Chris Johns
  • Member for 5 years, 11 months
  • Last seen more than a month ago
9 answers
83 votes
37k views
Why do glass windows still exist? (Why haven't they been replaced by plastics?)
110 votes

There are two main reasons why glass is still preferred over say PMMA. The first is durability. As long as it isn't broken, the glass in a window can easily last for hundreds of years in good ...

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6 answers
20 votes
7k views
Why did steam locomotives not transfer power by cogwheels?
36 votes

Steam piston engines can generate a lot of torque from stationary and the pistons can be physically remote from the boiler, so in most cases it is most convenient to have the pistons directly drive ...

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11 answers
24 votes
8k views
From an engineering point of view what are the downsides of a too accurate model?
26 votes

The most obvious downside is cost, all engineering projects have a finite budget and spending more money than you need to is clearly a bad thing not to mention wasting time. There can also be more ...

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2 answers
24 votes
6k views
Why would you launch a large ship by dropping it sideways?
Accepted answer
25 votes

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 ...

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1 answers
12 votes
2k views
Why was Freon used in spray cans before it was declared a threat to ozone?
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18 votes

Spray cans work by containing a propellant which has a boiling point a small amount below room temperature. When the can is sealed the liquid and vapour reach equilibrium at a relatively modest ...

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9 answers
44 votes
8k views
Is it possible for a bicycle chainwheel to have a fractional number of teeth?
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18 votes

I suspect that the answer to this is that, ultimately the gear ratio comes from the ratio of diameters of the gears rather than the number of teeth, although in most circumstances practicality ...

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4 answers
6 votes
3k views
Is it useful to stuff my glasses with newspaper to protect them?
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12 votes

In itself putting paper inside a glass will make very little difference. Glass is a brittle material and tends to fail by shock and point loading. Its static tensile strength is actually pretty good. ...

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4 answers
4 votes
3k views
Why wont Petrol engine run on Acetylene?
Accepted answer
12 votes

There are certainly a few reasons why acetylene is not a very practical fuel for an IC engine. Perhaps the most important is that it is inherently unstable and tends to explode under pressure, this ...

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4 answers
18 votes
30k views
What material is used to hold molten iron in a furnace?
11 votes

Molten ferrous metals are often handled in steel ladles with a refractory lining. It's only since about the 1860s that any ferrous metals other than cast iron (which has a significantly lower ...

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4 answers
3 votes
22k views
Why use bushings of different material than what they contact?
10 votes

As already mentioned in many contexts you will make one part essentially consumable in order to reduce wear on an expensive or difficult-to-replace part. Similarly if you have a situation like a ...

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3 answers
14 votes
131k views
Calculation of Clamping force from bolt torque
10 votes

That figure is about right for a low tensile bolt. See also this calculator and this table As a reality check if we approximate to a cross sectional area of 7 mm2 and a load of 1000 N that gives a ...

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7 answers
11 votes
16k views
Why do we use multiple reinforcing bars instead of one bar with a big diameter in reinforced concrete?
10 votes

The main purpose of rebar is to improve the tensile strength of concrete and in practice most of these loads come from bending rather than pure tension. When a beam in subject to bending forces the ...

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3 answers
12 votes
1k views
PID controller explanation
9 votes

In intuitive terms I have found the following explanation to be useful. For the sake of argument lets say that our system is filling a bucket with a hole in it water from a tap. We measure the depth ...

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5 answers
13 votes
3k views
Are there any materials known to be sound-insulating but thermally-conductive?
9 votes

I can think of a few possible ways to approach this. Probably the first thing to look at is the design of the case itself. They generally tend to be made up of thin, flat panels riveted to a frame. ...

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1 answers
3 votes
248 views
What's the name of the white lubricant that children's toys often use?
8 votes

It is lithium grease aka white grease. It is often sold in aerosol can packaging. It adheres well to moving parts and tends to work well for long term lubrication of low to moderate load mechanisms.

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2 answers
6 votes
4k views
Why manufacturers prefer Brass over Copper in electrical conductors?
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8 votes

Pure copper is a very soft metal and while it's ductility is good for wires and other flexible connectors it lacks the mechanical strength required for pins etc. Also brass can have somewhat better ...

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2 answers
2 votes
2k views
Is it safe to hold up a large heavy mirror with copper wire?
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8 votes

Copper wire is not ideal as it is soft, has low tensile strength and can creep under load over time and if you are reusing old electrical wire there is no way to know whether it has small cracks in ...

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4 answers
9 votes
7k views
Many small bolts, or few large bolts?
7 votes

There are a few principal advantages to having more bolts. The first is that the loads are more evenly distributed, especially when the rigidity of the fixture itself is a bit marginal and when it ...

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3 answers
8 votes
6k views
Heat pipes vs fluid cooling
Accepted answer
7 votes

Heat pipes are very useful when you have limited space to work with or you want an entirely sealed and self contained unit. An interesting example of this is that heat pipes are used to cool the ...

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2 answers
2 votes
64 views
Why conduct experiment for load distribution within three triangulated structural frameworks when you can calculate theoretically
7 votes

There are several general reasons for conducting practical tests in addition to theoretical analysis. to verify that the theoretical approach and associated assumptions are valid. Pretty much all ...

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2 answers
4 votes
760 views
When are Nyloc nuts indicated instead of cap nuts?
7 votes

Not all cap nuts are self locking, some are, although they can be a bit weird looking. Nylocs are generally the first choice for general use as they provide good resistance to loosening due to ...

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4 answers
8 votes
3k views
Do "non-iron steels" exist?
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7 votes

Iron and carbon have an interaction which make them different from most engineering alloys. This is to do with both the relative size of C and Fe atoms and their chemistry. Carbon atoms are just the ...

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1 answers
-1 votes
124 views
What would be the implications of a metal highway design?
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7 votes

Asphalt, which is essentially crushed aggregate bound with bitumen has a lot of advantages for road building. It is porous and so can drain by allowing water to pass through it as well as running ...

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4 answers
5 votes
768 views
How do I add a resistance to mechanical movement in both directions, apart from friction?
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7 votes

The two basic mechanical options are either a viscous damper or inertia or indeed a combination or both. Of the two a damper is probably the closest to what you want as it will provide a force which ...

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2 answers
3 votes
2k views
If gold is a worse electrical conductor than silver and copper, why are gold plated contacts considered "better" by the market?
Accepted answer
6 votes

Gold is used for connectors not so much for its conductivity as it's chemical inertness. Even ostensibly corrosion resistant materials like aluminium and stainless steel form a thin metal oxide ...

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4 answers
4 votes
3k views
How does an expansion tank work?
Accepted answer
6 votes

It is better to think of it in terms of volume than pressure. The key point is that air is compressible while water is not. If you heat water is has to expand. If it is in a closed container then it ...

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2 answers
-1 votes
137 views
Why are car bodies not made of titanium?
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6 votes

The bulk price of titanium doesn't tell the whole story. A big chunk of the cost of titanium parts is down to the fact that it is significantly more costly to make things out of titanium than steel. ...

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2 answers
2 votes
1k views
Why are coils used to generate magnetic fields
Accepted answer
6 votes

The current generated by a conductor moving through a magnetic field is related to the length of the conductor and the magnetic field strength. A coil is simply a convenient way to get a longer ...

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2 answers
5 votes
305 views
What is the correct pipe size for a 2inch pipe?
Accepted answer
6 votes

I would guess that the discrepancy is down to rounding fractional inch sizes to decimal inches ie 2.38 is not exactly 2 3/8" Note that 2.375 (two and three quarter eighth inches) is 60.325mm

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5 answers
1 votes
395 views
Why isn't it possible to build a car moved by wind power?
6 votes

The fundamental problem in this case is that the turbine in not powered by 'wind' as such but by the relative motion of the car through the air. This is not 'free' energy, because the turbine must ...

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