Their are many basic definitions a mechanical engineer has to know. here  i am making an attempt to cover most of the definitions below hope you guyz love it . n suggest me with your precious comments.

Torque or Turning Force:

Couple:

• Two forces that are equal in magnitude but opposite in direction (and acting along parallel lines), thus creating the turning effect of a torque or moment.

Moment:

• It is the amount of moving effect which is gained for action of turning force.

Stress:

• The internal distribution of force per unit area (pressure) within a body reacting to applied forces which causes strain or deformation  The amount of reactive force per unit area is called stress. e.g. Tensile Stress, Compressive Stress, Thermal Stress.

Strain:

• If a force acts on a substance, then in that case if the substance would deform. Then the amount of deformation per unit length of that substance is called strain.

Spring:

• It is one type of device which is being distorted under certain amount of load & also can also go to its original shapeafter the removal of that load.
• Its function:
• To store energy.
• To absorb energy.
• To control motion of two elements.

Stiffness:

• Stiffness is the rigidity of an object — the extent to which it resists deformation in response to an applied force. 
• The complementary concept is flexibility or pliability: the more flexible an object is, the less stiff it is.

• The property of a fluid that resists the force tending to cause the fluid to flow. 
• A liquid's viscosity depends on the size and shape of its particles and the attractions between the particles. 
• A fluid that has no resistance to shear stress is known as an ideal fluid or inviscid fluid. 
• Zero viscosity is observed only at very low temperatures, in superfluids. Otherwise all fluids have positive viscosity. 
•  A liquid whose viscosity is less than that of water is sometimes known as a mobile liquid, while a substance with a viscosity substantially greater than water is called a viscous liquid.

• The amount of resistance of one layer of fluid over other layer of fluid.

• It is the ratio of dynamic viscosity to density.

• When a body is immersed in a liquid, it is lifted up by a force equal to weight of liquid displaced by the body. The tendency of liquid to lift up an immersed body is buoyancy. The upward thrust of liquid to lift up the body is called buoyancy force.

• It measures discharge of fluid.

• It measures discharge of fluid.

• It measures discharge of fluid.

• It measures velocity of fluid.

• The ratio of the speed of a moving body to the speed of sound
• Mach number varies by the composition of the surrounding medium and also by local conditions, especially temperature and pressure. 

•  Quantity of fluid flowing per second.
  (through a section of pipe/ through a section of channel)
  
  
  Q=AV
  
  
  where, V= velocity of fluid,A= cross-sectional area of pipe/channel
  Note: 1m³ = 1000 L1 cusec = 1 ft³/sec1 ft = 0.3048 m

• Uses kinetic energy of fluid as a working medium
• example :Turbine,Pump,Compressor etc.

• It attaches with reaction turbine . Its function is to reduce energy loss from reaction turbine & it also reduce pressure at outlet which is must blow the atmospheric pressure.

• The science concerned with the relations between heat and mechanical energy or work, and the conversion of one into the other.

• If two body are in thermal equilibrium with a third body then these two body are also in thermal equilibrium with each other.

• In a closed system, work deliver to the surrounding is directly proportional to the heat taken away from the surrounding.And also, In a closed system, work done on a system is directly proportional to the heat deliver to the surrounding.

• It is impossible to make a system or an engine which can change 100 percent input energy to 100 percent output

• A Thermodynamic quantity representing the amount of energy in a system that is no longer available for doing mechanical work

• where, ds = change of entropy, dq = change of heat, T = Temperature.
• In adiabatic process, entropy can not change. Actually,lacking or mal-adroitness of tranfering energy of a system is entropy.

• The amount of heat produced by the complete combustion of a material or fuel. Measured in units of energy per amount of material, e.g. kJ/kg.

• A closed vessel in which water is heated to make steam for powering turbines, supplying heat, etc.

• It is a part of boiler. Its function is to heat feed water which is supplied to boiler.

• A superheater is a device used to convert saturated steam or wet steam into dry steam used in steam engines or in processes, such as steam reforming. 
• There are three types of superheaters namely: radiant, convection, and separately fired. 
• A superheater can vary in size from a few tens of feet to several hundred feet.

• It is a part of boiler. Its funtion is to preheats the air to be supplied to furnace and it recover heat from exhaust gas.

Boler Draught:

• It is an important term for boiler. It is the difference of pressure above and below the fire grate. This pressure difference have to maintain very carefully inside the bolier.
•  It actually maintaind the rate of steam generation. This depends on rate of fuel burning. Inside the boiler rate of fuel burning is maintained with rate of entry fresh air. 
• If proper amount of fresh air never entered into the boiler, then proper amount of fuel inside the boiler never be burnt. So, proper fresh air enters into the boiler only by maintaining boiler draught.

• A nozzle is a device designed to control the direction or characteristics of a fluid flow as it exits 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. 
• 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.

• Scavenging is the process of pushing exhausted gas-charge out of the cylinder and drawing in a fresh draught of air or fuel/air mixture for the next cycle

• Actually, power output of engine depends on what amount of air enter into the engine through intake manifold. Amount of entry air if increased, then must be engine speed will increased. Amount of air will be increased by increasing inlet air density. The process of increasing inlet air density is supercharging. 
• The device which is used for supercharging is called supercharger.Supercharger is driven by a belt from engine crankshaft. It is installed in intake system.

• Turbocharging is similar to the supercharging. But in that case tubocharger is installed in exhaust system whereas supercharger is installed in intake system. 
• The key difference between a turbocharger and a conventional supercharger is that the latter is mechanically driven from the engine, often from a belt connected to the crankshaft, whereas a turbocharger is driven by the engine's exhaust gas turbine
• Compared to a mechanically-driven supercharger, turbochargers tend to be more efficient but less responsive. Twincharger refers to an engine which has both a supercharger and a turbocharger. 

• Its function is to regulate mean speed of engine when there are variation in the load. 
• If load increases on the engine, then engine's speed must decrease. In that case supply of working fluid have to increase. In the other way, if load decrease on the engine, then engine' speed must increase. In that case supply of working fluid have to decrease.Governor automatically, controls the supply of working fluid to the engine with varying load condition.

• A flywheel is a rotating mechanical device that is used to store rotational energy. 
• Its main function id to store energy in the time of working stroke or expansion stroke. And, it releases energy to the crankshaft in the time of suction stroke, compression stroke & exhaust stroke

• Octane number : Octane number indicates ability of fuel to resist knock.

• Cetane Number : Cetane number indicates ability of ignition of diesel fuel. That means, how much fast diesel fuel  ignites .

• It is the chemically correct air-fuel ratio by volume. By which theoretically sufficient oxygen will be supplied to burn all combustible elements in fuel completely.

Heat Transfer:

• Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy and heat between physical systems. 

• It is the quantity of heat flows between two parts of solid material by conduction. In this case following consideration will be important fact-
• Time------ 1 sec
• Area of that solid material-------- 1 m²
• Thickness of that solid material------ 1m
• Temperature difference between two parts of that material------ 1k

• A device for transferring the heat of one substance to another, as from the exhaust gases to the incoming air in a furnace.
• The classic example of a heat exchanger is found in an internal combustion engine in which a circulating fluid known as engine coolant flows through radiator coils and air flows past the coils, which cools the coolant and heats the incoming air.
• More examples : intercooler, preheater, condenser, boiler etc.

• Refrigeration is a process in which work is done to move heat from one location to another

1 tonne of refrigeration:

• It is amount of refrigeration effect or cooling effect which is produced by uniform melting of 1 tonne ice in 24 hours from or at 0 degree centigrade or freezing 1 tonne water in 24 hours from or at 0 degree centigrade.

• The process of increasing the water vapour content of a gas.

Dehumidification:

• It is the removal of moisture from the air without change dry bulb temperature.

• A system of interconnected gears
• Some important features of gears and gear trains are: ⁕The ratio of the pitch circles of mating gears defines the speed ratio and the mechanical advantage of the gear set.

Heat Treatment:

• Heat treatment involves the use of heating or chilling, normally to extreme temperatures, to achieve a desired result such as hardening or softening of a material.

• It is the difference between basic dimension of mating parts. That means, minimum clearance between mating parts that can be allowed.

• It is the difference between upper limit of dimension. It is also the permissible variation above and below the basic size. That means maximum permissible variation in dimensions.

• It is the difference in size between mating parts. That means, in that case the outside dimension of the shaft is less than internal dimension of the hole.

• It is the ability to resist deformation.
• Stiffness is the rigidity of an object — the extent to which it resists deformation in response to an applied force. The complementary concept is flexibility

• It is the property to resist fracture.

• When a material is subjected to repeated stress below yield point stress, such type of failure is fatigue failure.

• A nuclear reaction in which a massive nucleus splits into smaller nuclei with the simultaneous release of energy

• It is also a nuclear reaction by which one big nucleolus will produced by adding two small nucleolus.

• It is the process of joining two similar or dissimilar metal by fusion.

• A machine tool is a machine for shaping or machining metal or other rigid materials, usually by cutting, boring, grinding, shearing or other forms of deformation. 

• It is the method of dividing periphery of job into equal number of division. Actually, it is the process of dividing circular or other shape of workpiece into equal space, division or angle.

• It is one type of device which hold & locate workpiece and also guide & control cutting tool. It uses in drilling, reaming and tapping.

• It is one type of device which hold and locate workpiece. It uses in miling, grinding, planning & turning.

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