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ME 202 Mechanical Engineering for Civil Engineers
Kinematics of Simple Machines Asanga Ratnaweera
Dept of Mechanical Engineering
ME 201: Mechanics of Machines I
Kinematics of Simple Machines Asanga Ratnaweera
Dept of Mechanical Engineering
23 October 2006 Asanga Ratnaweera, Department of Mechanical Engineering
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Kinematics of Simple Machines
What is mechanics:
Mechanics
Static Dynamics
Kinematics Kinetics
23 October 2006 Asanga Ratnaweera, Department of Mechanical Engineering
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Kinematics of Simple Machines
What is Kinematics:Kinematics deals with the study of motion without the reference to the forces which cause or are developed by the motion.[Deals with the relative motion between the parts of the machine]
What is Kinetics:Kinetics deals with the forces that produce motion[deals with both motion and the forces]
23 October 2006 Asanga Ratnaweera, Department of Mechanical Engineering
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Kinematics of Simple MachinesWhat is a machine:
Machines are used to convert energy into different forms and transmit energy. Machines are composed of different types of mechanisms.
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Kinematics of Simple MachinesSimple mechanisms:
A mechanism is a mechanical device used to transfer motion, force and power from a source to an output.
Ex: cams, gears, and pulleys & belts, Linkages.
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Kinematics of Simple MachinesLink:
A rigid body having two or more pairing elements which connect to other bodies for the purpose of transmitting force or motion
Link
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Kinematics of Simple MachinesCommon types of links
Binary link: two nodesTernary link: thee nodes
quaternary link: four nodes
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Kinematics of Simple MachinesKinematic link
A resistant body or an assembly which goes to make a part of a machine connecting other parts which have motion relative to it.
Rigid link
Kinematic link
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Kinematics of Simple MachinesKinematic chain
A combination of links and pairs without a fixed link.
No element is fixed to the ground or frame
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Kinematics of Simple MachinesJoint
In general called a kinematic pair, providing the connection between links that permit constrained relative motion. There are two types of kinematic pairs: Lower pair, Higher pair.
Joints
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Kinematics of Simple MachinesLower pair
Have surface contact between the elemental surfaces.
Revolute (pin) joints prismatic joints
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Kinematics of Simple MachinesLower pair
Helical joints
Cylindrical joint
Universal joint
Spherical joint
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Kinematics of Simple MachinesHigher pair
Have line and point contact between the pairing elements.
CAM Follower Gears
Pin and slot
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Kinematics of Simple MachinesLinkages
Refer to kinematic chains joined by only lower pairs.
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Kinematics of Simple MachinesMechanism
A set of moving parts in a machine that performs a task. Mechanism consists of linkages and joints
Fixed
MechanismsNo fixed link: not a mechanismDoes not perform a task
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LinkagesSome examples
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LinkagesKinematic (skeleton) diagram
It is used to represent the essential skeleton (structure) of a mechanism, (similar to electrical circuit diagram).Basic symbols
Lines or simple shapes are used to represent links, which are labeled by numbers.Circles or blocks are used to represent joints, which are labeled by letters (capital).
The terms such as space diagram and configuration diagram are also used for kinemtaic diagram
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LinkagesKinematic (Skeleton) diagram
Skeleton diagram
A
B
C
1
2
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Fixed
FixedCrank
Connecting rod
Piston
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LinkagesKinematic (Skeleton) diagram
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LinkagesKinematic (Skeleton) diagram
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LinkagesKinematic (Skeleton) diagram
Stewart platform
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LinkagesDegree of Freedom (DOF)
Number of independent movements a rigid body has
In 2D plane
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LinkagesDegree of freedom
Revolute Joint1 DOF(2 DOF restricted)Slider Joint
1 DOF(2 DOF restricted)
Pin in slot2 DOF(1 DOF restricted)
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LinkagesDegree of Freedom (DOF) of a linkage
Gruebler’s Equation
212)1(3 PPnDOF −−−=
wheren : Total number of links (including a fixed or single ground link)P1 : Total number of lower pairsP2 : Total number of higher pairs
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LinkagesDegree of Freedom (DOF) of a linkage
Ex: three bar linkageB
CA
Ex: Four bar linkage
A
B
C
D
Ex: Five bar linkage
A
B
C
D
E
212)1(3 PPnDOF −−−=
n = 3P1 = 3P2 = 0DOF = 0
n = 4P1 = 4P2 = 0DOF = 1
n = 5P1 = 5P2 = 0DOF = 2
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Four bar mechanism
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Four bar linkagesSlider crank mechanism
Skeleton diagramA
B
C
1
2
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Fixed
FixedCrank
Connecting rod
PistonFrame
This is a four bar mechanism
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Four bar linkages
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Four bar linkages
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Four bar linkages
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Four bar linkagesLinkage geometry
Class I : Shortest member can complete a full revolution
Class II :No link can make a full revolution
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Four bar linkagesLinkage geometry
Class I: a – b < c – d
Class II: a – b > c – d
a
cd
b
dcba +>+
ba >dc >
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Kinematic Analysis of Mechanisms
Analytical methodsDifficult when the motion is complicated
Graphical MethodsMajor concern of this course
Computer methodsBeyond the scope