36 votes
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How do ball bearings not crack from a load concentrated on an infinitesimally small point?

Nothing is rigid, the raceways and the ball in a ball-bearing are no exception. The contact area deflects and accommodates the ball in a small contact surface, not a point. Also, the balls take the ...
kamran's user avatar
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33 votes
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Why exactly does a washer help distribute the stress around a bolt?

I will expand on DKNguyen answer, because to my knowledge also the two reasons are: reduce contact/bearing stresses (having a significant effect on thin finishes live galvanisation) change the joint ...
NMech's user avatar
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27 votes

Why exactly does a washer help distribute the stress around a bolt?

It is for spreading out the stress. But it is also for giving the bolt a bearing surface to turn on. The washer always goes on the side (nut or bolt) that is being turned. It prevents it from marring ...
DKNguyen's user avatar
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17 votes

Why exactly does a washer help distribute the stress around a bolt?

To visualize part of Nmech's answer: in the image, the washer actually greatly increases the contact area of the bolt head. The bolt head looks pretty big: But most of that is the shaft, which ...
Mark Peschel's user avatar
17 votes

How do ball bearings not crack from a load concentrated on an infinitesimally small point?

TL;DR: ball bearings deform due to Hertzian pressure and therefore distribute the load However, the reason balls (rolling elements) don't easily crack is the ductility of the material (and to some ...
NMech's user avatar
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16 votes

Why exactly does a washer help distribute the stress around a bolt?

Except for special applications, most washers are made of dead soft steel, which deforms under the compressive load imposed by a tightened bolt head. As the washer smooshes, it minimizes stress ...
niels nielsen's user avatar
12 votes
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Why is the point of failure load lower than the peak load?

That is due to necking. When a bar is under tension, two opposing mechanisms take place: The bar loses cross-sectional area since it attempts to retain its volume while being stretched (see Poisson'...
Wasabi's user avatar
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12 votes

Do outer fillets|chamfers also reduce stress as inner ones do?

Sorry for not taking your "Thin Hollow Brick-Shaped Object" as an example - I felt a standard notched test specimen would illustrate the principle more clearly: It can be seen that adding external ...
Jonathan R Swift's user avatar
11 votes
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What is the difference between the Polar Moment of Inertia, $ I_P $ and the torsional constant, $ J_T $ of a cross section?

The torsion constant $J_T$ relates the angle of twist to applied torque via the equation: $$ \phi = \frac{TL}{J_T G} $$ where $T$ is the applied torque, $L$ is the length of the member, $G$ is ...
atom44's user avatar
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9 votes
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Do scaled models reflect strength characteristics of full size

Things scale, but not always in a nice simple way. For example consider the units of stiffness of part of the structure are $E$ (Young's Modulus) times some length, but the units of weight are $\rho$ ...
alephzero's user avatar
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8 votes

Why exactly does a washer help distribute the stress around a bolt?

Another important part of the answer is the symmetry of the stress pattern. The stress caused by a bolt head varies greatly between the points of the bolt head and the straight sides. As a result ...
Jeff Hester's user avatar
7 votes
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What's the best way to design a joint with two fasteners?

I think you might have a misconception regarding to how far the pressure from the fasteners extends. One subject you might want to have a look into is "bolt joint stiffness". The most ...
NMech's user avatar
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7 votes
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Does perfectly elastic and perfectly plastic material make any sense?

If I understood correctly you are only after the stress-strain curves. Figure: Stress strain curves for different types of materials (source What's pipping) Perfectly Elastic : (referred to as ...
NMech's user avatar
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6 votes

Difference between 'plastic yielding' , 'plastic yield strength', plastic strength' and 'plastic stress'?

All these terms refer to the effect of loading on the deformation of materials. Let us assume that we start with zero load and zero deformation. Elastic deformation If you increase the load you ...
Biswajit Banerjee's user avatar
6 votes
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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 ...
Wasabi's user avatar
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6 votes
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Which drawer design can hold more weight?

Using a tie-and-strut model makes it easier to explain the difference here. A tie-and-strut model is a simplification of the structure which describes where the structure is under tension, and where ...
Wasabi's user avatar
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6 votes

Why don't car engine piston-rods (or similar parts) fail as often as other car parts?

The main reason from a designer's point of view is the consequence of a failure. Almost always, something like battery failure or a puncture doesn't cause any collateral damage - you just replace the ...
alephzero's user avatar
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6 votes
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Calculate whether a polycarbonate plate can withstand a certain force

For impact force calculations you need some more information such as the shear modules and the material properties like toughness and tensile yield and shear yield. Also you need to define a physical ...
kamran's user avatar
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6 votes

Why is the Stress vs strain diagram preferable to the Load vs displacement diagram?

The Load-Displacement (or Load Extension) and stress strain diagrams are two diagrams identical in shape. See below. The main visible difference is the values on axis (which are at first glance ...
NMech's user avatar
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6 votes
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Creep of the materials

Yes, creep occurs at all temperatures, although the dropoff is exponential. The reason is that at any nonzero temperature (which is all temperatures), there’s a nonzero chance for a thermal defect ...
Chemomechanics's user avatar
5 votes

Glass panel engineering at work

Glass tends to contain quite a lot of residual stresses from the manufacturing process and these will tend to be concentrated near corners (depending on how the sheet was manufactured). There is also ...
Chris Johns's user avatar
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5 votes
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What really is so special about principal stresses or stress invariants?

I'll take your word for it that your quote is "recited everywhere". The point of it is not that you can "rotate your coordinate system" just for fun, but that the principal stresses are independent of ...
alephzero's user avatar
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5 votes
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Beginners material science for handling a load

Material Properties For the linear-elastic analysis you describe, the key pieces of material information are the elastic modulus (E) to calculate deflection and the yield stress (Fy) to check if the ...
CableStay's user avatar
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5 votes
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Why is it said that the flange of an I beam carries most of the moment?

What is really proportional to the distance from the neutral axis (let's denote it $z$) is the strain. For pure bending and a symmetric cross-section the equation for strain is given by $$\epsilon(z) ...
NMech's user avatar
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5 votes
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How exactly do forces transfer in different directions?

The direction of Forces isn't necessarily along the connecting element. If that happens depends a lot on the constraints between the different elements. For example see the following image: In the ...
NMech's user avatar
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5 votes
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Mechanics: example of less mass/volume gives more strength

I just found an answear, the keyword is "stress concentration". Sometimes, the material is removed to make a stress relief in an element. Some examples: The origin source I found it: https:/...
radoslav006's user avatar
4 votes
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Tensile strength to weight conversion

Pay attention to your unit analysis. You multiplied by the acceleration due to gravity, rather than dividing by it. Unit analysis would have shown this to be nonsense. $$13 \frac{N}{mm^2} \cdot 100 ...
Dave Tweed's user avatar
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4 votes

Difference between Precipitation Hardening and Tempered Martensite

Summary: Both processes depend on relative solubility of solute between high and low temperature solvent phases. How each process proceeds at sub-microscopic scale is very different. Precipitation ...
do-the-thing-please's user avatar
4 votes

A question on mechanical failure

It won't. Solid objects don't break in hydrostatic compression. The pressure is pushing all parts of the rod together, so there's nowhere for them to go except stay together. You can imagine an object ...
user1318499's user avatar

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