basic concepts - tom

7/29/2019 Basic Concepts - TOM

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Theory of Machines

(Basic Concepts)


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Machine and Mechanism

There is a difference between a machine and a mechanism:

All machines transform energy to do work, but only somemechanisms are capable of performing work. The termmachinerymeans an assembly that includes both machines

and mechanisms. Figure (a) illustrates a cross section of a machinean

internal combustion engine. The assembly of the piston,connecting rod, and crankshaft is a mechanism, termed aslider-crank mechanism.

The basic schematic drawing of that mechanism,Fig.(b),called a skeleton Outline, shows only its fundamentalstructure without the technical details explaining how it isconstructed.

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Actual I.C Engine and its Skeleton

Diagram



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Efficiency of Machine

The efficiency of a machine is the ratio of its

output to its input, if both input and output

are expressed in the same units ofenergy or

power. This ratio is always less than unity, and

it is usually expressed in percent by

multiplying the ratio by 100.



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Mechanical Advantage

The mechanical advantage of a mechanism or

system is the ratio of the load or weight W,

typically in pounds or kilograms, dividedby

the effort or force F exerted by the initiating

entity or operator, also in pounds or kilograms.

If friction has been considered or is known

from actual testing, the mechanicaladvantage, MA, of a machine is:



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Velocity Ratio

Machines and mechanisms are used to

translate a small amount of movement or

distance into a larger amount of movement or

distance. This property is known as the

velocity ratio: it is



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Concept of Mechanical Advantage -

MA



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Pulleys to achieve MA

The operator or other power source must exert a force F through a distance

S=4h so that the velocity ratio of s to h is 4.



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Screw Jack to achieve MA



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Levers and its class

Levers are the simplest of mechanisms; A lever is a rigid beam that can rotate about a fixed point

along its length called thefulcrum. Physical effort applied toone end of the beam will move a load at the other end.

Each is capable of providing a different level of mechanical

advantage. These levers are called Class 1, Class 2, and Class 3. The

differences

in the classes are determined by:

Position along the length of the lever where the effort isapplied

Position along the length of the lever where the load isapplied

Position along the length of the lever where the fulcrum or

pivot point is located07/16/2012 10Prof.Irfan Shaikh


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Class 1 - Lever



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Class 2 - Lever



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Class 3 - Lever



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Link or Linkage

Each part of a machine, which moves relative to someother part, is known as a element. A link may consist of several parts, which are rigidly fastened together

A linkage is a mechanism formed by connecting two ormore levers together. Linkages can be designed to changethe direction of a force or make two or more objects move

at the same time.

Linkages can be classified according to their primaryfunctions:

Function generation: the relative motion between the linksconnected to the frame

Path generation: the path of a tracer point

Motion generation: the motion of the coupler link



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Types of Links

1. Rigid link. A rigid link is one which does not undergoany deformation while transmitting motion. Strictlyspeaking, rigid links do not exist. However, as thedeformation of a connecting rod, crank etc. of a

reciprocating steam engine is not appreciable, they canbe considered as rigid links.

2. Flexible link. A flexible link is one which is partlydeformed in a manner not to affect the transmission ofmotion. For example, belts, ropes, chains and wires are

flexible links and transmit tensile forces only.3. Fluid link. A fluid link is one which is formed by having

a fluid in a receptacle and the motion is transmittedthrough the fluid by pressure or compression only, as inthe case of hydraulic presses, jacks and brakes.



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Machine and Structure

1. The parts of a machine move relative to oneanother, whereas the members of a structuredo not move relative to one another.

2. A machine transforms the available energyinto some useful work, whereas in a structureno energy is transformed into useful work.

3. The links of a machine may transmit bothpower and motion, while the members of astructure transmit forces only.



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Kinematic Pair

The two links or elements of a machine, when

in contact with each other, are said to form a

pair. If the relative motion between them is

completely or successfully constrained (i.e. ina definite direction), the pair is known as

kinematic pair.



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Completely constrained motion



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Incompletely constrained motion



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Successfully constrained motion



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Classification of Kinematic Pair

Sliding Pair

Turning Pair Turning Pair Screw Pair

Rolling Pair



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Kinematic Chain

When the kinematic pairs are coupled in such away that the last link is joined to the first link totransmit definite motion (i.e. completely orsuccessfully constrained motion), it is called a

kinematic chain. In other words, a kinematic chain may be defined

as a combination of kinematic pairs, joined insuch a way that each Link forms a part of two

pairs and the relative motion between the linksor elements is completely or successfullyconstrained.



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Equation of Kinematic Chain



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Definition of Mechanism

When one of the links of a kinematic chain is fixed, thechain is known as mechanism. It may be used fortransmitting or transforming motion e.g. engine indicators,typewriter etc.

A mechanism with four links is known as simple mechanism,

and the mechanism with more than four links is known ascompound mechanism.

When a mechanism is required to transmit power or to dosome particular type of work, it then becomes a machine. Insuch cases, the various links or elements have to be

designed to withstand the forces (both static and kinetic)safely.

A little consideration will show that a mechanism may beregarded as a machine in which each part is reduced to thesimplest form to transmit the required motion.



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Number of Degrees of Freedom for

Plane Mechanism



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Arrest of Degrees of Freedom of

Linkages



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Types of Kinematic Chain & Grashofs

law

1. Four bar chain or quadric cyclic chain,

2. Single slider crank chain, and

3. Double slider crank chain.

Grashofs law According to Grashofs law for a four bar

mechanism, the sum of the shortest and longestlink lengths should not be greater than the sum

of the remaining two link lengths if there is to becontinuous relative motion between the twolinks.



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Four bar Chain or Simple Planer

Linkages



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Beam Engine and Coupler Mechanism



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Single Slider Crank chain mechanism



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Pendulum Pump and Oscillating

Cylinder Engine

Pendulum Pump

Oscillating Cylinder Engine



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Rotary Internal Combustion Engine



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Crank and Slotted lever Quick Return

Motion Mechanism



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Theory of Quick Return



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Whitworth Quick Return Mechanism



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Elliptical Trammell



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Equation of elliptical trammel



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Scotch Yoke Mechanism



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Oldhams Coupling



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Straight Line Mechanisms

One of the most common forms of the constraintmechanisms is that it permits only relativemotion of an oscillatory nature along a straightline.

The mechanisms used for this purpose are calledstraight line mechanisms. These mechanisms areof the following two types:

1. In which only turning pairs are used, and

2. In which one sliding pair is used.

These two types of mechanisms may produceexact straight line motion or approximate straight

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Peaucellier mechanism



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Harts mechanism



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The tracing point P traces out an

approximate straight line over certain

positions of its

movement, if PB/PA = O1A/OB.


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Roberts mechanism.



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Grasshopper mechanism


OA = (AP)2/ AQ.


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Pantograph

A pantograph is an instrument used to

reproduce to an enlarged or a reduced scale

and as exactly as possible the path described

by a given point. It consists of a jointedparallelogram ABCD as shown in Fig.



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Pantograph

It is made up of bars connected by turningpairs. The bars BA and BC are extended to O

and E respectively, such that OA/OB = AD/BE



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Engine Indicator Diagram

The application of straight line motions is mostly found inthe engine indicators.

In these instruments, the cylinder of the indicator is indirect communication with the steam or gas inside thecylinder of an engine.

The indicator piston rises and falls in response to pressurevariation within the engine cylinder. The piston is resistedby a spring so that its displacement is a direct measure ofthe steam or gas pressure acting upon it.

The displacement is communicated to the pencil whichtraces the variation of pressure in the cylinder (also knownas indicator diagram) on a sheet of paper wrapped on theindicator drum which oscillates with angular motion aboutits axis, according to the motion of the engine piston.

The variation in pressure is recorded to an enlarged scale.07/16/2012 53Prof.Irfan Shaikh


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Watts Indicator


Universal or Hookes Joint


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Universal or Hooke s Joint



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Davis Steering Gear


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Davis Steering Gear



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