36
votes
Accepted
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 ...
- 21.6k
33
votes
Accepted
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 ...
- 24k
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 ...
- 4,922
17
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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 ...
- 171
17
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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 ...
- 24k
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 ...
- 13.5k
14
votes
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Why do we even use engineering stress?
We use engineering strain even though it is not the "correct" value because in most cases, specifically in the elastic regime, engineering strain differs negligibly from true strain.
For linear ...
- 4,031
13
votes
Accepted
Strength of a welded steel gate with vertical bars vs. crossed diagonal bars
As grfrazee said, you won't know for sure until you do a finite element analysis. I was intrigued by this question as a colleague and I got into a discussion about this. While we both agreed the ...
- 1,217
13
votes
Strength of a welded steel gate with vertical bars vs. crossed diagonal bars
Assuming the joints are welded, for the top gate to deform as you draw it the vertical bars will have to bend into an "S" shape. The flexibility in bending will be proportional to the cube of the ...
- 12.4k
12
votes
Why does prestressing in trees make them stronger?
How To Read the Graphs
The graphs are plots of stress versus position. Stress is the force per unit area exerted on a material. Positive values are tension and negative values are compression.
The ...
- 1,167
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 ...
- 7,614
11
votes
Accepted
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'...
- 13k
10
votes
Why do we even use engineering stress?
Adding to @starrise's answer:
With regards to your dismissal of reasons 1 and 2, you are forgetting to consider the cost-benefit analysis regarding them. As @starrise showed in their answer, the ...
- 13k
10
votes
Accepted
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 ...
- 2,631
9
votes
Accepted
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 $\rho$ ...
- 12.4k
8
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Strength of a welded steel gate with vertical bars vs. crossed diagonal bars
While you've described your problem pretty well, I don't think you're going to find a satisfactory answer without having to run a fairly complex finite element analysis on both structures.
The first ...
- 3,587
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
7
votes
Accepted
Interpretation of shear flow/shear center of L-profile
Firstly, the shear center is the point at which an applied load produces no torsion on the section. For a singly or doubly symmetric section, the shear center will lie on the axis of symmetry. For the ...
- 2,760
7
votes
Accepted
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 ...
- 24k
7
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 ...
- 24k
7
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 ...
- 71
7
votes
Accepted
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 ...
- 24k
6
votes
Is a distributed load in two parts equal to a full distributed load?
Splitting a uniform load into separate pieces that are still continuous will have no effect. This is frequently done.
As far as your question about bending in HE and EB, there shouldn't be any ...
- 10.6k
6
votes
Accepted
How do you select the correct area to consiser for first moment of area calculations?
You're looking for $\tau_{xz}$, which pertains to horizontal shear flow along the top flange. (Rather than the vertical shear flow considered in $\tau_{xy}$.) As Mark noted, the shear flow starts from ...
- 2,760
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
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 ...
- 1,433
6
votes
Accepted
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 ...
- 13k
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 ...
- 12.4k
6
votes
Accepted
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 ...
- 21.6k
5
votes
Accepted
Material with very small Young's modulus
You are asking for an unrealistic material. If you peruse the engineering toolbox you'll find that the Young's modulus of typical materials is measured in 109 Pa so the value you are asking for is 7 ...
- 6,386
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