dynamic of machinary

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Chapter 1Introduction

Dr. Aria Alasty, Sharif U. of Tech.

Mechanics

Statics Dynamics

Kinematics Kinetics


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Mechanism:Transforms motion to some desirable pattern.Develops very low forces.Transmits little power.

Machine:Contains mechanisms which are designed to provide significant forces and transmits significant power.


Link:An rigid body which possesses at least two nodes.

Node:Point for attachment to other links.

Binary link Ternary link Quaternary link

Nodes

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Joint:A connection between two or more links

a) Classification of joints:a) By the number of degrees of freedom.b) By the type of contact between the elements.c) By the type of physical closure of the joint.d) By the number of links joined.


One DOFThree DOFOne or Two DOF


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Lower pair (Full joint):Joints with surface contacts

Higher pair(Half joint):Joints with point or line contact


Form closed:Is kept together or closed By its geometry.

Force closed:Requires some externalforce to keep it together or closed.


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Second order pin joint

First order pin joint


Kinematic chain:An assemblage of links and joints,interconnected in a way to provide a controlled outputmotion In response to a supplied input motion.

Open kinematic chain Closed kinematic chain


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Constrained kinematic chain: A kinematic chain which one link has been grounded.

Mechanism: A closed constrained kinematic chain.


Inversion


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Planar mechanism:All particlesdescribe plane curves in space andall these curves lie in parallel planes.

Spatial mechanism:No restrictionon the relative motion of the particles.


Spherical mechanism: Each link has some point whichremains stationary as the linkage moves and in which thestationary points of all links lie at a common location.the locus of each point is a curve contained in a sphericalsurface , and the spherical surfaces defined by severalarbitrarily chosen points , are all concentric.


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Degree of freedom(DOF):The number of inputs which need to be provided in order to create a predictable output.

1

2

J :J :J :

J :Number of single degree of freedom pairs.

J :Number of two degree of freedom pairs.

n:Number of links.

3

4

5


Kutsbach criterion:


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Grublers criterion

Grublers criterion:


Some examples:

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Mechanism DOF=+1 Structure DOF=0

Preloaded structure DOF= -1 Dr. Aria Alasty, Sharif U. of Tech.

Paradoxes :Because the kutzbach criterion pays no attention to link sizes or shapes it can give misleading results in the face of unique geometric configurations .

Kutsbach predicts DOF=0DOF=0

Same connections




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4 binaries

and

2 ternaries

Isomers

Isomers

Watts sixbar isomer

Stephensons sixbar isomer An invalid sixbar isomerDr. Aria Alasty, Sharif U. of Tech.

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Introduction to some famous mechanisms


Intermittent motion:A sequence of motion and dwells.

Dwell :A period in which the output link remainsstationary while the input link continues to move.

Geneva mechanism.


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Cam and follower mechanism


Swinging or rocking mechanisms:The out put memberrocks or swings through angles which are generally less Than 360.o


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Reciprocating mechanisms:

FOffset

Workingstroke

Returnstroke


Time ratio =Q= Time of advance strokeTime of return stroke

Quick return mechanismsWith high Q


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The Grashof condition


In order for any four-bar linkageto operate,the following conditions must exist:

A

B

D C

AB < BC + CD + DABC < CD + DA + ABCD < DA + AB + BCDA < AB + BC + CD


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S

P

L

Q

S = Length of shortest linkL = Length of longest linkP = Length of one remaining linkQ = Length of other remaining link

L + S < P + QOne of the links will rotate

continuously relative tothe other three links .


L+S=P+Q

L+S=P+Q

L+SP+Q


dynamic of machinary

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