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42
votes
9answers
7k views

Is it possible for a bicycle chainwheel to have a fractional number of teeth?

Background In the world of bicycle motocross, also known as BMX racing, gearing is a hotly-debated topic. Since the bikes are all single-speed, gear ratio is a fixed number defined as ...
13
votes
2answers
6k views

Quantifying inertia on the electricity grid

A lot of discussion about the modernisation of electricity systems is about "inertia". This is usually a qualitative discussion about how turbines (in hydro, coal & gas plants) with lots of ...
44
votes
9answers
6k views

My customer wants to use my products to do something unsafe. What is my ethical obligation?

We sell products that attach to a motor drive's DC bus. We also formerly sold diode kits that let you hook one product up to multiple drives. We stopped selling those diode kits because they were ...
32
votes
4answers
8k views

Why dig out and then fill in before building a large structure?

I work in the middle of London, in an area full of large office blocks. Across the road from my office they have started construction of a large building (10 stories plus). Over the last few weeks, ...
22
votes
3answers
1k views

How to know whether the flow is supersonic in a nozzle?

For a project I had built a convergent divergent nozzle designed for Mach number = 3. In that project, I could know the flow has gone supersonic by seeing the manometer fixed between the throat and ...
14
votes
3answers
4k views

What happens when you put the wrong type of fuel in an internal combustion engine?

What happens to the working of petrol engine when it is emptied and filled with diesel or to a diesel engine emptied and filled with petrol? Will the engine be able to operate and, if not, why not?
12
votes
3answers
5k views

Can a magnetic lock be negated or overcome by a stronger electromagnet?

Magnetic locks seem to be fairly common on the doors of modern buildings, but I question their reliability when compared to a traditional, mechanical lock. I realize that the strength that it would ...
6
votes
1answer
636 views

efficiency and flow separation

I am studying about wind turbine and turbine. I have some questions, but first, I explain some cases as follow: Low velocity air over an airfoil at a zero-incidence angle does not initiate flow ...
3
votes
2answers
590 views

What is the strongest known *metallic* material which can be used on Earth?

The discovery of nanotubes and graphene has pushed the limit of material strength high enough that building a space elevator could be managable. But most of the new materials are created from carbon ...
20
votes
6answers
9k views

How to make smoke for a small wind tunnel?

I am making a small (desktop) wind tunnel for educational purposes, I want to have 10 fairly thick smoke-streams about 3cm apart. I have experimented with incense but the stream is not thick enough ...
15
votes
2answers
6k views

How do I size metal plates to get the correct dimensions after folding?

I am designing a metal plate that will be laser-cut (or machine-cut) and then folded. I want to know how to size the pre-folded plate in order to get the right dimensions after folding. My actual ...
12
votes
4answers
5k views

How do I calculate the forces on a desk and its legs?

I have a design for a desk, and I'd like to not just guess at how strong it'll be, but I can't find an explanation on how to figure out all the forces involved that doesn't assume I already know a lot ...
11
votes
1answer
2k views

How to calculate Power of motor when used as a generator?

How can you calculate how much power can be generated by a motor when used as a generator (like a motor used in a wind turbine)? Does the power depend on how fast the magnet is rotated? If so, is ...
10
votes
2answers
493 views

How is gas flow through (extremely long) pipelines monitored and controlled?

(This is closely linked with measuring the Mach number inside a nozzle but it is not regarding the supersonic flow) Friction and heat transfer have effects on the Mach number of the compressible flow ...
9
votes
4answers
2k views

Humidity in home is much higher than outside

We have a two year-old, two-story town house with: fiberglass insulation in the walls; a Tyvek vapor barrier under foam board on the exterior walls; blown cellulose insulation in the attic; stucco ...
9
votes
1answer
4k views

What is the difference between Bluetooth Low Energy and Bluetooth BR/EDR in Park mode?

It is known that Bluetooth Low Energy transmits data only during short time intervals called Connection Events. Connection events occur regullary with predefined period. The rest of the time Bluetooth ...
7
votes
2answers
728 views

How do I use FEM to derive the torsional constant of an arbitrary shape?

In this question I ask about how to perform a first-principle derivation of the torsional constant of a section. It appears that there is no such analytical derivation for torsional constant, so my ...
7
votes
1answer
9k views

How do train tracks handle really cold weather?

First of all, I'm interested in train track only, not the rolling stock. How are tracks built to cope with really cold weather? An example might be some place in Canada or Siberia. Ice would ...
7
votes
2answers
1k views

How do you model a real-life truss in structural analysis software?

I am trying to recreate the following model of a wooden roof truss provided to me by a truss manufacturer: My question is twofold: What is the common/correct way to model the boundary conditions and ...
6
votes
1answer
221 views

Hanging a steel mesh without significant deformation

I have an welded steel mesh, like in the following image. . Material: steel Diameter of the wires: 4mm Mesh: 15x15cm I found the following information about the welds: the minimal relative ...
4
votes
1answer
902 views

How to secure a rotating disc so that it has no wobble

For a 3D printing application, I need to have a spinning disc(that I will rotate via geared stepper motors) that will wobble as little as possible. By wobble, I mean that it will only rotate around it'...
4
votes
1answer
766 views

Velocity of flue gases through a pipe

I was wondering if there is any equation that helps to calculate flue gas velocity through a pipe? Can I use the Poiseuille Equation to solve this problem by assuming the flue gas is acting as a ...
4
votes
2answers
23k views

Physical meaning of shear lag [closed]

What is the physical meaning behind the concept of shear lag in fibre reinforced composite structures or the concept in general for any structure?
3
votes
1answer
253 views

How does a Levitron work?

I just watched this video: https://www.facebook.com/HigherPerspective/videos/1162007547164896/ I don't understand what I see. If the spinner's magnetic, the bowl is made of iron and that the wood ...
3
votes
1answer
1k views

How to design a cooling chimney for a computer server?

I'm building a server type computer consisting of integrated motherboard, CPU, RAM and hard disk. There will also be a standard power supply unit. This is to be a Steam Punked computer, so... I ...
3
votes
2answers
7k views

What is Saint Venant Principle?

I am having problem with the application of Saint Venant principle.I have heard that quite often we use that knowingly or unknowingly.I would be very greatful to anyone who would explain this very ...
3
votes
2answers
1k views

Is the reliability of automotive sensor technology impeding the success of self driving car?

The reality is that sci-fi KITT is almost here. Recently I read articles Delphi car driving itself across the country as well as Tesla cars will be self-driving this summer. As of 2013 four states (...
2
votes
1answer
867 views

Would a double helix spring be more or less effective than two springs?

Say I have a spring in the shape of the backbone of DNA. If I unwind the two parts which would have more potential energy when pushed all the way down, the double helical or both of the individual ...
0
votes
2answers
2k views

How would I find the Second Moment of Area of a Parallelogram?

How would I find the second moment of area of a parallelogram, as shown?
0
votes
1answer
88 views

How can I calculate the power and torque required for the motor on a wheeled robot/vehicle?

How can I calculate the power and torque required for the motor on a wheeled robot or vehicle if a particular acceleration or movement up an incline is required?
23
votes
2answers
7k views

Why doesn't a lightning strike destroy the lightning rod?

Lightning strikes have been known to cause massive amounts of damage. The stats on a lightning bolt are: current levels sometimes in excess of 400 kA, temperatures to 50,000 degrees F., and speeds ...
20
votes
4answers
24k views

Would it make sense to have a 12V lighting circuit in a house?

Modern LED bulbs must convert the standard household supply (for example $240\text{V AC}$ in the UK) into a DC supply at a lower voltage (usually $12\text{V DC}$ I think) for the LED array. This is ...
17
votes
1answer
2k views

How to design a house to be cooled passively? [duplicate]

I live in Louisiana these days, in an area that is known for its numerous antebellum plantation homes (circa early 1800s). While touring one of these homes it was clear that almost everything about ...
16
votes
4answers
5k views

Why does the microwave plate start in a random direction?

...or what type of motor is used there? I found this type of motor - usually powered with low-voltage AC (~12V), but at times with 230V, in several appliances that require very slow rotation and ...
14
votes
4answers
1k views

How should the public raise questions about unsafe structures in the United States?

In a program on NPR that I was listening to, there was a bit about a bridge that from the description sounded to a layman as unsound and is still in use. The program described it as an old wooden ...
13
votes
4answers
15k views

How much clearance does a car need when turning a corner?

I am contemplating buying a new car. However, the approach to the underground garage in my apartment has a 90 degree frustrating turn. Given the dimensions of the approach and car, what is the max ...
13
votes
1answer
599 views

What are the lightest springs?

I am wondering if there is a spring strong enough and light enough to inflate an airtight membrane so that it floats at Standard Temp & Pressure. As an example, a 10cm cylinder would displace ...
11
votes
1answer
928 views

How to determine the lateral earth pressure in a double-walled cofferdam?

The design of a retaining wall commonly involves determining the lateral earth pressure using either Rankine theory or Coulomb theory. Both theories involve mobilising the shear resistance of a ...
9
votes
5answers
431 views

Why is “regular” gasoline standard instead of something more knock-resistant?

The standard light petroleum distillate for vehicle engines, "regular gasoline," is (or is equivalent to) some mix of heptane (C7) and octane (C8). Higher proportions of C8 are more knock-resistant, ...
9
votes
3answers
5k views

Reading stepper motor datasheets to get torque and speed

I don't understand how to use the information in a stepper motor datasheet to understand how much torque it can generate. I was going to do a simple board and try to lift a small weight as an example ...
8
votes
3answers
1k views

Motor for a hydraulic pump in a hydraulic system

I am a complete beginner to hydraulic systems, and I've wanted to learn more about this area. I'm designing a hydraulic system that involves using hydraulics to push/pull objects using pistons. I have ...
8
votes
1answer
1k views

Large deflection of a cantilever beam with distributed normal load

I have a strip of stainless steel encastree'd at one end to which is applied a constant pressure on one side, and I need to know what the deflection equation y = f(x) is at equilibrium. If the ...
8
votes
5answers
15k views

What do you call the difference between the on and off temperatures in a simple thermostat?

A simple thermostat will turn on at one temperature and off at a higher temperature. This keeps the thermostat from cycling on and off too quickly. The difference between these values is sometimes ...
7
votes
8answers
2k views

Can I weaken a coil spring consisting of spring steel?

I have a steel coil spring that is used by compression. It is too strong, and I would like to reduce its strength by some fraction, ideally keeping its length. Reducing it to roughly half would be ...
7
votes
2answers
2k views

Are there standard vibration and shock specs for shipment of a product?

Suppose I want to ship my product via UPS, or whatever other professional carrier service. What vibration and shock forces should I design my product to withstand? And how can I effectively test my ...
7
votes
3answers
7k views

Why are concrete bridges more prevalent than steel bridges in the United States?

According to the United States 2014 National Bridge Inventory the number of concrete bridges far outstrips the number of steel bridges. Reinforced Concrete: 253,336 Prestressed Concrete: 148,333 ...
7
votes
1answer
1k views

Modeling Transient Heat Transfer between two 1-D materials

I have been working on creating a 1-D model of a material with a layer of heat resistant material stuck to it. For this I have been trying to use the implicit finite-differential method. The equations ...
7
votes
5answers
1k views

What was that?! Quantifying the impact of a martial arts strike

Measuring things like a martial artist's strike can be challenging because it is a complex motion and common vernacular will use imprecise or incorrect terms. This question is an attempt to identify ...
6
votes
1answer
427 views

Differential equation of the vertical displacement of a cable

Given a cable (totally flexible) fixed at both ends, subjected to a vertical force $f(x)$ in his plane, with variable area $A(x)$, and variable elasticity $E(x)$ I want to find the differential ...
6
votes
3answers
1k views

Truss manufacturer drawings unclear

I am currently looking at a set of truss layouts produced by our truss manufacturing partner for a residential building project. I have reason to believe these layouts are generated using Alpine (...

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