kinematics of machinery

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Kinematics of Machinery

III SEMESTER MECHANICAL ENGINEERING

Prepared by

John Martin .A,

Asst.Professor,

Department of Mechanical Engineering,

N.P.R.College of Engineering and Technology,

Natham.


ME1202 KINEMATICS OF MACHINERY L T P C

3 1 0 4

UNIT I BASICS OF MECHANISMS 7

Definitions Link Kinematic pair Kinematic chain Mechanism Machine

Degree of Freedom

Mobility Kutzbach criterion (Grueblers equation) Grashoff's law Kinematic

Inversions of

four-bar chain and slider crank chain Mechanical Advantage Transmission angle

Description of common Mechanisms Offset slider mechanism as quick return

mechanisms

Pantograph Straight line generators (Peaucellier and Watt mechanisms) Steering

gear for

automobile Hookes joint Toggle mechanism Ratchets Escapements

Indexing Mechanisms

UNIT II KINEMATIC ANALYSIS 10

Analysis of simple mechanisms (Single slider crank mechanism and four bar

mechanism) Graphical

Methods for displacement Velocity and Acceleration Shaping machine

mechanism Coincident

points Coriolis acceleration Analytical method of analysis of slider crank

mechanism and four bar

mechanism Approximate analytical expression for displacement, velocity and

acceleration of piston

of reciprocating engine mechanism.

UNIT III KINEMATICS OF CAMS 8

Classifications Displacement diagrams Parabolic, Simple harmonic and Cycloidal

motions

Graphical construction of displacement diagrams and layout of plate cam profiles

Circular arc and

tangent cams Pressure angle and undercutting.

UNIT IV GEARS 10

Classification of gears Gear tooth terminology Fundamental law of toothed

gearing and involute


gearing Length of path of contact and contact ratio Interference and undercutting

Gear trains

Simple Compound Epicyclic gear trains Differentials.

UNIT V FRICTION 10

Dry friction Friction in screw jack Pivot and collar friction Plate clutches Belt

and rope drives

Block brakes Band brakes

L: 45 T: 15 Total: 60

TEXT BOOKS

1. Ambekar, A.G., Mechanism and Machine Theory, Prentice Hall of India, 2007.

2. Uicker, J.J., Pennock, G.R. and Shigley, J.E., Theory of Machines and

Mechanisms(Indian

Edition), Oxford University Press, 2003.

REFERENCES

1. Thomas Bevan, Theory of Machines, CBS Publishers and Distributors, 1984.

2. Ramamurti, V., Mechanism and Machine Theory, 2nd Edition, Narosa Publishing

House,

2005.

3. Ghosh, A. and Mallick, A.K., Theory of Mechanisms and Machines, Affiliated

East-West

Pvt. Ltd., 1998.

BIS Codes of Practice/Useful Websites

1. IS 2458 : 2001, Vocabulary of Gear Terms Definitions Related to Geometry

2. IS 2467 : 2002 (ISO 701: 1998), International Gear Notation Symbols for

Geometric Data.

3. IS 5267 : 2002 Vocabulary of Gear Terms Definitions Related to Worm Gear

Geometry.

4. IS 5037 : Part 1 : 2004, Straight Bevel Gears for General

Engineering and Heavy Engineering - Part 1: Basic Rack.

5. IS 5037 : Part 2 : 2004, Straight Bevel Gears for General

Engineering and Heavy Engineering - Part 2: Module and Diametral Pitches.

Web site: www.howstuffworks.com


Objective: To study the mechanism, machine and the geometric aspect of motion.

Unit I BASICS OF MECHANISMS

Introduction:

The objective of kinematics is to develop various means

of transforming motion to achieve a specific kind needed in

applications. For example, an object is to be moved from point

A to point B along some path. The first question in solving this

problem is usually: What kind of a mechanism (if any) can be

used to perform this function? And the second question is: How

does one design such a mechanism?

The objective of dynamics is analysis of the behavior of a given machine or

mechanism when subjected to dynamic forces. For the above example, when

the mechanism is already known, then external forces are applied and its

motion is studied. The determination of forces induced in machine

components by the motion is part of this analysis.

As a subject, the kinematics and dynamics of machines and mechanisms is

disconnected from other subjects (except statics and dynamics) in the

Mechanical Engineering curriculum. This absence of links to other subjects

may create the false impression that there are no constraints, apart from the

kinematic ones, imposed on the design of mechanisms. Look again at the

problem of moving an object from A to B. In designing a mechanism, the

size, shape, and weight of the object all constitute input into the design

process.

All of these will affect the size of the mechanism. There are other

considerations as well, such as, for example, what the allowable speed of

approaching point B should be. The outcome of this inquiry may affect

either the configuration or the type of the mechanism. Within the subject of

kinematics and dynamics of machines and mechanisms such requirements

cannot be justifiably formulated; they can, however, be posed as a learning

exercise.


KINEMATICS AND DYNAMICS AS PART OF THE DESIGN PROCESS

The role of kinematics is to ensure the functionality of the mechanism, while

the role of dynamics is to verify the acceptability of induced forces in parts.

The functionality and induced forces are subject to various constraints

(specifications) imposed on the design. Look at the example of a cam

operating a valve

Fundamentals of Kinematics and Dynamics of Machines and Mechanisms

The design process starts with meeting the functional requirements of the

product.

The basic one in this case is the proper opening, dwelling, and closing of the

valve as a function of time. To achieve this objective, a corresponding cam

profile producing the needed follower motion should be found. The rocker

arm, being a lever, serves as a displacement amplifier/reducer. The timing of

opening, dwelling, and closing is controlled by the speed of the camshaft.

The function of the spring is to keep the roller always in contact with the

cam. To meet this requirement the inertial forces developed during the

followervalve system motion should be known, since the spring force must

be larger than these forces at any time. Thus, it follows that the

determination of component accelerations needed to find inertial forces is

important for the choice of the proper spring stiffness. Kinematical analysis

allows one to satisfy the functional requirements for valve displacements.

Dynamic analysis allows one to find forces in the system as a function of

time. These forces are needed to continue the design process. The design

process continues with meeting the constraints requirements, which in this

case are:

1. Sizes of all parts;

2. Sealing between the valve and its seat;

3. Lubrication;

4. Selection of materials;

5. Manufacturing and maintenance;

6. Safety;

7. Assembly, etc.

The forces transmitted through the system during cam rotation allow one to


determine the proper sizes of components, and thus to find the overall

assembly dimension. The spring force affects the reliability of the valve

sealing. If any of the requirements cannot be met with the given assembly

design, then another set of parameters should be chosen, and the kinematic

and dynamic analysis repeated for the new version.

Thus, kinematic and dynamic analysis is an integral part of the machine

design process, which means it uses input from this process and produces

output for its continuation.

IS IT A MACHINE, A MECHANISM, OR A STRUCTURE?

The term machine is usually applied to a complete product. A car is a machine, as is a

tractor, a combine, an earthmoving machine, etc. At the same time, each of these

machines may have some devices performing specific functions, like a windshield

wiper in a car, which are called mechanisms. An internal combustion engine is called

neither a machine nor a mechanism. It is clear that there is a historically established

terminology and it may not be consistent. What is important, as far as the subject of

kinematics and dynamics is concerned, is that the identification of something as a

machine or a mechanism has no bearing on the analysis to be done. And thus in the

following, the term machine or mechanism in application to a specific device will be

used according to the established custom. The distinction between the

machine/mechanism and the structure is more fundamental. The former must have

moving parts, since it transforms motion, produces work, or transforms energy. The

latter does not have moving parts; its function is purely structural, i.e., to maintain its

form and shape under given external loads,

like a bridge, a building, or an antenna mast.

Fundamentals of Kinematics and Dynamics of Machines and Mechanisms

chair, or a solar antenna, may be confusing. Before the folding chair can be used as a

chair, it must be unfolded. The transformation from a folded to an unfolded state is the

transformation of motion. Thus, the folding chair meets two definitions: it is a

mechanism during unfolding and a structure when unfolding is completed. Again, the

terminology should not affect the understanding of the substance of the matter.

Definitions : Link or Element, Pairing of Elements with degrees of freedom,

Grublers criterion (without derivation), Kinematic chain, Mechanism, Mobility of Mechanism, Inversions, Machine.


Kinematic Chains and Inversions : Recall that a kinematic chain becomes a mechanism when one of the links in

the chain becomes a frame. The process of choosing different links in the

chain as framesis known as kinematic inversion. In this way, for an n-link

chain n different mechanisms can be obtained. An example of a four-link

slider-crank chain shows how different mechanisms are obtained by fixing

different links functionally.

Steam engines

Beam engine, with twin connecting rods (almost vertical) between the horizontal

beam and the flywheel cranks

The first steam engines, Newcomen's atmospheric engine, was single-acting: its piston

only did work in one direction, and so these used a chain rather than a connecting rod.

Their output rocked back and forth, rather than rotating continuously.

Crosshead of a stationary steam engine: piston rod to the left, connecting rod to the

right

Steam engines after this are usually double-acting: their internal pressure works on

each side of the piston in turn. This requires a seal around the piston rod and so the


hinge between the piston and connecting rod is placed outside the cylinder, in a large

sliding bearing block called a crosshead.

Steam locomotive rods, the large angled rod being the connecting rod

Internal combustion engines

Compound rods

Articulated connecting rods in a WW1 aero-engine

BMW 132 radial engine rods

LINKS


Types Of Mechanisms:

i) Quick return motion mechanisms Drag link mechanism, Whitworth mechanism and Crank and slotted lever mechanism

ii) Straight line motion mechanisms Peaceliers mechanism and Roberts mechanism.

iii) Intermittent motion mechanisms Geneva mechanism and Ratchet & Pawl mechanism.

iv) Toggle mechanism, Pantograph, Hookes joint and Ackerman Steering gear mechanism.

1. Terminology and Definitions-Degree of Freedom, Mobility

Kinematics: The study of motion (position, velocity, acceleration). A major goal of understanding kinematics is to develop the ability to design a system

that will satisfy specified motion requirements. This will be the emphasis of

this class.

Kinetics: The effect of forces on moving bodies. Good kinematic design should produce good kinetics.

Mechanism: A system design to transmit motion. (low forces)

Machine: A system designed to transmit motion and energy. (forces involved)

Basic Mechanisms: Includes geared systems, cam-follower systems and linkages (rigid links connected by sliding or rotating joints). A mechanism

has multiple moving parts (for example, a simple hinged door does not qualify

as a mechanism).

Examples of mechanisms: Tin snips, vise grips, car suspension, backhoe, piston engine, folding chair, windshield wiper drive system, etc.


Key concepts:

Degrees of freedom: The number of inputs required to completely control a system. Examples: A simple rotating link. A two link system. A four-bar

linkage. A five-bar linkage.

Types of motion: Mechanisms may produce motions that are pure rotation, pure translation, or a combination of the two. We reduce the degrees of

freedom of a mechanism by restraining the ability of the mechanism to move

in translation (x-y directions for a 2D mechanism) or in rotation (about the z-

axis for a 2-D mechanism).

Link: A rigid body with two or more nodes (joints) that are used to connect to other rigid bodies. (WM examples: binary link, ternary link (3 joints),

quaternary link (4 joints))

Joint: A connection between two links that allows motion between the links. The motion allowed may be rotational (revolute joint), translational (sliding or

prismatic joint), or a combination of the two (roll-slide joint).

Kinematic chain: An assembly of links and joints used to coordinate an output motion with an input motion.

Link or element:

A mechanism is made of a number of resistant bodies out of which some may have

motions relative to the others. A resistant body or a group of resistant bodies with

rigid connections preventing their relative movement is known as a link.


A link may also be defined as a member or a combination of members of a

mechanism, connecting other members and having motion relative to them, thus a link may consist of one or more resistant bodies. A link is also known as Kinematic

link or an element.

Links can be classified into 1) Binary, 2) Ternary, 3) Quarternary, etc.

Kinematic Pair: A Kinematic Pair or simply a pair is a joint of two links having relative motion

between them.

Example:

In the above given Slider crank mechanism, link 2 rotates relative to link 1 and

constitutes a revolute or turning pair. Similarly, links 2, 3 and 3, 4 constitute turning

pairs. Link 4 (Slider) reciprocates relative to link 1 and its a sliding pair.

Types of Kinematic Pairs:

Kinematic pairs can be classified according to

i) Nature of contact.

ii) Nature of mechanical constraint.

iii) Nature of relative motion.

i) Kinematic pairs according to nature of contact :

a) Lower Pair: A pair of links having surface or area contact between the members is

known as a lower pair. The contact surfaces of the two links are similar.

Examples: Nut turning on a screw, shaft rotating in a bearing, all pairs of a slider-

crank mechanism, universal joint.

b) Higher Pair: When a pair has a point or line contact between the links, it is known

as a higher pair. The contact surfaces of the two links are dissimilar.

Examples: Wheel rolling on a surface cam and follower pair, tooth gears, ball and

roller bearings, etc.

ii) Kinematic pairs according to nature of mechanical constraint.

a) Closed pair: When the elements of a pair are held together mechanically, it is

known as a closed pair. The contact between the two can only be broken only by the

destruction of at least one of the members. All the lower pairs and some of the higher

pairs are closed pairs.

b) Unclosed pair: When two links of a pair are in contact either due to force of gravity

or some spring action, they constitute an unclosed pair. In this the links are not held

together mechanically. Ex.: Cam and follower pair.

iii) Kinematic pairs according to nature of relative motion.


a) Sliding pair: If two links have a sliding motion relative to each other, they form a

sliding pair. A rectangular rod in a rectangular hole in a prism is an example of a

sliding pair.

b) Turning Pair: When on link has a turning or revolving motion relative to the other,

they constitute a turning pair or revolving pair.

c) Rolling pair: When the links of a pair have a rolling motion relative to each other,

they form a rolling pair. A rolling wheel on a flat surface, ball ad roller bearings, etc.

are some of the examples for a Rolling pair.

d) Screw pair (Helical Pair): if two mating links have a turning as well as sliding

motion between them, they form a screw pair. This is achieved by cutting matching

threads on the two links.

The lead screw and the nut of a lathe is a screw Pair

e) Spherical pair: When one link in the form of a sphere turns inside a fixed link, it is

a spherical pair. The ball and socket joint is a spherical pair.

Degrees of Freedom: An unconstrained rigid body moving in space can describe the following independent

motions.

1. Translational Motions along any three mutually perpendicular axes x, y and z,

2. Rotational motions along these axes.

Thus a rigid body possesses six degrees of freedom. The connection of a link with

another imposes certain constraints on their relative motion. The number of restraints

can never be zero (joint is disconnected) or six (joint becomes solid).

Degrees of freedom of a pair is defined as the number of independent relative

motions, both translational and rotational, a pair can have.

Degrees of freedom = 6 no. of restraints. To find the number of degrees of freedom for a plane mechanism we have an equation

known as Grublers equation and is given by F = 3 ( n 1 ) 2 j1 j2 F = Mobility or number of degrees of freedom

n = Number of links including frame.

j1 = Joints with single (one) degree of freedom.

J2 = Joints with two degrees of freedom.

If F > 0, results in a mechanism with F degrees of freedom. F = 0, results in a statically determinate structure.

F < 0, results in a statically indeterminate structure.

Kinematic Chain: A Kinematic chain is an assembly of links in which the relative motions of the links is

possible and the motion of each relative to the others is definite (fig. a, b, and c.)


In case, the motion of a link results in indefinite motions of other links, it is a non-

kinematic chain. However, some authors prefer to call all chains having relative

motions of the links as kinematic chains.

Linkage, Mechanism and structure: A linkage is obtained if one of the links of kinematic chain is fixed to the ground. If

motion of each link results in definite motion of the others, the linkage is known as

mechanism. If one of the links of a redundant chain is fixed, it is known as a structure.

To obtain constrained or definite motions of some of the links of a linkage, it is

necessary to know how many inputs are needed. In some mechanisms, only one input

is necessary that determines the motion of other links and are said to have one degree

of freedom. In other mechanisms, two inputs may be necessary to get a constrained

motion of the other links and are said to have two degrees of freedom and so on.

The degree of freedom of a structure is zero or less. A structure with negative degrees

of freedom is known as a Superstructure.

Motion and its types:


The three main types of constrained motion in kinematic pair are,

1.Completely constrained motion : If the motion between a pair of links is limited to

a definite direction, then it is completely constrained motion. E.g.: Motion of a shaft

or rod with collars at each end in a hole as shown in fig.

2. Incompletely Constrained motion : If the motion between a pair of links is not

confined to a definite direction, then it is incompletely constrained motion. E.g.: A

spherical ball or circular shaft in a circular hole may either rotate or slide in the hole

as shown in fig.

Completely

Constrained

Motion

Partially

Constrained

Motion

Incompletely

Constrained

Motion


3. Successfully constrained motion or Partially constrained motion: If the motion

in a definite direction is not brought about by itself but by some other means, then it is

known as successfully constrained motion. E.g.: Foot step Bearing.

Machine:

It is a combination of resistant bodies with successfully constrained motion which is

used to transmit or transform motion to do some useful work. E.g.: Lathe, Shaper,

Steam Engine, etc.

Kinematic chain with three lower pairs It is impossible to have a kinematic chain consisting of three turning pairs only. But it

is possible to have a chain which consists of three sliding pairs or which consists of a

turning, sliding and a screw pair.

The figure shows a kinematic chain with three sliding pairs. It consists of a frame B,

wedge C and a sliding rod A. So the three sliding pairs are, one between the wedge C

and the frame B, second between wedge C and sliding rod A and the frame B.

This figure shows the mechanism of a fly press. The element B forms a sliding with A

and turning pair with screw rod C which in turn forms a screw pair with A. When link

A is fixed, the required fly press mechanism is obtained.


2. Kutzbach criterion, Grashoff's law Kutzbach criterion:

Fundamental Equation for 2-D Mechanisms: M = 3(L 1) 2J1 J2 Can we intuitively derive Kutzbachs modification of Grublers equation? Consider a rigid link constrained to move in a plane. How many degrees of

freedom does the link have? (3: translation in x and y directions, rotation

about z-axis)

If you pin one end of the link to the plane, how many degrees of freedom does it now have?

Add a second link to the picture so that you have one link pinned to the plane and one free to move in the plane. How many degrees of freedom exist

between the two links? (4 is the correct answer)

Pin the second link to the free end of the first link. How many degrees of freedom do you now have?

How many degrees of freedom do you have each time you introduce a moving link? How many degrees of freedom do you take away when you add a

simple joint? How many degrees of freedom would you take away by adding

a half joint? Do the different terms in equation make sense in light of this

knowledge?

Grashoff's law:

Grashoff 4-bar linkage: A linkage that contains one or more links capable of undergoing a full rotation. A linkage is Grashoff if: S + L < P + Q (where:

S = shortest link length, L = longest, P, Q = intermediate length links). Both

joints of the shortest link are capable of 360 degrees of rotation in a Grashoff

linkages. This gives us 4 possible linkages: crank-rocker (input rotates 360),

rocker-crank-rocker (coupler rotates 360), rocker-crank (follower); double

crank (all links rotate 360). Note that these mechanisms are simply the

possible inversions (section 2.11, Figure 2-16) of a Grashoff mechanism.

Non Grashoff 4 bar: No link can rotate 360 if: S + L > P + Q

Lets examine why the Grashoff condition works:

Consider a linkage with the shortest and longest sides joined together. Examine the linkage when the shortest side is parallel to the longest side (2

positions possible, folded over on the long side and extended away from the

long side). How long do P and Q have to be to allow the linkage to achieve

these positions?

Consider a linkage where the long and short sides are not joined. Can you figure out the required lengths for P and Q in this type of mechanism

3. Kinematic Inversions of 4-bar chain and slider crank chains:

Types of Kinematic Chain: 1) Four bar chain 2) Single slider chain 3) Double

Slider chain

Four bar Chain: The chain has four links and it looks like a cycle frame and hence it is also called

quadric cycle chain. It is shown in the figure. In this type of chain all four pairs will

be turning pairs.


Inversions:

By fixing each link at a time we get as many mechanisms as the number of links, then

each mechanism is called Inversion of the original Kinematic Chain. Inversions of four bar chain mechanism: There are three inversions: 1) Beam Engine or Crank and lever mechanism. 2)

Coupling rod of locomotive or double crank mechanism. 3) Watts straight line mechanism or double lever mechanism.

Beam Engine: When the crank AB rotates about A, the link CE pivoted at D makes vertical

reciprocating motion at end E. This is used to convert rotary motion to reciprocating

motion and vice versa. It is also known as Crank and lever mechanism. This

mechanism is shown in the figure below.

2. Coupling rod of locomotive: In this mechanism the length of link AD =

length of link C. Also length of link AB = length of link CD. When AB rotates about

A, the crank DC rotates about D. this mechanism is used for coupling locomotive

wheels. Since links AB and CD work as cranks, this mechanism is also known as

double crank mechanism. This is shown in the figure below.


3. Watts straight line mechanism or Double lever mechanism: In this mechanism, the links AB & DE act as levers at the ends A & E of these levers are

fixed. The AB & DE are parallel in the mean position of the mechanism and coupling

rod BD is perpendicular to the levers AB & DE. On any small displacement of the

mechanism the tracing point C traces the shape of number 8, a portion of which will be approximately straight. Hence this is also an example for the approximate

straight line mechanism. This mechanism is shown below.

2. Slider crank Chain:

It is a four bar chain having one sliding pair and three turning pairs. It is shown in the

figure below the purpose of this mechanism is to convert rotary motion to

reciprocating motion and vice versa.

Inversions of a Slider crank chain:

There are four inversions in a single slider chain mechanism. They are:

1) Reciprocating engine mechanism (1st

inversion)

2) Oscillating cylinder engine mechanism (2nd

inversion)

3) Crank and slotted lever mechanism (2nd

inversion)

4) Whitworth quick return motion mechanism (3rd

inversion)

5) Rotary engine mechanism (3rd

inversion)

6) Bull engine mechanism (4th

inversion)

7) Hand Pump (4th

inversion)

1. Reciprocating engine mechanism :

In the first inversion, the link 1 i.e., the cylinder and the frame is kept fixed. The fig

below shows a reciprocating engine.


A slotted link 1 is fixed. When the crank 2 rotates about O, the sliding piston 4

reciprocates in the slotted link 1. This mechanism is used in steam engine, pumps,

compressors, I.C. engines, etc.

2. Crank and slotted lever mechanism: It is an application of second inversion. The crank and slotted lever mechanism is

shown in figure below.

In this mechanism link 3 is fixed. The slider (link 1) reciprocates in oscillating slotted

lever (link 4) and crank (link 2) rotates. Link 5 connects link 4 to the ram (link 6). The

ram with the cutting tool reciprocates perpendicular to the fixed link 3. The ram with

the tool reverses its direction of motion when link 2 is perpendicular to link 4. Thus

the cutting stroke is executed during the rotation of the crank through angle and the return stroke is executed when the crank rotates through angle or 360 . Therefore, when the crank rotates uniformly, we get,

Time to cutting = = Time of return 360 This mechanism is used in shaping machines, slotting machines and in rotary engines.

3. Whitworth quick return motion mechanism:

Third inversion is obtained by fixing the crank i.e. link 2. Whitworth quick return

mechanism is an application of third inversion. This mechanism is shown in the figure

below. The crank OC is fixed and OQ rotates about O. The slider slides in the slotted

link and generates a circle of radius CP. Link 5 connects the extension OQ provided

on the opposite side of the link 1 to the ram (link 6). The rotary motion of P is taken

to the ram R which reciprocates. The quick return motion mechanism is used in

shapers and slotting machines. The angle covered during cutting stroke from P1 to P2

in counter clockwise direction is or 360 -2. During the return stroke, the angle covered is 2 or .


Therefore, Time to cutting = 360 -2 = 180 Time of return 2 = = . 360

4. Rotary engine mechanism or Gnome Engine: Rotary engine mechanism or gnome engine is another application of third inversion. It

is a rotary cylinder V type internal combustion engine used as an aero engine. But now Gnome engine has been replaced by Gas turbines. The Gnome engine has

generally seven cylinders in one plane. The crank OA is fixed and all the connecting

rods from the pistons are connected to A. In this mechanism when the pistons

reciprocate in the cylinders, the whole assembly of cylinders, pistons and connecting

rods rotate about the axis O, where the entire mechanical power developed, is

obtained in the form of rotation of the crank shaft. This mechanism is shown in the

figure below.

Double Slider Crank Chain: A four bar chain having two turning and two sliding pairs such that two pairs of the

same kind are adjacent is known as double slider crank chain.

Inversions of Double slider Crank chain: It consists of two sliding pairs and two turning pairs. They are three important

inversions of double slider crank chain. 1) Elliptical trammel. 2) Scotch yoke

mechanism. 3) Oldhams Coupling.

1. Elliptical Trammel: This is an instrument for drawing ellipses. Here the slotted link is fixed. The sliding

block P and Q in vertical and horizontal slots respectively. The end R generates an

ellipse with the displacement of sliders P and Q.

The co-ordinates of the point R are x and y. From the fig. cos = x. PR and Sin = y. QR Squaring and adding (i) and (ii) we get x

2 + y

2 = cos

2 + sin2

(PR)2

(QR)2


x2 + y

2 = 1

(PR)2

(QR)2

The equation is that of an ellipse, Hence the instrument traces an ellipse. Path traced

by mid-point of PQ is a circle. In this case, PR = PQ and so x2

+y2

=1 (PR)2

(QR)2

It is an equation of circle with PR = QR = radius of a circle.

2. Scotch yoke mechanism: This mechanism, the slider P is fixed. When PQ rotates above P, the slider Q reciprocates in the vertical slot. The mechanism is used

to convert rotary to reciprocating mechanism.

3. Oldhams coupling: The third inversion of obtained by fixing the link

connecting the 2 blocks P & Q. If one block is turning through an angle, the frame

and the other block will also turn through the same angle. It is shown in the figure

below.

An application of the third inversion of the double slider crank mechanism is

Oldhams coupling shown in the figure. This coupling is used for connecting two parallel shafts when the distance between the shafts is small. The two shafts to be

connected have flanges at their ends, secured by forging. Slots are cut in the flanges.

These flanges form 1 and 3. An intermediate disc having tongues at right angles and

opposite sides is fitted in between the flanges. The intermediate piece forms the link 4

which slides or reciprocates in flanges 1 & 3. The link two is fixed as shown. When

flange 1 turns, the intermediate disc 4 must turn through the same angle and whatever

angle 4 turns, the flange 3 must turn through the same angle. Hence 1, 4 & 3 must

have the same angular velocity at every instant. If the distance between the axis of the

shaft is x, it will be the diameter if the circle traced by the centre of the intermediate

piece. The maximum sliding speed of each tongue along its slot is given by

v=x where, = angular velocity of each shaft in rad/sec v = linear velocity in m/sec


4. Mechanical Advantage, Transmission angle:

The mechanical advantage (MA) is defined as the ratio of output torque to the input torque. (or) ratio of load to output.

Transmission angle.

The extreme values of the transmission angle occur when the crank lies along the line of frame.

5. Description of common mechanisms-Single, Double and offset slider mechanisms - Quick return mechanisms:

Quick Return Motion Mechanisms: Many a times mechanisms are designed to perform repetitive operations. During these

operations for a certain period the mechanisms will be under load known as working

stroke and the remaining period is known as the return stroke, the mechanism returns

to repeat the operation without load. The ratio of time of working stroke to that of the

return stroke is known a time ratio. Quick return mechanisms are used in machine

tools to give a slow cutting stroke and a quick return stroke. The various quick return

mechanisms commonly used are i) Whitworth ii) Drag link. iii) Crank and slotted

lever mechanism

1. Whitworth quick return mechanism: Whitworth quick return mechanism is an application of third inversion of the single

slider crank chain. This mechanism is shown in the figure below. The crank OC is

fixed and OQ rotates about O. The slider slides in the slotted link and generates a

circle of radius CP. Link 5 connects the extension OQ provided on the opposite side

of the link 1 to the ram (link 6). The rotary motion of P is taken to the ram R which

reciprocates. The quick return motion mechanism is used in shapers and slotting

machines.

The angle covered during cutting stroke from P1 to P2 in counter clockwise direction

is or 360 -2. During the return stroke, the angle covered is 2 or .

2. Drag link mechanism :


This is four bar mechanism with double crank in which the shortest link is fixed. If

the crank AB rotates at a uniform speed, the crank CD rotate at a non-uniform speed.

This rotation of link CD is transformed to quick return reciprocatory motion of the

ram E by the link CE as shown in figure. When the crank AB rotates through an angle

in Counter clockwise direction during working stroke, the link CD rotates through 180. We can observe that / >/ . Hence time of working stroke is / times more or the return stroke is / times quicker. Shortest link is always stationary link. Sum of the shortest and the longest links of the four links 1, 2, 3 and 4 are less than the sum of

the other two. It is the necessary condition for the drag link quick return mechanism.

3. Crank and slotted lever mechanism: It is an application of second inversion. The crank and slotted lever mechanism is

shown in figure below.

In this mechanism link 3 is fixed. The slider (link 1) reciprocates in oscillating slotted

lever (link 4) and crank (link 2) rotates. Link 5 connects link 4 to the ram (link 6). The

ram with the cutting tool reciprocates perpendicular to the fixed link 3. The ram with

the tool reverses its direction of motion when link 2 is perpendicular to link 4. Thus

the cutting stroke is executed during the rotation of the crank through angle and the return stroke is executed when the crank rotates through angle or 360 . Therefore, when the crank rotates uniformly, we get,

Time to cutting = = Time of return 360 This mechanism is used in shaping machines, slotting machines and in rotary engines.

6. Ratchets and escapements - Indexing Mechanisms - Rocking Mechanisms:

Intermittent motion mechanism:

1. Ratchet and Pawl mechanism: This mechanism is used in producing intermittent rotary motion member. A ratchet and Pawl mechanism consists of a

ratchet wheel 2 and a pawl 3 as shown in the figure. When the lever 4 carrying pawl is

raised, the ratchet wheel rotates in the counter clock wise direction (driven by pawl).

As the pawl lever is lowered the pawl slides over the ratchet teeth. One more pawl 5 is

used to prevent the ratchet from reversing. Ratchets are used in feed mechanisms,

lifting jacks, clocks, watches and counting devices.


2. Geneva mechanism: Geneva mechanism is an intermittent motion

mechanism. It consists of a driving wheel D carrying a pin P which engages in a slot

of follower F as shown in figure. During one quarter revolution of the driving plate,

the Pin and follower remain in contact and hence the follower is turned by one quarter

of a turn. During the remaining time of one revolution of the driver, the follower

remains in rest locked in position by the circular arc.

3. Pantograph: Pantograph is used to copy the curves in reduced or enlarged

scales. Hence this mechanism finds its use in copying devices such as engraving or

profiling machines.

This is a simple figure of a Pantograph. The links are pin jointed at A, B, C and D.

AB is parallel to DC and AD is parallel to BC. Link BA is extended to fixed pin O. Q

is a point on the link AD. If the motion of Q is to be enlarged then the link BC is

extended to P such that O, Q and P are in a straight line. Then it can be shown that the

points P and Q always move parallel and similar to each other over any path straight

or curved. Their motions will be proportional to their distance from the fixed point.

Let ABCD be the initial position. Suppose if point Q moves to Q1 , then all the links

and the joints will move to the new positions (such as A moves to A1 , B moves to


Q1, C moves to Q1 , D moves to D1 and P to P1 ) and the new configuration of the

mechanism is shown by dotted lines. The movement of Q (Q Q1) will be enlarged to

PP1 in a definite ratio.

4. Toggle Mechanism:

In slider crank mechanism as the crank approaches one of its dead centre position, the

slider approaches zero. The ratio of the crank movement to the slider movement

approaching infinity is proportional to the mechanical advantage. This is the principle

used in toggle mechanism. A toggle mechanism is used when large forces act through

a short distance is required. The figure below shows a toggle mechanism. Links CD

and CE are of same length. Resolving the forces at C vertically F Sin =P Cos 2 Therefore, F = P . (because Sin /Cos = Tan ) 2 tan Thus for the given value of P, as the links CD and CE approaches collinear position (O), the force F rises rapidly.

5. Hookes joint:

Hookes joint used to connect two parallel intersecting shafts as shown in figure. This can also be used for shaft with angular misalignment where flexible coupling does not

serve the purpose. Hence Hookes joint is a means of connecting two rotating shafts whose axes lie in the same plane and their directions making a small angle with each

other. It is commonly known as Universal joint. In Europe it is called as Cardan joint.

5. Ackermann steering gear mechanism:

This mechanism is made of only turning pairs and is made of only turning pairs wear

and tear of the parts is less and cheaper in manufacturing. The cross link KL connects


two short axles AC and BD of the front wheels through the short links AK and BL

which forms bell crank levers CAK and DBL respectively as shown in fig, the longer

links AB and KL are parallel and the shorter links AK and BL are inclined at an angle

. When the vehicles steer to the right as shown in the figure, the short link BL is turned so as to increase , where as the link LK causes the other short link AK to turn so as to reduce . The fundamental equation for correct steering is, CotCos = b / l In the above arrangement it is clear that the angle through which AK turns is less than the angle through which the BL turns and therefore the left front axle turns through a smaller angle than the right front axle. For different angle of turn , the corresponding value of and (Cot Cos ) are noted. This is done by actually drawing the mechanism to a scale or by calculations. Therefore for different value of

the corresponding value of and are tabulated. Approximate value of b/l for correct

steering should be between 0.4 and 0.5. In an Ackermann steering gear mechanism,

the instantaneous centre I does not lie on the axis of the rear axle but on a line parallel

to the rear axle axis at an approximate distance of 0.3l above it.

Three correct steering positions will be:

1) When moving straight. 2) When moving one correct angle to the right

corresponding to the link ratio AK/AB and angle . 3) Similar position when moving to the left. In all other positions pure rolling is not obtainable.

Some Of The Mechanisms Which Are Used In Day To Day Life.

BELL CRANK: GENEVA STOP:

BELL CRANK: The bell crank was originally used in large house to operate the

servants bell, hence the name. The bell crank is used to convert the direction of reciprocating movement. By varying the angle of the crank piece it can be used to

change the angle of movement from 1 degree to 180 degrees.

GENEVA STOP: The Geneva stop is named after the Geneva cross, a similar shape

to the main part of the mechanism. The Geneva stop is used to provide intermittent

motion, the orange wheel turns continuously, the dark blue pin then turns the blue

cross quarter of a turn for each revolution of the drive wheel. The crescent shaped cut

out in dark orange section lets the points of the cross past, then locks the wheel in

place when it is stationary. The Geneva stop mechanism is used commonly in film

cameras.


ELLIPTICAL TRAMMEL PISTON ARRANGEMENT

ELLIPTICAL TRAMMEL: This fascinating mechanism converts rotary motion to

reciprocating motion in two axis. Notice that the handle traces out an ellipse rather

than a circle. A similar mechanism is used in ellipse drawing tools.

PISTON ARRANGEMENT: This mechanism is used to convert between rotary

motion and reciprocating motion, it works either way. Notice how the speed of the

piston changes. The piston starts from one end, and increases its speed. It reaches

maximum speed in the middle of its travel then gradually slows down until it reaches

the end of its travel.

RACK AND PINION RATCHET

RACK AND PINION: The rack and pinion is used to convert between rotary and

linear motion. The rack is the flat, toothed part, the pinion is the gear. Rack and pinion

can convert from rotary to linear of from linear to rotary. The diameter of the gear

determines the speed that the rack moves as the pinion turns. Rack and pinions are

commonly used in the steering system of cars to convert the rotary motion of the

steering wheel to the side to side motion in the wheels. Rack and pinion gears give a

positive motion especially compared to the friction drive of a wheel in tarmac. In the

rack and pinion railway a central rack between the two rails engages with a pinion on

the engine allowing the train to be pulled up very steep slopes.

RATCHET: The ratchet can be used to move a toothed wheel one tooth at a time.

The part used to move the ratchet is known as the pawl. The ratchet can be used as a

way of gearing down motion. By its nature motion created by a ratchet is intermittent.

By using two pawls simultaneously this intermittent effect can be almost, but not

quite, removed. Ratchets are also used to ensure that motion only occurs in only one


direction, useful for winding gear which must not be allowed to drop. Ratchets are

also used in the freewheel mechanism of a bicycle.

WORM GEAR WATCH ESCAPEMENT.

WORM GEAR: A worm is used to reduce speed. For each complete turn of the

worm shaft the gear shaft advances only one tooth of the gear. In this case, with a

twelve tooth gear, the speed is reduced by a factor of twelve. Also, the axis of rotation

is turned by 90 degrees. Unlike ordinary gears, the motion is not reversible, a worm

can drive a gear to reduce speed but a gear cannot drive a worm to increase it. As the

speed is reduced the power to the drive increases correspondingly. Worm gears are a

compact, efficient means of substantially decreasing speed and increasing power.

Ideal for use with small electric motors.

WATCH ESCAPEMENT: The watch escapement is the centre of the time piece. It

is the escapement which divides the time into equal segments. The balance wheel, the

gold wheel, oscillates backwards and forwards on a hairspring (not shown) as the

balance wheel moves the lever is moved allowing the escape wheel (green) to rotate

by one tooth. The power comes through the escape wheel which gives a small 'kick' to

the palettes (purple) at each tick.

GEARS CAM FOLLOWER.

GEARS: Gears are used to change speed in rotational movement. In the example

above the blue gear has eleven teeth and the orange gear has twenty five. To turn the

orange gear one full turn the blue gear must turn 25/11 or 2.2727r turns. Notice that as

the blue gear turns clockwise the orange gear turns anti-clockwise. In the above

example the number of teeth on the orange gear is not divisible by the number of teeth

on the blue gear. This is deliberate. If the orange gear had thirty three teeth then every

three turns of the blue gear the same teeth would mesh together which could cause

excessive wear. By using none divisible numbers the same teeth mesh only every

seventeen turns of the blue gear.


CAMS: Cams are used to convert rotary motion into reciprocating motion. The

motion created can be simple and regular or complex and irregular. As the cam turns,

driven by the circular motion, the cam follower traces the surface of the cam

transmitting its motion to the required mechanism. Cam follower design is important

in the way the profile of the cam is followed. A fine pointed follower will more

accurately trace the outline of the cam. This more accurate movement is at the

expense of the strength of the cam follower.

STEAM ENGINE.

Steam engines were the backbone of the industrial revolution. In this common design

high pressure steam is pumped alternately into one side of the piston, then the other

forcing it back and forth. The reciprocating motion of the piston is converted to useful

rotary motion using a crank.

As the large wheel (the fly wheel) turns a small crank or cam is used to move the

small red control valve back and forth controlling where the steam flows. In this

animation the oval crank has been made transparent so that you can see how the

control valve crank is attached.

7. Straight line generators, Design of Crank-rocker Mechanisms:

Straight Line Motion Mechanisms: The easiest way to generate a straight line motion is by using a sliding pair but in

precision machines sliding pairs are not preferred because of wear and tear. Hence in

such cases different methods are used to generate straight line motion mechanisms:

1. Exact straight line motion mechanism.

a. Peaucellier mechanism, b. Hart mechanism, c. Scott Russell mechanism

2. Approximate straight line motion mechanisms

a. Watt mechanism, b. Grasshoppers mechanism, c. Roberts mechanism, d. Tchebicheffs mechanism

a. Peaucillier mechanism : The pin Q is constrained to move long the circumference of a circle by means of the

link OQ. The link OQ and the fixed link are equal in length. The pins P and Q are on

opposite corners of a four bar chain which has all four links QC, CP, PB and BQ of

equal length to the fixed pin A. i.e., link AB = link AC. The product AQ x AP remain

constant as the link OQ rotates may be proved as follows: Join BC to bisect PQ at F;

then, from the right angled triangles AFB, BFP, we have AB=AF+FB and

BP=BF+FP. Subtracting, AB-BP= AF-FP=(AFFP)(AF+FP) = AQ x AP . Since AB and BP are links of a constant length, the product AQ x AP is constant.

Therefore the point P traces out a straight path normal to AR.


b. Roberts mechanism: This is also a four bar chain. The link PQ and RS are of equal length and the tracing

pint O is rigidly attached to the link QR on a line which bisects QR at right angles. The best position for O may be found by making use of the instantaneous centre of

QR. The path of O is clearly approximately horizontal in the Roberts mechanism.

a. Peaucillier mechanism b. Hart mechanism

Oldham Coupling.

Below is exploded view of an Oldham Coupling.


An Oldham coupler is a method to transfer torque between two parallel but not

collinear shafts. It has three discs, one coupled to the input, one coupled to the output,

and a middle disc that is joined to the first two by tongue and groove. The tongue and

groove on one side is perpendicular to the tongue and groove on the other. Often

springs are used to reduce backlash of the mechanism. The coupler is much more

compact than, for example, two universal joints.

The coupler is named for John Oldham who invented it in Ireland, in 1820, to solve a

paddle placement problem in a steamship design. The middle disc rotates around its

center at the same speed as the input and output shafts. Its center traces a circular

orbit, twice per rotation, around the midpoint between input and output shafts.

Unit II KINEMATICS

Velocity and Acceleration analysis of mechanisms (Graphical Methods):

Velocity and acceleration analysis by vector polygons: Relative velocity and

accelerations of particles in a common link, relative velocity and accelerations of

coincident particles on separate link, Coriolis component of acceleration.

Velocity and acceleration analysis by complex numbers: Analysis of single slider

crank mechanism and four bar mechanism by loop closure equations and complex

numbers.

8. Displacement, velocity and acceleration analysis in simple mechanisms:

Important Concepts in Velocity Analysis 1. The absolute velocity of any point on a mechanism is the velocity of that point

with reference to ground.

2. Relative velocity describes how one point on a mechanism moves relative to

another point on the mechanism.

3. The velocity of a point on a moving link relative to the pivot of the link is given

by the equation: V = r, where = angular velocity of the link and r = distance from pivot.

Acceleration Components

Normal Acceleration: An = 2r. Points toward the center of rotation

Tangential Acceleration: At = r. In a direction perpendicular to the link

Coriolis Acceleration: Ac = 2(dr/dt). In a direction perpendicular to the link

Sliding Acceleration: As = d2r/dt2. In the direction of sliding.


A rotating link will produce normal and tangential acceleration components at any

point a distance, r, from the rotational pivot of the link. The total acceleration of

that point is the vector sum of the components.

A slider attached to ground experiences only sliding acceleration.

A slider attached to a rotating link (such that the slider is moving in or out along

the link as the link rotates) experiences all 4 components of acceleration. Perhaps

the most confusing of these is the coriolis acceleration, though the concept of

coriolis acceleration is fairly simple. Imagine yourself standing at the center of a

merry-go-round as it spins at a constant speed (). You begin to walk toward the outer edge of the merry-go-round at a constant speed (dr/dt). Even though you are

walking at a constant speed and the merry-go-round is spinning at a constant

speed, your total velocity is increasing because you are moving away from the

center of rotation (i.e. the edge of the merry-go-round is moving faster than the

center). This is the coriolis acceleration. In what direction did your speed

increase? This is the direction of the coriolis acceleration.

The total acceleration of a point is the vector sum of all applicable acceleration

components:

A = An + A

t + A

c + A

s

These vectors and the above equation can be broken into x and y components by

applying sines and cosines to the vector diagrams to determine the x and y

components of each vector. In this way, the x and y components of the total

acceleration can be found.

9. Graphical Method, Velocity and Acceleration polygons :

Graphical velocity analysis: It is a very short step (using basic trigonometry with sines and cosines) to convert the

graphical results into numerical results. The basic steps are these:

1. Set up a velocity reference plane with a point of zero velocity designated.

2. Use the equation, V = r, to calculate any known linkage velocities. 3. Plot your known linkage velocities on the velocity plot. A linkage that is

rotating about ground gives an absolute velocity. This is a vector that originates at

the zero velocity point and runs perpendicular to the link to show the direction of

motion. The vector, VA, gives the velocity of point A.

4. Plot all other velocity vector directions. A point on a grounded link (such as

point B) will produce an absolute velocity vector passing through the zero velocity

point and perpendicular to the link. A point on a floating link (such as B relative

to point A) will produce a relative velocity vector. This vector will be

perpendicular to the link AB and pass through the reference point (A) on the

velocity diagram.

5. One should be able to form a closed triangle (for a 4-bar) that shows the vector

equation: VB = VA + VB/A where VB = absolute velocity of point B, VA = absolute

velocity of point A, and VB/A is the velocity of point B relative to point A.

10. Velocity Analysis of Four Bar Mechanisms:

Problems solving in Four Bar Mechanisms and additional links.


11. Velocity Analysis of Slider Crank Mechanisms:

Problems solving in Slider Crank Mechanisms and additional links.

12. Acceleration Analysis of Four Bar Mechanisms:

Problems solving in Four Bar Mechanisms and additional links.

13. Acceleration Analysis of Slider Crank Mechanisms:

Problems solving in Slider Crank Mechanisms and additional links.

14. Kinematic analysis by Complex Algebra methods:

Analysis of single slider crank mechanism and four bar mechanism by loop closure equations and complex numbers.

15. Vector Approach:

Relative velocity and accelerations of particles in a common link, relative velocity and accelerations of coincident particles on separate link

16. Computer applications in the kinematic analysis of simple mechanisms:

Computer programming for simple mechanisms

17. Coincident points, Coriolis Acceleration:

Coriolis Acceleration: Ac = 2(dr/dt). In a direction perpendicular to the link.

A slider attached to ground experiences only sliding acceleration.

A slider attached to a rotating link (such that the slider is moving in or out along

the link as the link rotates) experiences all 4 components of acceleration. Perhaps

the most confusing of these is the coriolis acceleration, though the concept of

coriolis acceleration is fairly simple. Imagine yourself standing at the center of a

merry-go-round as it spins at a constant speed (). You begin to walk toward the outer edge of the merry-go-round at a constant speed (dr/dt). Even though you are

walking at a constant speed and the merry-go-round is spinning at a constant

speed, your total velocity is increasing because you are moving away from the

center of rotation (i.e. the edge of the merry-go-round is moving faster than the

center). This is the coriolis acceleration. In what direction did your speed

increase? This is the direction of the coriolis acceleration.


Unit III KINEMATICS OF CAM

Camshaft

For the fictional characters of the same name, see Camshaft (Transformers).

Computer animation of a camshaft operating valves

A camshaft is a shaft to which a cam is fastened or of which a cam forms an integral

part.

CAMPROFILE


Cams: Type of cams, Type of followers, Displacement, Velocity and acceleration time

curves for cam profiles, Disc cam with reciprocating follower having knife edge,

roller follower, Follower motions including SHM, Uniform velocity, Uniform

acceleration and retardation and Cycloidal motion.

Cams are used to convert rotary motion into reciprocating motion. The motion created

can be simple and regular or complex and irregular. As the cam turns, driven by the

circular motion, the cam follower traces the surface of the cam transmitting its motion

to the required mechanism. Cam follower design is important in the way the profile of

the cam is followed. A fine pointed follower will more accurately trace the outline of

the cam. This more accurate movement is at the expense of the strength of the cam

follower.

18. Classifications - Displacement diagrams

Cam Terminology:


Physical components: Cam, follower, spring

Types of cam systems: Oscilllating (rotating), translating

Types of joint closure: Force closed, form closed

Types of followers: Flat-faced, roller, mushroom

Types of cams: radial, axial, plate (a special class of radial cams).

Types of motion constraints: Critical extreme position the positions of the follower that are of primary concern are the extreme positions, with considerable

freedom as to design the cam to move the follower between these positions. This is

the motion constraint type that we will focus upon. Critical path motion The path by which the follower satisfies a given motion is of interest in addition to the extreme

positions. This is a more difficult (and less common) design problem.

Types of motion: rise, fall, dwell

Geometric and Kinematic parameters: follower displacement, velocity,

acceleration, and jerk; base circle; prime circle; follower radius; eccentricity; pressure

angle; radius of curvature.

19. Parabolic, Simple harmonic and Cycloidal motions:

Describing the motion: A cam is designed by considering the desired motion of the follower. This motion is specified through the use of SVAJ diagrams

(diagrams that describe the desired displacement-velocity-acceleration and

jerk of the follower motion)

20. Layout of plate cam profiles:

Drawing the displacement diagrams for the different kinds of the motions and the plate cam profiles for these different motions and different followers.

SHM, Uniform velocity, Uniform acceleration and retardation and Cycloidal motions

Knife-edge, Roller, Flat-faced and Mushroom followers.

21. Derivatives of Follower motion:

Velocity and acceleration of the followers for various types of motions.

Calculation of Velocity and acceleration of the followers for various types of motions.

22. High speed cams:

High speed cams

23. Circular arc and Tangent cams:

Circular arc

Tangent cam

24. Standard cam motion:

Simple Harmonic Motion

Uniform velocity motion

Uniform acceleration and retardation motion

Cycloidal motion

25. Pressure angle and undercutting:

Pressure angle


A camshaft

The relationship between the rotation of the camshaft and the rotation of the

crankshaft is of critical importance. Since the valves control the flow of air/fuel

mixture intake and exhaust gases, they must be opened and closed at the appropriate

time during the stroke of the piston. For this reason, the camshaft is connected to the

crankshaft either directly, via a gear mechanism, or indirectly via a belt or chain

called a timing belt or timing chain. In some designs the camshaft also drives the

distributor and the oil and fuel pumps. Some General Motors vehicles also have the

power steering pump driven by the camshaft. Also on early fuel injection systems,

cams on the camshaft would operate the fuel injectors.


In a two-stroke engine that uses a camshaft, each valve is opened once for each

rotation of the crankshaft; in these engines, the camshaft rotates at the same rate as the

crankshaft. In a four-stroke engine, the valves are opened only half as often; thus, two

full rotations of the crankshaft occur for each rotation of the camshaft.

The timing of the camshaft can be advanced to produce better low end torque or it can

be retarded to produce better high end torque.

Duration

Duration is the number of crankshaft degrees of engine rotation during which the

valve is off the seat. As a generality, greater duration results in more horsepower. The

RPM at which peak horsepower occurs is typically increased as duration increases at

the expense of lower rpm efficiency (torque).

Duration can often be confusing because manufacturers may select any lift point to

advertise a camshaft's duration and sometimes will manipulate these numbers. The

power and idle characteristics of a camshaft rated at .006" will be much different than

one rated the same at .002".

Many performance engine builders gauge a race profile's aggressiveness by looking at

the duration at .020", .050" and .200". The .020" number determines how responsive

the motor will be and how much low end torque the motor will make. The .050"

number is used to estimate where peak power will occur, and the .200" number gives

an estimate of the power potential.

A secondary effect of increase duration is increasing overlap, which is the number of

crankshaft degrees during which both intake and exhaust valves are off their seats. It

is overlap which most affects idle quality, inasmuch as the "blow-through" of the

intake charge which occurs during overlap reduces engine efficiency, and is greatest

during low RPM operation. In reality, increasing a camshaft's duration typically

increases the overlap event, unless one spreads lobe centers between intake and

exhaust valve lobe profiles.


Lift

The camshaft "lift" is the resultant net rise of the valve from its seat. The further the

valve rises from its seat the more airflow can be realised, which is generally more

beneficial. Greater lift has some limitations. Firstly, the lift is limited by the increased

proximity of the valve head to the piston crown and secondly greater effort is required

to move the valve's springs to higher state of compression. Increased lift can also be

limited by lobe clearance in the cylinder head construction, so higher lobes may not

necessarily clear the framework of the cylinder head casing. Higher valve lift can

have the same effect as increased duration where valve overlap is less desirable.

Higher lift allows accurate timing of airflow; although even by allowing a larger

volume of air to pass in the relatively larger opening, the brevity of the typical

duration with a higher lift cam results in less airflow than with a cam with lower lift

but more duration, all else being equal. On forced induction motors this higher lift

could yield better results than longer duration, particularly on the intake side. Notably

though, higher lift has more potential problems than increased duration, in particular

as valve train rpm rises which can result in more inefficient running or loss or torque.

Cams that have too high a resultant valve lift, and at high rpm, can result in what is

called "valve bounce", where the valve spring tension is insufficient to keep the valve

following the cam at its apex. This could also be as a result of a very steep rise of the

lobe and short duration, where the valve is effectively shot off the end of the cam

rather than have the valve follow the cams profile. This is typically what happens on

a motor over rev. This is an occasion where the engine rpm exceeds the engine

maximum design speed. The valve train is typically the limiting factor in determining

the maximum rpm the engine can maintain either for a prolonged period or

temporarily. Sometimes an over rev can cause engine failure where the valve stems

become bent as a result of colliding with the piston crowns.

Position

Depending on the location of the camshaft, the cams operate the valves either directly

or through a linkage of pushrods and rockers. Direct operation involves a simpler

mechanism and leads to fewer failures, but requires the camshaft to be positioned at


the top of the cylinders. In the past when engines were not as reliable as today this

was seen as too much bother, but in modern gasoline engines the overhead cam

system, where the camshaft is on top of the cylinder head, is quite common.

Number of camshafts

Main articles: overhead valve and overhead cam

While today some cheaper engines rely on a single camshaft per cylinder bank, which

is known as a single overhead camshaft (SOHC), most modern engine designs (the

overhead-valve or OHV engine being largely obsoleted from passenger vehicles), are

driven by a two camshafts per cylinder bank arrangement (one camshaft for the

intake valves and another for the exhaust valves); such camshaft arrangement is

known as a double or dual overhead cam (DOHC), thus, a V engine, which has two

separate cylinder banks, may have four camshafts (colloquially known as a quad-cam

engine[6]

).

More unusual is the modern W engine (also known as a 'VV' engine to distinguish

itself from the pre-war W engines) that has four cylinder banks arranged in a "W"

pattern with two pairs narrowly arranged with a 15 degree separation. Even when

there are four cylinder banks (that would normally require a total of eight individual

camshafts), the narrow-angle design allows the use of just four camshafts in total. For

the Bugatti Veyron, which has a 16 cylinder W engine configuration, all the four

camshafts are driving a total of 64 valves.

The overhead camshaft design adds more valvetrain components that ultimately incur

in more complexity and higher manufacturing costs, but this is easily offset by many

advantages over the older OHV design: multi-valve design, higher RPM limit and

design freedom to better place valves, ignition (Spark-ignition engine) and

intake/exhaust ports.

Maintenance

The rockers or cam followers sometimes incorporate a mechanism to adjust and set

the valve play through manual adjustment, but most modern auto engines have

hydraulic lifters, eliminating the need to adjust the valve lash at regular intervals as


the valvetrain wears, and in particular the valves and valve seats in the combustion

chamber.

Sliding friction between the surface of the cam and the cam follower which rides upon

it is considerable. In order to reduce wear at this point, the cam and follower are both

surface hardened, and modern lubricant motor oils contain additives specifically to

reduce sliding friction. The lobes of the camshaft are usually slightly tapered, causing

the cam followers or valve lifters to rotate slightly with each depression, and helping

to distribute wear on the parts. The surfaces of the cam and follower are designed to

"wear in" together, and therefore when either is replaced, the other should be as well

to prevent excessive rapid wear. In some engines, the flat contact surfaces are

replaced with rollers, which eliminate the sliding friction and wear but adds mass to

the valvetrain.

Alternatives

In addition to mechanical friction, considerable force is required to overcome the

valve springs used to close the engine's valves. This can amount to an estimated 25%

of an engine's total output at idle, reducing overall efficiency. Some approaches to

reclaiming this "wasted" energy include:

Springless valves, like the desmodromic system employed today by Ducati

Camless valvetrains using solenoids or magnetic systems have long been

investigated by BMW and Fiat, and are currently being prototyped by Valeo

and Ricardo

The Wankel engine, a rotary engine which uses neither pistons nor valves, best

known for being used by Mazda in the RX-7 and RX-8 sports cars.


Gallery

Components of a typical, four stroke cycle, DOHC piston

engine. (E) Exhaust camshaft, (I) Intake camshaft, (S) Spark

plug, (V) Valves, (P) Piston, (R) Connecting rod, (C)

Crankshaft, (W) Water jacket for coolant flow.

Double overhead cams control the opening and closing

of a cylinder's valves.

1. Intake

2. Compression

3. Power

4. Exhaust

Valve timing gears on a Ford Taurus V6 engine the small

gear is on the crankshaft, the larger gear is on the camshaft.

The gear ratio causes the camshaft to run at half the RPM of

the crankshaft.

Unit IV GEARS

For the gear-like device used to drive a roller chain, see Sprocket.

This article is about mechanical gears. For other uses, see Gear (disambiguation).

Two meshing gears transmitting rotational motion. Note that the smaller gear is

rotating faster. Although the larger gear is rotating less quickly, its torque is

proportionally greater.


A gear is a rotating machine part having cut teeth, or cogs, which mesh with another

toothed part in order to transmit torque. Two or more gears working in tandem are

called a transmission and can produce a mechanical advantage through a gear ratio

and thus may be considered a simple machine. Geared devices can change the speed,

magnitude, and direction of a power source. The most common situation is for a gear

to mesh with another gear, however a gear can also mesh a non-rotating toothed part,

called a rack, thereby producing translation instead of rotation.

The gears in a transmission are analogous to the wheels in a pulley. An advantage of

gears is that the teeth of a gear prevent slipping.

When two gears of unequal number of teeth are combined a mechanical advantage is

produced, with both the rotational speeds and the torques of the two gears differing in

a simple relationship.

In transmissions which offer multiple gear ratios, such as bicycles and cars, the term

gear, as in first gear, refers to a gear ratio rather than an actual physical gear. The

term is used to describe similar devices even when gear ratio is continuous rather than

discrete, or when the device does not actually contain any gears, as in a continuously

variable transmission.

Spur Gears


Miter Gears

Helical Gears


Miter Gears-Helical

Worm Gears


Planetary Gears

Non-Metal Gears

GEAR TRAINS


Gears are used to change speed in rotational movement.

In the example above the blue gear has eleven teeth and the orange gear has twenty

five. To turn the orange gear one full turn the blue gear must turn 25/11 or 2.2727r

turns. Notice that as the blue gear turns clockwise the orange gear turns anti-

clockwise. In the above example the number of teeth on the orange gear is not

divisible by the number of teeth on the blue gear. This is deliberate. If the orange gear

had thirty three teeth then every three turns of the blue gear the same teeth would

mesh together which could cause excessive wear. By using none divisible numbers

the same teeth mesh only every seventeen turns of the blue gear.

26. Spur gear Terminology and definitions:

Spur Gears:

External

Internal

Definitions

27. Fundamental Law of toothed gearing and Involute gearing:

Law of gearing

Involutometry and Characteristics of involute action


Path of Contact and Arc of Contact

Contact Ratio

Comparison of involute and cycloidal teeth

28. Inter changeable gears, gear tooth action, Terminology:

Inter changeable gears

Gear tooth action

Terminology

29. Interference and undercutting:

Interference in involute gears

Methods of avoiding interference

Back lash

30. Non standard gear teeth: Helical, Bevel, Worm, Rack and Pinion gears (Basics only)

Helical

Bevel

Worm

Rack and Pinion gears

Worm

Worm gear

Worm gears resemble screws. A worm gear is usually meshed with an ordinary

looking, disk-shaped gear, which is called the gear, wheel, or worm wheel.

Worm-and-gear sets are a simple and compact way to achieve a high torque, low

speed gear ratio. For example, helical gears are normally limited to gear ratios of less

than 10:1 while worm-and-gear sets vary from 10:1 to 500:1.[ A disadvantage is the

potential for considerable sliding action, leading to low efficiency.

Worm gears can be considered a species of helical gear, but its helix angle is usually

somewhat large (close to 90 degrees) and its body is usually fairly long in the axial


direction; and it is these attributes which give it its screw like qualities. The

distinction between a worm and a helical gear is made when at least one tooth persists

for a full rotation around the helix. If this occurs, it is a 'worm'; if not, it is a 'helical

gear'. A worm may have as few as one tooth. If that tooth persists for several turns

around the helix, the worm will appear, superficially, to have more than one tooth, but

what one in fact sees is the same tooth reappearing at intervals along the length of the

worm. The usual screw nomenclature applies: a one-toothed worm is called single

thread or single start; a worm with more than one tooth is called multiple thread or

multiple start. The helix angle of a worm is not usually specified. Instead, the lead

angle, which is equal to 90 degrees minus the helix angle, is given.

In a worm-and-gear set, the worm can always drive the gear. However, if the gear

attempts to drive the worm, it may or may not succeed. Particularly if the lead angle is

small, the gear's teeth may simply lock against the worm's teeth, because the force

component circumferential to the worm is not sufficient to overcome friction. Worm-

and-gear sets that do lock are called self locking, which can be used to advantage, as

for instance when it is desired to set the position of a mechanism by turning the worm

and then have the mechanism hold that position. An example is the machine head

found on some types of stringed instruments.

If the gear in a worm-and-gear set is an ordinary helical gear only a single point of

contact will be achieved. If medium to high power transmission is desired, the tooth

shape of the gear is modified to achieve more intimate contact by making both gears

partially envelop each other. This is done by making both concave and joining them at

a saddle point; this is called a cone-drive.

RACK AND PINION WORM GEAR


RACK AND PINION: The rack and pinion is used to convert between rotary and

linear motion. The rack is the flat, toothed part, the pinion is the gear. Rack and pinion

can convert from rotary to linear of from linear to rotary. The diameter of the gear

determines the speed that the rack moves as the pinion turns. Rack and pinions are

commonly used in the steering system of cars to convert the rotary motion of the

steering wheel to the side to side motion in the wheels. Rack and pinion gears give a

positive motion especially compared to the friction drive of a wheel in tarmac. In the

rack and pinion railway a central rack between the two rails engages with a pinion on

the engine allowing the train to be pulled up very steep slopes.

WORM GEAR: A worm is used to reduce speed. For each complete turn of the

worm shaft the gear shaft advances only one tooth of the gear. In this case, with a

twelve tooth gear, the speed is reduced by a factor of twelve. Also, the axis of rotation

is turned by 90 degrees. Unlike ordinary gears, the motion is not reversible, a worm

can drive a gear to reduce speed but a gear cannot drive a worm to increase it. As the

speed is reduced the power to the drive increases correspondingly. Worm gears are a

compact, efficient means of substantially decreasing speed and increasing power.

Ideal for use with small electric motors.

31. Gear trains:

Gear Train Basics

The velocity ratio, mV, of a gear train relates the output velocity to the input velocity.

For example, a gear train ratio of 5:1 means that the output gear velocity is 5 times the input gear velocity.

32. Parallel axis gear trains:

Simple Gear Trains A simple gear train is a collection of meshing gears where each gear is on its own axis. The train ratio for a simple gear train is

the ratio of the number of teeth on the input gear to the number of teeth on the

output gear. A simple gear train will typically have 2 or 3 gears and a gear

ratio of 10:1 or less. If the train has 3 gears, the intermediate gear has no

numerical effect on the train ratio except to change the direction of the output

gear.

Compound Gear Trains A compound gear train is a train where at least one shaft carries more than one gear. The train ratio is given by the ratio mV =

(product of number of teeth on driver gears)/(product of number of teeth on

driven gears). A common approach to the design of compound gear trains is to

first determine the number of gear reduction steps needed (each step is

typically smaller than 10:1 for size purposes). Once this is done, determine


the desired ratio for each step, select a pinion size, and then calculate the gear

size.

Reverted Gear Trains A reverted gear train is a special case of a compound gear train. A reverted gear train has the input and output shafts in line with one another. Assuming no idler gears are used, a reverted gear train can be

realized only if the number of teeth on the input side of the train adds up to the

same as the number of teeth on the output side of the train.

33. Epicyclic gear trains:

If the axis of the shafts over which the gears are mounted are moving relative

to a fixed axis , the gear train is called the epicyclic gear train.

Problems in epicyclic gear trains.

34. Differentials:

Used in the rear axle of an automobile.

To enable the rear wheels to revolve at different speeds when negotiating a curve.

To enable the rear wheels to revolve at the same speeds when going straight.

Rack and pinion

Rack and pinion gearing

A rack is a toothed bar or rod that can be thought of as a sector gear with an infinitely

large radius of curvature. Torque can be converted to linear force by meshing a rack

with a pinion: the pinion turns; the rack moves in a straight line. Such a mechanism is

used in automobiles to convert the rotation of the steering wheel into the left-to-right

motion of the tie rod(s). Racks also feature in the theory of gear geometry, where, for

instance, the tooth shape of an interchangeable set of gears may be specified for the

rack (infinite radius), and the tooth shapes for gears of particular actual radii then

derived from that. The rack and pinion gear type is employed in a rack railway.


Epicyclic

Epicyclic gearing

In epicyclic gearing one or more of the gear axes moves. Examples are sun and planet

gearing (see below) and mechanical differentials.

Sun and planet

Sun (yellow) and planet (red) gearing

Main article: Sun and planet gear

Sun and planet gearing was a method of converting reciprocal motion into rotary

motion in steam engines. It played an important role in the Industrial Revolution. The

Sun is yellow, the planet red, the reciprocating crank is blue, the flywheel is green and

the driveshaft is grey.


Harmonic drive

Harmonic drive gearing

A harmonic drive is a specialized proprietary gearing mechanism.

Cage gear

A cage gear, also called a lantern gear or lantern pinion has cylindrical rods for teeth,

parallel to the axle and arranged in a circle around it, much as the bars on a round bird

cage or lantern. The assembly is held together by disks at either end into which the

tooth rods and axle are set.

Nomenclature

General nomenclature

Rotational frequency, n

Measured in rotation over time, such as RPM.

Angular frequency,

Measured in radians per second. 1RPM = / 30 rad/second

Number of teeth, N

How many teeth a gear has, an integer. In the case of worms, it is the number

of thread starts that the worm has.

Gear, wheel

The larger of two interacting gears.

Pinion

The smaller of two interacting gears.


Path of contact

Path followed by the point of contact between two meshing gear teeth.

Line of action, pressure line

Line along which the force between two meshing gear teeth is directed. It has

the same direction as the force vector. In general, the line of action changes

from moment to moment during the period of engagement of a pair of teeth.

For involute gears, however, the tooth-to-tooth force is always directed along

the same linethat is, the line of action is constant. This implies that for

involute gears the path of contact is also a straight line, coincident with the

line of actionas is indeed the case.

Axis

Axis of revolution of the gear; center line of the shaft.

Pitch point, p

Point where the line of action crosses a line joining the two gear axes.

Pitch circle, pitch line

Circle centered on and perpendicular to th

kinematics of machinery

Documents

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mechanisms unit

theory of mechanisms

machine theory

reciprocating engine

watt mechanisms

basics of mechanisms

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