Lecture - 1 & 2.pptx of mechanical engineering subject Mechanics of Machines

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MECHANICS OF MACHINES
LECTURE 1 & 2

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COURSE INSTRUCTOR:
Waqas Asghar
Lecturer,
Mechanical Department, UET Taxila
PhD. (2017-2021): University of Chinese Academy of Sciences, China.
MSc. (2013-2016): University of Engineering & Technology, Taxila.
BSc. (2008-2012): University of Engineering & Technology, Taxila.
Email: waqas.asghar@uettaxila.edu.pk
Office: Faculty wing MED

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COURSE CONTENTS:
• Links & Mechanism
• Degree of Freedom
• Number Synthesis
• Inversion +Linkage Transformation
• Synthesis of 4-bar Mechanism
• Kinematic Analysis (Displacement, Velocity, Acceleration)
• Gear Trains
• Cam Design
• Mechanical Governors

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GRADING POLICY:
• Quizzes or Assignments: 12%
• Semester Project or CEP: 13%
• Mid-semester Exam: 25%
• Final Exam: 50%

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COURSE BOOKS:
• Design of Machinery by Robert L. Norton, 6th
Edition
• Machines & Mechanisms by David H. Myszka, 4th
Edition.
Reference Books:
• Theory of Machines & Mechanisms, 5th
edition by John,
Pennock, Shingley
• Theory of Machines by R Khurmi, 14th
edition.
• Theory of Machines by S S Rattan, 3rd
edition.

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MECHANICS
Branch of Physics that describes and predicts the conditions of rest or
motion of bodies under the action of forces.
(1) Statics:
Branch of mechanics which deals with the forces and their effects while the machine
parts are at rest
(2) Dynamics
Branch of mechanics which deals with the forces and their effects, acting on machine
parts which are in motion
BRANCHES OF MECHANICS:

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BRANCHES OF DYNAMICS
(2) Kinetics
Study of effect of forces (or torques) on systems in motion.
Especially those forces are important, which do not originate within the system itself
(e.g.to increase/decrease the speed of body or to give motion to static object.)
(1) Kinematics
Branch of mechanics which deals with the study of relative motion between the
various parts of the machines without regard to forces.
It is independent of size,shape,strength and nature of material used in machine parts.
Basic aim is to:
>> design the desired motions of mechanical parts then
>> mathematically computing the position,velocities & accelerations,which
those motions will create on parts.

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DEFINITION OF MECHANICS OF MACHINES
Branch of Mechanical Engineering that deals with the study of
relative motion between the various parts of a machine and forces
which act on them
PURPOSE OF COURSE:
 To explore the topics of Kinematics and Dynamics of machinery in respect to:
 Synthesis of mechanism to accomplish desire motion or task, and
 Analysis of mechanisms to determine their rigid body dynamic behavior.

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MACHINES AND MECHANISMS
 Mechanism: A system of moving parts connected together for the purpose of
transmitting forces or controlled output motion.
 Machine: Collection of mechanism arranged to transmit forces and to perform
some useful work.
 Analysis: Study of motion of various machine parts and forces acting on them is
called analysis.
 Synthesis: Designing the size, shape, strength and type of material of machine
parts is called synthesis.

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EXAMPLES OF MECHANISMS
Door Hinge Mechanism Automatic Door
Closer Mechanism
Folding Chair
Mechanism

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Jaw Crusher Machine
Can Crusher Machine Simple press

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Scissor Lift Platform Exercise Machine
Folding Fence
Mechanism

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Electric Staircase Gear Trains

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Excavator Front End Loader Cam Mechanism

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Hood Hinge
Mechanism
Wind Screen
Wiper
Car Steering Wheel
Mechanism

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Oil Well Pump Mechanism

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Aircraft Landing Gear
Mechanism
Conveyer Belt

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Spider Robot Industrial Robotic
Arm
Industrial Robotic
Manufacturing

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MECHANISMS GENERALLY CONSIST OF
a) Moving components such as
1. gears and gear trains
2. belt and chain drives
3. cam and follower mechanisms
4. linkages
b) Frictional devices such as
a) brakes
b)clutches
c) Structural components such as
a) frame and fasteners
b)bearing
c) springs
d)lubricants and seals
d) Specialized machine elements such as
a) splines
b)pins & keys

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MACHINES & THEIR IMPORTANCE
Machinery is a central part of life today.
Drill
Tractor
Car Tram
Exercise Bicycle

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DEFINITIONS
Machine:
Collection of mechanism arranged to transmit forces and to perform some
useful work.
Structure:
An assemblage/combination of various members having no relative motion
between them and are supposed to carry loads
 e.g.A railway bridge, a roof truss, machine frames etc.
Machine Structure
Relative motion b/w parts of m/c No relative motion b/w members
Transforms energy into some useful work No work done
Transmit forces & motion Transmit only forces

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VARIOUS STRUCTURES
Vehicle Frame High Rise Building Bridge Supporting
Roadway
Residential Dwelling
Structures can be seen all around us in our everyday lives.

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STRUCTURE OR MECHANISM?
Please identify Structure
and Mechanism within
this bicycle?
Frame = Structure
Chain and Sprocket = One
example of a mechanism

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INTRODUCTION
2.Wheelbarrow
1.Wrench
In each item, please identify the mechanisms and/or structure ?
3. Entrance 4. Moving
Hacksaw

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INTRODUCTION
Rack and Pinion Pulley Wheels Cam and Follower
Moving parts of machine are called –Mechanisms

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MECHANISMS AND MOTION
Mechanism
A system of moving parts that
performs some function (Collins
Dictionary).
Motion
The process of continual change
in the position of an object
(Collins Dictionary).

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TYPES OF MOTION
1. Plane Motion: Motion of a body confined to only one plane is called plane
motion. It has two types:
1. Rectilinear or Translatory Motion: Motion of body along a straight line path.
2. Curvilinear Motion: Motion of body along a curved path.
(Curvilinear motion confined to only 1-plane is called plane curvilinear
motion)
2. Oscillating (Periodic) Motion: Motion repeating itself or To & fro motion of an
object about its mean position.
3. Reciprocating Motion = Linear repetitive motion in up & down or back & forth
direction. E.g. sewing m/c needle, reciprocation engine, reciprocating pumps,
shaper machine etc.
4. Rotary or Circular Motion: Motion in which object spin or rotates about the axis
of rotation. E.g. earth rotation, wheels of moving car, fan blades, Gears, drill
machine, blender, Helicopter rotor blades. Etc.

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SPECIFY THE TYPE OF MOTION?
Shaft motion Power Hacksaw
Chuk of drill
m/c
Engine’s Piston & Valves Wheels of bicycle
Moving Swing
Pendulum
Moving
Train

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TYPES OF MOTION W.R.T MMC
 Pure Rotation
The body possesses one point i.e. center of rotation that has no motion with
respect to the "stationary" frame of reference. E.g. Shaft rotation, windmill.
 Pure Translation
All points on the body describe parallel (curvilinear or rectilinear) paths. E.g. Car
moving in straight line
 Complex Motion (Rotation + Translation)
Simultaneous combination of rotation and translation. E.g. wheels of a straightly
moving car, sphere rolling on ground (but not slipping).
Translation and rotation represent independent motions of the body. Each can exist
without the other.

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DEGREE OF FREEDOM (DOF) OR MOBILITY OF
MECHANICAL SYSTEM
 Number of independent coordinates (parameters) required to define the position
of Mechanical system.
OR
 Number of inputs that need to be provided in order to create a predictable output
 DOF is always defined w.r.t a selected frame of reference.

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DEGREE OF FREEDOM (DOF) OR MOBILITY

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DEGREE OF FREEDOM (DOF) OR MOBILITY
Rigid body in plane motion has 3DOF & any rigid body in 3D space has 6DOF.
How to define position of pencil in 2D?
2 linear coordinates (x and y) and 1 angular coordinate
(θ)= 3 DOF
How to define position of pencil in 3D?
Three lengths (x, y, z) and three angles (Φ, θ, ρ) =6 DOF

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SUMMARY OR FLOW CHART OF MACHINE

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DEFINITIONS
 Machine
 Collection of mechanism arranged to transmit forces and to perform some useful
work.
 Mechanism
 A kinematic chain having at least 1 link fixed to ground.
e.g. (1) In reciprocating engine, cylinder is ground
 Kinematic Chain
 Group of moving links connected together for the purpose of transmitting forces
or controlled output motion.

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LINKAGES AND LINKS
 Linkages or Kinematic Linkages
 Basic building blocks of all the mechanisms
 Linkages are made up of links and joints
 Kinematic Pairs of Joints
 Connection point b/w two or more links that allows relative motion
 Link
 Each part of machine which moves relative to some other part is known as link
or kinematic link or element.
 All the mechanical components like, cams, gears, belts, chains are represented
as a link.
 Link should be resistant or rigid enough to transmit the required forces with
negligible deformation.
 Links may be rigid, flexible or fluid type

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TYPES OF LINKS
 1. Rigid link: Link which does not undergoes any deformation while transmitting
motion. Strictly speaking, rigid links do not exist.
E.g. Crank shaft, piston etc.
 2. Flexible link: Link which undergoes partial deformation without affecting the
transmission of motion.
E.g. belts, ropes, chains, wires, springs etc.
 3. Fluid link: A fluid link contains the fluid inside the container and the motion is
transmitted through the fluid by pressure or compression only.
E.g. hydraulic presses, hydraulic jacks, hydraulic cranes, and brakes.

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TYPES OF LINKS
Rigid link
Flexible link
Fluid link

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LINKS
Node: Connection
points of any link are
called nodes
Link are assumed
to be incapable of
deformation and
massless for the
sake of kinematic
analysis

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CALCULATE THE NO. OF LINKS?
Binary = 2, 3, 4, 5
Ternary = 6, 1
Total = 6

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DEFINITION OF KINEMATIC PAIRS OF JOINTS
Joint or Kinematic pairs
 Connection point b/w two or more links that allows relative motion.
 Motion of Joints: Joints can allow rotation, translation or both between the
links joined

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EXAMPLE OF LINKS & JOINTS IN HUMAN ARM

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EXAMPLE OF LINKS & JOINTS IN ROBOTIC ARM

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LINKS & JOINTS IN QUADRUPED ROBOT

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SUMMARY OR FLOW CHART OF MACHINE

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CLASSIFICATION OF JOINTS
1. Type of contact between the links
a) Lower Pair (surface contact)
b) Higher Pair (line or point)
2. Number of degrees of freedom or nature of relative motion
a) Revolute (R) or Pin joint (Rotating)
b) Prismatic (P) or Slider Joint
c) Helical (H) Joint
d) Cylindric (C) Joint
e) Spherical (S) Joint
f) Planar (F) Joint
3. Type of physical closure of the joint
a) force closed
b) form closed
4. Number of links joined (Order of the joint)
a) First order joint
b) Second order joint etc.
Lower Pairs

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1. Type of contact between the elements
I. Lower pair
 Joint in which links are joined by surface contact
II. Higher pair
 Joint in which links are connected by point or line contact

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DIFFERENCE BETWEEN LOWER & HIGHER PAIRS
Lower Pairs Higher Pairs
Surface or Area contact Line or Point Contact
Similar contact surfaces Dissimilar contact surfaces
Pure sliding or turning Partly sliding or turning
e.g. sliding pair, turning pair, screw
pairs, universal joint, book sliding on
table etc.
e.g. cam & follower, gear drive,
wheel rolling on a road etc.

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Line contact Line contact
Surface contact

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(i) Revolute (R) or Pin joint (Rotating)
 1 DOF Joint
 Usable in planar mechanisms
 Examples
A shaft with collars at both ends, fitted into a circular
hole
the crankshaft in a journal bearing in an engine
lathe spindle supported in head stock
cycle wheels turning over their axles etc.
JOINTS CLASSIFICATION: SIX LOWER PAIRS

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(ii) Prismatic (P) or Slider Joint (Translating)
 1 DOF
 usable in planar mechanisms
 Examples:
 piston and cylinder,
 cross-head and guides of a reciprocating steam
engine,
 ram and its guides in shaper,
 tail stock on the lathe bed etc.

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(iii) Helical (H) Joint
 Anything having threads
 1 DOF
 contains RP
 Examples:
Lead screw of a lathe with nut,and
Bolt with a nut are examples of a screw pair

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(iv) Cylindric (C) Joint
 Similar to helical but without threads
 2 DOF
 contains RP
 Examples:
shaft inside the bearing
In prismatic we have only translation but in cylindric
we may have translation + rotation.

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(v) Spherical (S) Joint
 Rotation about 3 axis.
 Contains RRR
 3 DOF
 Example:
The ball and socket joint,
attachment of a car mirror,
pen stand etc.

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(vi) Planar (F) Joint
 3 DOF
 Contains RPP
 Capable of both rotation and translation

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Planar Joint
Prismatic Joint
Spherical Joint
Helical Joint
Revolute Joint
Cylindric Joint

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Interesting
• Motion of either the nut or the screw w.r.t each other is the result of helical motion.
• If the helix angle is made 0º, the nut rotates without advancing and it becomes the
pin joint.
• If the helix angle is made 90º, the nut will translate along the axis of the screw, and
it becomes the slider joint.
• Revolute (R) and the prismatic (P) Pairs are basic building blocks of all other pairs
• Revolute (R) and the prismatic (P) Pairs are used for planar mechanisms
• Remaining all the pairs are combination of revolute or prismatic joints & are used
in 3d mechanisms.

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3. Number of DOFs allowed at the joint
I. Full joints
 Those joints having 1 DOF
 Example: pin joint, translating/prismatic joint
II. Half Joints
 Those joints having more than 1 DOF
 It is also known as roll-slide joint
 Example: spherical joint

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4. Type of physical closure of the joint
I. Form-closed
 Joint which is kept together or closed by its geometry.
 e.g. pin in a hole
II. Force-closed
 Joint which is kept together by some external force
 (gravity, spring or any external means).
 e.g. cam-follower systems

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JOINT B/W THE ROAD AND THE WHEEL OF A CAR
• Wheel of straightly moving
car has 1DOF because
translatory motion is
dependent on rotary motion
of wheel.
• Automobile Tire rolling on
road is pure rolling joint.
• This joint converts to pure
sliding joint, when brakes
are applied.
• On icy road it may convert to
roll-slide joint.
Lower Pair or Higher Pair?
• Contact type= Line contact
• Higher Pair
Half Joint or Full Joint?
• Full Joint: If only roll or slide then
1DOF
• Half Joint: If road is frictionless, then
rolling + sliding both occur
independently of each oher then
2DOF (R+P)
• Friction determines the actual DOF
of this kind of joint.
Force Close or Form Close Joint?
• Force close (weight of car)
• If any force higher than car weight is
applied the joint or contact will be
removed.

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4. Order of the Joint
 Order of Joint = Number of links joined (L) – 1 = L - 1
 Plays a key role in determining the overall DOF of mechanisms
 Single joint is a combination of 2 links
 As additional links are placed on the same joint, the joint order is increased on a
one-for-one basis

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