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Machine and Mechanisms

Introduction to mechanismsand Kinematics

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Introduction

Determine appropriatemovement of the wipers

View range

Tandem or opposite

Wipe angle

Location of pivots

Timing of wipers

Wiping velocity

The force acting on the

machine

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Machines and mechanisms

Machine

A Combination ofresistant bodies soarranged that by theirmeans the mechanical

forces of nature can becompelled to do workaccompanied by certaindeterminate motion.

Devices used to alter,

transmit, and directforces to accomplish aspecific objective

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Mechanism An assemblage of resistant

bodies, connected bymovable joints, to form aclosed kinematics chain

with one link fixed andhaving the purpose oftransforming motion.

Mechanical portion of amachine that has the

function of transferringmotion and forces from apower source to an output

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Structure

Combination of resistant bodiesconnected by joints, but itspurpose is not to do work ortransform motion. A structure isintended to be rigid.

Linkage

Consist of links where rigidbodies are connected by jointssuch as pins to form open orclosed chains.

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Kinematics

Kinematics Deal with the way things move

Kinematic analysis Determine

Position, displacement, rotation, speed, velocity,acceleration

Provide

Geometry dimensions of the mechanism

Operation range

Dynamic analysis Power capacity, stability, member load

Planar mechanism motion in 2D space

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Mechanism terminology

Linkage

Frame

Links rigid body

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LINK OR ELEMENT

Any body (normally rigid) which has motion relative

to another

Binary link

Ternary link

Quaternary link

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Joint

A connection between two or more links (at theirnodes) which connection allows some motion, orpotential motion, between the connected links

Lower pair Joints

Joints with surface contact between the pairelements

Higher pair joints

Joints with point or line contact between the pairelements.

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Mechanism terminology cont

Joint

Primary joint (full joint) Revolute joint (pin or

hinge joint)pure

rotation Sliding joint (piston or

prism joint) linearsliding

Higher-order joint (halfjoint) Allow rotation and sliding

Cam joint

Gear connection

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Based on the nature of mechanicalconstraint

(a) Closed pair

(b) Unclosed or force closed pair

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Example for Mechanism

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Mechanism terminology cont

Simple link A rigid body contains

only two joints

Crank

Rocker

Complex link A rigid body contains

more than two joints

Rocker arm

Bellcrank

Point of interest Actuator

A power source link

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Kinematic diagrams

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Kinematic diagrams cont

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Example 1.1

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Example 1.1 cont

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Methodology

1. Identify the Frame

2. Identify all other links

3. Identify the joints4. Identify any points of interest

5. Draw the kinematics diagram

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Example 1.2

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Example 1.2cont

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Kinematics inversion

The selection of frame is arbitrary

For different frame, the relative motion is thesame

Selection of a different frame is termedkinematics inversion

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Degrees of freedom/mobility of amechanism

It is the number of inputs (number of

independent coordinates) required to describethe configuration or position of all the links ofthe mechanism, with respect to the fixed link atany given instant.

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mobility

Gruebler Condition,

F = 3n 2j 3G

In any real mechanism, if more than one link ofthe kinematic chain is grounded, the net effect willcreate one larger, higher-order ground link, asthere can be only one ground. Thus,

F = 3(n-1) -2j

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mobility

Mobility the degrees of freedom of amechanism (F) or (M)

Kutzbachs equation

hPjjnfreedomofdegreesF !! 2)1(3

n = total number of links in the mechanism

jP = total number of primary joints

jh = total number of high-order joints

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Mobility cont

Constrained mechanism one degree of freedom

Locked mechanism

Zero or negative degrees of freedom Unconstrained mechanism

More than one degree of freedom

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Examples - DOF

F = 3(n-1)-2l-h

Here, n = 4, l = 4 & h = 0.

F= 3(4-1)-

2(4) = 1

I.e., one input to any one link will result

in definite motion of all the links.

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F = 3(n-1)-2l-h

Here, n = 5, l = 5 and h = 0.

F = 3(5-1)-2(5) = 2

I.e., two inputs to any two links are required to yield

definite motions in all the links.

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Example 1.3

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Example 1.3cont

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Example 1.3cont

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Example 1.4

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Example 1.4cont

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Example 1.4cont

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Example 1.5

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Example 1.5cont

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Example 1.5cont

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Actuators and drivers

Electric motors (AC)

Electric motors (DC)

Engines

Servomotors Air or hydraulic motors

Hydraulic or pneumatic cylinders

Screw actuators

Manual

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Example 1.6