76
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 ...
55
votes
Accepted
Why are railroad tank cars bent in the middle?
As other have pointed out, the main utility is to allow easier cleaning of the tank cars from sediment (solid precipitates). Usually, there is a drain in the lowest point.
The reason that it is in ...
- 24k
44
votes
Accepted
Why are engine blocks so robust apart from containing high pressure?
If you consider only the static forces then indeed the thickness might seem over-engineered. However, engine blocks are not statically loaded. They operate in the range of a few hundred to a few ...
- 24k
43
votes
Why are bulldozers so slow?
I wonder why can't we engineer..
We CAN engineer a vehicle which is both fast, and has a lot of torque. (E.g. a tank, as you mentioned yourself)
The reason why we don't do this for a bulldozer (I.e. &...
- 736
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.
...
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38
votes
Accepted
Windmills in empty fields. Why no trees?
Trees increase the turbulence of the air that reaches the turbines. That creates all sorts of uneven, rapidly-shifting loads on the blades and structure. That increases the maintenance costs, ...
- 3,721
38
votes
Accepted
Why do heavy trucks use air brakes?
Biggest advantage is that when connecting to trailers, there are no bleeding issues.
Imagine having to connect hydraulic pipes and remove the air bubbles...
As for the amount of air - the compressor ...
- 15k
36
votes
How do bike wheels keep turning when the rider stops pedalling?
The soft clicking sound you hear when coasting is the pawl going over the ratchet as in the picture below.
- 361
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.
- 15k
35
votes
Why does this fence have a kink?
They're V-Mesh panels.
V-Mesh panel from Irish Wire.
The offset of the vee gives rigidity in the horizontal direction. The vertical rigidity is addressed by the fence posts. It's available in 2, 3 or ...
- 9,794
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
32
votes
Accepted
Does an electric vehicle going downhill recover energy?
It depends on how steep the hill is. On a slight hill, the energy added by gravity is still not enough to overcome rolling friction and air resistance, so the car still needs power to maintain speed. ...
- 1,022
32
votes
Accepted
Why is this bridge thickest above the support pillars instead of the mid-span where the bending stress is highest?
If I model this as a simply supported beam having load at mid span [...]
I suspect that this is where your analysis went awry.
First off, you should always model bridges with distributed loads, not a ...
- 13k
31
votes
Accepted
Why are rear wheels not placed at the extreme rear of a bus?
Some ideas:
Wheel Load Distribution: The load is greater on the rear wheels providing the power; more force on the front ones bring no benefit and would provide less traction.
Better manoeuvrability ...
- 940
29
votes
Is it possible for a bicycle chainwheel to have a fractional number of teeth?
Gear ratio
This is a Roller Chain drive system, and as such it's a timed system. The ratio between the two connected pieces depends entirely on the number of teeth on either end.
Even if you ...
- 1,116
27
votes
Accepted
Understanding required torque for a motor lifting a weight
You have the right concept, but slipped a decimal point. 5 cm = 0.05 m. The gravitational force on your 450 g mass is 4.4 N as you say, so the torque just to keep up with gravity is (4.4 N)(0.05 m) =...
- 11.3k
27
votes
Accepted
Why do car wheels have holes?
Car wheels have holes mostly due to weight and cost considerations. Each hole is a chunk of material that you aren't wasting and weighing down the wheel with.
As another bonus, the holes help with ...
- 5,741
27
votes
Why does moving air feel cold?
There are a couple reasons. First of all, it's important to note that the sensation of warmth or coolness is only indirectly related to temperature. The receptors in your skin that deal with ...
- 3,211
27
votes
Accepted
What limits the speed of a car?
There are a few simple reasons why the speed of a vehicle (road conditions notwithstanding) may be limited:
Gearing -- Production vehicles with conventional transmissions have a limited number of ...
- 402
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 ...
- 4,897
26
votes
How do bike wheels keep turning when the rider stops pedalling?
it is called a freewheel.
Inside the sprocket gear, there is a mechanism that lets the chain engage the rear wheel only when it is moving faster than the wheel is turning; else the wheel will turn ...
- 21.6k
26
votes
Why does "potential energy" have the word "potential" in it?
Because it has the potential to produce mechanical work.
- 24k
26
votes
Why are bulldozers so slow?
From memory, but pretty much realistic: a tank will use e.g. 1000 liters of diesel fuel just to move 100 km and will probably require a track maintenance afterwards. A truck with the same tank on its ...
- 499
25
votes
Accepted
Why would you launch a large ship by dropping it sideways?
A specific reason for doing this is simply when there isn't enough room to do a bow or stern first launch. This is often the case when a ship or boat is built in a yard on a river or canal either ...
- 15.1k
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 ...
- 13k
24
votes
Accepted
Why are most jigsaw blades designed to cut on the up stroke and have teeth that point towards the shank?
The photo you have is not for the standard jigsaw blade. Standard blades teeth point upward and cut upward for the obvious reasons: less wobble, more control, center of force near the handle.
This is ...
- 21.6k
24
votes
Accepted
How do vending machines prevent people from stealing goods?
The vending machines have different product delivery box implementations.
One of the most common ones -I've encountered- is the following:
Basically you have a box which has an opening, as shown in ...
- 24k
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 ...
- 24k
22
votes
Effective ways to drive a single shaft with multiple motors?
A differential is a mechanical device designed to do exactly what you propose. It will allow the two motors to spin at slightly different rates while still combining the power. The most common use ...
- 6,386
Only top scored, non community-wiki answers of a minimum length are eligible