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A screw thread, often shortened to thread, is a helical structure used to convert between rotational and linear movement or force. A screw thread is a ridge wrapped around a cylinder or cone in the form of a helix, with the former being called a straight thread and the latter called a tapered thread. More screw threads are produced each year than any other machine element.
The mechanical advantage of a screw thread depends on its lead, which is the linear distance the screw travels in one revolution. In most applications, the lead of a screw thread is chosen so that friction is sufficient to prevent linear motion being converted to rotary, that is so the screw does not slip even when linear force is applied so long as no external rotational force is present. This characteristic is essential to the vast majority of its uses. The tightening of a fastener's screw thread is comparable to driving a wedge into a gap until it sticks fast through friction and slight plastic deformation.
Screw threads have several applications:
- Moving objects linearly by converting rotary motion to linear motion, as in the leadscrew of a jack.
- Measuring by correlating linear motion to rotary motion (and simultaneously amplifying it), as in a micrometer.
- Both moving objects linearly and simultaneously measuring the movement, combining the two aforementioned functions, as in a leadscrew of a lathe.
In all of these applications, the screw thread has two main functions:
- It converts rotary motion into linear motion.
- It prevents linear motion without the corresponding rotation.
Every matched pair of threads, external and internal, can be described as male and female. For example, a screw has male threads, while its matching hole (whether in nut or substrate) has female threads. This property is called gender.
The helix of a thread can twist in two possible directions, which is known as handedness. Most threads are oriented so that a bolt or nut, seen from above, is tightened (the item turned moves away from the viewer) by turning it in a clockwise direction, and loosened (the item moves towards the viewer) by turning anti-clockwise. This is known as a right-handed (RH) thread, because it follows the right hand grip rule (often called, more ambiguously, "the right-hand rule"). Threads oriented in the opposite direction are known as left-handed (LH).
To determine if a particular thread is right or left-handed, look straight at the thread. If the helix of the thread is moving up to the right, it is a right-handed thread and conversely up to the left, a left-handed thread. This holds whether the thread is oriented up or down.
By common convention, right-handedness is the default handedness for screw threads. Therefore, most threaded parts and fasteners have right-handed threads. Left-handed thread applications include:
- Where the rotation of a shaft would cause a conventional right-handed nut to loosen rather than to tighten due to fretting induced precession. Examples include:
- In some gas supply connections to prevent dangerous misconnections, for example in gas welding the flammable gas supply uses left-handed threads.
- In a situation where neither threaded pipe end can be rotated to tighten/loosen the joint, e.g. in traditional heating pipes running through multiple rooms in a building. In such a case, the coupling will have one right-handed and one left-handed thread
- In some instances, for example early ballpoint pens, to provide a "secret" method of disassembly.
- In mechanisms to give a more intuitive action as:
The term chiralitycomes from the Greek word for "hand" and concerns handedness in many other contexts.
The cross-sectional shape of a thread is often called its form or threadform (also spelled thread form). It may be square, triangular, trapezoidal, or other shapes. The terms form and threadform sometimes refer to all design aspects taken together (cross-sectional shape, pitch, and diameters).
Most triangular threadforms are based on an isosceles triangle. These are usually called V-threads or vee-threads because of the shape of the letter V. For 60Â° V-threads, the isosceles triangle is, more specifically, equilateral. For
An urban area is characterized by higher population density and vast human features in comparison to areas surrounding it. Urban areas may be cities, towns or conurbations, but the term is not commonly extended to rural settlements such as villages and hamlets. Urban areas are created and further developed by the process of urbanization. Measuring the extent of an urban area helps in analyzing population density and urban sprawl, and in determining urban and rural populations. Unlike an urban area, a metropolitan area includes not only the urban area, but also satellite cities plus intervening rural land that is socio-economically connected to the urban core city, typically by employment ties through commuting, with the urban core city being the primary labor market. In fact, urbanized areas agglomerate and grow as the core population/economic activity center within a larger metropolitan area or envelope. In the US, Metropolitan areas tend to be defined using counties or county sized political units as building blocks. Counties tend to be stable political boundaries; economists prefer to work with economic and social statistics based on metropolitan areas. Urbanized areas are a more relevant statistic for determining per capita land usage and densities. Definitions They vary somewhat amongst different nations. European countries define urbanized areas on the basis of urban-type land use, not allowing any gaps of typically more than 200 m, and use satellite imagery instead of census blocks to determine the boundaries of the urban area. In less developed countries, in addition to land use and density requirements, a requirement that a large majority of the population, typically 75%, is not engaged in agriculture and/or fishing is sometimes used. Australia In Australia, urban areas are referred to as "urban centres" and are defined as population clusters of 1000 or more people, with a density of at least 200/km2. Canada According to Statistics Canada, an urban area in Canada is an area with a population of at least 1,000 people where the density is no fewer than 400 persons per square km2. If two or more urban areas are within of each other by road, they are merged into a single urban area, provided they do not cross census metropolitan area or census agglomeration boundaries. China In China, an urban area is an urban district, city and town with a population density higher than 1,500/km2. As for urban districts with a population density lower than that number, only the population that lives in streets, town sites, and adjacent villages is counted as urban population. France In France, an urban area is a zone (aire urbaine) encompassing an area of built-up growth (called an "urban unit" (unitÃ© urbaine) - close in definition to the North American urban area) and its commuter belt (couronne pÃ©riurbaine). Although the official INSEE translation of aire urbaine is "urban area", most North Americans would find the same as being similar in definition to their metropolitan area. Japan In Japan urbanized areas are defined as contiguous areas of densely inhabited districts (DIDs) using census enumeration districts as units with a density requirement of 4000|PD/sqkm|PD/sqmi. New Zealand Statistics New Zealand defines New Zealand urban areas for statistical purposes as a settlement with a population of a thousand people or more. Norway Statistics Norway defines urban areas ("tettsteder") similarly to the other Nordic countries. Unlike in Denmark and Sweden, the distance between each building has to be of less than 50 m, although exceptions are made due to parks, industrial areas, rivers, and similar. Groups of houses less than 400 m from the main body of an urban area are included in the urban area. Philippines With an estimated population of 16,300,000, Metro Manila is the most populous metropolitan area in the Philippines and the 11th in the world. However, the greater urban area is the 5th largest in the world with a population of 20,654,307 people (2010 estimate). The Philippines has twelve more metropolitan areas as defined by the National Economic and Development Authority (NEDA). Metro Angeles, Metro Bacolod, Metro Baguio, Metro Batangas, Metro Cagayan de Oro, Metro Cebu, Metro Dagupan, Metro Davao, Metro Iloilo-Guimaras, Metro Naga, Metro Olongapo. Poland In Poland, official "urban" population figures simply refer to those localities which have the status of towns (miasta). The "rural" population is that of all areas outside the boundaries of these towns. This distinction may give a misleading impression in some cases, since some localities with only village status may have acquired larger and denser populations than many smaller towns. Russia In Russia, only the population residing in cities/towns and urban-type settlements is considered to be "urban". The city/town/urban-settlement designation means usually that the majority of the population is employed in areas other than agriculture, but the exact definitions vary from one federal subject to another. Sweden Urban areas in Sweden (tÃ¤torter) are statistically defined localities, totally independent of the administrative subdivision of the country. There are 1940 such localities in Sweden, with a population ranging from 200 to 1,252,000 inhabitants. United Kingdom The United Kingdom's Office for National Statistics has produced census results from urban areas since 1951, since 1981 based upon the extent of irreversible urban development indicated on Ordnance Survey maps. The definition is an extent of at least 20 ha and at least 1,500 census residents. Separate areas are linked if less than 200 m (220 yd) apart. Included are transportation features. The 20 largest urban areas are Greater London Urban Area, West Midlands Urban Area, Greater Manchester Urban Area, West Yorkshire Urban Area, Greater Glasgow, Tyneside, Liverpool Urban Area, Nottingham Urban Area, Sheffield Urban Area, Bristol Urban Area, Greater Belfast, Brighton/Worthing/Littlehampton, Edinburgh, Portsmouth Urban Area, Leicester Urban Area, Bournemouth Urban Area, Reading/Wokingham Urban Area, Teesside, The Potteries Urban Area and Coventry/Bedworth Urban Area United States In the United States there are two categories of urban area. The term urbanized area denotes an urban area of 50,000 or more people. Urban areas under 50,000 people are called urban clusters. Urbanized areas were first delineated in the United States in the 1950 census, while urban clusters were added in the 2000 census. There are 1,371 urban areas and urban clusters with more than 10,000 people. The U.S. Census Bureau defines an urban area as: "Core census block groups or blocks that have a population density of at least 1,000 people per square mile (386 per square kilometer) and surrounding census blocks that have an overall density of at least 500 people per square mile (193 per square kilometer)." The concept of Urbanized Areas as defined by the U.S. Census Bureau is often used as a more accurate gauge of the size of a city, since in different cities and states the lines between city borders and the urbanized area of that city are often not the same. For example, the city of Greenville, South Carolina has a city population under 60,000 and an urbanized area population of over 300,000, while Greensboro, North Carolina has a city population over 200,000 and an urbanized area population of around 270,000 — meaning that Greenville is actually "larger" for some intents and purposes, but not for others, such as taxation, local elections, etc. The largest urban area in the United States is that of New York City, with its city proper population exceeding 8 million and its metropolitan area population almost 19 million. The next four largest urban areas in the U.S. are those of Los Angeles, Chicago, Miami and Philadelphia. About 79 percent of the population of the United States lives within the boundaries of urbanized area as of April, 2000 . Combined, these areas occupy about 2 percent of the United States. The majority of urbanized area residents are suburbanites; core central city residents make up about 30 percent of the urbanized area population (a
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Answers:WOW that's so impressive, I don't think I would be able to handle it. Maybe if I was as smart as you....
Answers:Good choice of wire gauge and breaker for 4 outlets. Run wires as you stated to the first outlet using the bottom screws. Start the run for the next outlet using the top screws and do the same at every box. Do not use the "convenient" stick-in connections, use the screws for better continuity and voltage transfer. Last but not least, make sure the ground is firmly connected to each outlet and to the ground bar of the breaker box. Buy a "lighted" plug-in circuit tester at the hardware store and check the continuity of every outlet after wiring. If you have the polarity off or a ground not connected, it will tell you very quickly. They are only a few bucks and well worth it for troubleshooting circuits.
Answers:No, colleges don't see any of these scores. Nor do they see the PSAT/NMSQT (Practice SAT/National Merit Scholar Qualifying Test) that you'll probably take early high school, or any state-administered mandatory tests. All they ask for is the SAT Reasoning test (normal SATs), the SAT Subject tests (optional, but if you're applying to the Ivies or other first tier schools, you should definitely take at least two, in the subjects you're strongest in), and the ACT if you take it (more popular among non-New England schools, so the Ivies generally prefer the SATs.) If you're planning on trying for an Ivy, look at this as a learning experience. Look at the subjects you didn't do well in, and make sure you pay extra attention to improving in those areas. Because while this one test may not have any bearing on your acceptance, if you're only receiving proficient scores in class instead of advanced scores (aka, A's), that will. As a side piece of advice: Take the PSAT twice. It only counts for the Merit Scholar competition your junior year, and most people only take it then, then their SATs senior year, but if you're smart enough to be shooting for Ivy's, you're smart enough to be shooting for National Merit Scholars, as well. So take it sophomore year (if your school lets you, mine restricted which sophomores were allowed to take it) so you'll do better as a junior when it counts. Similarly, take your first SAT as a junior instead of as a senior, as long as you feel prepared. That way, if you're not satisfied with your scores, you have more time to improve instead of rushing to take it twice as a senior.
Answers:Basically, the parameter deciding how appropriate an instrument is before being used in a particular situation is its LEAST COUNT. The usual least count of a vernier calipers is usually 0.1mm, so when we are dealing with situations involving measurements not more minute than 0.1 mm we can safely avail of its use. The least count of a GENERAL vernier caliper can be easily determined in the following manner. A vernier caliper consists of a fixed main scale and a sliding vernier scale which, as the name suggests, slides on the main scale. The value of one division on a main scale is 1mm and on the vernier scale there are ten divisions which are equal to nine divisions of a main scale. So the value of each vernier scale mark is 0.9mm(9X1/10). The formula for calculation of the least count of a vernier scale is : Value of one M.S. division - value of one V.S. division, which comes out to be 0.1 mm, hence my statement earlier. Along with this V.C. is also used to measure the internal and external diameters of cylinders, bottles etc. There are instruments which are even more accurate than V.C. such as screw gauge which has a least count of 0.01mm.