section-b hydraulic turbines & · pdf filesection-b hydraulic turbines & pumps :...

LECTURE NOTES OF BME

World Institute Of Technology

8km milestone ,Sohna Palwal Road , NH-71 B ,Sohna , Gurgaon ,Haryana.

Website : www.wit.net.in E-mail : [email protected]

SECTION-B

Hydraulic Turbines & Pumps :

Hydraulic turbines Hydraulic turbines are the machines which use the energy of water and

convert it into mechanical energy. The mechanical energy developed by a turbine is used in

running an an electric generator which couple to the turbine. According to the type of energy at

the inlet or action of water flowing through the turbine runners, turbines classified as

1. Impulse turbine

2. Reaction turbine

Also turbines are classified according to the direction of fluid flow or flow path

1. Radial flow turbine

2. Axial flow turbine

3. Mixed flow turbine

Impulse turbine In the turbine all the available energy of water is converted into kinetic energy

or velocity head by passing it through a converging nozzle provided at the end of penstock.

Penstock is the pipe which carries water from the dam to power station.

The water coming out of the nozzle is forced into a free jet which impinges on a series of

buckets of the runner, thus causing it to revolve. The runner is a circular frame with series of

buckets. These buckets are shaped like a double hemispherical cup. The buckets are made up of

cast iron , steel or bronze. The term impulse means that the force that turns the turbine comes

from the impact of the jet on the blades.

Working The water from the reservoir enters the nozzle through penstock. Nozzle

converts its pressure energy into Kinetic energy. Water leaves the nozzle in the form of jet

and impinges the buckets of runner, thus causing it to revolve. Eg: Pelton turbine

Reaction turbine In reaction turbine at the entrance to the runner, only a part of the

available energy of water is converted into kinetic energy and substantial part of pressure

energy remains. As the water flows through the runner the change from pressure energy

to K.E. takes place gradually.

Francis Turbine is an Inward Flow Reaction Turbine having Radial Discharge at Outlet. .

Modern Francis Turbine is a mixed flow type turbine (i.e. Water enters the runner of the





turbine in the radial direction and leaves the runner in the axial direction). Radial Flow

Turbines are those turbines in which the water flows in the Radial Direction. In Francis

Turbine the water flows from outwards to inwards through the runner (Inward Flow

Radial Turbine). Reaction Turbine means that the water at the in let of the Turbine

possesses Kinetic Energy as well as Pressure Energy.

CONSTRUCTION: - The main parts of Francis Turbine are: CASING The runner is

completely enclosed in an air-tight spiral casing. The casing and runner are always full of

water.

GUIDE MECHANISM/GUIDE VANE It consists of a circular wheel all round the runner of

the turbine. The stationary guide vanes are fixed on the guide wheel. The guide vanes

allow the water to strike the vanes fixed on the runner without shock at inlet. Also width

between the two adjacent vanes can be altered so that amount of water striking the

runner can be varied.





RUNNER It is a circular wheel on which a series of Radial Curved Vanes are fixed. The

vanes are so shaped that the water enters and leaves the runner without shock.

DRAFT TUBE The pressure at the exit of the runner of Reaction Turbine is generally less

than atmospheric pressure. The water at exit cannot be directly discharged to the tail

race. A tube or pipe of gradually increasing area is used for discharging water from the

exit of turbine to the tail race. This tube of increasing area is called Draft Tube. One end

of the tube is connected to the outlet of runner while the other end is sub-merged below

the level of water in the tail-race.

Radial flow turbines In this turbine water flows along the radial direction and remins

mainly in the plane normal to the axis of rotation, as it passes through the runner. A

radial flow turbine may be either inward radial flow or outward radial flow type. Old

francis turbine is an example for the inward radial flow turbine Axial flow turbine In this

flow of water through the runner is wholly and mainly along the direction parallel to the

axis of rotation. Eg: propeller turbine, Kaplan Turbine Mixed flow turbine Water enters

the runner at the outer periphery in radial direction and leaves the turbine at the centre





in the direction parallel to the axis of rotation. Eg: modern Francis turbine

Pumps In general pumps may be defined as a mechanical device which when connected in a

pipeline, converts the mechanical energy supplied to it from some external sources(normally

electric motor) into hydraulic energy and transfer the same to the liquid through the pipeline.

Thereby increasing the energy of flowing fluid. Normally pumps are used to transfer liquid from

one place to another as well as lower level to higher level. Pumps broadly classified into two

1. Positive displacement pumps

2. Rotodynamic pumps

Positive displacement pumps are those pumps in which liquid is sucked and then it is

pushed or displaced due to the thrust exerted on it, by a moving member. Most common

example is reciprocating pumps The rotodynamic pumps have a rotating element, called

impeller through which liquid passes. During this motion its angular momentum

changes, due to which the pre. Energy of liquid is increased . here pump does not push

the liquid as in the case of positive displacement pump. Eg: centrifugal pump

Reciprocating pump





A reciprocating pump essentially consist of a piston or plunger which moves to and fro inside a

cylinder. The cylinder is connected to suction and delivery tube each of which provide with a

non return valve called suction valve and delivery valve. The piston connected to the crank by

means of a connecting rod. Crank rotated by an engine or motor. When the crank rotates θ=0˚

to θ=180˚ piston moves from extreme left position to extreme right position Working During

the motion of piston from left to right(refer fig.) a partial vacuum created inside the cylinder.

Because of this low pressure water will rise from well through suction tube and fill the cylinder

by forcing to open the suction valve. This operation is known as suction stroke.(motion of

piston from left to right). In this stroke crank rotates θ=0˚ to θ=180˚. Also delivery valve will be

closed and suction valve will be open during this stroke.

When the crank rotates from θ=180˚ to θ=360˚ piston moves inwardly from position right to

left. Now piston exerts pressure on the liquid and due to which suction valve closes and

delivery valve opens.the liquid is then foced up through delivery pipe. This stroke is known as

delivery stroke. Now the pump has completed one cycle. The same cycle repeated as the

crank rotates.





Work done by reciprocating pump The volume of liquid pumped is known as discharge. Here

discharge in one cycle equals the volume of cylinder.





Centrifugal pumps

The basic principle on which a centrifugal pump work is that when a certain mass of

liquid is made to rotate by an external force. It is thrown away from the central axis of

rotation and a centrifugal head is developed which enables it to rise to higher levels may

be ensured. Since in these pumps the lifting of the liquid is due to the centrifugal action,

these pumps are called centrifugal pumps. In addition to centrifugal action, liquid passes

through revolving impeller, its angular momentum changes which also results in

increasing the pressure of the liquid

Components and construction





Impeller: it is a wheel or rotor which is provided with a series of backward curved

blades or vanes. it is mounted on a shaft which is coupled to an external source of

energy(electric motor) Suction pipe: it is a pipe connects its upper end to the inlet of pipe

and lower end dips into water Delivery pipe: pipe which is connected at its lower end to

the outlet of the pump and it delivers liquid to required height Working The first step is

priming. It is the operation in which suction pipe, casing of pump and portion of delivery

tube are completely filled with the liquid which is to be pumped, so that all the air from

this portion of the sump is driven out and no air pocket is left. If there is any air pocket, it

result in no delivery of liquid from pump. The necessity of priming a centrifugal pump is

due to the fact that the pressure generated in a centrifugal pump impeller is directly

proportional to the density of fluid. After the pump is primed, electric motor started to

rotate the impeller. Due to rotation impeller rotation, produces a vortex which imparts a

centrifugal head to liquid. Then the liquid starts to flow in an outward radial direction

therby leaving the vanes of impeller. At the centr of impeller a partial vacuum is created ,

causes the liquid from sump or well to rush through suction pipe to the eye of impeller.





Multi stage pumps Normally a pump with a single impeller can be used to deliver the required

discharge against a maximunhead of about 100m. but ifthe liquid is required to be delivered





against a lrger head then it can be done by using two or more pums in series. This arrangement

can be replaced by a multi stage pump.

A multistage pump consist of two or more identical impellers mounted on the same shaft and

enclosed in the same casing. All impellers are connected in series so that liquid discharged with

increased pressure. Total head developed H= n X Hm Where n= no of stages and Hm= head

developed in each stage

Advantages of centrifugal pumps over reciprocating pumps

1. Greater discharging capacity





2. Centrifugal pump can pump high viscous fluids but reciprocating pumps can handle water

or low viscous fluid

3. Operated at very high speed

4. Speed of reciprocating engine is limited

5. Maintenance cost is low for centrifugal pump.

Vane pump

A rotary vane pump is a positive-displacement pump that consists of vanes mounted on a

rotor that rotates inside of a cavity. In vane pump, the vanes slide in and out of the rotor

during the operation of the device. This combination of actions creates a seal on the

interior of the cavity, and effectively forms a series of small chambers within the larger

chamber. Liquid is captured in each of these chambers and is forced through the system

by the resulting pressure of the rotation. Essentially, there is atmospheric pressure on the

intake side of the pump that helps to suck in the liquid, while the pressure created by the

rotating action help to move and discharge the collected liquid from the outtake or

discharge side of the pump. The rotor helps to keep the flow of the liquid uniform

throughout the process.

Gear pump

External gear pumps are similar in pumping action to internal gear pumps in that two

gears come into and out of mesh to produce flow. However, the external gear pump uses

two identical gears rotating against each other -- one gear is driven by a motor and it in





turn drives the other gear. Each gear is supported by a shaft with bearings on both sides

of the gear.

1. As the gears come out of mesh, they create expanding volume on the inlet side of the

pump. Liquid flows into the cavity and is trapped by the gear teeth as they rotate.

2. Liquid travels around the interior of the casing in the pockets between the teeth and

the casing -- it does not pass between the gears.

3. Finally, the meshing of the gears forces liquid through the outlet port under pressure.

Because the gears are supported on both sides, external gear pumps are quiet-running and

are routinely used for high-pressure applications such as hydraulic applications. With no

overhung bearing loads, the rotor shaft can't deflect and cause premature wear.

Refrigeration: It can be defined as the process of transferring heat from a low temperature region to a high temperature region. In other words it is the process of cooling a substance. This can be achieved only if the heat is removed from that substance. Principle of refrigeration: The principle of refrigeration is based on second law of thermodynamics. It sates that heat does not flow from a low temperature body to a high temperature body without the help of an external work. In refrigeration process, since the heat has to be transferred from a low temperature body to a high temperature body some external work has to be done according to the second law of thermodynamics as shown. This external work is done by means of compressor, condenser etc. The machine, which works under this





principle and serves the purpose of refrigeration is called a Refrigerator Terms in refrigeration: 1. Refrigerator: It is a machine used to extract heat from a body at low temperature and reject this heat to a body at high temperature. Thus it cools the body. 2. Refrigerant: It is substance, which is used as a working fluid in refrigerators. The refrigerant has low boiling point, which means that it vaporizes at low temperature and takes away the heat from a substance. Examples: Freon 12 used in Domestic refrigerators. Freon 22 used in Air Conditioners. Properties of good refrigerant: 1. Have low freezing and boiling point 2. Have high COP 3. Be non toxic and non corrosive to metal 4. Be non explosive 5. Easily be liquefied 3. Capacity of Refrigerator: It is defined as the rate at which heat can be removed from the cold body. Simply it is the rate at which refrigeration can be produced. Its unit is expressed in terms of Ton of Refrigeration. One ton of refrigeration is defined as the quantity of heat removed to freeze one ton of water into ice at 0oC in 24 hours. Its value is 3.5 KW.

4. Refrigeration Effect: It is defined as the ratio of the quantity of heat removed to the time taken. Refrigeration Effect = Heat removed / Time taken 5. Coefficient of Performance (COP): It is defined as the ratio of heat absorbed in a given time (Refrigeration Effect) to the work done COP = Refrigeration Effect/Work done Types of Refrigerators: 1. Vapor Compression Refrigerators 2. Vapor Absorption Refrigerators Vapor Compression Refrigeration System: This type of refrigeration system is the most commonly used system in domestic refrigerators. In VCRS the vapor alternatively undergoes a change of phase from vapor to liquid and vice versa during a cycle. Construction:





Vapor compression refrigeration system has the following components at its basic parts. 1. Compressor: The function of the compressor is to compress the input refrigerant of low pressure and low temperature. As a result the pressure and the temperature of the refrigerant increases. Generally reciprocating compressors are used in a refrigeration system. An external motor is used to drive the compressor. 2. Condenser: The condenser is a coil of tubes, which are made of copper. This is used to condense the refrigerant which is in the form of vapor. And convert into liquid. 3. Expansion Valve: this is otherwise called throttle valve. This valve is used to control the flow rate of refrigerant and also to reduce the pressure of the refrigerant. 4. Evaporator: This is the part in which the cooling takes place. This is kept in the space where cooling is required. It is a coil of tubes made up of copper. Working Principle: The refrigerant, which is at low pressure and low temperature flows into the compressor. In the compressor the refrigerant is compressed and converted into a high pressure and high temperature refrigerant. This high pressure and high temperature refrigerant in vapor form then passes through the condenser where it is condensed into high pressure liquid refrigerant. The high pressure liquid refrigerant thus produced passes through the expansion valve. In the expansion valve the pressure and temperature of the refrigerant drops and it partly evaporates. It is the allowed to flow into the evaporator at a controlled rate. In the evaporator, the partly liquid and vapor refrigerant is mostly evaporated and converted into a low pressure vapor. During this process, the refrigerant absorbs its latent heat of vaporization from the material that is to be cooled. Thus the body is cooled in the evaporator . Then the low pressure vapor refrigerant enters the compressor and the cycle is repeated. Thus a material is cooled in vapor compression system.





The Psychrometric Chart The properties and processes described above are conveniently related on a graph commonly known as a psychrometric chart. Although close examination of the skeleton chart of and the detailed chart in the Appendix shows that it is not quite a rectangular coordinate plot, for most purposes it may be thought of as such, with humidity ratio as ordinate and dry





bulb temperature as abscissa. A psychrometric chart is constructed for a specified value of barometric pressure.

.

Unit of refrigeration

The units of refrigeration are always a unit of power. Domestic and commercial refrigerators may be rated in kJ/s, or Btu/h of cooling. For commercial and industrial refrigeration systems most of the world uses the kilowatt (kW) as the basic unit refrigeration. Typically, commercial and industrial refrigeration systems North America are rated in tons of refrigeration (TR). Historically, one TR was defined as the energy removal rate that will freeze one short ton of water at 0 °C (32 °F) in one day.





Air Conditioning: It is the process of controlling and maintaining the properties of air like temperature, humidity, purity, direction of flow etc in a closed space. One can have the desired condition around him using air conditioning. Terms in Air Conditioning:

Psychrometry: It is the study of the properties of moist air. The properties of the air and water vapor mixture are called psychometric properties.

Dry Air: Atmospheric air without presence of water vapor is called dry air. It is combination of 79% of nitrogen and 21% of oxygen by weight.

Moist Air: It is the mixture of dry air and water vapor. The amount of water vapor present varies according to the temperature.

Dry Bulb Temperature (DBT): It is the temperature of the air measured using an ordinary thermometer. This temperature is not affected by the water vapor present in the air.

Wet Bulb Temperature (WBT): It is the temperature measured by ordinary thermometer when its bulb is covered with wet cloth and exposed to air. It is always less than DBT.

Wet Bulb Depression (WBD): The difference between the dry bulb temperature and the wet bulb temperature. If the air is fully saturated then the wet bulb depression is zero.

Dew Point Temperature (DPT): The temperature at which the water vapor in the air begins to condense when the temperature of the air is continuously reduced.

Humidity: the quantity of water vapor present in the air is known as humidity. It depends on the temperature of the air and is independent of the pressure of the air.

Relative Humidity: It is defined as the ratio of mass of water vapor present in a given volume of air at a given temperature to the mass of water vapor present in the same volume and temperature of the air when it is fully saturated.





Working of a Air conditioning system It consists of dampers, air filter, cooling coil, spray type humidifier, heating coil and a fan. Atmospheric air flows through the dampers. The quantity of air depends upon the "load and the dampers control it. Air then passes through the Air filter. The filter removes dirt, dust and other impurities. The air now passes over a cooling coil. So when air is cooled below its dew point temperature, the water vapour is removed from the air in the form of water droplets. The surface temperature of the cooling coil has to be maintained below the dew-point temperature of the atmospheric air to accomplish dehumidification. The quantity of water removed from air is collected in the sump and is drained. The temperature of air leaving the cooling coil is lower than the ambient temperature for comfort. During the dry weather the spray type humidifier is used to increase the humidity of the conditioned air. During wet weather condition the relative humidity of the air is high, is controlled by the heating coil. For the comfort condition required is DBT around 23O C and relative humidity 60%. So the air is to be cooled and humidified to the comfort condition. Now the conditioned air is supplied to the conditioned space by a fan and ducts.

Section C

Power Transmission Methods and Devices AND Stresses and

Strains

Power transmission systems In mechanical industries, power from the engines or electric motor

are transmitted to the machines using the following drives 1.belt drive 2. chain drive 3. gear

drive

Belt drives





A belt is a loop of flexible material used to link two or more rotating shafts mechanically. Belts

are looped over pulleys. In a two pulley system, the belt can either drive the pulleys in the same

direction, or the belt may be crossed, so that the direction of the shafts is opposite. The shaft

from which power is transmitted is called driver shaft and the shaft to which power is

transmitted is called driven shaft.

Types of belts Based on arrangement of shafts and belt

Open belt drive: in this the direction of rotation is same for both driver and driven shaft. See

the fig. Above. The driver pulley pulls the belt from one side and delivers the same belt to the

other side . hence the tension on the former side will be greater than the later side. The side

where tension is more is called tight side and the other side is called slack side.

Cross belt drive: in this driver and driven pulley have different direction of rotation. At the point

where belt crosses, it rubs against itself and wears. In order to reduce this shaft should be

placed at a minimum distance of 20 d, where ‘d’ is the width of belt.

Based on shape of cross section

Flat belt- it used to transmit moderate amount of power for a long distane between

shafts(upto 10m). Flat belts are again classifie as open belt drive and cross belt drive





Vee/ V- belt- these are used to transmit large amout of power between two shafts for a

short distance

Circular belt/rope- these belts are used to transmit large amount of power for large

distance(>8m) Flat-belt drives are simple and convenient. They permit the use of

ordinary pulleys with smooth surfaces, and they can be operated at speeds as high as 40–

50 m/sec and more. However, they are bulky in design and low in strength. V-belt drives

provide improved attachment of the belt to the pulleys, permit shortening of the centre

distances, and allow a decrease in the size of the drive. Round-belt drives are now rare

and are used only in mechanisms of low power, such as those in sewing machines. The

advantages of belt drives are their simplicity of design, relative low cost, capacity to

transmit power over significant distances (up to 10 m and more), and smooth and

noiseless operation. It can be used with very high speed drives. In addition, the elastic

properties of the belt and its ability to slip on the pulleys help prevent overload. The

disadvantages include the short lifetime of the belts, relatively large size, heavy stress on

the shafts and bearings, and variation in the tension ratio caused by the inevitable

slipping of the belt. Belts made of highly elastic, strong synthetic materials like leather,

cotton and rubber. Belt drives are widely used in agricultural machines, electric

generators, certain machine tools, and textile machines. They are ordinarily used for

transmitting power up to 30–50 kilowatts.]

Power transmitted by belt drive





Here the driving pulley pulls the belt from the lower side to the upper side. Thus the

tension in the lower side will be greater than the tension in the upper side. The upper

side is called the slack side and lower side is the tight side. T1 - tension in the tight side of

the belt is Newton T2 – Tension in the slack side of the belt is Newton d1 – diameter of

the driving pulley d2 – diameter of the follower v = linear velocity of the belt in m/sec The

driven pulley rotates because of the difference in tensions in the tight and slack side of

the belt. Therefore the force causing the rotation is the difference between the two

tensions the belt exerts a force on the pulley. Let F is the net force acting on the belt So F

= (T1-T2)

Relation between belt tension and friction





In the picture angle of contact between belt and pulley is θ. Let the tight side tension be

T1 and slack side tension T2. Consider a short length MN of belt, which subtends an angle

δθ at pulley centre. Let T be tension at M and (T + δT) be the tension at N. The frictional

force depends normal reaction R. Suppose the belt is in equilibrium. Then ΣX=0 and

ΣY=0. Here x direction is horizontal(radial direction) and y direction is tangential at point

P.

(T + δT) cos (δθ / 2)- T cos (δθ / 2) - μR =0

Since δθ is very small, cos (δθ / 2) = 1 (T + δT) + T-μR = 0

μR = δT ........................................… (1) and resolving the force is radial reaction.

T sin (δθ / 2) + (T + δT) sin (δθ/2)- R =0 Since, δθ is very small, sin (δθ/2)= (δθ/2) T δθ/2 +

(T + δT) δθ/2 R = 0 (δT. δθ / 2 is neglected)

T δθ=R................................................…(2) From equations (1) and (2)

μ(T δθ) = δT δT / T = μ δθ On integration,

we get ∫T1T2 δT/T = ∫oθ μdθ

log T1 / T2 = μθ

T1 / T2 = (e)μθ





Gears and gear train

Toothed wheels are known as gears. A gear is a rotating machine part having cut teeth, which

mesh with another toothed part in order to transmit torque. Gears having high efficiency and

high accuracy. It is having less maintenance cost. There are different types of gears. Gears may

be classified according to the relative position of the axis of revolution. The axis may be

1. spur,

2. helical

3. bevel gear

4. worm and worm wheel

5. Rack and pinion.

Spur Gears Spur Gears





Spur gears are the most commonly used gear type. They are characterized by teeth which are

perpendicular to the face of the gear or teeth are parallel to the axis of rotation. Spur gears are

by far the most commonly available, and are generally the least expensive. The basic

descriptive geometry for a spur gear is shown in the figure. Limitations: Spur gears generally

cannot be used when a direction change between the two shafts is required. Advantages: Spur

gears are easy to find, inexpensive, and efficient.

Helical Gears

Helical gears are similar to the spur gear except that the teeth are at an angle to the shaft/axis

of rotation. The resulting teeth are longer than the teeth on a spur gear of equivalent pitch

diameter. The longer teeth cause helical gears to have the following differences from spur gears

of the same size :

1. Tooth strength is greater because the teeth are longer,

2. Greater surface contact on the teeth allows a helical gear to carry more load than a spur gear

3. The longer surface of contact reduces the efficiency of a helical gear relative to a spur gear

Helical gears may be used to mesh two shafts that are parallel.the angle between tooth

and axis of rotation is called helix angle. Limitations: Helical gears have the major

disadvantage that they are expensive . Helical gears are also slightly less efficient than a

spur gear of the same size Advantages: Helical gears can be used on non parallel and even

perpendicular shafts, and can carry higher loads than spur gears.





Bevel Gears

Bevel gears are primarily used to transfer power between intersecting shafts. The teeth of

these gears are formed on a conical surface. Standard bevel gears have teeth which are cut

straight and are all parallel to the line pointing the apex of the cone on which the teeth

are based. One of the most common applications of bevel gears is the automobile

differential system, Limitations: Limited availability. Cannot be used for parallel shafts.

Can become noisy at high speeds. Advantages: Excellent choice for intersecting shaft

systems.

Gear terminology:





Pitch circle: it is an imaginary circle which by pure rolling action, would give the same motion as

the actual gear

Addendum circle: A circle drawn through the top of the teeth and is concentric with pitch circle.

Root (or dedendum) circle: The circle drawn through the bottom of the teeth.

Addendum: The radial distance between the pitch circle and the addendum circle.

Dedendum: The radial distance between the pitch circle and the dedendum circle.

Clearance: The difference between the dedendum of one gear and the addendum of the mating

gear.

Face of a tooth: That part of the tooth surface lying outside the pitch surface.

Flank of a tooth: The part of the tooth surface lying inside the pitch surface.

Circular thickness (also called the tooth thickness) : The thickness of the tooth measured on the

pitch circle. It is the length of an arc and not the length of a straight line.

Tooth space: The distance between adjacent teeth measured on the pitch circle.

Backlash: The difference between the circle thickness of one gear and the tooth space of the

mating gear.

Circular pitch p: The width of a tooth and a space, measured on the pitch circle.

Diametral pitch P: The number of teeth of a gear per inch of its pitch diameter. A toothed gear

must have an integral number of teeth. The circular pitch, therefore, equals the pitch

circumference divided by the number of teeth. The diametral pitch is, by definition, the number

of teeth divided by the pitch diameter.

Module m: Pitch diameter divided by number of teeth. The pitch diameter is usually specified in

inches or millimeters; in the former case the module is the inverse of diametral pitch.





Fillet : The small radius that connects the profile of a tooth to the root circle.

Velocity ratio: The ratio of the number of revolutions of the driving (or input) gear to the

number of revolutions of the driven (or output) gear, in a unit of time.

Pitch point: The point of tangency of the pitch circles of a pair of mating gears.

Line of action: A line normal to a pair of mating tooth profiles at their point of contact.

Path of contact: The path traced by the contact point of a pair of tooth profiles.

Pressure angle : The angle between the common normal at the point of tooth contact and the

common tangent to the pitch circles. It is also the angle between the line of action and the

common tangent.

Base circle :An imaginary circle used in involute gearing to generate the involutes that form the

tooth profiles.

Gear Trains

Gear trains consist of two or more gears for the purpose of transmitting motion and

power from one shaft to another. Figure a shows a simple ordinary gear train in which

there is only one gear for each axis. In Figure b a compound gear train is, in which two or

more gears may rotate about a single axis





Stress is the internal resistance, or counterfource, of a material to the distorting effects of an external force or load. These counterforces tend to return the atoms to their normal positions. The total resistance developed is equal to the external load. This resistance is known as stress. Although it is impossible to measure the intensity of this stress, the external load and the area to which it is applied can be measured. Stress (σ) can be equated to the load per unit area or the force (F) applied per cross-sectional area (A) perpendicular to the force as shown in Equation (2-1).





TYPES OF STRESS Stress intensity within the body of a component is expressed as one of three basic types of internal load. They are known as tensile, compressive, and shear. Figure 1 illustrates the different types of stress. Mathematically, there are only two types of internal load because tensile and compressive stress may be regarded as the positive and negative versions of the same type of normal loading.





The quantity E, the ratio of the unit stress to the unit strain, is the modulus of elasticity of the material in tension or compression and is often called Young's Modulus.





a.designation of the material under test

b. original cross section dimensions of the specimen within the gage length c. original gage length





d. a series of frequent readings identifying the load and the corresponding gage length dimension e. final average diameter of the minimum cross section f. final gage length g. description of the appearance of the fracture surfaces (for example, cup-cone, wolf's ear, diagonal, start)





SECTION – A : Introduction to Machine Tool to Commonly used

Machine Tools in a Workshop, Basic concept of

thermodynamics, Properties of Steam & Steam Generator

Basic functional principles of machine tool operations Machine Tools produce desired geometrical surfaces on solid bodies (preformed blanks) and for that they are basically comprised of;

• Devices for firmly holding the tool and work • Drives for providing power and motions to the tool and work • Kinematic system to transmit motion and power from the sources to the tool-work • Automation and control systems • Structural body to support and accommodate those systems with sufficient

strength and rigidity.

For material removal by machining, the work and the tool need relative movements and those motions and required power are derived from the power source(s) and transmitted through the kinematic system(s) comprised of a number and type of mechanisms.

Configuration of Basic Machine Tools and their use • Centre lathes

- configuration

Fig. 2.9 shows the general configuration of center lathe. Its major parts are:

o Head stock: it holds the blank and through that power and rotation

are transmitted to the job at different speeds o tailstock: supports longer blanks and often accommodates tools

like drills, reamers etc for hole making.

o carriage: accommodates the tool holder which in turn holds the moving tools o bed: ∆ headstock is fixed and tailstock is clamped on it. Tailstock has a

provision to slide and facilitate operations at different locations





∆ carriage travels on the bed

o columns: on which the bed is fixed o work-tool holding devices

⎯ uses of center lathes

Centre lathes are quite versatile being used for various operations: ⎯ turning taper ⎯ facing, centering, drilling, recessing and parting ⎯ thread cutting; external and internal ⎯ knurling.

Some of those common operations are shown in Fig. 2.10. Several other operations can also be done in center lathes using suitable attachments.

• Shaping machine

Fig. 2.11 shows the general configuration of shaping machine. Its major parts are:

o Ram: it holds and imparts cutting motion to the tool through reciprocation o Bed: it holds and imparts feed motions to the job (blank) o Housing with base: the basic structure and also accommodate the drive

mechanisms

o Power drive with speed and feed change mechanisms. Shaping machines are generally used for producing flat surfaces, grooving, splitting etc. Because of poor productivity and process

capability these machine tools are not widely used now-a-days for production





� Planing machine The general configuration is schematically shown in Fig. This machine tool also does the same operations like shaping machine but the major differences are:

o In planing the job reciprocates for cutting motion and the tool moves slowly for the feed motions unlike in shaping machine.

o Planing machines are usually very large in size and used for large jobs and heavy duty work.

• Drilling machine

Fig. shows general configuration of drilling machine, column drill in particular. The salient parts are

o Column with base: it is the basic structure to hold the other parts o Drilling head: this box type structure accommodates the

power drive and the speed and feed gear boxes. o Spindle: holds the drill and transmits rotation and axial

translation to the tool for providing cutting motion and feed motion – both to the drill. Drilling machines are available in varying size and configuration such as pillar drill, column drill, radial drill, micro-drill etc. but in working principle all are more or less the same.





Drilling machines are used: o Mainly for drilling (originating or enlarging cylindrical holes)

o Occasionally for boring, counter boring, counter sinking etc. o Also for cutting internal threads in parts like nuts using suitable attachment

� Milling machine The general configuration of knee type conventional milling machine with horizontal arbour is shown in Fig. 2.14. Its major parts are

o Milling arbour: to hold and rotate the cutter o Ram: to support the arbour o Machine table: on which job and job holding devices are mounted to

provide the feed motions to the job. o Power drive with Speed and gear boxes: to provide power and motions

to the tool-work o Bed: which moves vertically upward and downward and accommodates the various drive mechanisms o Column with base: main structural body to support other parts.





Milling machines are also quite versatile and can do several operations like o making flat surfaces o grooving, slitting and parting o helical grooving





Specification of Machine Tools. A machine tool may have a large number of various features and characteristics. But only some specific salient features are used for specifying a machine tool. All the manufacturers, traders and users must know how are machine tools specified. The methods of specification of some basic machine tools are as follows:

o Centre lathe • Maximum diameter and length of the jobs that can be accommodated • Power of the main drive (motor) • Range of spindle speeds • Range of feeds • Space occupied by the machine.





o Shaping machine • Length, breadth and depth of the bed • Maximum axial travel of the bed and vertical travel of the bed / tool • Maximum length of the stroke (of the ram / tool) • Range of number of strokes per minute • Range of table feed • Power of the main drive • Space occupied by the machine

o Drilling machine (column type)

• Maximum drill size (diameter) that can be used • Size and taper of the hole in the spindle • Range of spindle speeds

� Range of feeds • Power of the main drive • Range of the axial travel of the spindle / bed • Floor space occupied by the machine

o Milling machine (knee type and with arbour)

• Type; ordinary or swiveling bed type • Size of the work table • Range of travels of the table in X-Y-Z directions • Arbour size (diameter) • Power of the main drive • Range of spindle speed • Range of table feeds in X-Y-Z directions • Floor space occupied.





THERMODYNAMIC PROPERTIES: Mass and Weight The mass (m) of a body is the measure of the amount of material present in that body. The weight (wt) of a body is the force exerted by that body when its mass is accelerated in a gravitational field. Mass and weight are related as shown in Equation 1-1. wt = mg /gc where: wt = weight (lbf) m = mass (lbm) g = acceleration of gravity = 32.17 ft/sec2 gc = gravitational constant = 32.17 lbm-ft/lbf-sec2 Note that gc has the same numerical value as the acceleration of gravity at sea level, but is not the acceleration of gravity. Rather, it is a dimensional constant employed to facilitate the use of Newton’s Second Law of Motion with the English system of units. The weight of a body is a force produced when the mass of the body is accelerated by a gravitational acceleration. The mass of a certain body will remain constant even if the gravitational acceleration acting upon that body changes. Rev.

Specific Volume The specific volume (ν) of a substance is the total volume (V) of that substance divided by the total mass (m) of that substance (volume per unit mass). It has units of cubic feet per pound-mass (ft3/lbm). ν= V /m where: ν = specific volume (ft3/lbm) V = volume (ft3) m = mass (lbm) Density The density (ρ ) of a substance is the total mass (m) of that substance divided by the total volume (V) occupied by that substance (mass per unit volume). It has units of pound-mass per





cubic feet (lbm/ft3). The density (ρ ) of a substance is the reciprocal of its specific volume (ν). ρ =m /V=1/ν where: ρ = density (lbm/ft3) m = mass (lbm) V = volume (ft3) ν = specific volume (ft3/lbm)

Specific Gravity Specific gravity (S.G.) is a measure of the relative density of a substance as compared to the density of water at a standard temperature. Physicists use 39.2°F (4°C) as the standard, but engineers ordinarily use 60°F. In the International System of Units (SI Units), the density of water is 1.00 g/cm3 at the standard temperature. Therefore, the specific gravity (which is dimensionless) for a liquid has the same numerical value as its density in units of g/cm3. Since the density of a fluid varies with temperature, specific gravities must be determined and specified at particular temperatures. Humidity Humidity is the amount of moisture (water vapor) in the air. It can be expressed as absolute humidity or relative humidity. Absolute humidity is the mass of water vapor divided by a unit volume of air (grams of water/cm3 of air). Relative humidity is the amount of water vapor present in the air divided by the maximum amount that the air could contain at that temperature. Relative humidity is expressed as a percentage. The relative humidity is 100% if the air is saturated with water vapor and 0% if no water vapor is present in the air at all. Intensive and Extensive Properties Thermodynamic properties can be divided into two general classes, intensive and extensive properties. An intensive property is independent of the amount of mass. The value of an extensive property varies directly with the mass. Thus, if a quantity of matter in a given state is divided into two equal parts, each part will have the same value of intensive property as the original and half the value of the extensive property. Temperature, pressure, specific volume, and density are examples of intensive properties. Mass and total volume are examples of extensive properties. Temperature Temperature is a measure of the molecular activity of a substance. The greater the movement of molecules, the higher the temperature. It is a relative measure of how "hot" or "cold" a substance is and can be used to predict the direction of heat transfer. Temperature Scales The two temperature scales normally employed for measurement purposes are the Fahrenheit (F) and Celsius (C) scales. These scales are based on a specification of the number of increments between the freezing point and boiling point of water at standard atmospheric pressure. The





Celsius scale has 100 units between these points, and the Fahrenheit scale has 180 units. The zero points on the scales are arbitrary.

°F = 32.0 + (9/5)°C °C = (°F - 32.0)(5/9)

Pressure Pressure is a measure of the force exerted per unit area on the boundaries of a substance (or system). It is caused by the collisions of the molecules of the substance with the boundaries of the system. As molecules hit the walls, they exert forces that try to push the walls outward. The forces resulting from all of these collisions cause the pressure exerted by a system on its surroundings. Pressure is frequently measured in units of lbf/in2 (psi). Pressure Scales When pressure is measured relative to a perfect vacuum, it is called absolute pressure (psia); when measured relative to atmospheric pressure (14.7 psi), it is called gauge pressure (psig). The latter pressure scale was developed because almost all pressure gauges register zero when open to the atmosphere. Therefore, pressure gauges measure the difference between the pressure of the fluid to which they are connected and that of the surrounding air. Pabs = Patm + Pgauge Pabs = Patm – Pvac Energy Energy is defined as the capacity of a system to perform work or produce heat. Potential Energy Potential energy (PE) is defined as the energy of position. Using English system units, it is defined by Equation. PE =mgz /gc where: PE = potential energy (ft-lbf) m = mass (lbm) z = height above some reference level (ft) g = acceleration due to gravity (ft/sec2) gc = gravitational constant = 32.17 ft-lbm/lbf-sec2 Kinetic Energy Kinetic energy (KE) is the energy of motion. Using English system units, it is defined by Equation KE mv 2 /2gc where:





KE = kinetic energy (ft-lbf) m = mass (lbm) v = velocity (ft/sec) gc = gravitational constant = 32.17 ft-lbm/lbf-sec2 Specific Enthalpy Specific enthalpy (h) is defined as h = u + Pν, where u is the specific internal energy (Btu/lbm) of the system being studied, P is the pressure of the system (lbf/ft2), and ν is the specific volume (ft3/lbm) of the system. Enthalpy is usually used in connection with an "open" system problem in thermodynamics. Enthalpy is a property of a substance, like pressure, temperature, and volume, but it cannot be measured directly. Normally, the enthalpy of a substance is given with respect to some reference value. For example, the specific enthalpy of water or steam is given using the reference that the specific enthalpy of water is zero at .01°C and normal atmospheric pressure. The fact that the absolute value of specific enthalpy is unknown is not a problem, however, because it is the change in specific enthalpy (∆h) and not the absolute value that is important in practical problems. Steam tables include values of enthalpy as part of the information tabulated. Work Kinetic energy, potential energy, internal energy, and P-V energy are forms of energy that are properties of a system. Work is a form of energy, but it is energy in transit. Work is not a property of a system. Work is a process done by or on a system, but a system contains no work. This distinction between the forms of energy that are properties of a system and the forms of energy that are transferred to and from a system is important to the understanding of energy transfer systems. Work is defined for mechanical systems as the action of a force on an object through a distance. It equals the product of the force (F) times the displacement (d). W = Fd Heat Heat, like work, is energy in transit. The transfer of energy as heat, however, occurs at the molecular level as a result of a temperature difference. The symbol Q is used to denote heat. In engineering applications, the unit of heat is the British thermal unit (Btu). Specifically, this is called the 60 degree Btu because it is measured by a one degree temperature change from 59.5 to 60.5°F. q= Q /m where: q = heat transferred per unit mass (Btu/lbm) Q = heat transferred (Btu) m = mass (lbm) Entropy (S) is a property of a substance, as are pressure, temperature, volume, and enthalpy. Because entropy is a property, changes in it can be determined by knowing the initial and final conditions of a substance. Entropy quantifies the energy of a substance that is no longer





available to perform useful work. Because entropy tells so much about the usefulness of an amount of heat transferred in performing work, the steam tables include values of specific entropy (s = S/m) as part of the information tabulated. Entropy is sometimes referred to as a measure of the inability to do work for a given heat transferred. Entropy is represented by the letter S and can be defined as ∆S in the following relationships. ∆S= ∆Q/Tabs ∆s= ∆q/Tabs where: ∆S = the change in entropy of a system during some process (Btu/°R) Thermodynamic Systems and Surroundings Thermodynamics involves the study of various systems. A system in thermodynamics is nothing more than the collection of matter that is being studied. A system could be the water within one side of a heat exchanger, the fluid inside a length of pipe, or the entire lubricating oil system for a diesel engine. Determining the boundary to solve a thermodynamic problem for a system will depend on what information is known about the system and what question is asked about the system. Everything external to the system is called the thermodynamic surroundings, and the system is separated from the surroundings by the system boundaries. These boundaries may either be fixed or movable. In many cases, a thermodynamic analysis must be made of a device, such as a heat exchanger, that involves a flow of mass into and/or out of the device. The procedure that is followed in such an analysis is to specify a control surface, such as the heat exchanger tube walls. Mass, as well as heat and work (and momentum), may flow across the control surface. Types of Thermodynamic Systems Systems in thermodynamics are classified as isolated, closed, or open based on the possible transfer of mass and energy across the system boundaries. An isolated system is one that is not influenced in any way by the surroundings. This means that no energy in the form of heat or work may cross the boundary of the system. In addition, no mass may cross the boundary of the system. A thermodynamic system is defined as a quantity of matter of fixed mass and identity upon which attention is focused for study. A closed system has no transfer of mass with its surroundings, but may have a transfer of energy (either heat or work) with its surroundings. An open system is one that may have a transfer of both mass and energy with its surroundings. Thermodynamic Equilibrium When a system is in equilibrium with regard to all possible changes in state, the system is in thermodynamic equilibrium. For example, if the gas that comprises a system is in thermal equilibrium, the temperature will be the same throughout the entire system. Control Volume A control volume is a fixed region in space chosen for the thermodynamic study of mass and





energy balances for flowing systems. The boundary of the control volume may be a real or imaginary envelope. The control surface is the boundary of the control volume. Steady State Steady state is that circumstance in which there is no accumulation of mass or energy within the control volume, and the properties at any point within the system are independent of time. Thermodynamic Process Whenever one or more of the properties of a system change, a change in the state of the system occurs. The path of the succession of states through which the system passes is called the thermodynamic process. One example of a thermodynamic process is increasing the temperature of a fluid while maintaining a constant pressure. Another example is increasing the pressure of a confined gas while maintaining a constant temperature. Thermodynamic processes will be discussed in more detail in later chapters. Cyclic Process When a system in a given initial state goes through a number of different changes in state (going through various processes) and finally returns to its initial values, the system has undergone a cyclic process or cycle. Therefore, at the conclusion of a cycle, all the properties have the same value they had at the beginning. Steam (water) that circulates through a closed cooling loop undergoes a cycle. Reversible Process A reversible process for a system is defined as a process that, once having taken place, can be reversed, and in so doing leaves no change in either the system or surroundings. In other words the system and surroundings are returned to their original condition before the process took place. In reality, there are no truly reversible processes; however, for analysis purposes, one uses reversible to make the analysis simpler, and to determine maximum theoretical efficiencies. Therefore, the reversible process is an appropriate starting point on which to base engineering study and calculation. Although the reversible process can be approximated, it can never be matched by real processes. One way to make real processes approximate reversible process is to carry out the process in a series of small or infinitesimal steps. For example, heat transfer may be considered reversible if it occurs due to a small temperature difference between the system and its surroundings. For example, transferring heat across a temperature difference of 0.00001 °F "appears" to be more reversible than for transferring heat across a temperature difference of 100 °F. Therefore, by cooling or heating the system in a number of infinitesamally small steps, we can approximate a reversible process. Although not practical for real processes, this method is beneficial for thermodynamic studies since the rate at which processes occur is not important. Irreversible Process An irreversible process is a process that cannot return both the system and the surroundings to





their original conditions. That is, the system and the surroundings would not return to theiroriginal conditions if the process was reversed. For example, an automobile engine does not give back the fuel it took to drive up a hill as it coasts back down the hill. There are many factors that make a process irreversible. Four of the most common causes of irreversibility are friction, unrestrained expansion of a fluid, heat transfer through a finite temperature difference, and mixing of two different substances. These factors are present in real, irreversible processes and prevent these processes from being reversible. Adiabatic Process An adiabatic process is one in which there is no heat transfer into or out of the system. The system can be considered to be perfectly insulated. Isentropic Process An isentropic process is one in which the entropy of the fluid remains constant. This will be true if the process the system goes through is reversible and adiabatic. An isentropic process can also be called a constant entropy process. Polytropic Process When a gas undergoes a reversible process in which there is heat transfer, the process frequently takes place in such a manner that a plot of the Log P (pressure) vs. Log V (volume) is a straight line. Or stated in equation form PVn = a constant. This type of process is called a polytropic process. An example of a polytropic process is the expansion of the combustion gasses in the cylinder of a water-cooled reciprocating engine. Laws of thermodynamics:

Zeroth law of thermo dynamics:





First law of thermodynamics:-

The First Law of Thermodynamics states: Energy can neither be created nor destroyed, only altered in form. For any system, energy transfer is associated with mass and energy crossing the control boundary, external work and/or heat crossing the boundary, and the change of stored energy within the control volume. The mass flow of fluid is associated with the kinetic, potential, internal, and "flow" energies that affect the overall energy balance of the system. The exchange of external work and/or heat complete the energy balance.

The First Law of Thermodynamics is referred to as the Conservation of Energy principle, meaning that energy can neither be created nor destroyed, but rather transformed into various forms as the fluid within the control volume is being studied. The energy balance spoken of here is maintained within the system being studied. The system is a region in space (control volume) through which the fluid passes. The various energies associated with the fluid are then observed





as they cross the boundaries of the system and the balance is made. As discussed in previous chapters of this module, a system may be one of three types: isolated, closed, or open. The open system, the most general of the three, indicates that mass, heat, and external work are allowed to cross the control boundary. The balance is expressed in words as: all energies into the system are equal to all energies leaving the system plus the change in storage of energies within the system. Recall that energy in thermodynamic systems is composed of kinetic energy (KE), potential energy (PE), internal energy (U), and flow energy (PL); as well as heat and work processes. Σ (all energies in) = Σ (all energies out) + ∆(energy stored in system)

Second law of thermodynamics:-

One of the earliest statements of the Second Law of Thermodynamics was made by R. Clausius in 1850. He stated the following. It is impossible to construct a device that operates in a cycle and produces no effect other than the removal of heat from a body at one temperature and the absorption of an equal quantity of heat by a body at a higher temperature. With the Second Law of Thermodynamics, the limitations imposed on any process can be studied to determine the maximum possible efficiencies of such a process and then a comparison can be made between the maximum possible efficiency and the actual efficiency achieved. One of the areas of application of the second law is the study of energy-conversion systems. For example, it is not possible to convert all the energy obtained from a nuclear reactor into electrical energy. There must be losses in the conversion process. The second law can be used to derive an expression for the maximum possible energy conversion efficiency taking those losses into account. Therefore, the second law denies the possibility of completely converting into work all of the heat supplied to a system operating in a cycle, no matter how perfectly designed the system may be. The concept of the second law is best stated using Max Planck’s description: It is impossible to construct an engine that will work in a complete cycle and produce no other effect except the raising of a weight and the cooling of a heat reservoir. The Second Law of Thermodynamics is needed because the First Law of Thermodynamics does not define the energy conversion process completely. The first law is used to relate and to evaluate the various energies involved in a process. However, no information about the direction of the process can be obtained by the application of the first law. Early in the development of





the science of thermodynamics, investigators noted that while work could be converted completely into heat, the converse was never true for a cyclic process. Certain natural processes were also observed always to proceed in a certain direction (e.g., heat transfer occurs from a hot to a cold body). The second law was developed as an explanation of these natural phenomena.

Third law of thermodynamics:-

Steam Tables Steam tables consist of two sets of tables of the energy transfer properties of water and steam: saturated steam tables and superheated steam tables. Portions of the tables are shown in Figure A-2 of Appendix A. Both sets of tables are tabulations of pressure (P), temperature (T), specific volume (ν), specific enthalpy (h), and specific entropy (s). The following notation is used in steam tables. Some tables use v for ν (specific volume) because there is little possibility of confusing it with velocity. T = temperature (°F) P = pressure (psi) ν = specific volume (ft3/lbm) νf = specific volume of saturated liquid (ft3/lbm) νg = specific volume of saturated vapor (ft3/lbm)

Property Diagrams The phases of a substance and the relationships between its properties are most commonly shown on property diagrams. A large number of different properties have been defined, and there are some dependencies between properties. For example, at standard atmospheric pressure and





temperature above 212°F, water exists as steam and not a liquid; it exists as a liquid at temperatures between 32°F and 212°F; and, it exists as ice at temperatures below 32°F. In addition, the properties of ice, water, and steam are related. Saturated steam at 212°F and standard atmospheric pressure has a specific volume of 26.8 ft3/lbm. At any other temperature and pressure, saturated steam has a different specific volume. For example, at 544°F and 1000 psia pressure, its specific volume is 0.488 ft3/lbm. There are five basic properties of a substance that are usually shown on property diagrams. These are: pressure (P), temperature (T), specific volume (ν), specific enthalpy (h), and specific entropy (s). When a mixture of two phases, such as water and steam, is involved, a sixth property, quality (x), is also used. There are six different types of commonly encountered property diagrams. These are: Pressure- Temperature (P-T) diagrams, Pressure-Specific Volume (P-ν) diagrams, Pressure-Enthalpy (P-h) diagrams, Enthalpy-Temperature (h-T) diagrams, Temperature-entropy (T-s) diagrams, and Enthalpy-Entropy (h-s) or Mollier diagrams.

SECTION-D

Introduction to Manufacturing Systems

NUMERICAL CONTROL Numerical control (NC) is the operation of a machine tool by a series of coded instructions consisting of numbers, letters of the alphabet, and symbols that the machine control unit (MCU) can understand. These instructions are changed into electrical pulses of current that the machine's motors and controls follow to carry out manufacturing operations on a workpiece. The numbers, letters, and symbols are coded instructions that refer to specific distances, positions, functions, or motions, that the machine tool can understand as it machines the workpiece.

ADVANTAGE:-

NC Machine

Numerical Control (NC) refers to the method of controlling the manufacturing operation

by means

of directly inserted coded numerical instructions into the machine tool.

Numerical control (NC) refers to the automation of machine tools that are operated by

abstractly programmed commands encoded on a storage medium, as opposed to





manually controlled via handwheels or levers, or mechanically automated via cams alone.

The major advantages of NC over conventional methods of machine control are as

follows:

v higher precision: NC machine tool are capable of machining at very close tolerances, in

some operations as small as 0.005 mm;

v machining of complex three-dimensional shapes: this is discussed in Section 6.2 in

connectionwith the problem of milling of complex shapes;

v better quality: NC systems are capable of maintaining constant working conditions

forall parts in a batch thus ensuring less spread of quality characteristics;v higher

productivity: NC machine tools reduce drastically the non machining time.Adjusting the

machine tool for a different product is as easy as changing the computerprogram and tool

turret with the new set of cutting tools required for the particular part.

v multi-operational machining: some NC machine tools, for example machine centers,

are capable of accomplishing a very high number of machining operations thus reducing

significantly the number of machine tools in the workshops.

v low operator qualification: the role of the operation of a NC machine is simply to upload

the workpiece and to download the finished part. In some cases, industrial robots are

employed for material handling, thus eliminating the human operator.

Disadvantages of conventional NC

There are a lot of important setbacks in the conventional NC machines that is worth to be

mentioned because this was the reason that lead to the development of improved

numerical control machines. The most important are:

1. Part programming mistakes. Mistakes in the part programming were quite often and a

program should be rewritten more than two times to function correctly.

2. Paper tape. The paper tape is by its nature durable and fragile and cannot be used

repeatedly without wear and eventually tear. More durable materials have been used later

on, however, these material are relatively expensive.

3. Tape reader. This is the least reliable component of the NC system. When a machine

breaks down the first to be checked is the tape reader.

4. Controller. The hard-wired NC controller unit means that the control features cannot

be easily altered to incorporate improvements to the unit.

5. Management information. No timely information on operational performance can be





provided to management.

6. No optimal speeds and feeds. Since the function of the conventional NC is just to

control the position of the tool relative to the work there is no attempt to optimize speeds

and feeds. As a result the part programmer must plan the cutting conditions

conservatively and this reduces productivity.

Direct numerical control (DNC)

Direct numerical control is defined as a manufacturing system in which a number of NC

machines are controlled by a computer through direct connection in real time. The tape

reader is omitted from the system. The information or the part program is being

transferred directly to the machine tool through communication lines from the main

computer. This is done in real time and the communication is two way, both the

computer and the machine tool can send information to each other. The computer sends

information to the machine tool upon request of the latter and when this occurs the

request for instructions must be satisfied almost instantaneously. The computer stores

the information in a bulk memory and can control more than 100 machine tools. In

addition in cases where the computational capability of the main computer is not enough

to satisfy the needs of the vast number of the machine tools additional computers are

used linked to the main server to satisfy group of machine tools. These smaller computers

are called satellite computers.

FUNDAMENTALS OF NC:-

Numerical Control (NC) refers to the method of controlling the manufacturing operation by means of directly inserted coded numerical instructions into the machine tool. It is important to realize that NC is not a machining method, rather, it is a concept of machine control. Although the most popular applications of NC are in machining, NC can be applied to many other operations, including welding, sheet metalworking, riveting, etc. Because of the introductory character of this chapter, we will restrict our discussion only to twodimensional machining operations (e.g. turning), which are among the most simple applications of





NC. Nevertheless, most of the principles and conclusions here are also valid for more advanced NC applications. The major advantages of NC over conventional methods of machine control are as follows: 1. higher precision: NC machine tool are capable of machining at very close tolerances, in some operations as small as 0.005 mm; 2. machining of complex three-dimensional shapes: this is discussed in Section 6.2 in connection with the problem of milling of complex shapes; 3. better quality: NC systems are capable of maintaining constant working conditions for all parts in a batch thus ensuring less spread of quality characteristics; 4. higher productivity: NC machine tools reduce drastically the non machining time. Adjusting the machine tool for a different product is as easy as changing the computer program and tool turret with the new set of cutting tools required for the particular part. 5. multi-operational machining: some NC machine tools, for example machine centers, are capable of accomplishing a very high number of machining operations thus reducing significantly the number of machine tools in the workshops. 6. low operator qualification: the role of the operation of a NC machine is simply to upload the workpiece and to download the finished part. In some cases, industrial robots are employed for material handling, thus eliminating the human operator.

Types of NC systems Machine controls are divided into three groups, 1.traditional numerical control (NC); 2. computer numerical control (CNC); 3.distributed numerical control (DNC). The original numerical control machines were referred to as NC machine tool. They have “hardwired” control, whereby control is accomplished through the use of punched paper (or plastic) tapes or cards. Tapes tend to wear, and become dirty, thus causing misreadings. Many other problems arise from the use of NC tapes, for example the need to manual reload the NC tapes for each new part and the lack





of program editing abilities, which increases the lead time. The end of NC tapes was the result of two competing developments, CNC and DNC. CNC refers to a system that has a local computer to store all required numerical data. While CNC was used to enhance tapes for a while, they eventually allowed the use of other storage media, magnetic tapes and hard disks. The advantages of CNC systems include but are not limited to the possibility to store and execute a number of large programs (especially if a three or more dimensional machining of complex shapes is considered), to allow editing of programs, to execute cycles of machining commands, etc. The development of CNC over many years, along with the development of local area networking, has evolved in the modern concept of DNC. Distributed numerical control is similar to CNC, except a remote computer is used to control a number of machines. An off-site mainframe host computer holds programs for all parts to be produced in the DNC facility. Programs are downloaded from the mainframe computer, and then the local controller feeds instructions to the hardwired NC machine. The recent developments use a central computer which communicates with local CNC computers (also called Direct Numerical Control)

Difference between CNC & NC machines

In a Numerical Control machine, the program is fed to the machine through magnetic tapes or other such media. The original NC machines were essentially basic machine tools which were modified to have motors for movement along the axes. In a Computer Numerical Controlled machine, the machines are interfaced with computers. This makes them more versatile in the sense that, suppose a change in dimension of a part is required. In a NC machine, you would have had to change the program in the tape and then feed it to the





machine again. But in a CNC machine, you just change a variable in the computer and your modification is done!!

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