Experiment no 3

Slotted Link Mechanism Experiment

Objective
To study the motion of a slotted link and to see if the piston rod moves with simple harmonic
motion.

Apparatus
Slotted link mechanism experiment apparatus

Theory
Slotted link mechanism is a reciprocating motion mechanism, 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 location of
the piston versus time is a sine wave of constant amplitude, and constant frequency given a
constant rotational speed.

Applications:
This setup is most commonly used in control valve actuators in high-pressure oil and gas
pipelines.
Although not a common metalworking machine nowadays, crude shapers can use Slotted link
mechanism. Almost all those use a Whitworth linkage, which gives a slow speed forward
cutting stroke and a faster return.
It has been used in various internal combustion engines, such as the Bourke engine, SyTech
engine, and many hot air engines and steam engines.
The term scotch yoke continues to be used when the slot in the yoke is shorter than the
diameter of the circle made by the crank pin. For example, the side rods of a locomotive may
have slotted link mechanism to permit vertical motion of intermediate driving axles.
 Uses in internal combustion engine:
Under ideal engineering conditions, force is applied directly in the line of travel of the
assembly. The sinusoidal motion, sinusoidal velocity, and sinusoidal acceleration (assuming
constant angular velocity) result in smoother operation. The higher percentage of time spent
at top dead Centre (dwell) improves theoretical engine efficiency of constant volume
combustion cycles. It allows the elimination of joints typically served by a wrist pin, and near
elimination of piston skirts and cylinder scuffing, as side loading of piston due to sine
of connecting rod angle is mitigated. The longer the distance between the piston and the yoke,
the less wear that occurs, but greater the inertia, making such increases in the piston rod
length realistically only suitable for lower RPM (but higher torque) applications
The Scotch yoke is not used in most internal combustion engines because of the rapid wear of
the slot in the yoke caused by sliding friction and high contact pressures. This is mitigated by
a sliding block between the crank and the slot in the piston rod. Also, increased heat loss
during combustion due to extended dwell at top dead center offsets any constant volume
combustion improvements in real engines.[5] In an engine application, less percent of the time
is spent at bottom dead center when compared to a conventional piston and crankshaft
mechanism, which reduces blow down time for two-stroke engines. Experiments have shown
that extended dwell time does not work well with constant volume combustion Otto cycle
engines. Gains might be more apparent in Otto cycle engines using a stratified direct injection
(diesel or similar) cycle to reduce heat losses.
Procedure
i. The crank was set to 0° and the piston displacement was recorded.
ii. The crank was moved by 20° and the displacement is again recorded.
iii. Repeat step 1 and 2 for one complete revolution of the crank.
iv. The theoretical piston displacement was also calculated.
Observation and Result

Linear Position
Crank angle
(degrees) (mm)
 Experiment No 4
Simple Four-Bar Linkage Mechanism
Objective
The experiment is designed to give a better understanding of the performance of the four-bar
linkages in its different conditions according to its geometry. Measuring the Dead point
angles φ and transmission angles β at two positions when the constant bar a at two different
positions 4˝ and 6˝

Apparatus
Four bar linkage mechanism

Theory
A four-bar linkage, also called a four-bar, is the simplest movable closed chain linkage. It
consists of four bodies, called bars or links, connected in a loop by four joints. Generally, the
joints are configured so the links move in parallel planes, and the assembly is called a planar
four-bar linkage.

If the linkage has four hinged joints with axes angled to intersect in a single point, then the
links move on concentric spheres and the assembly is called a spherical four-bar linkage.
Bennett's linkage is a spatial four-bar linkage with hinged joints that have their axes angled in
a particular way that makes the system movable.

Animated four-bar linkage

Planar four-bar linkages are constructed from four links connected in a loop by four one
degree of freedom joints. A joint may be either a revolute, that is a hinged joint, denoted by
R, or a prismatic, as sliding joint, denoted by P.

A link connected to ground by a hinged joint is usually called a crank. A link connected to
ground by a prismatic joint is called a slider. Sliders are sometimes considered to be cranks
that have a hinged pivot at an extremely long distance away perpendicular to the travel of the
slider.

The link that connects two cranks is called a floating link or coupler. A coupler that connects
a crank and a slider, it is often called a connecting rod.

Definition of four bar linkage

In the range of planar mechanisms, the simplest groups of lower pair mechanisms are

four bar linkages. A four bar linkage comprises four bar-shaped links and four turning

pairs as shown in Figure

 Figure : Four bar linkage

The link opposite the frame is called the coupler link, and the links which are hinged to

the frame are called side links. A link which is free to rotate through 360 degree with

respect to a second link will be said to revolve relative to the second link (not necessarily

a frame). If it is possible for all four bars to become simultaneously aligned, such a state

is called a change point.

Some important concepts in link mechanisms are:

1. Crank: A side link which revolves relative to the frame is called a crank.

2. Rocker: Any link which does not revolve is called a rocker.

3. Crank-rocker mechanism: In a four bar linkage, if the shorter side link revolves

And the other one rocks (i.e., oscillates), it is called a crank-rocker mechanism.

4. Double-crank mechanism: In a four bar linkage, if both of the side links revolve,

it is called a double-crank mechanism.

5. Double-rocker mechanism: In a four bar linkage, if both side links rock, it is

Called a double-rocker mechanism.

Classification
Before classifying four-bar linkages, we need to introduce some basic nomenclature. In a

four-bar linkage, we refer to the line segment between hinges on a given link as a bar

where

s = length of shortest bar

l = length of longest bar

p, q = lengths of intermediate bar

Grashof's theorem states that a four-bar mechanism has at least one revolving link if

s + l <= p + q(1.1)

and all three mobile links will rock if

s + l > p + q (1.2)

The inequality 1.1 is Grashof's criterion.

A mechanism to have a crank, the sum of the length of its shortest and longest links must be
less than or equal to the sum of the length of the other two links. However, this condition is
necessary but not sufficient. Mechanisms satisfying this condition fall into the following
three categories:

1. When the shortest link is a side link, the mechanism is a crank-rocker mechanism.

2. When the shortest link is the frame of the mechanism, the mechanism is a double crank
mechanism.

3. When the shortest link is the coupler link, the mechanism is a double-rocker mechanism.

Description of the Experiment

The experimental setup consists of two four-bar linkage mechanism trains. Careful
examination of the setup should result in the correct categorization of the linkages. There is
an arm following the coupler curve trace, a software generated linkage similar to the actual
linkage is studied using the Working Model simulation package.

Experimental Procedure
i. Set the fixed bar at 4˝.
ii. Observe the movement of the four bar linkage mechanism.
iii. Find the first dead point, then measure Dead point angle φ and transmission angle β.
iv. Repeat step 3 for the second dead point.
v. Set the fixed bar a at 6˝.
vi. Repeat the steps 2, 3, 4.

Observation and Results

Dead point angle φ Transmission angle β
(degrees) (degrees)