Roots-type supercharger

Roots-type supercharger Техника

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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. A screw thread is the essential feature of the screw as a simple machine and also as a threaded fastener.

Roots-type supercharger

Screw thread, used to convert torque into the linear force in the flood gate. The operator rotates the small vertical bevel gear in the center. Through mechanical advantage this eventually causes the horizontal bevel gears (at far left and far right, with threaded center holes) to rotate. Their rotation raises or lowers the two long vertical threaded shafts — as they are not free to rotate.

Screw threads have several applications:

In all of these applications, the screw thread has two main functions:

Every matched pair of threads, external and internal, can be described as male and female. Generally speaking, the threads on an external surface are considered male, while the ones on an internal surface are considered 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. Assembling a male-threaded fastener to a female-threaded one is called mating.

Right- and left-handed screw threads

The right-hand rule of screw threads

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:

Different (and incompatible) threads including (from left) M12 left hand, standard M12, M12x1.5 (fine), M12x1.25 (fine), 1/2″ UNF, 1/2″ UNC, 1/2″ BSW, and 1/2″ BSF

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), but commonly refer to the standardized geometry used by the screw. Major categories of threads include machine threads, material threads, and power threads.

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 60° V-threads, the isosceles triangle is, more specifically, equilateral. For buttress threads, the triangle is scalene.

The theoretical triangle is usually truncated to varying degrees (that is, the tip of the triangle is cut short). A V-thread in which there is no truncation (or a minuscule amount considered negligible) is called a sharp V-thread. Truncation occurs (and is codified in standards) for practical reasons—the thread-cutting or thread-forming tool cannot practically have a perfectly sharp point, and truncation is desirable anyway, because otherwise:

In ball screws, the male-female pairs have bearing balls in between. Roller screws use conventional thread forms and threaded rollers instead of balls.

The included angle characteristic of the cross-sectional shape is often called the thread angle. For most V-threads, this is standardized as 60 degrees, but any angle can be used.
The cross section to measure this angle lies on a plane which includes the axis of the cylinder or cone on which the thread is produced.

Lead, pitch, and starts

Lead and pitch for two screw threads; one with one start and one with two starts

Up to four starts are labeled with different colors in this example.

Lead () and pitch are closely related concepts. They can be confused because they are the same for most screws. Lead is the distance along the screw’s axis that is covered by one complete rotation of the screw thread (360°). Pitch is the distance from the crest of one thread to the next one at the same point.

Whereas metric threads are usually defined by their pitch, that is, how much distance per thread, inch-based standards usually use the reverse logic, that is, how many threads occur per a given distance. Thus, inch-based threads are defined in terms of threads per inch (TPI). Pitch and TPI describe the same underlying physical property—merely in different terms. When the inch is used as the unit of measurement for pitch, TPI is the reciprocal of pitch and vice versa. For example, a -20 thread has 20 TPI, which means that its pitch is inch (0.050 in or 1.27 mm).

As the distance from the crest of one thread to the next, pitch can be compared to the wavelength of a wave. Another wave analogy is that pitch and TPI are inverses of each other in a similar way that period and frequency are inverses of each other.

Coarse versus fine

Coarse threads are those with larger pitch (fewer threads per axial distance), and fine threads are those with smaller pitch (more threads per axial distance). Coarse threads have a larger threadform relative to screw diameter, where fine threads have a smaller threadform relative to screw diameter. This distinction is analogous to that between coarse teeth and fine teeth on a saw or file, or between coarse grit and fine grit on sandpaper.

Camshaft cover stud threaded

-20 UNC (left, for aluminium cylinder head) and

-28 UNF (right, for steel nut; from a 1960s Jaguar XK engine)

The common V-thread standards (ISO 261 and Unified Thread Standard) include a coarse pitch and a fine pitch for each major diameter. For example, -13 belongs to the UNC series (Unified National Coarse) and -20 belongs to the UNF series (Unified National Fine). Similarly, M10 (10 mm nominal outer diameter) as per ISO 261 has a coarse thread version at 1.5 mm pitch and a fine thread version at 1.25 mm pitch.

The term coarse here does not mean lower quality, nor does the term fine imply higher quality. The terms when used in reference to screw thread pitch have nothing to do with the tolerances used (degree of precision) or the amount of craftsmanship, quality, or cost. They simply refer to the size of the threads relative to the screw diameter.

The three diameters that characterize threads

Sign ⌀ in a technical drawing

There are three characteristic diameters (⌀) of threads: major diameter, minor diameter, and pitch diameter: Industry standards specify minimum (min.) and maximum (max.) limits for each of these, for all recognized thread sizes. The minimum limits for external (or bolt, in ISO terminology), and the maximum limits for internal (nut), thread sizes are there to ensure that threads do not strip at the tensile strength limits for the parent material. The minimum limits for internal, and maximum limits for external, threads are there to ensure that the threads fit together.

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The major diameter of threads is the larger of two extreme diameters delimiting the height of the thread profile, as a cross-sectional view is taken in a plane containing the axis of the threads. For a screw, this is its outside diameter (OD). The major diameter of a nut cannot be directly measured (as it is obstructed by the threads themselves) but it may be tested with go/no-go gauges.

The major diameter of external threads is normally smaller than the major diameter of the internal threads, if the threads are designed to fit together. But this requirement alone does not guarantee that a bolt and a nut of the same pitch would fit together: the same requirement must separately be made for the minor and pitch diameters of the threads. Besides providing for a clearance between the crest of the bolt threads and the root of the nut threads, one must also ensure that the clearances are not so excessive as to cause the fasteners to fail.

The minor diameter is the lower extreme diameter of the thread. Major diameter minus minor diameter, divided by two, equals the height of the thread. The minor diameter of a nut is its inside diameter. The minor diameter of a bolt can be measured with go/no-go gauges or, directly, with an optical comparator.

As shown in the figure at right, threads of equal pitch and angle that have matching minor diameters, with differing major and pitch diameters, may appear to fit snugly, but only do so radially; threads that have only major diameters matching (not shown) could also be visualized as not allowing radial movement. The reduced material condition, due to the unused spaces between the threads, must be minimized so as not to overly weaken the fasteners.

Variants of snug fit. Only threads with matched PDs are truly snug, axially as well as radially.

The pitch diameter (PD, or D2) of a particular thread, internal or external, is the diameter of a cylindrical surface, axially concentric to the thread, which intersects the thread flanks at equidistant points, when viewed in a cross-sectional plane containing the axis of the thread, the distance between these points being exactly one half the pitch distance. Equivalently, a line running parallel to the axis and a distance D2 away from it, the «PD line,» slices the sharp-V form of the thread, having flanks coincident with the flanks of the thread under test, at exactly 50% of its height. We have assumed that the flanks have the proper shape, angle, and pitch for the specified thread standard. It is generally unrelated to the major (D) and minor (D1) diameters, especially if the crest and root truncations of the sharp-V form at these diameters are unknown. Everything else being ideal, D2, D, & D1, together, would fully describe the thread form. Knowledge of PD determines the position of the sharp-V thread form, the sides of which coincide with the straight sides of the thread flanks: e.g., the crest of the external thread would truncate these sides a radial displacement D − D2 away from the position of the PD line.

Provided that there are moderate non-negative clearances between the root and crest of the opposing threads, and everything else is ideal, if the pitch diameters of a screw and nut are exactly matched, there should be no play at all between the two as assembled, even in the presence of positive root-crest clearances. This is the case when the flanks of the threads come into intimate contact with one another, before the roots and crests do, if at all.

However, this ideal condition would in practice only be approximated and would generally require wrench-assisted assembly, possibly causing the galling of the threads. For this reason, some allowance, or minimum difference, between the PDs of the internal and external threads has to generally be provided for, to eliminate the possibility of deviations from the ideal thread form causing interference and to expedite hand assembly up to the length of engagement. Such allowances, or fundamental deviations, as ISO standards call them, are provided for in various degrees in corresponding classes of fit for ranges of thread sizes. At one extreme, no allowance is provided by a class, but the maximum PD of the external thread is specified to be the same as the minimum PD of the internal thread, within specified tolerances, ensuring that the two can be assembled, with some looseness of fit still possible due to the margin of tolerance. A class called interference fit may even provide for negative allowances, where the PD of the screw is greater than the PD of the nut by at least the amount of the allowance.

The pitch diameter of external threads is measured by various methods:

Classes of fit

The way in which male and female fit together, including play and friction, is classified (categorized) in thread standards. Achieving a certain class of fit requires the ability to work within tolerance ranges for dimension (size) and surface finish. Defining and achieving classes of fit are important for interchangeability. Classes include 1, 2, 3 (loose to tight); A (external) and B (internal); and various systems such as H and D limits.

The pitch diameter of a thread is measured where the radial cross section of a single thread equals half the pitch, for example: 16 pitch thread =  in = 0.0625 in the pitch actual pitch diameter of the thread is measured at the radial cross section measures 0.03125 in.

Screw threads are almost never made perfectly sharp (no truncation at the crest or root), but instead are truncated, yielding a final thread depth that can be expressed as a fraction of the pitch value. The UTS and ISO standards codify the amount of truncation, including tolerance ranges.

A perfectly sharp 60° V-thread will have a depth of thread («height» from root to crest) equal to 0.866 of the pitch. This fact is intrinsic to the geometry of an equilateral triangle — a direct result of the basic trigonometric functions. It is independent of measurement units (inch vs mm). However, UTS and ISO threads are not sharp threads. The major and minor diameters delimit truncations on either side of the sharp V.

The nominal diameter of Metric (e.g. M8) and Unified (e.g.  in) threads is the theoretical major diameter of the male thread, which is truncated (diametrically) by of the pitch from the dimension over the tips of the «fundamental» (sharp cornered) triangles. The resulting flats on the crests of the male thread are theoretically one eighth of the pitch wide (expressed with the notation p or 0.125p), although the actual geometry definition has more variables than that. A full (100%) UTS or ISO thread has a height of around 0.65p.

Threads can be (and often are) truncated a bit more, yielding thread depths of 60% to 75% of the 0.65p value. For example, a 75% thread sacrifices only a small amount of strength in exchange for a significant reduction in the force required to cut the thread. The result is that tap and die wear is reduced, the likelihood of breakage is lessened and higher cutting speeds can often be employed.

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This additional truncation is achieved by using a slightly larger tap drill in the case of female threads, or by slightly reducing the diameter of the threaded area of workpiece in the case of male threads, the latter effectively reducing the thread’s major diameter. In the case of female threads, tap drill charts typically specify sizes that will produce an approximate 75% thread. A 60% thread may be appropriate in cases where high tensile loading will not be expected. In both cases, the pitch diameter is not affected. The balancing of truncation versus thread strength is similar to many engineering decisions involving the strength, weight and cost of material, as well as the cost to machine it.

Tapered threads are used on fasteners and pipe. A common example of a fastener with a tapered thread is a wood screw.

The threaded pipes used in some plumbing installations for the delivery of fluids under pressure have a threaded section that is slightly conical. Examples are the NPT and BSP series. The seal provided by a threaded pipe joint is created when a tapered externally threaded end is tightened into an end with internal threads. For most pipe joints, a good seal requires the application of a separate sealant into the joint, such as thread seal tape, or a liquid or paste pipe sealant such as pipe dope.

An example of M16, ISO metric screw thread

Additional product standards identify preferred thread sizes for screws and nuts, as well as corresponding bolt head and nut sizes, to facilitate compatibility between spanners (wrenches) and other tools.

ISO standard threads

The most common threads in use are the ISO metric screw threads (M) for most purposes, and BSP threads (R, G) for pipes.

These were standardized by the International Organization for Standardization (ISO) in 1947. Although metric threads were mostly unified in 1898 by the International Congress for the standardization of screw threads, separate metric thread standards were used in France, Germany, and Japan, and the Swiss had a set of threads for watches.

Other current standards

In particular applications and certain regions, threads other than the ISO metric screw threads remain commonly used, sometimes because of special application requirements, but mostly for reasons of backward compatibility:

History of standardization

Graphic representation of formulas for the pitches of threads of screw bolts

A good summary of screw thread standards in current use in 1914 was given in Colvin FH, Stanley FA (eds) (1914): American Machinists’ Handbook, 2nd ed, New York and London, McGraw-Hill, pp. 16–22. USS, metric, Whitworth, and BA standards are discussed. The SAE series was not mentioned—at the time this edition of the Handbook was being compiled, they were either still in development or just newly introduced.

Survey results on the use of SAE standards (including screw size standards), reported in the journal Horseless Age, 1916

Meanwhile, in Britain, the British Association screw threads were also developed and refined for small instrumentation and electrical equipment. These were based on the metric Thury thread, but like Whitworth etc. were defined using Imperial units.

During this era, in continental Europe, the British and American threadforms were well known, but also various metric thread standards were evolving, which usually employed 60° profiles. Some of these evolved into national or quasi-national standards. They were mostly unified in 1898 by the International Congress for the standardization of screw threads at Zurich, which defined the new international metric thread standards as having the same profile as the Sellers thread, but with metric sizes. Efforts were made in the early 20th century to convince the governments of the U.S., UK, and Canada to adopt these international thread standards and the metric system in general, but they were defeated with arguments that the capital cost of the necessary retooling would drive some firms from profit to loss and hamper the economy.

Sometime between 1912 and 1916, the Society of Automobile Engineers (SAE) created an «SAE series» of screw thread sizes reflecting parentage from earlier USS and American Society of Mechanical Engineers (ASME) standards.

During the late 19th and early 20th centuries, engineers found that ensuring the reliable interchangeability of screw threads was a multi-faceted and challenging task that was not as simple as just standardizing the major diameter and pitch for a certain thread. It was during this era that more complicated analyses made clear the importance of variables such as pitch diameter and surface finish.

However, internationally, the metric system was eclipsing inch-based measurement units. In 1947, the ISO was founded; and in 1960, the metric-based International System of Units (abbreviated SI from the French Système International) was created. With continental Europe and much of the rest of the world turning to SI and ISO metric screw thread, the UK gradually leaned in the same direction. The ISO metric screw thread is now the standard that has been adopted worldwide and is slowly displacing all former standards, including UTS. In the U.S., where UTS is still prevalent, over 40% of products contain at least some ISO metric screw threads. The UK has completely abandoned its commitment to UTS in favour of ISO metric threads, and Canada is in between. Globalization of industries produces market pressure in favor of phasing out minority standards. A good example is the automotive industry; U.S. auto parts factories long ago developed the ability to conform to the ISO standards, and today very few parts for new cars retain inch-based sizes, regardless of being made in the U.S.

Even today, over a half century since the UTS superseded the USS and SAE series, companies still sell hardware with designations such as «USS» and «SAE» to convey that it is of inch sizes as opposed to metric. Most of this hardware is in fact made to the UTS, but the labeling and cataloging terminology is not always precise.

Another common inspection point is the straightness of a bolt or screw. This topic comes up often when there are assembly issues with predrilled holes as the first troubleshooting point is to determine if the fastener or the hole is at fault. ASME B18.2.9 «Straightness Gage and Gaging for Bolts and Screws» was developed to address this issue. Per the scope of the standard, it describes the gage and procedure for checking bolt and screw straightness at maximum material condition (MMC) and provides default limits when not stated in the applicable product standard.

The Roots-type blower is a
positive displacement lobe pump which operates by pumping a fluid with a pair of meshing lobes resembling a set of stretched gears. Fluid is trapped in pockets surrounding the lobes and carried from the intake side to the exhaust. The most common application of the Roots-type blower has been the induction device on two-stroke diesel engines, such as those produced by Detroit Diesel and Electro-Motive Diesel. Roots-type blowers are also used to supercharge four-stroke Otto cycle engines, with the blower being driven from the engine’s crankshaft via a toothed or V-belt, a roller chain or a gear train.

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A Roots blower with two-lobed rotors. Most real Roots blowers’ rotors have three or four lobes. (animated version) Key:

  • Rotary vane 1
  • Rotary vane 2

Animation showing the flow direction through a three-lobe Roots blower with a slight twist to the rotors

The superchargers used on top fuel engines, funny cars, and other dragsters, as well as hot rods, are in fact derivatives of General Motors Coach Division blowers for their industrial diesel engines, which were adapted for automotive use in drag racing. The model name of these units delineates their size — the once-commonly used 4–71 and 6–71 blowers were designed for 71 series diesels. Current competition dragsters use aftermarket GMC variants similar in design to the 71 series, but with the rotor and case length increased for added capacity; hot rodders also use reproduction 6-71s.

Some civil defense sirens used Roots blowers to pump air to the rotor (chopper) so as to drastically increase its sound output through all pitch ranges. The most well known are the Federal Signal Thunderbolt Series, and ACA (now American Signal Corporation) Hurricane. These sirens are known as «supercharged sirens».

Roots blowers are also used in reverse to measure the flow of gases or liquids, for example, in gas meters.

Construction of a two-lobed cycloidal rotor. The red curve is an epicycloid and the blue curve is a hypocycloid. The smaller generating circles (red and blue) are one quarter the diameter of the larger generating circle (black). The rotor profile is the thick line.

The simplest form of a Roots blower has cycloidal rotors, constructed of alternating tangential sections of hypocycloidal and epicycloidal curves. For a two-lobed rotor, the smaller generating circles are one-quarter the diameter of the larger. Real Roots blowers may have more complex profiles for increased efficiency. The lobes on one rotor will not drive the other rotor with minimal free play in all positions, so that a separate pair of gears provide the phasing of the lobes.

Because rotary lobe pumps need to maintain a clearance between the lobes, a single stage Roots blower can pump gas across only a limited pressure differential. If the pump is used beyond its specification, the compression of the gas generates enough heat so that the lobes expand to the point that they jam, damaging the pump.

Roots pumps are capable of pumping large volumes but, as they only achieve moderate compression, it is not uncommon to see multiple Roots blower stages, frequently with heat exchangers (intercoolers) in between to cool the gas. The lack of oil on the pumping surfaces allows the pumps to work in environments where contamination control is important. The high pumping rate for hydrocarbons allows the Roots pump to provide an effective isolation between oiled pumps, such as rotary compression pumps, and the vacuum chamber.

A variant uses claw-shaped rotors for higher compression.

Roots efficiency map

The Roots-type blower may achieve an efficiency of approximately 70% while achieving a maximum pressure ratio of two. Higher pressure ratios are achievable but at decreasing efficiency. Because a Roots-type blower pumps air in discrete pulses (unlike a screw compressor), pulsation noise and turbulence may be transmitted downstream. If not properly managed (through outlet piping geometry) or accounted for (by structural reinforcement of downstream components), the resulting pulsations can cause fluid cavitation and/or damage to components downstream of the blower.

Roots supercharger efficiency map. Generalized blower efficiency map shows how a blower’s efficiency varies with speed and boost.

For any given Roots blower running under given conditions, a single point will fall on the map. This point will rise with increasing boost and will move to the right with increasing blower speed. It can be seen that, at moderate speed and low boost, the efficiency can be over 90%. This is the area in which Roots blowers were originally intended to operate, and they are very good at it.

Boost is given in terms of pressure ratio, which is the ratio of absolute air pressure before the blower to the absolute air pressure after compression by the blower. If no boost is present, the pressure ratio will be 1.0 (meaning 1:1), as the outlet pressure equals the inlet pressure. 15psi boost is marked for reference (slightly above a pressure ratio of 2.0 compared to atmospheric pressure). At 15 psi (100 kPa) boost, Roots blowers hover between 50% and 58%. Replacing a smaller blower with a larger blower moves the point to the left. In most cases, as the map shows, this will move it into higher efficiency areas on the left as the smaller blower likely will have been running fast on the right of the chart. Usually, using a larger blower and running it slower to achieve the same boost will give an increase in compressor efficiency.

The volumetric efficiency of the Roots-type blower is very good, usually staying above 90% at all but the lowest blower speeds. Because of that, a blower running at low efficiency will still mechanically deliver the intended volume of air to the engine, but that air will be hotter. In drag racing applications, where large volumes of fuel are injected with that hot air, vaporizing the fuel absorbs the heat. That functions as a kind of liquid aftercooler system and goes a long way to negating the inefficiency of the Roots design in that application.

Rotary lobe blowers, commonly called boosters in high vacuum application, are not used as a stand-alone pump. In high vacuum applications, the boosters’ pumping speed can be used towards reducing the end pressure and increasing the pumping speed.

With a low increase in pressure, fans are commonly used to move substantial quantities of gas. They’re typically employed for the circulation of air in buildings, machine ventilation, cooling equipment and other industrial applications.

Blowers create medium air pressure with moderate pressure levels. They are used in applications where the pressure need is higher than fans.

Compressors generate higher air pressures in industrial applications generally between 8 and 12 bars with less amount of air flow rates.

The term «blower» is commonly used to define a device placed on engines with a functional need for additional airflow using a direct mechanical link as its energy source. The term blower is used to describe different types of superchargers. A screw type supercharger, Roots-type supercharger, and a centrifugal supercharger are all types of blowers. Conversely, a turbocharger, using exhaust compression to spin its turbine, and not a direct mechanical link, is not generally regarded as a «blower» but simply a «turbo».

Перевод, синонимы, произношение, примеры предложений, антонимы, транскрипция

noun: корень, источник, причина, основа, корнеплоды, основание, корнеплод, вершина, прародитель, отпрыск

adjective: коренной, основной

verb: укореняться, корениться, пускать корни, внедрять, ободрять, укоренять, приковывать, пригвождать, подрывать корни, рыть землю рылом

noun: винт, болт, шнек, шуруп, червяк, гребной винт, тюремщик, скряга, кляча, поворот винта

verb: крутить, привинчивать, завинчивать, вертеть, трахаться, завинчиваться, скаредничать, щурить, навинчивать, скреплять винтами

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