether they are used primarily for transferring: motion: transferring angular position and angular ...
  • 22.9k
17 votes

Why are screw holes in most of the door hinges in zigzag orientation?

Staggering the screws will give a better chance for some of them to penetrate into solid grain. and discourage toilet paper perforation pattern. Provide for larger torque resistance. keeping the ...
  • 21.2k
11 votes
Accepted

What is the advantage of statically indeterminate structures?

For example, take a look at the following static systems. Assume they have the same length and the same (constant) cross-section. Thus an equal allowed bending moment $M_u$. The first system is ...
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9 votes
Accepted

How to calculate lever force when lever has uniformed distributed load?

Since the mass is 5k kg and the lever is 5m, this makes it quite easy to simplify because it is exactly 1k kg per m. The leftmost 2k kg (2m) of the mass has its center of mass exactly above the ...
  • 5,960
9 votes
Accepted

Where did I go wrong conceptually when attempting to calculate the maximum force on a truss at a given joint?

The reason is that you assumed that the elements around node $\text{D}$ will be the first to fail. That is not the case. Indeed, it is the elements under compression ($\text{AB}$ and $\text{BC}$) that ...
  • 13k
9 votes

Determine the moment of inertia of a filled circular sector

Consider an infinitesimal element of area $r d\theta dr$ which is at a distance $r \sin (\theta)$ from the $x$ axis. Its moment of inertia is $r d\theta dr (r \sin (\theta ))^2$. The moment of ...
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9 votes
Accepted

Why does the minimum force have to be parallel to the incline?

The minimum force is in the direction the object would move without that force. Clearly the object would move along the inclined plain, down and right in your diagram. Put another way, its the ...
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8 votes
Accepted

What angle - for a strut - provides the greatest vertical strength/support for a cantilever?

Assumptions The angle between the wall and the strut is $\theta$ $a$ is the depth of the table top $P$ is the weight on the table top, applied at the edge furthest from the wall The strut will fail ...
  • 356
8 votes

How to calculate lever force when lever has uniformed distributed load?

In any continuous situation, you simply use integration. The linear mass density of your block is $\lambda=\frac{m}{\ell}=$1000 kg/m. Now you can express the torque due to an infinitesimal slice of ...
8 votes
Accepted

Does the length of a beam change upon deflection?

The dimensions of the beam and magnitude of the deflection are important here. In most structural applications, it's reasonable to assume the length of a beam is unchanged by a small deformation. One ...
  • 3,211
8 votes

Why do we assume the moment is zero when analyzing a simply supported beam?

This rule is typically applied when studying statics. Static means that your structure or object does not move. If the moments didn't all add up to zero, that would mean there was a net force action ...
  • 4,501
8 votes

Why does a gear have extra holes?

In addition to the other answers such as weight reduction and inertia, there are other possibilities: Often there can be a precision machined hole for timing purposes. A common solution for some / ...
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7 votes
Accepted

Mathematical representation of hinge and pin connection

I believe you have chosen a poor reference. Indeed, that link has absolutely nothing to do with structural analysis, but rather a specific implementation in steel structures. A ...
  • 13k
7 votes
Accepted

Given the tensile strength of a rod, how do you calculate the max weight that can be hung from it?

You're hanging it from a rod supported by both ends - and need to use the bending equations. For this case (case 7 in the link), the max weight is: $$W_{max} = \frac{\pi d^3 \sigma}{8L}$$ $\sigma$ ...
  • 5,323
7 votes

is bending moment on roller supports at beams zero?

no. The change in moment is zero, as you can see on your plot. I think you can see if you imagine sectioning the beam slightly to the right of the support and constructing a free body diagram, the ...
  • 979
7 votes

Why does a gear have extra holes?

It could be for combination of reasons. lubrication: the holes will both pump and let the lubricant pass through. Reduced angular momentum reduces backlash and adds to gearbox responsiveness. ...
  • 21.2k
7 votes

Why does the upper wing skin fail in buckling and lower wing skin does not?

In a wing the normal situation is that the aerodynamic force is upwards (resisting gravity). You are right that there is some shear and a fair amount of torsion, but the result is that: the top side ...
  • 22.9k
6 votes

Does the length of a beam change upon deflection?

Adding to @Air's answer, there's also the issue of boundary conditions. A simple span where neither support allows for axial displacements will have a slight gain in length, including along the "...
  • 13k
6 votes
Accepted

Why is there a moment at the cross section end when you cut a body?

To put it simply your loads are causing the beam/bar to bend right? So at the cut we need to consider the internal bending moment ($M$). Like you said: the professor has cut the bar to calculate the ...
  • 1,217
6 votes
Accepted

Maximum Shear stress in Beams

You've got your terms confused. The maximum shear stress at the midpoint is equal to $$\tau_{max} = 1.5\frac{V}{A} = 1.5\overline\tau$$ where $\dfrac{V}{A}=\overline\tau$, which is the average ...
  • 13k
6 votes

Why does a simple pin-ended triangular (arched/curved) beam deflect more due to secondary effects (non-linear analysis)?

Play around with a simple version of this structure, made from a sheet of paper fixed in a slight curve, and see what happens when you apply a load to the mid point. If the first example, any ...
  • 12.5k
6 votes
Accepted

Why do we ignore horizontal reactions at roller supports?

If a bar is only supported by roller supports (which allow for horizontal displacements) and a horizontal force is applied, the bar will not suffer any internal forces because the entire structure (...
  • 13k
6 votes
Accepted

Isn't a pin support supposed to prevent translation both horizontally and vertically?

It's simple: while B is a pinned support, C is a roller. You can see that in the figure, where B is drawn clearly bolted to the "ground" while C isn't. In fact, the contact between C and the "wall" ...
  • 13k
6 votes
Accepted

Calculating moment of inertia for a hollow discontinuous circle?

Edit: There seems to be a much easier way I overlooked, which I'll explain. My first answer is kept below for reference. Your assembly consists of a small sector subtracted from a larger sector as ...
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6 votes

Does a washer really increase static friction force?

No it doesn't, and it isn't meant for that either. A standard flat washer, that is. It's meant to give the nuts and bolts a better support, and it protects the material against the nut which may ...
  • 1,164
6 votes

Why does a gear have extra holes?

Also someone may have to pick the machine up and carry it. Reducing weight is not just related to the intrinsic function of the machine, its shafts and so on.
5 votes
Accepted

How to analyze the free body diagram of a pair of scissors?

Your general idea is correct. Your fingers apply a force which can be translated to a moment at the rotation axis. The reaction forces applied by the object to be cut must generate an equal moment at ...
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