CAPILLARY ACTION EXPLAINED !!

Capillary action (or capillarity) describes the ability of a liquid to flow against gravity in a narrow space such as a thin tube.
This spontaneous rising of a liquid is the outcome of two opposing forces:

Cohesion – the attractive forces between similar molecules or atoms, in our case the molecules or atoms of the liquid. Water, for example, is characterized by high cohesion since each water molecule can form four hydrogen bonds with neighboring molecules.

Adhesion – the attractive forces between dissimilar molecules or atoms, in our case the contact area between the particles of the liquid and the particles forming the tube.


The capillarity of the liquid is said to be high when adhesion is greater than cohesion, and vice versa. Hence, knowledge of the liquid is not sufficient to determine when capillary action will occur, since we must also know the chemical composition of the tube. These two, together with the contact area (the tube's diameter), comprise the key variables. For example, water in a thin glass tube has strong adhesive forces due to the hydrogen bonds that form between the water molecules and the oxygen atoms in the tube wall (glass = silica = SiO2). In contrast, mercury is characterized by stronger cohesion, and hence its capillarity is much lower.

So what's going on here?
In case the forces of adhesion are greater than those of cohesion and gravity (when it exists), the molecules of the liquid cling to the wall of the tube. We will observe that the upper surface of the liquid becomes concave (the height of the liquid at the contact area is higher than its height at the center of the tube). The cohesive forces between the molecules of the liquid are "attempting" to reduce the surface tension (i.e. to flatten the upper surface of the liquid and thus prevent the increased surface area in the concave state). In doing so, the molecules keep climbing up until a steady state between cohesion and adhesion is achieved (with or without the gravity component)
.
This also explains why this phenomenon occurs exclusively in thin tubes (also in the absence of gravity). In wider vessels, only a small fraction of the liquid comes into contact with the vessel walls, and so adhesive forces are negligible and there is hardly any rising of the liquid.

Many everyday phenomena are a result of capillary action, including:
(1) A kerosene lamp or a candle "sucking up" oil or liquid wax, respectively.
(2) Water climbing up the microscopic fibers of paper towels.
(3) Located at the inner ends of each eye, the lacrimal ducts drain our tears using capillary action.

TECHNICAL TERMS OF GAS TURBINE EXPLAINED !!

1.)NET JET THRUST – That part of the thrust of a turbojet engine which is available for climb and acceleration.

2.)NOZZLE – A flow passage specially shaped to produce kinetic energy at the expense of other forms of energy (available thermal energy).

3.)NOZZLE EFFICIENCY – The ratio of the actual kinetic energy produced on discharge (or between any two points in a nozzle) to that obtainable by assuming an isentropic expansion in the nozzle.

4.)OVERALL EFFICIENCY OF PROPELLER UNIT – Product of the propeller (propulsion) efficiency, thermal efficiency of the engine (power turbine) and the transmission efficiency from prime mover to propeller shaft.

5.)POLYTROPIC EFFICIENCY – The isentropic efficiency of an elemental stage of the compression which is constant throughout the process.Also called the SMALL STAGE EFFICIENCY.

6.)POWER RATIO – Ratio of useful or net horse power of the cycle compared with the power developed by the turbine of the system. Also called WORK RATIO.

7.)PREWHIRL – Whirl velocity (tangential component of the absolute velocity at intake), imparted to the air that enters the centrifugal compressor impeller, by allowing the air to be drawn into the impeller eye over curved inlet guide vanes attached to the impeller casing.

8.)PRIMARY AIR – Part of the air which flows through the core of the combustion chamber, in just sufficient quantity for combustion.

9.)PROPULSION EFFICIENCY – Ratio of thrust power to the jet power.

10.)RAM EFFECT – The effect which causes an increase of temperature and pressure of the air that enters the compressor of an aircraft gas turbine unit due to aircraft speed. Sometimes called RAM.

TECHNICAL TERMS OF GENERAL MECHANICAL ENGINEERING !!


1.)FILLET – A concave surface connecting the two surfaces meeting at an angle.

2.)FLANGE– A metal part which is spread out like a rim, the action of working
a piece or part to spread out.

3.)FLANK (Side of thread) – The straight part of the thread which connects the crest with the root.

4.)INVOLUTE GEAR TOOTH – A curved tooth generated by unwinding a string from a cylinder to form the curve.

5.)JOURNAL – The part of a shaft or axle that has been machined or finished to fit into a bearing.

6.)KEYS – Metal pieces of various designs that fit into a slot in a shaft and project above the shaft to fit into a mating slot in the center hole of a gear or pulley to provide a positive drive between the shaft and the gear or pulley.

7.)LOCK NUT – A type of nut that is prevented from loosening under vibration.The locking action is accomplished by squeezing, gripping or jamming
against the bolt threads.

8.)LOOSE PULLEY – A pulley which turns freely on a shaft so that a belt can be shifted from the driving pulley to the loose pulley in order to stop a machine driven by an overhead belt drive. 

9.)QUILL – A hollow shaft that revolves on a solid shaft carrying pulleys, gears or clutches. When the clutch is closed, the quill and shaft revolve together.

10.)RACK – A straight metal strip having teeth that mesh with those of a gear to convert rotary into reciprocating motion or just the opposite.

SURFACE TENSION EXPLAINED !!

Surface tension is a phenomenon in which the surface of a liquid, where the liquid is in contact with gas, acts like a thin elastic sheet. This term is typically used only when the liquid surface is in contact with gas (such as the air). If the surface is between two liquids (such as water and oil), it is called "interface tension."



Simply we can say,it is  property of a liquid surface displayed by its acting as if it were a stretched elastic membrane. This phenomenon can be observed in the nearly spherical shape of small drops of liquids and of soap bubbles. 

CAUSES OF SURFACE TENSION

Various intermolecular forces, such as Van der Waals forces, draw the liquid particles together. Along the surface, the particles are pulled toward the rest of the liquid, as shown in the picture to the right.



Surface tension depends mainly upon the forces of attraction between the particles within the given liquid and also upon the gassolid, or liquid in contact with it. The molecules in a drop of water, for example, attract each other weakly. Water molecules well inside the drop may be thought of as being attracted equally in all directions by the surrounding molecules. However, if surface molecules could be displaced slightly outward from the surface, they would be attracted back by the nearby molecules. The energy responsible for the phenomenon of surface tension may be thought of as approximately equivalent to the work or energy required to remove the surface layer of molecules in a unit area. Surface tension may be expressed, therefore, in units of energy (joules) per unit area (square metres). Water has a surface tension of 0.07275 joule per square metre at 20 °C (68 °F). In comparison, organic liquids, such as benzene and alcohols, have lower surface tensions, whereas mercury has a higher surface tension. An increase in temperature lowers the net force of attraction among molecules and hence decreases surface tension.

Units of Surface Tension
Surface tension is measured in SI units of N/m (newton per meter), although the more common unit is the cgs unit dyn/cm (dyne per centimeter).

In order to consider the thermodynamics of the situation, it is sometimes useful to consider it in terms of work per unit area. The SI unit in that case is the J/m2 (joules per meter squared). The cgs unit is erg/cm2.
These forces bind the surface particles together. Though this binding is weak - it's pretty easy to break the surface of a liquid after all - it does manifest in many ways.

EXAMPLES OF SURFACE TENSION:


Drops of water. When using a water dropper, the water does not flow in a continuous stream, but rather in a series of drops.
The shape of the drops is caused by the surface tension of the water. The only reason the drop of water isn't completely spherical is because of the force of gravity pulling down on it. In the absence of gravity, the drop would minimize the surface area in order to minimize tension, which would result in a perfectly spherical shape.

Insects walking on water. Several insects are able to walk on water, such as the water strider.

Their legs are formed to distribute their weight, causing the surface of the liquid to become depressed, minimizing the potential energy to create a balance of forces so that the strider can move across the surface of the water without breaking through the surface. This is similar in concept to wearing snow shoes to walk across deep snowdrifts without your feet sinking.

Needle (or paper clip) floating on water. Even though the density of these objects are greater than water, the surface tension along the depression is enough to counteract the force of gravity pulling down on the metal object. Click on the picture to the right, then click "Next," to view a force diagram of this situation or try out the Floating Needle trick for yourself.
Surface tension is also viewed as the result of forces acting in the plane of the surface and tending to minimize its area. On this basis, surface tension is often expressed as an amount of force exerted in the surface perpendicular to a line of unit length.

TECHNICAL TERMS OF ENGINE !!



1.)CRITICAL COMPRESSION RATIO – Lowest compression ratio at which any particular fuel will ignite by compression under prescribed test procedure. The lower the critical compression ratio the better ignition qualities the fuel has.

2.)DEGREE OF ATOMIZATION– is indicated by the smallness of the size of the particles in the spray and also by the smallness of the variation in the size of the particles.

3.)DELAY PERIOD – Time interval between the start of injection and begining of combustion as indicated by a rise in the pressure crank angle curve, from the curve which represents compression and expansion of air while motoring. Also called IGNITION DELAY.

4.)DIRECT INJECTION ENGINES– have a single open combustion chamber into which the entire quantity of fuel is injected directly.

5.)OPTIMUM INJECTION ADVANCE – Fuel injection timing before TDC which will result in minimum ignition delay.

6.)RESIDUAL PRESSURE – The pressure at which the fuel is retained in the fuel line when the injector needle valve and the pump delivery valve are in the closed position.

7.)SAC VOLUME– is the dead volume between the nozzle seat and the end of the spray holes, in a multi hole injector.

8.)SOLID INJECTION SYSTEM – The system which injects only the metered quantity of fuel by means of a pumping device. Also called AIRLESS INJECTION SYSTEM.

9.)UNIT INJECTOR – A combined fuel injection pump and fuel nozzle.

10.)VALVE CLOSING PRESSURE– is the fuel pressure at which the fuel injector needle valve snaps back on its seat. For the differential valve stem, this is less than the nozzle opening pressure.

COUPLING AND ITS TYPE EXPLAINED !!

Couplings are used to connect two shafts for torque transmission in varied 
applications. It may be to connect two units such as a motor and a  generator or it may be to form a long line shaft by connecting shafts of standard lengths say 6-8 m by couplings. Coupling may be rigid or they may provide flexibility and compensate for misalignment. They may also reduce shock loading and vibration.



However there are two main types of couplings: 

(a.)Rigid couplings 
(b.)Flexible couplings 
Rigid couplings are used for shafts having no misalignment while the flexible couplings can absorb some amount of misalignment in the shafts to be connected.

Rigid couplings:

1.Sleeve coupling 

One of the simple type of rigid coupling is a sleeve coupling which consists of a cylindrical sleeve keyed to the shafts to be connected.Normally sunk keys are used and in order to transmit the torque safely it is important to design the sleeve and the key properly. The key design is usually based on shear and bearing stresses.


2.Clamp coupling 

A typical clamp coupling is shown in  figure. It essentially 
consists of two half cylinders which are placed over the ends of the shafts 
to be coupled and are held together by through bolt.

3.Ring compression type couplings 


The coupling consists of two cones which are placed on 
the shafts to be coupled and a sleeve that fits over the cones. Three bolts are used to draw the cones towards each other and thus wedge them  firmly between the shafts and the outer sleeve.

4.Flange coupling 

It is a very widely used rigid coupling and consists of two flanges keyed to 
the shafts and bolted.

Flexible coupling
As discussed earlier these couplings can accommodate some 
misalignment and impact. A large variety offlexible couplings are available 
commercially and principal features of only a few will be discussed here.

1.Oldham coupling 

These couplings can accommodate both lateral and angular misalignment 
to some extent. An Oldham coupling consists of two flanges with slots on 
the faces and the flanges are keyed orscrewed to the shafts. A cylindrical piece, called the disc, has a narrow rectangular raised portion running 
across each face but at right angle to each other. The disc is placed 
between the flanges such that the raised portions fit into the slots in the 
flanges. The disc may be made of flexible materials and this absorbs 
some misalignment.

2.Universal joints 

These joints are capable of handling relatively large angular misalignment 
and they are widely used in agricultural machinery, machine tools and 
automobiles.

3.Pin type flexible coupling 
One of the most commonly used flexible coupling is a pin type flexible 
flange coupling in which torque is transmitted from one flange to the other 
through a flexible bush put around the bolt.These are used when excessive misalignment is not expected such as a coupling between a motor and a generator or a pump mounted on a common base plate.

TECHNICAL TERMS OF SURFACE TREATMENT EXPLAINED !!


1.)CHROMIUM PLATING – Electrolytic deposition of chromium on a metal surface, as a protection against corrosion, to provide improved wearing properties, or to build up an undersize part.

2.)CHROMIZING – Similar to carburizing. Low carbon steel parts are packed with a mixture of alumina and chromium powder and heated in a hydrogen atmosphere, forming a surface layer of chromized material of 10 to 20% chromium, according to time and temperature of heating

3.)CALORIZING – Rust proofing process for ferrous metals in which an aluminium film is formed on the surface of the metal. Means of protecting iron from oxidation at elevated temperatures.

4.)ELECTROLYTIC POLISHING – Method of polishing metals in which the work forms the anode of an electrical circuit, and is suspended in a suitable bath of acid.

5.)ELECTROPLATING – Deposition of a metal on a surface by electrolytic action.

6.)FLAME HARDENING – Process of hardening by which steel or cast iron is raised to a high temperature by a gas torch flame and then almost immediately quenched.

7.)GALVANIZING – Rust prevention treatment which consists of coating the metal (iron or steel) with a fairly thick film of zinc.

8.)HARDENING – Process of increasing the hardness of a ferrous alloy by austenitizing and quenching, also the process of increasing the hardness of some stainless steels and non-ferrous alloys by solution heat treatment and precipitation.

9.)INDUCTION HARDENING – Heating the surface of cast iron or tool steel by means of electromagnetic currents followed by a quench.

10.)LACQUERING – A protective coat given to an article to prevent the polished surface from tarnishing, to prevent oxidation or to improve the general appearance and make the article more pleasing to the eye, and hence more saleable.