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  • saurabhjain

    Calling Mechanical Engineers to collaborate on Twitter

    By saurabhjain

    If you are a mechanical engineering professional and have a twitter account .. we invite you in our mechanical engineering  campaign to collaborate on twitter.. Retweet the following status on https://twitter.com/mechportal/status/646544243649961985 Look forward for your presence. Regards Mechanical Engineeirng forum
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  • DrD

    #20 -- A Question of Stability (Revised)

    By DrD

        Mechanics Corner
        A Journal of Applied Mechanics and Mathematics by DrD, #20
        © Machinery Dynamics Research, 2015
    A Question of Stability Introduction     The word stability in its several forms is widely used in nontechnical communication. A person whose life it highly consistent from day to day is said to have a stable life. When the political situation in a particular area appears to be unlikely to change, it is said to be stable. A person who is well balanced and unlikely to be easily provoked to anger is said to be a stable person. When the medical condition of a sick or injured person ceases to get worse, the person is said to be stabilized. A company on the verge of bankruptcy is said to be an unstable company. But what does the word stability mean in a technical context? Each of the foregoing examples hints at the technical meaning without really being explicit about it.   A factor g = accel of gravity was missing in the potential energy expression. That is now corrected.
        
     
        Stability.pdf
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  • DrD

    A Question for Readers

    By DrD

    Many of you have asked me various questions, so now it is my turn. Let me lay a bit of background first, and then the questions.   I have had some conversations recently with JAG (one of the other writers here at ME Forums) regarding the choice of software for 3D modeling and analysis. JAG has made some excellent suggestions, specifically a cloud based program called Onshape. Unfortunately, for reasons that are unclear, my computer cannot run Onshape; I have worked with their help people for several hours, all to no avail. JAG recommends this in part because there is a "free version for the hobbyist" and a relatively inexpensive "full version for the professional." That is pretty attractive, but since I can't run it, I'm stuck.   I gather that virtually all engineering colleges these days are teaching some sort of 3D modeling and analysis software, but that raises a few questions in my mind. 1. If your college teaches brandX 3D software, what will you do when you go to work for a small company that cannot afford anything more than 2D drafting (simple CAD), with no analysis capability at all? How will you do your job then? You probably have your own pocket calculator, but will you have your own copy of ANSYS or Pro-E? 2. What software does your school teach (every students should have an answer to this question, so I expect lots of replies on this one!)? 3. If you have used software extensively for analysis of engineering problems (beam deflections, stress analysis, fluid flow, heat transfer, etc), are you confident  that you will be able to work all of those problems if there is no such software available to you on the job?   I might add, as sort of a postscript, most of you know that I am older than dirt (I just had another birthday, so the situation is even worse!), so I tend to look at things from an elderly perspective. One of my great fears as a working engineer was "What will happen when I'm ask to do something that I don't know how to do?" It happened more than once, and it usually resulted in a flurry of intense research to come up to speed on whatever topic was involved. I could usually do that because I have a pretty good library, and I knew how to use a university library as well. But in terms of software, I was always concerned that I had no FEA program, so how could I do problems that others were doing by FEA? I have come up with some interesting work-arounds, including writing my own FEA for some problems, but I never wanted to be dependent on software that I could not afford to own. So, back to my questions about: How are you going to buy your own copy of ANSYS? DrD
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  1. Crystalline semiconductors such as silicon can catch photons and convert their energy into electron flows. New research shows that a little stretching could give one of silicon's lesser-known cousins its own place in the sun.

    Nature loves crystals. Salt, snowflakes and quartz are three examples of crystals – materials characterized by the lattice-like arrangement of their atoms and molecules.

    Industry loves crystals, too. Electronics are based on a special family of crystals known as semiconductors, most famously silicon.

    To make semiconductors useful, engineers must tweak their crystalline lattice in subtle ways to start and stop the flow of electrons.

    Semiconductor engineers must know precisely how much energy it takes to move electrons in a crystal lattice.

    This energy measure is the band gap. Semiconductor materials such as silicon, gallium arsenide and germanium each have a band gap unique to their crystalline lattice. This energy measure helps determine which material is best for which electronic task.

    Now an interdisciplinary team at Stanford has made a semiconductor crystal with a variable band gap. Among other potential uses, this variable semiconductor could lead to solar cells that absorb more energy from the sun by being sensitive to a broader spectrum of light.

    Artificial-crystal-color.thumb.jpg.0c670

    A colorized image, enlarged 100,000 times, shows an ultrathin layer of molybdenum disulfide stretched over the peaks and valleys of part of an electronic device. Just 3 atoms thick, this semiconductor material is stretched in ways to enhance its electronic potential to catch solar energy.

    The material itself is not new. Molybdenum disulfide, or MoS2, is a rocky crystal, like quartz, that is refined for use as a catalyst and a lubricant.

    But in Nature Communications, Stanford mechanical engineer Xiaolin Zheng and physicist Hari Manoharan proved that MoS2 has some useful and unique electronic properties that derive from how this crystal forms its lattice.

    Molybdenum disulfide is what scientists call a monolayer: A molybdenum atom links to two sulfurs in a triangular lattice that repeats sideways like a sheet of paper. The rock found in nature consists of many such monolayers stacked like a ream of paper. Each MoS2 monolayer has semiconductor potential.

    "From a mechanical engineering standpoint, monolayer MoS2 is fascinating because its lattice can be greatly stretched without breaking," said Zheng, an associate professor.

    By stretching the lattice, the Stanford researchers were able to shift the atoms in the monolayer. Those shifts changed the energy required to move electrons. Stretching the monolayer made MoS2 something new to science and potentially useful in electronics: an artificial crystal with a variable band gap.

    "With a single, atomically thin semiconductor material we can get a wide range of band gaps," Manoharan said. "We think this will have broad ramifications in sensing, solar power and other electronics."

    Scientists have been fascinated with monolayers since the Nobel Prize-winning discovery of graphene, a lattice made from a single layer of carbon atoms laid flat like a sheet of paper.

    In 2012, nuclear and materials scientists at Massachusetts Institute of Technology devised a theory that involved the semiconductor potential of monolayer MoS2. With any semiconductor, engineers must tweak its lattice in some way to switch electron flows on and off. With silicon, the tweak involves introducing slight chemical impurities into the lattice.

    In their simulation, the MIT researchers tweaked MoS2 by stretching its lattice. Using virtual pins, they poked a monolayer to create nanoscopic funnels, stretching the lattice and, theoretically, altering MoS2's band gap.

    Band gap measures how much energy it takes to move an electron. The simulation suggested the funnel would strain the lattice the most at the point of the pin, creating a variety of band gaps from the bottom to the top of the monolayer.

    The MIT researchers theorized that the funnel would be a great solar energy collector, capturing more sunlight across a wide swath of energy frequencies.

    When Stanford postdoctoral scholar Hong Li joined the Department of Mechanical Engineering in 2013, he brought this idea to Zheng. She led the Stanford team that ended up proving all of this by literally standing the MIT theory on its head.

    Instead of poking down with imaginary pins, the Stanford team stretched the MoS2 lattice by thrusting up from below. They did this – for real rather than in simulation – by creating an artificial landscape of hills and valleys underneath the monolayer.

    They created this artificial landscape on a silicon chip, a material they chose not for its electronic properties, but because engineers know how to sculpt it in exquisite detail. They etched hills and valleys onto the silicon. Then they bathed their nanoscape with an industrial fluid and laid a monolayer of MoS2 on top.

    Evaporation did the rest, pulling the semiconductor lattice down into the valleys and stretching it over the hills.

    Alex Contryman, a PhD student in applied physics in Manoharan's lab, used scanning tunneling microscopy to determine the positions of the atoms in this artificial crystal. He also measured the variable band gap that resulted from straining the lattice this way.

    The MIT theorists and specialists from Rice University and Texas A&M University contributed to the Nature Communications paper.

    Team members believe this experiment sets the stage for further innovation on artificial crystals.

    "One of the most exciting things about our process is that is scalable," Zheng said. "From an industrial standpoint, MoS2 is cheap to make."

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    VKT0526
    Latest Entry

    Atomic Number : 13

    Density (20oC) : 2.70 g/cm3

    Atomic Weight : 26.98

    Melting point : 660o C

    Boiling point : 2467o C

    GENERAL

    Aluminum finds use as a deoxidizer, grain refiner, nitride former and alloying agent in steels. Its ability to scavenge nitrogen led to its widespread use in drawing quality steels, especially for automotive applications. Since aluminum is so often added to high quality steels.

    AVAILABLE FORMS
    Metallic aluminum is the most common addition agent. It is sold in the form of notch bars, or stick, and as shot, cones, small ingots, chopped wire, “hockey pucks”, briquettes and other convenient forms such as coiled machine fed wire. These standard products are supplied in bulk or packaged in bags or drums. Purity for deoxidation grades is usually over 95%, the major tramp elements being zinc, tin, copper, magnesium, lead and manganese. Coiled aluminum wire is normally made to 99% minimum specification.
    Ferroaluminum, a dense and highly efficient aluminum addition, contains 30-40% Al. It is supplied in lump form, 8 in. x 4 in., 5 in. x 2 in., 5 in. x D, and 2 in. x D, and nominal 12 lb. and 25 lb. pigs, packed in drums and pallet boxes.

    HEAT TREATMENT
    Aluminum has a weak effect on hardenability (it is never added for this purpose) and, because of its grain refining properties, actually detracts from deep hardening. Heat treatable steels made to fine grain practice require slightly extra alloying to counteract this phenomenon. Aluminum is, however, a ferrite former and promotes graphitization during long-term holding at elevated temperatures. It also enhances creep, probably because of its grain refining property. Aluminum, therefore, should not be used in Cr-Mo or Cr-Mo-V steels specified for boiler or high temperature pressure vessel applications. Perversely, aluminum is otherwise beneficial to such materials since it reduces scaling through the formation of a more tightly adhering oxide film, particularly if chromium is present as well.

    APPLICATIONS

    Beyond its important functions in deoxidation and grain size control, aluminum has several applications as an alloying agent. Nitriding steels, such as the Nitralloy family, contain up to 1.5% Al to produce a case with hardness as high as 1100 VHN (70 RC). The outer layer of this case must, however, be removed by grinding to prevent spalling in service. The oxidation (scaling) resistance imparted by aluminum is exploited in some stainless steels and various high temperature alloys. Precipitation hardening stainless steels (17/7 PH, 15/7 PH, etc.) make use of aluminum’s ability to form strength-inducing particles of intermetallic compounds. Aluminum is found in many superalloys for the same reason.

    Aluminum combines very readily with nitrogen, and this effect has important commercial uses. Aluminum killed deep drawing steels will be nonaging since AlN is extremely stable. Such steels will not exhibit stretcher strains (Lüder’s lines) or a yield point, even after prolonged holding after cold rolling. Aluminum is also added to nitriding steels for its ability to form an extremely hard case.

    Aluminum is an important addition to some HSLA steels, and AlN was the first nitride used to control grain size in normalised and heat treated steels. Again, Al removes nitrogen from solution and provides grain refinement. Both of these effects promote high toughness, especially at low temperatures.

    Mention should be made of the effect of aluminum on nonmetallic inclusions, since these will always be present in AK steel. Because aluminum is among the strongest deoxidizers known, it can combine with, and partially or totally reduce, any other oxides present in steel. The subject is quite complex and depends not only on aluminum, but also on oxygen, nitrogen, sulfur, manganese, silicon, and calcium contents. For ordinary steels, however, the pattern is generally as follows: unkilled steels will contain oxides of iron, manganese and silicon, to the extent they are present. Steels deoxidized with silicon and aluminum will contain complex inclusions containing silica, alumina and manganese and iron oxides. As aluminum is increased, it gradually replaces silicon in the inclusions, and the principal inclusions in aluminum killed steels will be alumina and iron-manganese aluminates. Calcium-aluminum deoxidized steels will contain calcium aluminates, the composition and properties of which will depend on oxygen content (see Calcium). The residual Al
    2O3 in a ladle aluminum deoxidized steel will usually be in the range of 0.015-0.020%. This alumina range will be present regardless of the amount of aluminum used for deoxidation. It is assumed that the remaining alumina of iron aluminate is slagged off.

    Aluminum also has a profound effect on the structure of sulfide inclusions. The three basic types of sulfides present in steels have been designated as Type I (fine, randomly distributed spheroids, usually oxysulfides), Type II (intergranular chains which are most harmful to mechanical properties) and Type III (large, globular particles with complex, multiphase structures). Incomplete deoxidation with aluminum results in Type I inclusions; complete, but not excessive deoxidation produces Type II inclusions, while excessive aluminum addition leads to the formation of the Type III particles.

    High aluminum contents also promote the generation of interdendritic alumina galaxies, which can impair machinability. Aluminum is added in some stainless grades to improve machinability.

    Aluminum as alumina in calcium aluminate slags has found extensive use as slag conditioners at LMF stations. These are used to remove sulfur and inclusions, to lower costs of dolomitic lime, fluorspar, aluminum and calcium carbide additions, to protect the refractory lining, and to improve castability. Applications include both aluminum- and silicon-killed steels.

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  2. What is the basic difference between Shaft and axle in automobile..?

    looking for some valid and good answer. Hope you can do this.

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    Oil has always been the most beneficial of natural resources, as much as it is useful as a source of power and energy as fuel, like LP gas or Petrol, there are other alternative methods for harnessing its power. This is evident by the abundance of machinery and equipments using this "Fluid Power" as a source for getting useful work done. Isn't it amazing to think that it can also be used to power structures like Cranes and Presses.

    Welcome to the amazing world of fluid power.

    One of the best places to get up close and personal lesson on hydraulics are machines at a construction site. The thing that is most amazing about these machines is their sheer size. From backyard log splitters to the huge machines you see on construction sites, hydraulic equipment is amazing in its strength and agility. On any construction site you see hydraulically-operated machinery in the form of bulldozers, backhoes, shovels, loaders, fork lifts and cranes. In most other hydraulic systems, hydraulic cylinders and pistons are connected through valves to a pump supplying high-pressure oil .

    The word "HYDRAULICS" is derived from the Greek "HYDRO" meaning water. (See Hydraulics at Wikipedia). There are many instances in the past where water was employed for useful work in the distant past to suit our needs. Modern hydraulics, especially in the industrial context is mainly concentrated on Oil as power transmission mediums.

    why fluids...?

    Fluids like water or oil are the best substances to work using pressure.

    "Pressure" is the epicenter of all hydraulic system design calculations. Pressure arises because of the sheer density of water, as we know which is about 1000 kg/cu. meter. The weight of water column increases in proportion to the depth, as in the case of the ocean. The deeper we dive, the higher we feel the pressure.

    The same pressure term can be used, when we are analyzing it in a context of a hydraulic press, which of course is an equipment found in a factory, on land, rather than the sea. And it also does not need lots of water to accomplish the "Pressing". The pressure in a hydraulic press is induced by means of other compression media, pumps for example, which are more compact.

    But... Still what next. How does it still help for oil/ liquid to be used for the working of hydraulic systems?

    The reason, quite simply put it, Fluids (oils talking in industrial context, i.e., everything with specific gravity value less than or equal to 1) are incompressible. (It is known that oils are lighter, specific gravity< 1).

    This brings us to what Pascal discovered, combining pressure and incompressibility, which we know today as the universal law stating...

    "Any change of pressure on an incompressible fluid in an enclosed space is transmitted equally and undiminished in all directions on to the surface of the container".

    What supports the statement above is that fluids, like water and oil, unlike gases(gases can also be termed fluids because they can flow) are the least compressible. Talking of compressibility, it is known that intermolecular forces are at the least among gases. They are like...free to move about in every direction, spread if allowed to(The open perfume bottle in a room trick) but also compressible into closed containers upto pressures possibly unthinkable. These gases, viz.,atmospheric air, can also be used for power transmission, but their use is limited. Pneumatics, or "air power" as an engineering entity is quite similar to hydraulics using the same fundamentals of pressure causing displacement or useful work, but with a lesser efficiency. Still, gases under pressure do follow the Pascal's Law. It is to be understood that both pneumatics and hydraulics employ fluid power, only difference being the use of medium for power transmission.

    Following Pascal's law...As liquids or oils are incompressible, let us consider any closed container, containing a liquid, such as a piston cylinder assembly. If it is subjected to force along the piston rod, ideally the fluid trapped in would have equal reactive forces acting normally to the inner walls of the cylinder, and also opposing the piston, or in other words....the cylinder walls and piston will the stressed uniformly from within. The fluid pressure acts equally on all the surfaces it comes in contact with.

    We may have another physical interpretation....

    Imagine the Tyre of a vehicle being inflated...It gets all puffed up and stiff from all sides and corners, doesn't it?. All the sides inflate uniformly.Same Logic applies since gases are also fluids. As earlier stated, it is Pneumatic power that is used, but it still works as per law.

    This concludes our first basic insight into how a hydraulic system works.

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    Hypothetically, Knowledge-based Design Automation can automate any simple or complex engineering or manufacturing-related design tasks. They enable real-time design of custom products and allow for complete product design, thereby, speeding up the product development cycle. They also enable reduction of engineering time for custom products from 70% to 80% over traditional CAD approaches. Knowledge-based Design Automation can be full or partial in nature.

    Practically, I would like to understand and explore the feasibility and scope of the MDA..........I request SME / Professionals to share and contribute to achieve my objective.

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