Chapter 2 Content

1. The Four-bar Mechanisms

2. Slider-crank Mechanisms
3. Straight-line Mechanisms
4. Parallelogram Mechanisms
5. Quick-Return Mechanisms
6. Scotch Yoke Mechanisms
7. Intermittent Motion Mechanisms
– The Geneva Mechanism
– Ratchet and Paul Mechanism
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 Common Mechanisms
The four-bar Mechanism
– The simplest and most common linkage.
– It is a combination of four links, one being
designated as the frame and connected by four
pin joints.

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 Example 1: Automotive rear-window
wiper system
• a four-bar mechanism composed of four links
connected by four pin joints and one link is unable
to move (frame).

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 The inversions of four-bar mechanism
Crank-Rocker (two non-distinct inversions)

• It has the shortest link (crank) configured adjacent to the frame

(ground either link adjacent to the shortest link).
• If this shortest link is continuously rotated, the output link
(rocker) will oscillate between limits.
• The wiper system in slide 3 is designed to be a crank-rocker.
– As the motor continuously rotates with the input link, the output link
oscillates, or “rocks.” The wiper arm and blade are firmly attached to
the output link, oscillating the wiper across a windshield. 4
The inversions of four-bar mechanism
Crank-Rocker Mechanism

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 The inversions of four-bar mechanism
Double Crank

• A double crank, or crank-crank (drag link) has the shortest link

configured as the frame. https://youtu.be/SLgKmlFrCf8
• If one of the pivoted links is rotated continuously, the other
pivoted link will also rotate continuously as does the coupler.
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 Grashof ’s Criterion
• Double Crank Mechanism

4 Double Crank Mechanism.mp4

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 The inversions of four-bar mechanism
Double Rocker

• The double rocker, or rocker-rocker mechanism has the link

opposite the shortest link configured as the frame.
• In this configuration, neither link connected to the frame will
be able to complete a full revolution.
• Both input and output links are constrained to oscillate
between limits and are called rockers.
• However, the coupler is able to complete a full revolution. 8
 Grashof ’s Criterion
• Double rocker mechanism

2 Double rocker mechanism explained.mp4

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 Change Point Mechanism
• If l + s = p + q, the same four mechanisms exist, but, change-
point condition occurs where the centerlines of all links become
collinear and the mechanism can toggle.
• All inversions will be either double-cranks or crank-rockers but
will have "change points“ twice per revolution of the input crank
when the links all become collinear.
• At these change points the output behavior will become
indeterminate (unpredictable) as it may assume either of two
configurations. https://youtu.be/zMa4cbnBQWY
• Its motion must be limited to avoid reaching the change points or
an additional, out-of-phase link provided to guarantee a "carry
through" of the change points.

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 Triple Rocker Linkage
If l + s > p + q, four non-distinct inversions of a non-Grashof
linkage mechanisms exist, depending on which is the ground
link, but continuous rotation is not possible.

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 Grashof ’s Criterion
• Double Rocker Mechanism Vs Triple Rocker Mechanism

3 Double Rocker Mechanism vs Triple Rocker Mechanism.mp4

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 Grashof ’s Criterion
• The Grashof condition is a very simple relationship which
predicts the rotation behavior of a four-bar linkage's
inversions based on the link lengths.
– s = length of the shortest link
– l = length of the longest link
– p = length of one of the intermediate length links
– q = length of the other intermediate length links
Grashof’s theorem states that “the sum of the shortest (s) and
longest (l) links of a planar four-bar linkage cannot be greater
than the sum of the remaining two links (p, q) if there is to be
continuous relative motion between two links and at least one
rotating link relative to the frame”. s + l <= p + q
Conversely, the three non-fixed links will merely rock if:
s+l>p+q
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 Example on Grashof ’s Criterion
• The Grashof Criterion

4 Grashof Law_ Example.mp4

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 Example 2: Aircraft Landing Gear
A nose assembly for a small aircraft is shown below. Classify the
motion of this four-bar mechanism based on the configuration
of the links.

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 Example 2: Solution
1. Distinguish the Links Based on Length
• The motion of the wheel assembly would be determined relative
to the body of the aircraft. Therefore, the aircraft body will be
designated as the frame.
• Kinematic diagram for the wheel assembly, numbering and
labeling the links. The tip of the wheel was designated as point
of interest X.

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 Example 2: Solution
• The lengths of the links are:
s = 12 in.; l = 32 in.; p = 30 in.; q = 26 in.
2. Compare to Criteria
• The shortest link is adjacent to the frame. According to
the criteria, this mechanism can be either a crank-
rocker, change point, or a triple rocker.
• The criteria for the different categories of four-bar
mechanisms should be reviewed.
3. Check the Crank-Rocker (Case 2) Criteria
• Is s + l < p + q? (12 + 32) < (30 + 26); 44 < 56 : {yes}
• Because the criteria for a crank-rocker are valid, the
nose-wheel assembly is a crank-rocker mechanism.17
 Slider Crank Mechanism
• A mechanism found in IC (Internal Combustion) Engines.
• Cylinder and Piston – rod arrangement.

How Slider Crank Mechanism.mp4

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 Slider Crank Mechanism
• A slider-crank mechanism consists of a combination of four
links, with one being designated as the frame.
• This mechanism is connected by three pin joints and one sliding
joint.
• A mechanism that drives a manual water pump (hand pump) and
the corresponding kinematic diagram is given below.

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 Slider Crank Mechanism
Hand Pump Mechanism

Hand Pump.mp4

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 Inversions of Slider-crank Linkage
Inversion of Slider - Crank

Inversion Slider Crank.avi

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 Inversions of Slider-crank Linkage
Inversion #1, with link 1 as ground and its slider block in pure
translation, is the most commonly seen and is used for piston
engines and piston pumps. Slider block translates.

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 Inversions of Slider-crank Linkage
Inversion #2 is obtained by grounding link 2 and gives the
Whitworth or Crank-shaper quick-return mechanism, in which
the slider block has complex motion.

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 Inversions of Slider-crank Linkage
Inversion #3 is obtained by grounding link 3 and gives the slider
block pure rotation. Slider block oscillates.

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 Inversions of Slider-crank Linkage
Inversion #4 is obtained by grounding the slider link 4 and is used
in hand operated well pump mechanisms, in which the handle is
link 2 (extended) and link 1 passes down the well pipe to mount
a piston on its bottom. The slider block is stationary.

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 Straight – Line Mechanism
Straight-line mechanisms cause a point to travel in a
straight line without being guided by a flat surface.
E.g. Watt linkage and Peaucellier Straight-line linkages

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 Straight – Line Mechanism
Straight-line mechanisms

Straight Line Mechanisms.mp4

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 Straight – Line Mechanism
Peaucellier Straight-line mechanisms

Peaucellier Straight Line Motion Mechanism.mp4

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 Parallelogram Mechanisms
Mechanisms are often comprised of links that form parallelograms
to move an object without altering its pitch.
These mechanisms create parallel motion for applications such as
balance scales, swings, and windows, scissor linkages and
drafting machine linkage

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 Parallelogram Mechanisms
Parallelograms mechanism

Parallelogram Mechanism.mp4

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Double-Parallelogram Mechanisms
Double-Parallelogram Mechanism

Double parallelogram mechanism 1.mp4

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 Quick – Return Mechanisms
Quick-return mechanisms exhibit a faster stroke in one direction
than the other when driven (input) at constant speed.
Commonly used on machine tools that require a slow cutting
stroke and a fast return stroke.
The KDs of two different quick-return mechanisms:
(a) Offset slider-crank linkage and (b) Crank-shaper linkage.

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 Quick – Return Mechanisms
Quick-return Motion Mechanisms

Quick Return Mechanism.mp4

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 Quick – Return Mechanisms
Quick-return Motion Mechanisms

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 Quick – Return Mechanisms
Quick-return Motion Mechanisms

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 Quick – Return Mechanisms
Application of Quick-return Mechanisms

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 Scotch Yoke Mechanism
• The Scotch Yoke Mechanism is the reciprocating
mechanism that converts rotational motions to linear
sliding/reciprocating motion or vice-versa.
• Mechanism also knowns as Slotted Link Mechanism.
• The mechanism is an inversion of the double slider
crank mechanism.

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 Scotch Yoke Mechanism
• A pin on a rotating link is engaged in the slot of a sliding
yoke.
• Regarding the input and output motion, the scotch yoke is
similar to a slider-crank, but the linear sliding motion in
scotch yoke mechanism is pure sinusoidal or SHM.
Compared to the slider-crank, the scotch yoke mechanism can
experience rapid wear in the slot.

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SIMPLE HARMONIC MOTION

Instructor: Robel Metiku

Scotch Yoke Mechanism

1 Scotch Yoke Mechanism.mp4

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Scotch Yoke Mechanism

4 Scotch Yoke Mechanism.mp4

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 Intermittent Motion Mechanisms
Intermittent motion is a sequence of motions and dwells.
A dwell is a period in which the output link remains
stationary while the input link continues to move.
There are many applications in machinery which require
intermittent motion. Two applications are:

1. Geneva Mechanism
2. Ratchet and Pawl Mechanism

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 Geneva Mechanism
• This is a transformed four-bar linkage in which the coupler has
been replaced by a half joint.
• The input crank is typically motor driven at a constant speed.
• The Geneva wheel is fitted with at least three equi-spaced, radial
slots.
• The crank has a pin that enters a radial slot and causes the
Geneva wheel to turn through a portion of a revolution.
• When the pin leaves that slot, the Geneva wheel remains
stationary until the pin enters the next slot. The result is
intermittent rotation of the Geneva wheel.
• The crank is also fitted with an arc segment, which engages a
matching cutout on the periphery of the Geneva wheel when the
pin is out of the slot.
• This keeps the Geneva wheel stationary and in the proper
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location for the next entry of the pin.
 Geneva Mechanism
• The number of slots determines the number of "stops" of the
mechanism, where stop is synonymous with dwell.
• A Geneva wheel needs a minimum of three stops to work.
• The maximum number of stops is limited only by the size of the
wheel.

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 Intermittent motion mechanisms
Geneva wheel mechanism
 Intermittent motion mechanisms
Geneva wheel mechanism
 Linear Geneva Mechanism
This is also a variation of the Geneva mechanism which has linear
translational output.
The mechanism is analogous to an open Scotch yoke device with
multiple yokes. It can be used as an intermittent conveyor drive
with the slots arranged along the conveyor chain or belt.
It can be used with a reversing motor to get linear, reversing
oscillation of a single slotted, put slider. https://youtu.be/fHqnOldbsVY

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 Ratchet and Pawl Mechanism
An arm pivots about the center of the toothed ratchet wheel and is moved back
and forth to index the wheel.
The driving pawl rotates the ratchet wheel (or ratchet) in the counterclockwise
direction and does no work on the return (clockwise) trip.
The locking pawl prevents the ratchet from reversing direction while the driving
pawl returns. Both pawls are usually spring-loaded against the ratchet. This
mechanism is widely used in devices such as "ratchet" wrenches, winches, etc.

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 Ratchet and Pawl Mechanism
Ratchet – and – Paul Mechanism

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 Ratchet and Pawl Mechanism
Ratchet – and – Paul Mechanism

Ratchet and Pawl Mechanism 2.mp4

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 Ratchet and Pawl Mechanism
Application of Ratchet – and – Paul Mechanism

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 Additional Mechanisms
Mechanisms

Mechanisms 0.mp4

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 Additional Mechanisms
Mechanisms

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 Additional Mechanisms
Mechanisms

Mechanisms 1.mp4

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 Additional Mechanisms
Mechanisms

Mechanisms 2.mp4

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 Additional Mechanisms
Oldham Coupling Mechanisms

Elliptical trammel to oldham coupling.mp4

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 Additional Mechanisms
Mid-Rise Movable Scissor Mechanisms

Mid-Rise movable scissor lift.mp4

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 Additional Mechanisms
Mechanisms

MOST AMAZING Marble Machines.mp4

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