oSabbiatrici Modelli

AIR COMPRESSOR

ALLUMINIUM OXIDE

NOZZLE

ROUGHNESS

SANDBLASTING

SHOT-PEENING

TUNGSTEN CARBIDE

VENTURI EFFECT

AIR COMPRESSOR
From Wikipedia, the free encyclopedia

A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.

Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas. Liquids are relatively incompressible, so the main action of a pump is to transport liquids.


Centrifugal compressors
Main article: Centrifugal compressor

Figure 1: A single stage centrifugal compressorCentrifugal compressors use a rotating disk or impeller in a shaped housing to force the gas to the rim of the impeller, increasing the velocity of the gas. A diffuser (divergent duct) section converts the velocity energy to pressure energy. They are primarily used for continuous, stationary service in industries such as oil refineries, chemical and petrochemical plants and natural gas processing plants. Their application can be from 100 hp (75 kW) to thousands of horsepower. With multiple staging, they can achieve extremely high output pressures greater than 10,000 psi (69 MPa).

Many large snow-making operations (like ski resorts) use this type of compressor. They are also used in internal combustion engines as superchargers and turbochargers. Centrifugal compressors are used in small gas turbine engines or as the final compression stage of medium sized gas turbines.


Diagonal or mixed-flow compressors
Main article: Diagonal or mixed-flow compressor
Diagonal or mixed-flow compressors are similar to centrifugal compressors, but have a radial and axial velocity component at the exit from the rotor. The diffuser is often used to turn diagonal flow to the axial direction. The diagonal compressor has a lower diameter diffuser than the equivalent centrifugal compressor.


Axial-flow compressors
Main article: Axial-flow compressor

An animation of an axial compressor.Axial-flow compressors are dynamic rotating compressors that use arrays of fan-like aerofoils to progressively compress the working fluid. They are used where there is a requirement for a high flows or a compact design.

The arrays of aerofoils are set in rows, usually as pairs: one rotating and one stationary. The rotating aerofoils, also known as blades or rotors, accelerate the fluid. The stationary aerofoils, also known as a stators or vanes, turn and decelerate the fluid; preparing and redirecting the flow for the rotor blades of the next stage. Axial compressors are almost always multi-staged, with the cross-sectional area of the gas passage diminishing along the compressor to maintain an optimum axial Mach number. Beyond about 5 stages or a 4:1 design pressure ratio, variable geometry is normally used to improve operation.

Axial compressors can have high efficiencies; around 90% polytropic at their design conditions. However, they are relatively expensive, requiring a large number of components, tight tolerances and high quality materials. Axial-flow compressors can be found in medium to large gas turbine engines, in natural gas pumping stations, and within certain chemical plants.


Reciprocating compressors

A motor-driven six-cylinder reciprocating compressor that can operate with two, four or six cylinders.Main article: Reciprocating compressor
Reciprocating compressors use pistons driven by a crankshaft. They can be either stationary or portable, can be single or multi-staged, and can be driven by electric motors or internal combustion engines. Small reciprocating compressors from 5 to 30 horsepower (hp) are commonly seen in automotive applications and are typically for intermittent duty. Larger reciprocating compressors well over 1000 hp are still commonly found in large industrial and petroleum applications. Discharge pressures can range from low pressure to very high pressure (>5000 psi or 35 MPa). In certain applications, such as air compression, multi-stage double-acting compressors are said to be the most efficient compressors available, and are typically larger, noisier, and more costly than comparable rotary units.


Rotary screw compressors

Diagram of a rotary screw compressorMain article: Rotary screw compressor
Rotary screw compressors use two meshed rotating positive-displacement helical screws to force the gas into a smaller space. These are usually used for continuous operation in commercial and industrial applications and may be either stationary or portable. Their application can be from 3 hp (2.24 kW) to over 500 hp (375 kW) and from low pressure to very high pressure (>1200 psi or 8.3 MPa).


Rotary vane compressors
See also: Rotary vane pump
Rotary vane compressors consist of a rotor with a number of blades inserted in radial slots in the rotor. The rotor is mounted offset in a larger housing which can be circular or a more complex shape. As the rotor turns, blades slide in and out of the slots keeping contact with the outer wall of the housing. Thus, a series of decreasing volumes is created by the rotating blades. Rotary Vane compressors are, with piston compressors one of the oldest of compressor technologies.

With suitable port connections, the devices may be either a compressor or a vacuum pump. They can be either stationary or portable, can be single or multi-staged, and can be driven by electric motors or internal combustion engines. Dry vane machines are used at relatively low pressures (e.g., 2 bar) for bulk material movement whilst oil-injected machines have the necessary volumetric efficiency to achieve pressures up to about 13 bar in a single stage. A rotary vane compressor is well suited to electric motor drive and is significantly quieter in operation than the equivalent piston compressor.


Scroll compressors
Main article: Scroll compressor

Mechanism of a scroll pumpA scroll compressor, also known as scroll pump and scroll vacuum pump, uses two interleaved spiral-like vanes to pump or compress fluids such as liquids and gases. The vane geometry may be involute, archimedean spiral, or hybrid curves. They operate more smoothly, quietly, and reliably than other types of compressors in the lower volume range

Often, one of the scrolls is fixed, while the other orbits eccentrically without rotating, thereby trapping and pumping or compressing pockets of fluid or gas between the scrolls.


Diaphragm compressors
Main article: Diaphragm compressor
A diaphragm compressor (also known as a membrane compressor) is a variant of the conventional reciprocating compressor. The compression of gas occurs by the movement of a flexible membrane, instead of an intake element. The back and forth movement of the membrane is driven by a rod and a crankshaft mechanism. Only the membrane and the compressor box come in touch with the gas being compressed.

Diaphragm compressors are used for hydrogen and compressed natural gas (CNG) as well as in a number of other applications.


A three-stage diaphragm compressorThe photograph included in this section depicts a three-stage diaphragm compressor used to compress hydrogen gas to 6,000 psi (41 MPa) for use in a prototype compressed hydrogen and compressed natural gas (CNG) fueling station built in downtown Phoenix, Arizona by the Arizona Public Service company (an electric utilities company). Reciprocating compressors were used to compress the natural gas.

The prototype alternative fueling station was built in compliance with all of the prevailing safety, environmental and building codes in Phoenix to demonstrate that such fueling stations could be built in urban areas.

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ALLUMINIUM OXIDE
From Wikipedia, the free encyclopedia

NOZZLE
From Wikipedia, the free encyclopedia

Rocket nozzle.
Water nozzle.A nozzle is a mechanical device or orifice designed to control the characteristics of a fluid flow as it exits (or enters) an enclosed chamber or pipe.

A nozzle is often a pipe or tube of varying cross sectional area, and it can be used to direct or modify the flow of a fluid (liquid or gas). Nozzles are frequently used to control the rate of flow, speed, direction, mass, shape, and/or the pressure of the stream that emerges from them.


Jets
A gas jet, fluid jet, or hydro jet is a nozzle intended to eject gas or fluid in a coherent stream into a surrounding medium. Gas jets are commonly found in gas stoves, ovens, or barbecues. Gas jets were commonly used for light before the development of electric light. Other types of fluid jets are found in carburetors, where smooth calibrated orifices are used to regulate the flow of fuel into an engine, and in jacuzzis or spas.

Another specialized jet is the laminar jet. This is a water jet that contains devices to smooth out the flow, and gives laminar flow, as its name suggests. This gives better results for fountains.

High velocity nozzles
Frequently the goal is to increase the kinetic energy of the flowing medium at the expense of its pressure energy and/or internal energy.

Nozzles can be described as convergent (narrowing down from a wide diameter to a smaller diameter in the direction of the flow) or divergent (expanding from a smaller diameter to a larger one). A de Laval nozzle has a convergent section followed by a divergent section and is often called a convergent-divergent nozzle.

Convergent nozzles accelerate subsonic fluids. If the nozzle pressure ratio is high enough the flow will reach sonic velocity at the narrowest point (i.e. the nozzle throat). In this situation, the nozzle is said to be choked.

Increasing the nozzle pressure ratio further will not increase the throat Mach number beyond unity. Downstream (i.e. external to the nozzle) the flow is free to expand to supersonic velocities. Note that the Mach 1 can be a very high speed for a hot gas; since the speed of sound varies as the square root of absolute temperature. Thus the speed reached at a nozzle throat can be far higher than the speed of sound at sea level. This fact is used extensively in rocketry where hypersonic flows are required, and where propellant mixtures are deliberately chosen to further increases the sonic speed.

Divergent nozzles slow fluids, if the flow is subsonic, but accelerate sonic or supersonic fluids.

Convergent-divergent nozzles can therefore accelerate fluids that have choked in the convergent section to supersonic speeds. This CD process is more efficient than allowing a convergent nozzle to expand supersonically externally. The shape of the divergent section also ensures that the direction of the escaping gases is directly backwards, as any sideways component would not contribute to thrust.

Propelling nozzles
Main article: Propelling nozzle
A jet exhaust produces a net thrust due to the energy obtained from combusting fuel which is added to the inducted air. This hot air is passed through a high speed nozzle, a propelling nozzle greatly increasing its kinetic energy.

For a given mass flow, greater thrust is obtained with a higher exhaust velocity. However, the best energy efficiency is obtained when the exhaust speed is well matched with the airspeed, but greater mass flows are needed to give similar thrust. However, no jet aircraft can fly exceed its exhaust jet speed very much due momentum considerations, and so supersonic jet engines, like those employed in fighters and SST aircraft (e.g. Concorde), need high exhaust speeds which in turn implies relatively high nozzle pressure ratios. Therefore supersonic aircraft very typically use a CD nozzle despite weight and cost penalties. Subsonic jet engines employ relatively low, subsonic, exhaust velocities. They thus have modest nozzle pressure ratios and employ simple convergent nozzles. In addition, bypass nozzles are employed giving even lower speeds.

Rocket motors use convergent-divergent nozzles with very large area ratios so as to maximise thrust and exhaust velocity and thus extremely high nozzle pressure ratios are employed. Mass flow is at a premium since all the propulsive mass is carried with vehicle, and the very highest exhaust speeds are usually desirable.

Nozzles used on feeding hot blast in a blast furnace or forge are called tuyeres.


Magnetic nozzles
Magnetic nozzles have also been proposed for some types of propulsion, such as VASIMR, in which the flow of plasma is directed by magnetic fields instead of walls made of solid matter.


Spray nozzles
Main article: Spray nozzle
Many nozzles produce a very fine spray of liquids.

Atomizer nozzles are used for spray painting, perfumes, carburettors for internal combustion engines, spray on deodorants, antiperspirants and many other uses.
Air-Aspirating Nozzle-uses an opening in the cone shaped nozzle to inject air into a stream of water based foam (CAFS/AFFF/FFFP) to make the concentrate "foam up". Most commonly found on foam extinguishers and foam handlines.
Swirl nozzles inject the liquid in tangentially, and it spirals into the center and then exits through the central hole. Due to the vortexing this causes the spray to come out in a cone shape.

Vacuum nozzles
Vacuum cleaner nozzles come in several different shapes.


Shaping nozzles
Some nozzles are shaped to produce a stream that is of a particular shape. For example Extrusion molding is a way of producing lengths of metals or plastics or other materials with a particular cross-section. These nozzles are typically referred to as a die.

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ROUGHNESS
From Wikipedia, the free encyclopedia

Roughness is a measure of the texture of a surface. It is quantified by the vertical deviations of a real surface from its ideal form. If these deviations are large, the surface is rough; if they are small the surface is smooth. Roughness is typically considered to be the high frequency, short wavelength component of a measured surface (see surface metrology).

Roughness plays an important role in determining how a real object will interact with its environment. Rough surfaces usually wear more quickly and have higher friction coefficients than smooth surfaces (see tribology). Roughness is often a good predictor of the performance of a mechanical component, since irregularities in the surface may form nucleation sites for cracks or corrosion.

Although roughness is usually undesirable, it is difficult and expensive to control in manufacturing. Decreasing the roughness of a surface will usually increase exponentially its manufacturing costs. This often results in a trade-off between the manufacturing cost of a component and its performance in application.

Measurement

Principle of a contacting stylus instrument profilometer: A cantilever is holding a small tip that is sliding along the horizontal direction over the object's surface. Following the profile the cantilever is moving vertically. The vertical position is recorded as the measured profile  shown in light green.Roughness may be measured using contact or non-contact methods. Contact methods involve dragging a measurement stylus across the surface; these instruments include profilometers. Non-contact methods include interferometry, confocal microscopy, electrical capacitance and electron microscopy.


Sketch depicting how a probe stylus travels over a surface.For 2D measurements, the probe usually traces along a straight line on a flat surface or in a circular arc around a cylindrical surface. The length of the path that it traces is called the measurement length. The wavelength of the lowest frequency filter that will be used to analyze the data is usually defined as the sampling length. Most standards recommend that the measurement length should be at least seven times longer than the sampling length, and according to the Nyquist–Shannon sampling theorem it should be at least ten times longer than the wavelength of interesting features. The assessment length or evaluation length is the length of data that will be used for analysis. Commonly one sampling length is discarded from each end of the measurement length.

For 3D measurements, the probe is commanded to scan over a 2D area on the surface. The spacing between data points may not be the same in both directions.

In some cases, the physics of the measuring instrument may have a large effect on the data. This is especially true when measuring very smooth surfaces. For contact measurements, most obvious problem is that the stylus may scratch the measured surface. Another problem is that the stylus may be too blunt to reach the bottom of deep valleys and it may round the tips of sharp peaks. In this case the probe is a physical filter that limits the accuracy of the instrument.

There are also limitations for non-contact instruments. For example instruments that rely on optical interference cannot resolve features that are less than some fraction of the frequency of their operating wavelength. This limitation can make it difficult to accurately measure roughness even on common objects, since the interesting features may be well below the wavelength of light. The wavelength of red light is about 650 nm, while the Ra of a ground shaft might be 2000 nm.


Analysis
In the past, surface finish was usually analyzed by hand. The roughness trace would be plotted on graph paper, and an experienced machinist decided what data to ignore and where to place the mean line. Today, the measured data is stored on a computer, and analyzed using methods from signal analysis and statistics.

The first step of roughness analysis is often to filter the raw measurement data to remove very high frequency data since it can often be attributed to vibrations or debris on the part surface. Next, the data is separated into roughness, waviness and form. This can be accomplished using reference lines, envelope methods, digital filters, fractals or other techniques. Finally the data is summarized using one or more of the roughness parameters, or a graph.


Illustration of the effect of different form removal techniques on surface finish analysis.
Plots showing how filter cutoff frequency affects the separation between waviness and roughness.
Illustration showing how the raw profile from a surface finish trace is decomposed into a primary profile, form, waviness and roughness.
Illustration showing the effect of using different filters to separate a surface finish trace into waviness and roughness.



Specification
In the United States, surface finish is usually specified based on the ASME Y14.36M-1996 standard. Other standards also exist, including ISO 1302:2001.


Illustration of how to specify surface finish on a manufacturing drawing.

Lay Patterns

Example surface finish lay patterns.A lay pattern is a repetitive impression created on the surface of a part. It is often representative of a specific manufacturing operation. A product designer may specify a lay pattern on a part because the directionality the lay affects the part's function. Unless otherwise specified, roughness is measured perpendicular to the lay.


Roughness Parameters
Each of the roughness parameters is calculated using a formula for describing the surface.

There are many different roughness parameters in use, but Ra is by far the most common. Other common parameters include Rz, Rq, and Rsk. Some parameters are used only in certain industries or within certain countries. For example, the Rk family of parameters is used mainly for cylinder bore linings, and the Motif parameters are used primarily within France.

Since these parameters reduce all of the information in a profile to a single number, great care must be taken in applying and interpreting them. Small changes in how the raw profile data is filtered, how the mean line is calculated, and the physics of the measurement can greatly affect the calculated parameter.

By convention every 2D roughness parameter is a capital R followed by additional characters in the subscript. The subscript identifies the formula that was used, and the R means that the formula was applied to a 2D roughness profile. Different capital letters imply that the formula was applied to a different profile. For example, Ra is the arithmetic average of the roughness profile, Pa is the arithmetic average of the unfiltered raw profile, and Sa is the arithmetic average of the 3D roughness.

Each of the formulas listed in the tables assumes that the roughness profile has been filtered from the raw profile data and the mean line has been calculated. The roughness profile contains n ordered, equally spaced points along the trace, and yi is the vertical distance from the mean line to the ith data point. Height is assumed to be positive in the up direction, away from the bulk material.


Amplitude Parameters
Amplitude parameters characterize the surface based on the vertical deviations of the roughness profile from the mean line. Many of them are closely related to the parameters found in statistics for characterizing population samples. For example, Ra is the arithmetic average of the absolute values and Rt is the range of the collected roughness data points.

The amplitude parameters are by far the most common surface roughness parameters found in the United States on mechanical engineering drawings and in technical literature. Part of the reason for their popularity is that they are straightforward to calculate using a digital computer.

Parameter Description Formula
Ra, Raa, Ryni arithmetic average of absolute values
Rq, RRMS root mean squared
Rv maximum valley depth Rv = miniyi
Rp maximum peak height Rp = maxiyi
Rt Maximum Height of the Profile Rt = Rp − Rv
Rsk skewness
Rku kurtosis
RzDIN, Rtm average distance between the highest peak and lowest valley in each sampling length, ASME Y14.36M - 1996 Surface Texture Symbols , where s is the number of sampling lengths, and Rti is Rt for the ith sampling length.
RzJIS Japanese Industrial Standard for Rz, based on the five highest peaks and lowest valleys over the entire sampling length. , where Rpi Rvi are the ith highest peak, and lowest valley respectively.


Slope, Spacing, and Counting Parameters
Slope parameters describe characteristics of the slope of the roughness profile. Spacing and counting parameters describe how often the profile crosses certain thresholds. These parameters are often used to describe repetitive roughness profiles, such as those produced by turning on a lathe.

Parameter Description Formula
Rdq, R?q the RMS slope of the profile within the sampling length


Bearing Ratio Curve Parameters
These parameters are based on the bearing ratio curve (also known as the Abbott-Firestone curve.) This includes the Rk family of parameters.


Sketches depicting surfaces with negative and positive skew. The roughness trace is on the left, the amplitude distribution curve is in the middle, and the bearing area curve (Abbott-Firestone curve) is on the right.
[edit] Fractal theory
The mathematician Benoît Mandelbrot has pointed out the connection between surface roughness and fractal dimension.


Engineering
In most cases, roughness is considered to be detrimental to part performance. As a consequence, most manufacturing prints establish an upper limit on roughness, but not a lower limit.

It can be difficult to quantify the relationship between roughness and part performance because there are so many different ways to characterize the surface.


Tribology
Roughness is often closely related to the friction and wear properties of a surface. A surface with a large Ra value, or a positive Rsk, will usually have high friction and wear quickly.

Deep valleys in the roughness profile are also important to tribology because they may act as lubricant reservoirs.

The peaks in the roughness profile are not always the points of contact. The form and waviness must also be considered.


Manufacturing
Many factors contribute to the surface roughness in manufacturing. When molding or forming a surface, the impression of the mold or die on the part is usually the principle factor in the surface roughness. In machining, and abrasive processes the interaction of the cutting edges and the microstructure of the material being cut both contribute to the roughness.

Just as different manufacturing processes produce parts at various tolerances, they are also capable of different roughnesses. Generally these two characteristics are linked: manufacturing processes that are dimensionally precise create surfaces with low roughness. In other words, if a process can manufacture parts to a narrow dimensional tolerance, the parts will not be very rough.
Surface finishes produced by common manufacturing processes.

Cost
In general, the cost of manufacturing a surface increases greatly as the roughness tolerance decreases.


Other Applications
International Roughness Index (IRI) - a dimensionless quantity used for measuring road roughness and proposed as a world standard by the World Bank. Typically IRI is presented as an average value over 20 m, 100 m, 400 m, 1 mile etc. IRI is not an excellent indicator on ride quality. Consider two 10 cm high and arc-shaped traffic calming speed bumps, one "spinebreaker" being 1 m long and the other being as much as 10 m long and thus too smooth for calming city traffic. Both give an IRI20 of about 8 mm/m. Not being able to distinguish between two bumps that obviously give dramatically different ride quality, one can really question IRI as a pavement performance indicator.
Manning's n-value - used by geologists to characterise river channels.

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SANDBLASTING
From Wikipedia, the free encyclopedia

Sandblasting or bead blasting is a generic term for the process of smoothing, shaping and cleaning a hard surface by forcing solid particles across that surface at high speeds; the effect is similar to that of using sandpaper, but provides a more even finish with no problems at corners or crannies. Sandblasting can occur naturally, usually as a result of particles blown by wind causing eolian erosion, or artificially, using compressed air. An artificial sandblasting process was patented by Benjamin Chew Tilghman on October 18, 1870.

Historically, the material used for artificial sandblasting was sand that had been sieved to a uniform size. The silica dust produced in the sandblasting process caused silicosis after sustained inhalation of dust. Several countries and territories now regulate sandblasting such that it may only be performed in a controlled environment using ventilation, protective clothing and breathing air supply (as shown in the top image).

Other materials for sandblasting have been developed to be used instead of sand; for example, carborundum grit, steel shots, copper slag, powdered slag, glass beads (bead blasting), metal pellets, dry ice, garnet[1], powdered abrasives of various grades, and even ground coconut shells, corncobs, walnut shells, and baking soda (sodablasting) have been used for specific applications and can produce distinct surface finishes. Some commercial grade blasters are specially designed to handle multiple blast abrasives. These blasters are commonly referred as multi-media blasters.

Sandblasting can also be used to produce three dimensional signage. This type of signage is considered to be a higher end product as compared to the flat signs. These signs often incorporate gold leaf overlay and sometimes crushed glass backgrounds which is called smalts.

Sandblasting can be used to refurbish buildings or create works of art (carved or frosted glass). Modern masks and resists facilitate this process, producing accurate results.

Sandblasting technique is used for cleaning boat hulls, bricks, and concrete work. Sandblasting which is also known as blast cleaning is used for cleaning industrial as well as commercial structures.

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SHOT PEENING

From Wikipedia, the free encyclopedia

Shot peening is a process used to produce a compressive residual stress layer and modify mechanical properties of metals. It entails impacting a surface with shot (round metallic, glass or ceramic particles) with force sufficient to create plastic deformation. It is similar to sandblasting, except that it operates by the mechanism of plasticity rather than abrasion: each particle functions as a ball-peen hammer. In practice, this means that less material is removed by the process, and less dust created.

Peening a surface spreads it plastically, causing changes in the mechanical properties of the surface. Shot peening is often called for in aircraft repairs to relieve tensile stresses built up in the grinding process and replace them with beneficial compressive stresses. Depending on the part geometry, part material, shot material, shot quality, shot intensity, shot coverage, shot peening can increase fatigue life from 0%-1000%.

Plastic deformation induces a residual compressive stress in a peened surface, along with tensile stress in the interior. Surface compressive stresses confer resistance to metal fatigue and to some forms of corrosion. The tensile stresses deep in the part are not as problematic as tensile stresses on the surface because cracks are less likely to start in the interior.

Shot peening may be used for cosmetic effect. The surface roughness resulting from the overlapping dimples causes light to scatter upon reflection. Because peening typically produces larger surface features than sand-blasting, the resulting effect is more pronounced.

Shot peening was originally developed by John Almen when he was working for Buick Motor Division of General Motors Cooperation. He noticed that shot blasting, as it was called back when he was working, made the side of the sheet metal that was exposed begin to bend and stretch. John Almen also created the Almen Strip to measure the comprehensive stresses in the strip created by the ball peening operation. One can obtain what is referred to as the "Intensity of the Blast Stream" by measuring the deformation on the Almen strip that is in the shot peening operation. As the strip reaches a 10% deformation, the Almen strip is then hit with the same intensity for twice the amount of time. If the strip deforms another 10%, then you have the, "Intensity of the Blast Stream."

A study done through the SAE Fatigue Design and Evaluation Committee showed what shot peening can do for welds compared to welds that didn't have this operation done. The study claimed that the regular welds would fail after 250,000 cycles when welds that had been shot peened would fail after 2.5 million cycles, and outside the weld area. This is part of the reason that shot peening is a popular operation with aerospace parts. However, the beneficial prestresses can anneal out at higher temperatures.

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SURFACE TREATMENT
From Wikipedia, the free encyclopedia

Surface finishing is used to describe a number of industrial processes that can be applied to improve the surface of a manufactured item. The major reason to apply these processes is to improve appearance, improve adhesion or ink wettability, corrosion protection, wear resistance and friction control also are areas where performance can be enhanced by these treatments. In limited cases some of these techniques can be used to restore original dimensions to salvage or repair an item.

Generally, surface finishing consists of one of the following:

Removing material, or reshaping the item
Adding material to the item's surface, or chemically altering it

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TUNGSTEN CARBIDE
From Wikipedia, the free encyclopedia

Tungsten carbide, WC, or tungsten semicarbide, W2C, is a chemical compound containing tungsten and carbon, similar to titanium carbide. Colloquially, tungsten carbide is often simply called carbide.

Chemical properties
There are two well characterized compounds of tungsten and carbon, WC and W2C. Both compounds may be present in coatings and the proportions can depend on the coating method.

WC can be prepared by reaction of tungsten metal and carbon at 1400–2000 °C.[2] Other methods include a patented fluid bed process that reacts either tungsten metal or blue WO3 with CO/CO2 mixture and H2 between 900 and 1200 °C.[3] Chemical vapor deposition methods that have been investigated include:

tungsten hexachloride with hydrogen, as reducing agent and methane as the source of carbon at 670 °C (1,238 °F)
WCl6 + H2 + CH4 → WC + 6HCl
reacting tungsten hexafluoride with hydrogen as reducing agent and methanol as source of carbon at 350 °C (662 °F)
WF6 + H2 + CH3OH → WC + 6HF + H2O
At high temperatures WC decomposes to tungsten and carbon and this can occur during high temperature thermal spray, e.g. high velocity oxygen fuel (HVOF) and high energy plasma (HEP) methods.
Oxidation of WC starts at 500–600 °C. It is resistant to acids and is only attacked by hydrofluoric acid/nitric acid (HF/HNO3) mixtures above room temperature.[2] It reacts with fluorine gas at room temperature and chlorine above 400 °C (752 °F) and is unreactive to dry H2 up to its melting point.
WC has been investigated for its potential use as a catalyst and it has been found to resemble platinum in its catalysis the production of water from hydrogen and oxygen at room temperature, the reduction of tungsten trioxide by hydrogen in the presence of water, and the isomerization of 2,2-dimethylpropane to 2-methylbutane. It has been proposed as a replacement for the iridium catalyst in hydrazine powered satellite thrusters.


Physical properties
Tungsten carbide is high melting, 2,870 °C (5,200 °F), extremely hard 8.5–9.0 Mohs scale[citation needed] at 22 GPa Vickers hardness with low electrical resistivity (1.7–2.2x10-7 ohm-m), comparable with metals (e.g vanadium 1.99x10-7 ohm-m).

WC is readily wetted by both molten nickel and cobalt. Investigation of the phase diagram of the W-C-Co system shows that WC and Co form a pseudo binary eutectic. The phase diagram also shows that there are so-called η-carbides with composition (W,Co)6C that can be formed and the fact that these phases are brittle is the reason why control of the carbon content in WC-Co hard metals is important.


Structure
There are two forms of WC, a hexagonal form, α-WC, and a cubic high temperature form, β-WC, which has the rock salt structure. The hexagonal form can be visualized as made up of hexagonally close packed layers of metal atoms with layers lying directly over one another, with carbon atoms filling half the interstices giving both tungsten and carbon a regular trigonal prismatic, 6 coordination. From the unit cell dimensions the following bond lengths can be determined; the distance between the tungsten atoms in an hexagonally packed layer is 291 pm, the shortest distance between tungsten atoms in adjoining layers is 284 pm, and the tungsten carbon bond length is 220 pm. The tungsten-carbon bond length is therefore comparable to the single bond in W(CH3)6 (218pm) in which there is strongly distorted trigonal prismatic coordination of tungsten.

Molecular WC has been investigated and this gas phase species has a bond length of 171 pm for 184W12C.


Toxicity
The primary health risks associated with carbide relate to inhalation of dust, leading to fibrosis.


Applications

Machine tools
Carbide cutting surfaces are often useful when machining through materials such as carbon steel or stainless steel, as well as in situations where other tools would wear away, such as high-quantity production runs. Most of the time, carbide will leave a better finish on the part, and allow faster machining. Carbide tools can also withstand higher temperatures than standard high speed steel tools. The material is usually tungsten-carbide cobalt, also called "cemented carbide", a metal matrix composite where tungsten carbide particles are the aggregate and metallic cobalt serves as the matrix. The process of combining tungsten carbide with cobalt is referred to as sintering or Hot Isostatic Pressing (HIP). During this process cobalt eventually will be entering the liquid stage and WC grains (>> higher melting point) remain in the solid stage. As a result of this process cobalt is embedding/cementing the WC grains and thereby creates the metal matrix composite with its distinct material properties. The naturally ductile cobalt metal serves to offset the characteristic brittle behavior of the tungsten carbide ceramic, thus raising its toughness and durability. Such parameters of tungsten carbide can be changed significantly within the carbide manufacturers sphere of influence, primarily determined by grain size, cobalt content, dotation (e.g. alloy carbides) and carbon content.

Machining with carbide can be difficult, as carbide is more brittle than other tool materials, making it susceptible to chipping and breaking. To offset this, many manufacturers sell carbide inserts and matching insert holders. With this setup, the small carbide insert is held in place by a larger tool made of a less brittle material (usually steel). This gives the benefit of using carbide without the high cost of making the entire tool out of carbide. Most modern face mills use carbide inserts, as well as some lathe tools and endmills.

To increase the life of carbide tools, they are sometimes coated. Four such coatings are TiN (titanium nitride), TiC (titanium carbide), Ti(C)N (titanium carbide-nitride), and TiAlN (Titanium Aluminum Nitride). (Newer coatings, known as DLC (Diamond Like Coating) are beginning to surface, enabling the cutting power of diamond without the unwanted chemical reaction between real diamond and iron.) Most coatings generally increase a tool's hardness and/or lubricity. A coating allows the cutting edge of a tool to cleanly pass through the material without having the material gall (stick) to it. The coating also helps to decrease the temperature associated with the cutting process and increase the life of the tool. The coating is usually deposited via thermal CVD and, for certain applications, with the mechanical PVD method. However if the deposition is performed at too high temperature, an eta phase of a Co6W6C tertiary carbide forms at the interface between the carbide and the cobalt phase, facilitating adhesion failure of the coating.


Military
Tungsten carbide is often used in armor-piercing ammunition, especially where depleted uranium is not available or not politically acceptable. The first use of W2C projectiles occurred in Luftwaffe tank-hunter squadrons, which used 37 mm autocannon equipped Ju-87G Stuka attack planes to destroy Soviet T-34 tanks in WWII. Owing to the limited German reserves of tungsten, W2C material was reserved for making machine tools and small numbers of projectiles for the most elite combat pilots, like Hans Rudel. It is an effective penetrator due to its high hardness value combined with a very high density.

Tungsten carbide ammunition can be of the sabot type (a large arrow surrounded by a discarding push cylinder) or a subcaliber ammunition, where copper or other relatively soft material is used to encase the hard penetrating core, the two parts being separated only on impact. The latter is more common in small-caliber arms, while sabots are usually reserved for artillery use.

Tungsten carbide is also an effective neutron reflector and as such was used during early investigations into nuclear chain reactions, particularly for weapons. A criticality accident occurred at Los Alamos National Laboratory on 21 August 1945 when Harry K. Daghlian, Jr. accidentally dropped a tungsten carbide brick onto a plutonium sphere causing the sub-critical mass to go critical with the reflected neutrons.


Sports
Hard carbides, especially tungsten carbide, are used by athletes, generally on poles which impact hard surfaces. Trekking poles, used by many hikers for balance and to reduce pressure on leg joints, generally use carbide tips in order to gain traction when placed on hard surfaces (like rock); such carbide tips last much longer than other types of tips.

While ski pole tips are generally not made of carbide, since they do not need to be especially hard even to break through layers of ice, rollerski tips usually are. Roller skiing emulates cross country skiing and is used by many skiers to train during warm weather months.

Sharpened carbide tipped spikes (known as studs) can be inserted into the drive tracks of snowmobiles. These studs enhance traction on icy surfaces. Longer v-shaped segments fit into grooved rods called wear rods under each snowmobile ski. The relatively sharp carbide edges enhance steering on harder icy surfaces. The carbide tips and segments reduce wear encountered when the snowmobile must cross roads and other abrasive surfaces.

Some tire manufacturers, such as Nokian and Schwalbe, offer bicycle tires with tungsten carbide studs for better traction on ice. These are generally preferred over steel studs because of their wear resistance.


Domestic
Tungsten carbide is used as the rotating ball in the tips of ballpoint pens to disperse ink during writing.

Tungsten carbide can now be found in the inventory of some jewelers, most notably as the primary material in men's wedding bands. When used in this application the bands appear with a lustrous dark hue often buffed to a mirror finish. The finish is highly resistant to scratches and scuffs, holding its mirror-like shine for years.

A common misconception held concerning tungsten carbide rings is they cannot be removed in the course of emergency medical treatment, requiring the finger to be removed instead. Emergency rooms are usually equipped with jewelers' saws that can easily cut through gold and silver rings without injuring the patient when the ring cannot be slipped off easily. However, these saws are incapable of cutting through tungsten carbide. Although standard ring cutting tools cannot be used due to the hardness of this material, there are specialty cutters available that are just as effective on tungsten carbide as they are on gold and platinum. Tungsten carbide rings may be removed in an emergency situation by cracking them into pieces with standard vice grip–style locking pliers.

Many manufacturers of this emerging jewelry material state that the use of a cobalt binder may cause unwanted reactions between the cobalt and the natural oils on human skin. Skin oils cause the cobalt to leach from the material. This is said to cause possible irritation of the skin and permanent staining of the jewelry itself. Many manufacturers now advertise that their jewelry is "cobalt free". This is achieved by replacing the cobalt with nickel as a binder.

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VENTURI EFFECT
From Wikipedia, the free encyclopedia

The Venturi effect is the fluid pressure that results when an incompressible fluid flows through a constricted section of pipe. The Venturi effect may be derived from a combination of Bernoulli's principle and the equation of continuity. The fluid velocity must increase through the constriction to satisfy the equation of continuity, while its pressure must decrease due to conservation of energy: the gain in kinetic energy is supplied by a drop in pressure or a pressure gradient force.

The limiting case of the Venturi effect is choked flow, in which a constriction in a pipe or channel limits the total flow rate through the channel, because the pressure cannot drop below zero in the constriction. Choked flow is used to control the delivery rate of water and other fluids through spigots and other valves.

Referring to the diagram to the right, using Bernoulli's equation in the special case of incompressible fluids (such as the approximation of a water jet), the theoretical pressure drop (P1 − P2) at the constriction would be given by .

The Venturi effect is named after Giovanni Battista Venturi, (1746–1822), an Italian physicist.


Experimental apparatus

This is a Venturi tube demonstration apparatus built out of PVC pipe and operated with a vacuum pump.Venturi tubes
The simplest apparatus, as shown in the photograph and diagram, is a tubular setup known as a Venturi tube or simply a venturi. Fluid flows through a length of pipe of varying diameter. To avoid undue drag, a venturi tube typically has an entry cone of 30 degrees and an exit cone of 5 degrees.
A venturi can also be used to mix a fluid with air. If a pump forces the fluid through a tube connected to a system consisting of a venturi to increase the water speed (the diameter decreases), a short piece of tube with a small hole in it, and last a venturi that decreases speed (so the pipe gets wider again), air will be sucked in through the small hole because of changes in pressure. At the end of the system, a mixture of fluid and air will appear. See aspirator and pressure head for a discussion of this type of siphon.
Orifice plate
Venturi tubes are more expensive to construct than a simple orifice plate which uses the same principle as a tubular scheme, but the orifice plate causes significantly more permanent energy loss.
Aortic insufficiency is a chronic heart condition that occurs when the aortic valve's initial large stroke volume is released and the Venturi effect draws the walls together, which obstructs blood flow, which leads to a Pulsus Bisferiens.


Practical uses
The Venturi effect may be observed or used in the following:

The capillaries of the human circulatory system, where it indicates aortic regurgitation
Large cities where wind is forced between buildings
Inspirators that mix air and flammable gas in grills, gas stoves, Bunsen burners and airbrushes
Water aspirators that produce a partial vacuum using the kinetic energy from the faucet water pressure
Steam siphons using the kinetic energy from the steam pressure to create a partial vacuum
Atomizers that disperse perfume or spray paint (i.e. from a spray gun).
Foam firefighting nozzles and extinguishers
Carburetors that use the effect to suck gasoline into an engine's intake air stream
Protein skimmers (filtration devices for saltwater aquaria)
In automated pool cleaners that use pressure-side water flow to collect sediment and debris
The modern-day barrel of the clarinet, which uses a reverse taper to speed the air down the tube, enabling better tone, response and intonation
Compressed air operated industrial vacuum cleaners
Venturi scrubbers used to clean flue gas emissions
Injectors (also called ejectors) used to add chlorine gas to water treatment chlorination systems
Sand blasters used to draw fine sand in and mix it with air
Emptying bilge water from a moving boat through a small waste gate in the hull—the air pressure inside the moving boat is greater than the water sliding by beneath
A scuba diving regulator to assist the flow of air once it starts flowing
Modern vaporizers to optimize efficiency
In Venturi masks used in medical oxygen therapy
In recoilless rifles to decrease the recoil of firing
Ventilators
A simple way to demonstrate the Venturi effect is to squeeze and release a flexible hose that is normal shape: the partial vacuum produced in the constriction is sufficient to keep the hose collapsed.

Venturi tubes are also used to measure the speed of a fluid, by measuring pressure changes at different segments of the device. Placing a liquid in a U-shaped tube and connecting the ends of the tubes to both ends of a Venturi is all that is needed. When the fluid flows though the Venturi the pressure in the two ends of the tube will differ, forcing the liquid to the "low pressure" side. The amount of that move can be calibrated to the speed of the fluid flow.

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