Fundamentals and Types of Mechanism

2011-2012 DIPLOMA IN Mechnical Engineering

FUNDAMENTALS AND TYPE OF MECHANISM

Mr.Vaibhav V Naik Mechanical Engineering 2011-2012

THEORY OF MACHINE

FUNDAMENTALS AND TYPES OF MECHANISMS

A. Kinematics of Machines: Machines: - Definition Definition of Kinematics, Kinematics, Dynamics, Dynamics, Statics, Kinetics, Kinematic link, Kinematic Pair and its types, constrained motion and its types, Kinematic chain and its types, Mechanism, inversion, machine and structure. B. Inversions of Kinematic Chain.

(Marks- 6)

(Marks- 8)

Inversion of four bar chain, 

Coupled wheels of Locomotive &



Pentograph.

Inversion of Single Slider Crank chain

Rotary I.C. Engines mechanism,



Whitworth quick return mechanism,



Crank and Slotted lever quick return mechanism.

Inversion of Double Slider Crank Chain 

Scotch Yoke Mechanism &



Oldham’s Coupling.

C. Common Mechanisms.

(Marks- 4)



Bicycle free wheel Sprocket mechanism.



Geneva Mechanism.





Ackerman’s Steering gear mechanism. Foot operated air pump mechanism.

FUNDAMENTALS AND TYPES OF MECHANISM

Vaibhav Vithoba Naik

THEORY OF MACHINE

THEORY OF MACHINE

Theory of machine is Branch of engineering science which deals with the study of of relative motion and forces between the various machine elements

THEORY OF MACHINE

STATICS

DYNAMICS

KINETICS

KINEMATICS

:

Figure: Diagram of Theory of Machine

1. Statics

It is the branch of theory of machine, which deals with the forces and its effect on the machine part while the latter is at rest. 2. Dynamics of machine

It is the branch of of theory of machine, which deals with the forces and effect of forces on the machine component when they act on them. 3. Kinetics

It is the branch of of theory of machine, which deals deals with the inertia forces, which are formed due to combined action of mass and motion of machine elements. 4. Kinematics of machine

It is the branch of theory of machine, which deals with the relative motion between the various parts of the machine, without considering the forces



THEORY OF MACHINE

1. Kinematic Link

a. Links are individual parts of a mechanism. b. Each parts of a machine which which has a relative motion to some other parts is known as Kinematic link. Characteristics of the Kinematic link

1. It should have relative motion and 2. It should be a resistant body

Classification of the LINK

A. Depending Depending upon the the nature of the resistant resistant bodies bodies 1. Rigid link. 2. Flexible link. 3. Fluid link.

B. Depending upon the points at which which the link is attached to mechanism 1. Binary link 2. Ternary link 3. Quaternary link



THEORY OF MACHINE

A. Depending upon the nature of the resistant bodies 1. Rigid link

The links which do not undergo appreciable transformation while transmitting the required motion and forces is known as rigid link. Example: a. Piston, b. Connecting Rod of Steam engine.

2. Flexible Link

The links which are partly deformed transformation while transmitting the required motion and forces in such a way that they don effect the required transmission of motion is known as Flexible link. Example: a. Chain, b. Belt and Spring

3. Fluid Link

The link which transmit the motion by fluid pressure as compression are called fluid link. Example: a. Liquid used in Hydraulic presses



THEORY OF MACHINE

B. Depending upon the points at which the link is attached to mechanism 1. Binary link The link which is attached to two points in the mechanism is called Binary

link.

2. Ternary link The link which is attached to three points in the mechanism is called Ternary

link.

3. Quaternary link The link which is attached to four points in the mechanism is called

Quaternary link.



THEORY OF MACHINE

2. Kinematic Pair

The two links of the machine when in contact with each other are said to form a pair if the relative motion between them is successfully constrained.

CLASSIFICATION OF KINEMATIC PAIR A. According to the relative motion between them

1. Sliding pair. S 2. Turning pair. 3. Rolling pair. 4. Screw pair. 5. Spherical pair. B. According to Nature of the contact

1. Lower pair. 2. Higher pair. C. According to type of closure

1. Self closed pair. 2. Force closed pair.



THEORY OF MACHINE

A. According to the relative motion between them 1. Sliding pair.

A sliding pair is said said to be formed by two links links such a manner that that one link link is constrained to have a sliding motion relative to other link.

Figure: Sliding Pair

Example: 1. Piston and Cylinder, 2. Cross head and guides of Steam engine.



THEORY OF MACHINE

2. Turning pair

A turning pair is said said to be formed by two links such a manner manner that one one link is constrained to turn or revolve relative to other link.

Figure: Turning Pair

Example: Shaft with collar in circular hole.



THEORY OF MACHINE

3. Rolling pair . A Rolling pair is said said to be formed by two links such a manner manner that one one link is constrained to have a rolling motion over the other link.

Figure: Rolling Pair

Example: Ball bearing forms Rolling pair.



THEORY OF MACHINE

4. Screw pair.

When the two elements of pairs are connected in such a way that one element can turn about the other by means of screw threads is known as Screw Pair.

Figure: Screw Pair

Example: Bolt and nut is example of Screw Pair.



THEORY OF MACHINE

5. Spherical pair.

When the two elements of pairs are connected in such a way that one element turn or swivels about the other fixed element, the pair formed is known as Screw Pair.

Figure:Spherical Pair

Example: Ball and socket is example of Spherical Pair.



THEORY OF MACHINE

B. According to Nature of the contact 1. Lower pair.

When the two element of a pair have a surface contact when the relative motion takes place and the surface of one element slides over the other, the pair forced is known as lower pair.

Figure: Lower Pair

Example: a. The motion of piston and cylinder in the engine. b. Shaft rotating in a bearing and c. Nut turning on a screw.



THEORY OF MACHINE

2. Higher pair .

When the two element of a pair have a line or point contact when the relative motion takes place and the motion between the two element is partially turning and partly sliding then the pair formed is know as Higher pair.

Figure: Higher Pair

Example: The motion of cam and follower



THEORY OF MACHINE

C. Kinematic pairs according to nature of mechanical constraint 1. Self Closed pair

When the elements of a pair are held together mechanically in such a way that only the required kind of relative motion occurs , all the lower pairs and some of the higher pairs are closed pairs. Example: The lower pair is known as closed pair.

2. Force Closed pair or Unclosed pair

When two links of a pair are not connected mechanically but kept in contact either due to force of gravity or some spring action, they constitute an unclosed pairs. Example: The motion of cam and follower



THEORY OF MACHINE

TYPE OF CONSTARINED MOTIONS.

1. Completely constrained motion. 2. Incompletely constrained motion. 3. Successfully constrained motion.

1. Completely constrained motion.

When the motion between a pair is limited to definite direction irrespective of the direction of force applied, then the motion is said to be a completely constrained motion.

Figure: Complete Constrained Motion

Example: 1. The motion of the square bar in square hole. 2. The motion of shaft with collars at each end in a circular hole.



THEORY OF MACHINE

2. Incompletely constrained motion.

When the motion between a pair can take place in more than one direction, then the motion is said to be a completely constrained motion.

Figure: Incomplete Constrained Motion

Example: 1. The motion of the circular bar in circular hole.



THEORY OF MACHINE

3. Successfully constrained constr ained motion

When the relative motion between the link is not completely constrained by itself but it is made by some other means is called successfully constrained motion.

Figure: Successfully Constrained Motion

Example: 1. Shaft in Footstep Bearing 2. The motion IC engine valve and piston in reciprocating inside an engine cylinder.



THEORY OF MACHINE

3. Kinematic Chain

When the kinematic pairs are coupled in such a way that the last linked is joined the the first link link to transmit transmit the definite definite motion it is called called a kinematic kinematic chain. chain.

To determine the given assemblage of links forms the kinematic chain or not:

The two equations are: 1. l = 2p - 4 2. J = 3/2 * l - 2 

l = number number of links links



p = number of pairs



j = number number of joints joints

Three possible cases are 1. If L.H.S=R.H.S then it is Constrained kinematic chain 2. If L.H.S>R.H.S then it is Locked Chain 3. If L.H.S


THEORY OF MACHINE

1. Arrangement of three links

Figure :Three Links

2. Arrangement of four links

Figure :Four Links



THEORY OF MACHINE

3. Arrangement of five links

Figure :Five Links

4. Arrangement of six links

Figure :Sic Links



THEORY OF MACHINE



THEORY OF MACHINE



THEORY OF MACHINE

Types of Joints in the Chain

1. Binary joints

When the two links are joined at the same connections, the joint is known as binary joint

Figure: Binary Joints W.Klien’s criterion of constraint






h = number of higher pairs



j = number of binary joints

j=4; l=4; h=0 h=0 j+h/2 = 3/2*l 3/2*l - 2 4+0 = 3/2*4 - 2 4

=4

L.H.S=R.H.S



THEORY OF MACHINE

2. Ternary Joints

When the three links are joined at the same connections, the joint is known as ternary joints.

Figure: Ternary Joints

A.W.Klien’s criterion of constraint









j = number number of binary joints

j=7; l=6; h=0 h=0 j+h/2 =3/2*l =3/2*l – 2 7+0 =3/2*6 – =3/2*6 – 2 7

=7

L.H.S=R.H.S



THEORY OF MACHINE

3. Quaternary Joints

When the four links are joined at the same connections, the joint is known as Quaternary joints.

Figure: Quaternary Joints

W.Klien’s criter ion ion of constraint









j = number of binary joints

j=12; l=9; h=0 j+h/2 = 3/2*l – 3/2*l – 2 12+0 = 3/2*9 – 3/2*9 – 2 12

= 11.5

L.H.S>R.H.S



THEORY OF MACHINE



THEORY OF MACHINE

Types of Kinematic chain

1. Four bar chain 2. Single slider crank chain. 3. Double slider crank chain Inversion of the following types 1. Four bar chain

a) Beam engine. b) Coupling rod of Locomotives. c) Watts’ indicator mechanism. d) Pantograph. 2. Single slider crank chain.

a) Pendulum pump or Bull engine. b) Oscillating cylinder engine. c) Rotary IC engine. d) Crank and slotted lever quick return mechanism. e) With worth worth quick quick return mechanism. 3. Double slider crank chain.

a) Elliptical trammel b) Scotch yoke mechanism. c) Oldham’s coupling.



THEORY OF MACHINE

3. MECHANISM

a. When one of the links of a kinematic chain is fixed, the chain is known as mechanism. b. It may be used for transmitting or transforming motion. Example: 1. Engine indicator, 2. Typewriter. Types of Mechanism

a. Simple mechanism (a mechanism with four links) b. Compound mechanism (a mechanism with more than four link) Differentiate between Machine and Mechanism

NO 1.

MACHINE It is like the human body, it transforms energy into useful work.

MECHANISM It is like a frame work and has a definite motion between various links.

2

It relates to energy only.

It relates to motion.

3

It has many link.

It also has many links.

4

Ex: lathe, shaper.

Ex. watt indicator,typwriter indicator,typwrite r

5



THEORY OF MACHINE

4. Structure

It is the assemblage of a number of resistant bodies having no relative motion between then and meant for carrying loads having a straining action.

Figure: Structure

Example: 1. A Railway bridge. 2. Roof Trusses 3. bridges, 4. buildings 5. machine frames



THEORY OF MACHINE

Differentiate between Machine and Structure.

NO

MACHINE

STRUCTURE

1

Has relative motion between the members

Has no relative motion

2

Transforms available energy into possible work.

Does not transform.

3

Members are meant to transmit motion and forces.

Members are meant to accept load

4

Example Shaper, lathe

Example :Bridge.

5. Inversion

The method of obtaining the different mechanism by fixing the different links in a kinematic chain is known as inversion of the mechanism.



THEORY OF MACHINE

Important Terminologies and Definition 1. Machine 





It is device which takes in the available energy and converts it into useful work. When the mechanism is required to transmit the power or to do some particular type of work, it then becomes a Machine. Example: Shaper, Lathe machine

2. Mechanics( Theory of Machine) 





Branch of engineering which deals with the relative motion and forces between the various machine elements . Kinematics of machine Deals with the relative motion without considering the forces Dynamics of machine Deals with the forces and effect of forces on the machine component when they act on them.

3. Kinetics of machine 

Deals with the forces, which are formed due to combined action of mass and motion of machine elements.

4. Statics 

Deals with the forces and its effect on the machine part while the latter is at rest.

5. Resistant body body is said to be resistant body if it is able to transmit the forces with the least possible deformation. 

A

Example: spring, belt oils in hydraulic presses

6. Kinematic Link  

Links are individual parts of a mechanism. Each parts of a machine which has a relative motion to some other parts is known as Kinematic link.



THEORY OF MACHINE

7. Kinematic Chain

When the kinematic pairs are coupled in such a way that the last linked is joined the the first link link to transmit the definite definite motion it is called called a kinematic kinematic chain. chain. 8. Kinematic Pair

The two links of the machine when in contact with each other are said to form a pair if the relative motion between them is successfully constrained. 9. Inversion

The method of obtaining the different mechanism by fixing the different links in a kinematic chain is known as inversion of the mechanism. 10. Structure

It is the assemblage of a number of resistant bodies having no relative motion between then and meant for carrying loads having a straining action. Example: a railway bridge.

11. Mechanism 



When one of the links of a kinematic chain is fixed, the chain is known as mechanism. It may be used for transmitting or transforming motion. Example: Engine indicator, Typewriter.



THEORY OF MACHINE

Grashof's Law The sum of the length of shortest and longest link must be equal or less than the sum of the length of the other two link length, if there is to be a continuous relative motion between the two links.

According According to Grashofs Grashofs law

s+l
l = length of the longest link.



s = length of the shortest link.



p & q = length of the other two links

In a four-bar linkage, we refer to the line segment between hinges on a given link as a bar where:



s = length of shortest bar



l = length of longest bar



p, q = lengths of intermediate bar

Grashof's theorem states that a four-bar mechanism has at least one revolving

link if

s + l <= p + q

s+l>p+q

(1) and all three mobile links will rock if

(2)

The inequality (1) is Grashof's criteria 

The Link opposite to the frame is called Coupler Link and the link which are hinged to the frame are called side link.



The link which is free to rotate through 360 0 with respect to second link will be said to revolve relative to the second link (not necessarily a frame).



THEORY OF MACHINE



If it is possible for all four bar to become simultaneously aligned, such a state is called change point. Case

Criterion

Shortest link

Category

1

s+l

<

p+q

frame

double crank

2

s+l

<

p+q

slide

crank rocker

3

s+l

<

p+q

coupler

4

s+l

=

p+q

any

change point

5

s+l

<

p+q

any

triple rocker

double rocker

From the table we can see that for a mechanism to have a crank, the sum of the length of shortest and longest link must be equal or less than the sum of the length of the other two link. However this condition is necessary but not sufficient. Mechanism satisfying this condition falls into following three categories.

1) When the shortest link is the side link, the mechanism is crank rocker rocker mechanism. The shortest link is the CRANK in the mechanism.

2) When the shortest link is the Frame, the mechanism is double crank mechanism.

3) When the shortest link is the coupler link, the mechanism is Double rocker mechanism



THEORY OF MACHINE

Inversion of the following types 1.

Four bar chain a. Beam engine. b. Coupling rod of Locomotives. c. Watts’ indicator mechanism. d. Pantograph.

2.

Single slider crank chain. a. Pendulum pump or Bull engine. b. Oscillating cylinder engine. c. Rotary IC engine. d. Crank and slotted lever quick return mechanism. e. Whit worth worth quick return mechanism.

3.

Double slider crank chain. a. Elliptical trammel b. Scotch yoke mechanism. c. Oldham’s coupling.



THEORY OF MACHINE

INVERSION OF FOUR BAR CHAIN MECHANISM



THEORY OF MACHINE

a. BEAM ENGINE

1. Beam engine is also called as Crank and lever mechanism. 2. It consist consist of four four link a. Link A – A –Frame Frame b. Link AB – AB –Crank Crank c. Link BC – BC –Coupler Coupler –Lever d. Link CE –Lever

Figure: Beam Engine

3. In this mechanism, w when hen the crank rotated about the fixed centre A, the lever oscillates about the fixed center D. 4. The end E of the lever CDE CDE is connected to piston rod which reciprocates due to the rotation of the crank. 5. Purpose: Convert rotary motion into reciprocating motion.



THEORY OF MACHINE

b. Coupling rod of locomotive

1. The coupling rod of the locomotive consists of four links as shown in figure. 2. It consist of four link a. Link AD – AD – frame b. Link AB – AB –crank crank c. Link BC – BC –connecting connecting rod –crank d. Link CD –crank 3. In this mechanism, the link AD and BC act as crank and are connected to the respective wheel.

Figure: Coupling Rod of locomotive

4. The link CD act as a coupling rod and the link AB is fixed in order to maintain a constrain motion between them. 5. Purpose: meant for transmitting rotary motion from one wheel to another wheel.



THEORY OF MACHINE

c. Watts indicator mechanism

A watt indicator indicator mechanism mechanism has a four link as shown shown in figure. figure.

Figure: Watts Indicator Mechanism

1. The four links are fixed link at A, link AC, link CE, and link BFD. 2. It may be noted that BF and FD forms one link because two parts have no relative motion between them. 3. The link CE and BFD act as lever. 4. The displacement of link BFD is directly proportional to the pressure of gas or steam which acts on the indicator plunger. 5. On any small displacement of the mechanism, the tracing point E at the end of the link CE traces out approximately straight line. 6. The initial position of the mechanism is shown in full line whereas the dotted lines shows the position of mechanism when the gas or steam pressure acts on the indicator plunger.



THEORY OF MACHINE

d. Pantograph

A pantograph pantograph is an an instrument instrument used to reproduce reproduce an enlarged enlarged or reduce scale and as exactly as possible the path described by a given point.

Figure: Pantograph

1. It is a based on four based kinematic chain. 2. It consists of jointed parallelogram ABCD. 3. It is made up of bar connected by turning pair. It has a four turning pair. 4. Link BA and BC are extended to O and E respectively such that OA/OE = AD/BE. 5. For all relative position of bars the triangle OAD OAD and OBE are similar and the point O,D,E are in straight line. 6. The point E describes the same path as described described by point D.



THEORY OF MACHINE

INVERSION OF SINGLE SINGLE SLIDER SLIDER CRANK CRANK CHAIN MECHANISM 1. A single slider crank chain is modification of the basic four bar chain. 2. It consist a. One Sliding pair and b. Three Turning pair

Figure: Single Slider Crank Chain Mechanism 3. Link 1 - Frame of engine; Link 2 - Crank Link 3 - Connecting rod Link 4 - Cross head 4. As the crank rotates the cross head reciprocates in the guides and thus the piston reciprocates in the cylinder 5. It is usually found in reciprocating steam engine mechanism. 6. This type of mechanism converts rotary motion into reciprocating motion and vice versa



THEORY OF MACHINE

a. Pendulum pump or Bull engine.

1. In this mechanism, the inversion is obtained by fixing the link 4 or cylinder (sliding pair). 2. It consist of four link a. Link 1 – Piston rod b. Link 2 – AB-Crank c. Link 3 -BC -connecting rod d. Link 4 –Cylinder. –Cylinder.

Figure: Pendulum pump or Bull engine

3. In this case, the crank rotates the connecting rod oscillates about the pin pivoted to the fixed link. 4. The piston attached to the piston rod reciprocates reciprocates inside the cylinder.



THEORY OF MACHINE

b. Oscillating cylinder engine.

1. In this mechanism, the inversion is obtained by fixing the link 3 or connecting rod (turning pair). 2. It consist of four link e. Link 1 -AB- Piston rod f. Link 2 -BD-Crank g. Link 3 -AD -Connecting rod h. Link 4 –Cylinder. –Cylinder.

Figure: Oscillating Cylinder Engine

3. The link 3 (AD-connecting rod) is corresponds to the connecting rod of the reciprocating steam engine mechanism. 4. When the link 2 rotates (BD-Crank) rotates, the piston attached to the piston rod reciprocates and the cylinder oscillates about the pin pivoted to the fixed link at A.



THEORY OF MACHINE

d. Rotary IC engine.

1. In this mechanism, the inversion is obtained by fixing the link 2 or crank. 2. It consist of four link a. Link 1 - Cylinder. b. Link 2 -Fixed Crank c. Link 3 -Piston d. Link 4 – Connecting rod

Figure: Rotary IC Engine

3. The link 4 (AD-connecting rod) rotates, the piston reciprocates inside the cylinder forming link1.



THEORY OF MACHINE

a. Crank and slotted lever quick return mechanism

1. In this mechanism, the inversion is obtained by fixing the link 3 or turning pair. 2. It consist of four link a. Link 1 -Slider b. Link 2 – Driving Crank c. Link 3 -Fixed link d. Link 4 –Slotted –Slotted lever or Slotted bar

Figure: Crank and slotted lever quick return mechanism

3. The driving crank AC revolves with uniform angular speed about fixed centre. 4. The sliding bock is attached to crank pin at A slides in the slotted bar AP and thus causes AP to oscillate about the point A. 5. A short link PR transmits the motion from AP to the Ram to which the cutting tool is fixed 6. The motion of the tool is constrained along the line produced



THEORY OF MACHINE

b. Whitworth quick return mechanism.

1. In this mechanism, the inversion is obtained by fixing the link 3 or turning pair. 2. It consist of four link a. Link 1 -Slider b. Link 2 - Crank c. Link 3 -Fixed link d. Link 4 –Slotted –Slotted lever

Figure: With worth Quick Return mechanism

3. The driving crank BC revolves with uniform angular speed about fixed centre. 4. The sliding bock is attached to crank pin at B slides in the slotted bar AP and thus causes AP to oscillate about the point A. 5. A short link PQ transmits the motion from AP to the Ram Ram to which which the cutting tool is fixed 6. The motion of the tool is constrained along the line produced.



THEORY OF MACHINE

INVERSION OF DOUBLE DOUBLE SLIDER SLIDER CRANK CRANK CHAIN MECHANISM 1. A kinematic chain which consists of of two turning pair and two sliding pair is called double slider crank chain. 2. It consist a. Two Sliding pair and b. Two Turning pair 3. The inversion of double slider crank chain mechanism are a. Elliptical trammel b. Scotch yoke mechanism. c. Oldham’s coupling.



THEORY OF MACHINE

a. Elliptical trammel

1. In this mechanism, the inversion is obtained by fixing the link 4 or slotted plate. 2. It consist of four link a. Link 1 – Sliders b. Link 2 – Bar c. Link 3 – Sliders d. Link 4 –Slotted –Slotted Plate

Figure: Elliptical trammel

3. The looted plate has two grooves cut in it at right angles to each other. 4. The link 1 and known as a sliders and form sliding pair with the link 4. 5. The link AB is a bar which forms the turning pair with the links 1 and link 3. 6. When the link 1 and link 3 slides along the respected grooves ,any point of the link 2 such as point P traces out the ellipse on the surface.



THEORY OF MACHINE

b. Scotch yoke mechanism.

1. In this mechanism, the inversion is obtained by fixing link 1 or link3. 2. It consist consist of four four link e. Link 1 – Fixed Link f. Link 2 – Crank g. Link 3 – Slider h. Link 4 – Frame (Piston and Cylinder arrangement )

Figure: Scotch Yoke Mechanism

3. When the link 2 rotates about the about B as a centre the link 4 reciprocates.(Piston reciprocates inside the cylinder) 4. The fixed links guides the frame .



THEORY OF MACHINE

c. Oldham’s coupling.

1. In this mechanism, the inversion is obtained by fixing link 2. 2. It consist consist of four four link a. Link 1 – Flanges b. Link 2 – Supporting Frame c. Link 3 – Flanges d. Link 4 –Intermediate –Intermediate Piece.

Figure: Oldham’s Coupling

3. The flanges (link 1 and link 3) forms turning pair with with the link 2. 4. When the driving shaft is rotated, the flange (link 1) causes the intermediated piece (link 4) to rotate at the same angle through which the flange (link 1) is rotated. 5. It further rotates the flange (link 3) at the same same angle through which the flange (link 1) is rotated. Hence link 1,3,4, have same angular velocity. 6. A little consideration there is a sliding motion between link 4 and link 1 and 3.



THEORY OF MACHINE

COMMON MECHANISM

1. BICYCLE FREE WHEEL SPROCKET MECHANISM 2. GENEVA MECHANISM 3. ACKERMAN’S STEERING GEAR MECHANISM 4. FOOT OPERATED AIR PUMP MECHANISM



THEORY OF MACHINE

1. BICYCLE FREE WHEEL SPROCKET MECHANISM or automotive engineering, a freewheel or overrunning clutch 1. Mechanical or automotive is a device in a transmission that disengages the driveshaft from the driven shaft when the driven shaft rotates faster than the driveshaft. An overdrive is sometimes mistakenly called a freewheel, but is otherwise unrelated. 2. The condition of a driven shaft spinning faster than its driveshaft exists in

most bicycles when the rider holds his or her feet still, no longer pushing the pedals. Without a freewheel the rear wheel would drive the pedals around.

Figure: Bicycle Free Wheel Sprocket Mechanism



THEORY OF MACHINE

3. In the past, such freewheel mechanisms have included an inner freewheel

body which engages threads on a rear wheel hub, and an outer freewheel body, including an integral sprocket for engagement with the roller chain. least one pawl pawl spring spring have been disposed disposed between between 4. A pair of pawls, and at least said inner and outer freewheel bodies, whereby forward rotation of the outer freewheel body would cause the pawls to engage and drive the inner freewheel body and rear wheel. Also, the pawls would allow the rear wheel to rotate in a forward direction when the outer freewheel body was rotating more slowly or was stopped.



THEORY OF MACHINE

2. GENEVA MECHANISM

1) In the mechanism shown in figure, link A is driver and it contains a pin which engages with the slots in the driven link B. 2) The slots are positioned in such a manner, that the pin enters and leaves them tangentially avoiding impact loading during transmission of motion.

Figure: Geneva mechanism

3) In the mechanism shown, shown, the driven member makes one-fourth of a revolution for each revolution of the driver. 4) The locking plate, which which is mounted on the driver, driver, prevents the driven member from rotating except during the indexing period.



THEORY OF MACHINE

5. The Geneva mechanism is a timing device. 6. Geneva mechanism consists of a rotating disk with a pin and another rotating disk with slots (usually four) into which the pin slides 7. In the most common arrangement, the driven wheel has four slots and thus advances for each rotation of the drive wheel by one step of 90 °. If the driven wheel has n slots, it advances by 360/n° per full rotation of the drive wheel.

Figure: Geneva mechanism

8.

One application of the Geneva drive is in movie projectors. Geneva wheels having the form of the driven wheel were also used in mechanical watches. Other applications of the Geneva drive include the pen change mechanism mechanism in plotters, automated sampling devices, indexing tables in assembly lines, tool changers for CNC machines, and so on.



THEORY OF MACHINE

3. Ackerman’s Steering Gear Mechanism.

1) The steering gear mechanism is used for changing the direction direct ion of two or more of the wheel axles with reference to the chassis, so as to move the automobile in any desired path. 2) When the vehicle takes a turn the front wheels along with the respective axles turn about the respective pivoted points. The back wheels remain straight and do not turn. Therefore steering is done by front wheels only. 3) In order to avoid skidding the two front wheels must turn about the same Instantaneous center I which lies on the axis of the back axle. If the ICR of the two front wheels do not coincide with the ICR of the back wheels skidding will take place, which causes wear and tear of tires. 4) Thus the condition for correct steering is that the entire four wheel must turn about the same ICR. The axis of the inner wheel makes a larger turning angle θ than the angle Φ subtended by the axis of outer wheel. Let a= wheel track b=wheel base c=distance between the pivots A and b of the front axle Now from triangle IBP Cot θ=BP θ=BP IP and from triangle IAP Cot Φ = AP = AB + BP BP = AB + BP = c + Cot θ IP

IP

IP

IP

b

Cot Φ - Cot θ =c/b



THEORY OF MACHINE

5) This is the fundamental equation for correct steering. If this condition is satisfied there will be no skidding of the wheels when vehicle takes a turn.

Figure: Ackerman’s Steering gear Mechanism.

6)

In Ackerman steering gear the mechanism ABCD is a four bar crank chain. The shorter link BC and AD are of equal length and are connected by hinge joints with front wheel axle.The longer link AB and CD are of unequal length.



THEORY OF MACHINE

7) The following are three positions for correct steering a. When vehicle moves along a straight path, the longer link AB and CD are parallel and shorter link BC and AD are equally inclined to the longitudinal axis of the vehicle. b. When the vehicle is steering to the left, the position of the gear as shown by dotted lines. In this position the lines of the front wheel axle intersect on the back wheel axle at I for correct steering. c. When the vehicle is steering to the right the similar position may be obtained.



THEORY OF MACHINE

4. Foot operated Air Air Pump Mechanism.

1) It consists of a cylinder which can oscillate. A piston is mounted in the cylinder. The cylinder is connected to the foot rest. 2) The arms connected to the foot rest can oscillate.

Figure: Foot Operated Air Pump Mechanism

3) A retrieving spring can bring back the foot rest back to initial position’s position’s the foot rest is pressed the cylinder oscillates. 4) It creates reciprocating motion of the piston in the cylinder. Therefore suction and delivery stroke can be obtained. This is also called as oscillating cylinder mechanism.



Fundamentals and Types of Mechanism

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