CHAPTER 1
INTRODUCTION
A mechanism is a combination of number
of bodies assembled in such a way that the motion of one causes constrained and
predictable motion to the other. Mechanisms are used to convert the given
motions into useful mechanical motions. Mechanisms transmit or transform
motion.
Four bar chain mechanism is a commonly used
mechanism. It has got four links forming a kinematic chain. It is used in many
machines because of its simplicity compared to other mechanisms. Different
configuration for this mechanism can be obtained by fixing different links of
the mechanism. This process of fixing the links is known as inversion of a
mechanism.
This scotch yoke mechanism
is used to convert the rotary motion of the crank into sliding motion. As the
crank rotates, the horizontal portion of the link slides or reciprocates in the
fixed link. Scotch yoke mechanism is obtained when one of the sliding links of
a double slider-crank chain is fixed. The advantage of scotch yoke mechanism
over the slider crank mechanism is that it has lesser moving parts and smoother
operation. This mechanism is most commonly used in control valve actuators in
high pressure oil and gas pipelines. Now a day it is also used in various
internal combustion engines such as Bourke engine, syTech engine and many hot
air engines and steam engines.
From the literature survey the
information available regarding the inversion of four bar chain is
insufficient. Hence the project is proposed to study and fabrication of scotch
yoke mechanism. The following are the main aspects of the project
1.
To study four bar chain, slider crank and double slider crank.
2.
To study its inversions
3.
To fabricate scotch yoke mechanism
4.
To analyze its motion and discuss its advantages, disadvantages and
applications.
CHAPTER.2
MECHANISM
A mechanism is a
combination of resistant bodies, so interconnected that by applying force or motion to one or more of those
bodies, some of those bodies are caused to perform desired work accompanied by
desired motions. Mechanisms are used to convert one type of motion into
another. The connections present in a mechanism are called kinematic pairs.
Kinematic pairs can be classified as higher pair or lower pair.
A mechanism is usually a
piece of a larger process or mechanical device. Sometimes an entire machine may be referred to as a mechanism.
Examples are the steering
mechanism in a car, or the winding mechanism of a wristwatch. Multiple mechanisms are machines.
Kinematics link or element:
Each part
of machine which moves relative to some other part is known as kinematic link
or element. A link may have consisted of several parts which are manufactured
as separate unit.
Characteristic of link:
It must be a resistance
body.
It should have relative motion.
Kinematics pair:
Kinematics pair has two elements or link
together which have relative motion between them. The motion between them
completely or successfully constrained.
Classification of Kinematics pair:
1.
On the basis of
contact:
Lower pair: A pair of links
having surface or area contact between the members is known as a lower pair.
The contact surface of the two links is similar.
Example: shaft rotating in a bearing
Higher pair: A pair of links
having point or line contact between the links is known as a higher pair. The
contact surface of the two links is dissimilar.
Example: wheel rolling on a surface
2. On the basis of type of constraint:
Closed pair: In a closed pair, the
elements of the pair are held together mechanically.
Example: screwpair
Unclosed pair: An unclosed pair will form when two links of a pair are in
contact either due to force of gravity or some spring action.
Example: cam and a follower pair
3. On the basis of type of relative motion:
Sliding pair: Two links
have sliding motion relative to each other. A rectangular rod in a rectangular
hole in a prism forms a sliding pair.
Screw pair: screw pair is formed when two mating links have a turning as
well as sliding motion between them.
Turning pair: A turning
pair is formed when one link has a turning or revolving motion relative to the
other. Circular shaft revolving inside a bearing is a turning pair.
Spherical pair: It is
formed when one link in the form of a sphere turns inside a fixed link. The
ball and socket joint is a spherical pair.
Rolling
pair: It is formed when the links of a pair have a rolling motion relative to
each other.
Inversion of mechanism:
When one
of the links is fixed in a kinematics chain ,it is called inversion of a
mechanism .so we can obtain as many mechanisms as the number of link in a
kinematic chain by fixing ,in turn , different link in a kinematics chain .
This method of obtaining different mechanism by fixing different links in a
kinematic chain is known as inversion.
Types of inversion:
1.
Four bar chain or quadric cycle chain:
Crank-Rocker
mechanism
Double crank
mechanism
Double rocker
mechanism
2.
Single slider crank chain:
Bull engine or
pendulum engine
Oscillating
cylinder engine
Rotary engine
Crank & slotted
quick return mechanism
Whitworth quick
return mechanism
3.
Double slider crank:
Elliptical
trammels
Scotch yoke
mechanism
Oldham’s
coupling
CHAPTER.3
FOUR BAR MECHANISM
A four-bar linkage is the simplest
movable linkage. It consists of four rigid
bodies (called bars or links), each attached to two others by single joints
or pivots to form a closed loop.
Four-bars
are simple mechanisms common in mechanical engineeringmachinedesign and fall under the study of kinematics.
If each joint has one rotationaldegree of freedom (i.e., it is a pivot), then
the mechanism is usually planar, and the
4-bar is determinate if the positions of any two bodies are known (although
there may be two solutions). One body typically does not move (called the ground link, fixed link, or the frame),
so the position of only one other body is needed to find all positions. The two
links connected to the ground link are called grounded links. The remaining link, not directly connected to the
ground link, is called the coupler link.
In terms of mechanical action, one of the grounded links is selected to be the input link, i.e., the link to
which an external force is applied to rotate it. The second grounded link is
called the follower link, since
its motion is completely determined by the motion of the input link
Planar
four-bar linkages perform a wide variety of motions with a few simple parts.
They were also popular in the past due to the ease of calculations, prior to
computers, compared to more complicated mechanisms.
CHAPTER 4
INVERSION OF FOUR BAR MECHANISM
Crank-Rocker
mechanism
Link
1 is taken as the base link or frame. In this configuration the shortest link
is jointed to the base link and this joint can fully rotate and hence called as
crank. The other link jointed to the base link oscillates and called as a
rocker. This configuration of the four-bar kinematic chain is called as
Crank-Rocker mechanism.
Double-Crank
mechanism
Link
2 is fixed as the base link. In this configuration shortest link is the base
and both joints to the base can rotate completely. It is thus called as
Double-Crank or a Drag-Link.
Crank-Rocker
mechanism
Link 3 is fixed as the base link. It
can be observed that this configuration is same as the Crank-Rocker mechanism.
Double-Rocker
mechanism
Link 4 is fixed as the base link. In
this configuration shortest link is the coupler and both the links connected to
the base link cannot rotate fully, both oscillate. In this configuration the
four-bar kinematic chain is called as Double-Rocker mechanism.
CHAPTER 5
SLIDER CRANK MECHANISM
A crank is an arm attached at right
angles to a rotating shaft by which circular motion is imparted to or received from the shaft. It is used
to change circular into reciprocating motion, or reciprocating into circular motion. The arm may be a bent portion of the shaft, or a separate
arm attached to it. Attached to the end of the crank by a pivot is a rod,
usually called a connecting rod. The end of the rod attached to the crank moves in a
circular motion, while the other end is usually constrained to move in a linear sliding motion, in and out.
The
term often refers to a human-powered crank which is used to manually turn an
axle, as in a bicyclecrankset or a brace and bit drill. In this case a
person's arm or leg serves as the connecting rod, applying reciprocating force
to the crank. Often there is a bar perpendicular to the other end of the arm,
often with a freely rotatable handle on it to hold in the hand, or in the case
of operation by a foot (usually with a second arm for the other foot), with a
freely rotatable pedal.
CHAPTER 6
INVERSION OF SLIDER CRANK
MECHANISM
In this mechanism, the inversion is obtained by fixing the cylinder or link
4 (i.e. sliding pair), as shown in Fig. In this case, when the crank (link 2)
rotates, the connecting rod (link 3) oscillates about a pin pivoted to the
fixed link 4 at A and the piston attached to the piston rod (link 1)
reciprocates. The duplex pump which is used to supply feed water to boilers
have two pistons attached to link 1, as shown in Fig.
The arrangement of
oscillating cylinder engine mechanism, as shown in Fig. is used to convert
reciprocating motion into rotary motion. In this mechanism, the link 3 forming
the turning pair is fixed. The link 3 corresponds to the connecting rod of a
reciprocating steam engine mechanism. When the crank (link 2) rotates, the
piston attached to piston rod (link 1) reciprocates and the cylinder (link 4)
oscillates about a pin pivoted to the fixed link at A.
Sometimes back, rotary internal combustion engines were used in aviation.
But now-a-days gas turbines are used in its place. It consists of seven
cylinders in one plane and all revolves about fixed centre D, as shown in Fig.
while the crank (link 2) is fixed. In this mechanism, when the connecting rod
(link 4) rotates, the piston (link 3) reciprocates inside the cylinders forming
link 1.
4) Inversions of Single Slider Crank Chain
When
the link 1 of the mechanism is fixed another inversion of slider crank chain is
obtained. This is mainly used in reciprocating engines to obtain rotary motion
from reciprocating motion of the piston.
CHAPTER 7
DOUBLE SLIDER CRANK MECHANISM
The double slider-crank linkage has
four links joined in a kinematic chain consisting of two revolute joints and
two sliding or prismatic joints. In this double slider, one sliding constraint
is perpendicular to the other. Such devices can be used to convert a circular
motion of the crank into an exact sinusoidal motion of the link moving in the
fixed linear constraint. A four bar chain having two turning and two sliding
pairs such that two pairs of the same kind are adjacent is known as a double
slider crank chain.
CHAPTER 8
INVERSION OF DOUBLE SLIDER CRANK MECHANISM
It is an instrument used
for drawing ellipses. This inversion is obtained by fixing the slotted plate
(link 4), as shown in Fig. The fixed plate or link 4 has two straight grooves
cut in it, at right angles to each other. The link 1 and link 3, are known as
sliders and form sliding pairs with link 4. The link AB (link 2) is a bar which
forms turning pair with links 1 and 3. When the links 1 and 3 slide along their
respective grooves, any point on the link 2 such as P traces out an ellipse on
the surface of link 4, as shown in Fig (a). A little consideration will show
that AP and BP are the semi-major axis and semi-minor axis of the ellipse
respectively.
This mechanism is used for
converting rotary motion into a reciprocating motion. The inversion is obtained
by fixing either the link 1 or link 3. In Fig, link 1 is fixed. In this
mechanism, when the link 2 (which corresponds to crank) rotates about B as
centre, the link 4 (which corresponds to a frame) reciprocates. The fixed link
1 guides the frame.
An Oldham’s coupling is
used for connecting two parallel shafts whose axes are at a small distance
apart. The shafts are coupled in such a way that if one shaft rotates, the
other shaft also rotates at the same speed. This inversion is obtained by
fixing the link 2, as shown in Fig (a). The shafts to be connected have two
flanges (link 1 and link 3) rigidly fastened at their ends by forging.
The link
1 and link 3 form turning pairs with link 2. These flanges have diametrical
slots cut in their inner faces, as shown in Fig (b). The intermediate piece
(link 4) which is a circular disc, have two tongues (i.e. diametrical
projections) T1 and T2 on each face at right angles to each other, as shown in
Fig (c). The tongues on the link 4 closely fit into the slots in the two
flanges (link 1 and link 3). The link 4 can slide or reciprocate in the slots
in the flanges.
CHAPTER 9
SCOTCH YOKE MECHANISM
Scotch
yoke is a mechanism for converting the linear motion of a slider into
rotational motion or vice-versa. The piston
or other reciprocating part is directly coupled to a sliding yoke with a slot that engages a
pin on the rotating part. The shape of the motion of the piston is a pure sine wave over time given a
constant rotational speed. The double slider crank mechanism is a mechanism
having two sliding pairs and two turning pairs. Scotch yoke mechanism is formed
when one of the two sliding pairs in a double slider crank mechanism is fixed.
It has got two turning pairs, one sliding pair and a fixed link.
CHAPTER 10
CONSTRUCTION
The scotch yoke mechanism is
constructed with iron bars. Here the crank is made in some length (say 5cm) and
the yoke is also made using the same material. It is noted that the minimum
length of the yoke should be double the length of the crank. The crank and yoke
is connected with a pin. Iron bars are welded to both sides of the yoke to get
the reciprocating motion. The yoke with
the iron bars is fixed on the display board with the help of c clamp. Now the
crank is welded to the end of the shaft of the motor. Now the pin on the crank
is connected to the yoke. The pin used to connect yoke and crank is a bolt. The
whole setup displayed in a plywood board.
CHAPTER 11
WORKING
When the power is supplied to the
12v dc motor, shaft and crank attached to the shaft start rotating. As the
crank rotates the pin slides inside the yoke and also moves the yoke forward.
When the crank rotates through
in clockwise direction the yoke will get a displacement in the
forward direction. The maximum displacement will be equal to the length of the
crank. When the crank completes the next
of rotation the yoke
comes back to its initial position. For the next
of rotation, yoke moves
in the backward direction. When the crank completes a full rotation the yoke
moves back to the initial position. For a complete rotation of crank the yoke
moves through a length equal to double the length of the crank. The
displacement of the yoke can be controlled by varying the length of the crank.
CHAPTER 12
ADVANTAGES
- Fewer moving parts.
- Smoother operation.
- Higher percentage of the time spent at top dead center (dwell) improving theoretical engine efficiency of constant volume combustion cycles, though actual gains have not been demonstrated.
- In an engine application,use of connecting rod is eliminated when compared to slider crank mechanism and thus reducing the vibrations produced on the connecting rod
CHAPTER 13
DISADVANTAGES
The disadvantages are:
- Rapid wear of the slot in the yoke caused by sliding friction and high contact pressures.
- Increased heat loss during combustion due to extended dwell at top dead center offsets any constant volume combustion improvements in real engines.
- Lesser percentage of the time spent at bottom dead center reducing blow down time for two stroke engines, when compared with a conventional piston and crankshaft mechanism.
CHAPTER 14
APPLICATIONS
It
has been used in various internal combustion engines, such as the Bourke engine, SyTech engine, and many hot air engines and steam engines.
In
internal combustion engines, scotch yoke mechanism is connected to the piston
instead of using the slider crank mechanism. It results in elimination of
connecting rod which reduces the vibrations caused in the connecting rod. It
has got extended dwell times. Experiments have shown that extended dwell time
will not work well with constant volume combustion (Otto, Bourke or similar)
cycles. Gains might be more apparent using a stratified direct injection
(diesel or similar) cycle to reduce heat loss
CHAPTER 15
CONCLUSION
The scotch yoke
mechanism is made and its advantages and disadvantages are discussed. Its
motion characteristics are studied. It is concluded that this mechanism is a
good choice to convert rotating motion into reciprocating motion because of
fewer moving parts and smoother operation. It can be used in direct injection
engines like diesel engines.
CHAPTER 1
INTRODUCTION
A mechanism is a combination of number
of bodies assembled in such a way that the motion of one causes constrained and
predictable motion to the other. Mechanisms are used to convert the given
motions into useful mechanical motions. Mechanisms transmit or transform
motion.
Four bar chain mechanism is a commonly used
mechanism. It has got four links forming a kinematic chain. It is used in many
machines because of its simplicity compared to other mechanisms. Different
configuration for this mechanism can be obtained by fixing different links of
the mechanism. This process of fixing the links is known as inversion of a
mechanism.
This scotch yoke mechanism
is used to convert the rotary motion of the crank into sliding motion. As the
crank rotates, the horizontal portion of the link slides or reciprocates in the
fixed link. Scotch yoke mechanism is obtained when one of the sliding links of
a double slider-crank chain is fixed. The advantage of scotch yoke mechanism
over the slider crank mechanism is that it has lesser moving parts and smoother
operation. This mechanism is most commonly used in control valve actuators in
high pressure oil and gas pipelines. Now a day it is also used in various
internal combustion engines such as Bourke engine, syTech engine and many hot
air engines and steam engines.
From the literature survey the
information available regarding the inversion of four bar chain is
insufficient. Hence the project is proposed to study and fabrication of scotch
yoke mechanism. The following are the main aspects of the project
1.
To study four bar chain, slider crank and double slider crank.
2.
To study its inversions
3.
To fabricate scotch yoke mechanism
4.
To analyze its motion and discuss its advantages, disadvantages and
applications.
CHAPTER.2
MECHANISM
A mechanism is a
combination of resistant bodies, so interconnected that by applying force or motion to one or more of those
bodies, some of those bodies are caused to perform desired work accompanied by
desired motions. Mechanisms are used to convert one type of motion into
another. The connections present in a mechanism are called kinematic pairs.
Kinematic pairs can be classified as higher pair or lower pair.
A mechanism is usually a
piece of a larger process or mechanical device. Sometimes an entire machine may be referred to as a mechanism.
Examples are the steering
mechanism in a car, or the winding mechanism of a wristwatch. Multiple mechanisms are machines.
Kinematics link or element:
Each part
of machine which moves relative to some other part is known as kinematic link
or element. A link may have consisted of several parts which are manufactured
as separate unit.
Characteristic of link:
It must be a resistance
body.
It should have relative motion.
Kinematics pair:
Kinematics pair has two elements or link
together which have relative motion between them. The motion between them
completely or successfully constrained.
Classification of Kinematics pair:
1.
On the basis of
contact:
Lower pair: A pair of links
having surface or area contact between the members is known as a lower pair.
The contact surface of the two links is similar.
Example: shaft rotating in a bearing
Higher pair: A pair of links
having point or line contact between the links is known as a higher pair. The
contact surface of the two links is dissimilar.
Example: wheel rolling on a surface
2. On the basis of type of constraint:
Closed pair: In a closed pair, the
elements of the pair are held together mechanically.
Example: screwpair
Unclosed pair: An unclosed pair will form when two links of a pair are in
contact either due to force of gravity or some spring action.
Example: cam and a follower pair
3. On the basis of type of relative motion:
Sliding pair: Two links
have sliding motion relative to each other. A rectangular rod in a rectangular
hole in a prism forms a sliding pair.
Screw pair: screw pair is formed when two mating links have a turning as
well as sliding motion between them.
Turning pair: A turning
pair is formed when one link has a turning or revolving motion relative to the
other. Circular shaft revolving inside a bearing is a turning pair.
Spherical pair: It is
formed when one link in the form of a sphere turns inside a fixed link. The
ball and socket joint is a spherical pair.
Rolling
pair: It is formed when the links of a pair have a rolling motion relative to
each other.
Inversion of mechanism:
When one
of the links is fixed in a kinematics chain ,it is called inversion of a
mechanism .so we can obtain as many mechanisms as the number of link in a
kinematic chain by fixing ,in turn , different link in a kinematics chain .
This method of obtaining different mechanism by fixing different links in a
kinematic chain is known as inversion.
Types of inversion:
1.
Four bar chain or quadric cycle chain:
Crank-Rocker
mechanism
Double crank
mechanism
Double rocker
mechanism
2.
Single slider crank chain:
Bull engine or
pendulum engine
Oscillating
cylinder engine
Rotary engine
Crank & slotted
quick return mechanism
Whitworth quick
return mechanism
3.
Double slider crank:
Elliptical
trammels
Scotch yoke
mechanism
Oldham’s
coupling
CHAPTER.3
FOUR BAR MECHANISM
A four-bar linkage is the simplest
movable linkage. It consists of four rigid
bodies (called bars or links), each attached to two others by single joints
or pivots to form a closed loop.
Four-bars
are simple mechanisms common in mechanical engineeringmachinedesign and fall under the study of kinematics.
If each joint has one rotationaldegree of freedom (i.e., it is a pivot), then
the mechanism is usually planar, and the
4-bar is determinate if the positions of any two bodies are known (although
there may be two solutions). One body typically does not move (called the ground link, fixed link, or the frame),
so the position of only one other body is needed to find all positions. The two
links connected to the ground link are called grounded links. The remaining link, not directly connected to the
ground link, is called the coupler link.
In terms of mechanical action, one of the grounded links is selected to be the input link, i.e., the link to
which an external force is applied to rotate it. The second grounded link is
called the follower link, since
its motion is completely determined by the motion of the input link
Planar
four-bar linkages perform a wide variety of motions with a few simple parts.
They were also popular in the past due to the ease of calculations, prior to
computers, compared to more complicated mechanisms.
CHAPTER 4
INVERSION OF FOUR BAR MECHANISM
Crank-Rocker
mechanism
Link
1 is taken as the base link or frame. In this configuration the shortest link
is jointed to the base link and this joint can fully rotate and hence called as
crank. The other link jointed to the base link oscillates and called as a
rocker. This configuration of the four-bar kinematic chain is called as
Crank-Rocker mechanism.
Double-Crank
mechanism
Link
2 is fixed as the base link. In this configuration shortest link is the base
and both joints to the base can rotate completely. It is thus called as
Double-Crank or a Drag-Link.
Crank-Rocker
mechanism
Link 3 is fixed as the base link. It
can be observed that this configuration is same as the Crank-Rocker mechanism.
Double-Rocker
mechanism
Link 4 is fixed as the base link. In
this configuration shortest link is the coupler and both the links connected to
the base link cannot rotate fully, both oscillate. In this configuration the
four-bar kinematic chain is called as Double-Rocker mechanism.
CHAPTER 5
SLIDER CRANK MECHANISM
A crank is an arm attached at right
angles to a rotating shaft by which circular motion is imparted to or received from the shaft. It is used
to change circular into reciprocating motion, or reciprocating into circular motion. The arm may be a bent portion of the shaft, or a separate
arm attached to it. Attached to the end of the crank by a pivot is a rod,
usually called a connecting rod. The end of the rod attached to the crank moves in a
circular motion, while the other end is usually constrained to move in a linear sliding motion, in and out.
The
term often refers to a human-powered crank which is used to manually turn an
axle, as in a bicyclecrankset or a brace and bit drill. In this case a
person's arm or leg serves as the connecting rod, applying reciprocating force
to the crank. Often there is a bar perpendicular to the other end of the arm,
often with a freely rotatable handle on it to hold in the hand, or in the case
of operation by a foot (usually with a second arm for the other foot), with a
freely rotatable pedal.
CHAPTER 6
INVERSION OF SLIDER CRANK
MECHANISM
In this mechanism, the inversion is obtained by fixing the cylinder or link
4 (i.e. sliding pair), as shown in Fig. In this case, when the crank (link 2)
rotates, the connecting rod (link 3) oscillates about a pin pivoted to the
fixed link 4 at A and the piston attached to the piston rod (link 1)
reciprocates. The duplex pump which is used to supply feed water to boilers
have two pistons attached to link 1, as shown in Fig.
The arrangement of
oscillating cylinder engine mechanism, as shown in Fig. is used to convert
reciprocating motion into rotary motion. In this mechanism, the link 3 forming
the turning pair is fixed. The link 3 corresponds to the connecting rod of a
reciprocating steam engine mechanism. When the crank (link 2) rotates, the
piston attached to piston rod (link 1) reciprocates and the cylinder (link 4)
oscillates about a pin pivoted to the fixed link at A.
Sometimes back, rotary internal combustion engines were used in aviation.
But now-a-days gas turbines are used in its place. It consists of seven
cylinders in one plane and all revolves about fixed centre D, as shown in Fig.
while the crank (link 2) is fixed. In this mechanism, when the connecting rod
(link 4) rotates, the piston (link 3) reciprocates inside the cylinders forming
link 1.
4) Inversions of Single Slider Crank Chain
When
the link 1 of the mechanism is fixed another inversion of slider crank chain is
obtained. This is mainly used in reciprocating engines to obtain rotary motion
from reciprocating motion of the piston.
CHAPTER 7
DOUBLE SLIDER CRANK MECHANISM
The double slider-crank linkage has
four links joined in a kinematic chain consisting of two revolute joints and
two sliding or prismatic joints. In this double slider, one sliding constraint
is perpendicular to the other. Such devices can be used to convert a circular
motion of the crank into an exact sinusoidal motion of the link moving in the
fixed linear constraint. A four bar chain having two turning and two sliding
pairs such that two pairs of the same kind are adjacent is known as a double
slider crank chain.
CHAPTER 8
INVERSION OF DOUBLE SLIDER CRANK MECHANISM
It is an instrument used
for drawing ellipses. This inversion is obtained by fixing the slotted plate
(link 4), as shown in Fig. The fixed plate or link 4 has two straight grooves
cut in it, at right angles to each other. The link 1 and link 3, are known as
sliders and form sliding pairs with link 4. The link AB (link 2) is a bar which
forms turning pair with links 1 and 3. When the links 1 and 3 slide along their
respective grooves, any point on the link 2 such as P traces out an ellipse on
the surface of link 4, as shown in Fig (a). A little consideration will show
that AP and BP are the semi-major axis and semi-minor axis of the ellipse
respectively.
This mechanism is used for
converting rotary motion into a reciprocating motion. The inversion is obtained
by fixing either the link 1 or link 3. In Fig, link 1 is fixed. In this
mechanism, when the link 2 (which corresponds to crank) rotates about B as
centre, the link 4 (which corresponds to a frame) reciprocates. The fixed link
1 guides the frame.
An Oldham’s coupling is
used for connecting two parallel shafts whose axes are at a small distance
apart. The shafts are coupled in such a way that if one shaft rotates, the
other shaft also rotates at the same speed. This inversion is obtained by
fixing the link 2, as shown in Fig (a). The shafts to be connected have two
flanges (link 1 and link 3) rigidly fastened at their ends by forging.
The link
1 and link 3 form turning pairs with link 2. These flanges have diametrical
slots cut in their inner faces, as shown in Fig (b). The intermediate piece
(link 4) which is a circular disc, have two tongues (i.e. diametrical
projections) T1 and T2 on each face at right angles to each other, as shown in
Fig (c). The tongues on the link 4 closely fit into the slots in the two
flanges (link 1 and link 3). The link 4 can slide or reciprocate in the slots
in the flanges.
CHAPTER 9
SCOTCH YOKE MECHANISM
Scotch
yoke is a mechanism for converting the linear motion of a slider into
rotational motion or vice-versa. The piston
or other reciprocating part is directly coupled to a sliding yoke with a slot that engages a
pin on the rotating part. The shape of the motion of the piston is a pure sine wave over time given a
constant rotational speed. The double slider crank mechanism is a mechanism
having two sliding pairs and two turning pairs. Scotch yoke mechanism is formed
when one of the two sliding pairs in a double slider crank mechanism is fixed.
It has got two turning pairs, one sliding pair and a fixed link.
CHAPTER 10
CONSTRUCTION
The scotch yoke mechanism is
constructed with iron bars. Here the crank is made in some length (say 5cm) and
the yoke is also made using the same material. It is noted that the minimum
length of the yoke should be double the length of the crank. The crank and yoke
is connected with a pin. Iron bars are welded to both sides of the yoke to get
the reciprocating motion. The yoke with
the iron bars is fixed on the display board with the help of c clamp. Now the
crank is welded to the end of the shaft of the motor. Now the pin on the crank
is connected to the yoke. The pin used to connect yoke and crank is a bolt. The
whole setup displayed in a plywood board.
CHAPTER 11
WORKING
When the power is supplied to the
12v dc motor, shaft and crank attached to the shaft start rotating. As the
crank rotates the pin slides inside the yoke and also moves the yoke forward.
When the crank rotates through
in clockwise direction the yoke will get a displacement in the
forward direction. The maximum displacement will be equal to the length of the
crank. When the crank completes the next
of rotation the yoke
comes back to its initial position. For the next
of rotation, yoke moves
in the backward direction. When the crank completes a full rotation the yoke
moves back to the initial position. For a complete rotation of crank the yoke
moves through a length equal to double the length of the crank. The
displacement of the yoke can be controlled by varying the length of the crank.
CHAPTER 12
ADVANTAGES
- Fewer moving parts.
- Smoother operation.
- Higher percentage of the time spent at top dead center (dwell) improving theoretical engine efficiency of constant volume combustion cycles, though actual gains have not been demonstrated.
- In an engine application,use of connecting rod is eliminated when compared to slider crank mechanism and thus reducing the vibrations produced on the connecting rod
CHAPTER 13
DISADVANTAGES
The disadvantages are:
- Rapid wear of the slot in the yoke caused by sliding friction and high contact pressures.
- Increased heat loss during combustion due to extended dwell at top dead center offsets any constant volume combustion improvements in real engines.
- Lesser percentage of the time spent at bottom dead center reducing blow down time for two stroke engines, when compared with a conventional piston and crankshaft mechanism.
CHAPTER 14
APPLICATIONS
It
has been used in various internal combustion engines, such as the Bourke engine, SyTech engine, and many hot air engines and steam engines.
In
internal combustion engines, scotch yoke mechanism is connected to the piston
instead of using the slider crank mechanism. It results in elimination of
connecting rod which reduces the vibrations caused in the connecting rod. It
has got extended dwell times. Experiments have shown that extended dwell time
will not work well with constant volume combustion (Otto, Bourke or similar)
cycles. Gains might be more apparent using a stratified direct injection
(diesel or similar) cycle to reduce heat loss
CHAPTER 15
CONCLUSION
The scotch yoke
mechanism is made and its advantages and disadvantages are discussed. Its
motion characteristics are studied. It is concluded that this mechanism is a
good choice to convert rotating motion into reciprocating motion because of
fewer moving parts and smoother operation. It can be used in direct injection
engines like diesel engines.
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