5. Topics To Cover :
Introduction to Pulley.
Construction and Design.
Types:
I) Single Fixed Pulley.
II) Movable Pulley.
III) Block And Tackle
Applications
6. Introduction:
History:
Starting with the basic topic,pulley is
originally a simple machine.It was possibly
created by Archimedes in B.C times. It's
original use was to lift the buckets of water.
The term "sheave" can also be used to refer to a pulley.
What Are Pulleys:
Pulleys can be found everywhere to help make tasks easier. In
addition to lifting things, pulleys can be used singly or with
many pulleys working together in order
to transport people or things.
Pulleys mostly changes the direction of force
to make things work easier
7. It is a collection of one or more wheels over which
you loop a rope to make it easier to lift things
If you want to lift a really heavy weight, there's only so
much force your muscles can supply, when if you're the
world's strongest man but if you use a simple machine
such as a pulley you can effectively multiply the force your
body produces.
Pulleys can be used everywhere to help make tasks easier.
In addition to lifting things pulleys can also be used
together with the other pulleys to transport the heavy
objects .They can also be used to provide power from one
shaft to another.
8. Construction:
PARTS:
Pulleys are nothing but,
Gears without teeth.
and instead of running together
directly they are made to drive
one another by
Cords
Ropes,
Cables
Belting of some kinds.
To be a pulley a simple machine must
have a rope or belt.
A Simple pulley is made up of TWO MAIN PARTS:
I. A pulley wheel that can spin around,
II. A rope that goes around it.
One cannot work without the other.
a pulley has a groove around its rim. This stops
the rope from sliding from side to side. The
groove also helps to grip the rope. Pulleys are
modified levers; their fulcrum is at their center.
9. The Main Part Of Pulley is BELT.
Belts are looped over pulleys and may
have a twist between the pulleys and
the shafts need not be parallel. In
TWO PULLEY SYSTEM
the belt can either drive the
pulleys normally in one direction
(the same if on parallel shafts),
or the belt may be crossed, so that
the direction of the driven shaft is
reversed.
A conveyor belt is one application where the belt is adapted to continuously
carry a load between two points.
10. TYPES:
FLAT BELT
ROUND BELT
V-BELT
PULLEY WITHOUT WHEEL:
You can make a pulley without a wheel. However there would be a
friction between the rope and the metal and the rope would not be
easily moved.
11. Forces Involved:
The weight of the load does not change, it is the
same for each trial.
The output force will be the force required to hold
the load still– it does not change since the weight
remains the same
As more strings are added, the input force required
to achieve the same output force decreases.
12. Types Of Pulley:
There are three types of pulleys
Fixed pulley
Movable pulley and
block and tackle,
I will be describing the first type.
The main aim of fixed pulley is it
changes the direction of the force
you exert by pulling, so you can pull
down to move an object up.
In this fixed pulley system, you pull down on one side
of the rope and the other side goes up. You use the same
amount of force to pull down as you would to lift the
object yourself. However, you have changed the direction
of the force..
13. A fixed pulley is useful because when
you pull down, you can use your body’s own
weight to add to the push.
In contrast, when you exert a force upwards by
lifting something, you also have to lift the
weight of
your own arms. Now I will describe the
mechanical
advantage of fixed pulley but first we have to
know
what is mechanical advantage. As the name
suggest it
involve machine.
14. Mechanical Advantage:
Work done in a task is the force applied multiplied by the distance over
which it was applied. The work required to complete a particular task is
fixed. Machines provide us a mechanical advantage by decreasing the
input force but increasing the distance. The formula for mechanical
advantage is:
Mechanical Advantage = Output Force/Input Force
A mechanical advantage of 1 means that the input force is equal to the output
force. A mechanical advantage greater than 1 means that we can accomplish the
work with lesser force but exerting it for a greater distance.
15. Movable Pulley:
A movable pulley is a pulley that is free to move up and down, and is
attached to a ceiling or other object by two lengths of the same rope.
Examples of movable pulleys include construction cranes, modern
elevators, and some types of weight lifting machines at the gym.
For Example:
Using the screws and your screwdriver or drill, attach one fixed pulley
to a point slightly off-center on one of your 2”x4” plank.
Feed the rope through the pulley.
Feed one end of the rope through a moveable pulley.
Attach the end of the rope fed though the movable pulled to the 2x4
plank, about 1 foot away from the fixed pulley.
Set the wooden plank between two high surfaces so the pulleys are
facing down. The movable pulley will hang down like this.
Attach the handle of the toolbox to the movable pulley.
Place different objects in the toolbox until it is heavy. Weigh the
toolbox on the scale.
16. Results will be: The fixed pulley will make the toolbox most difficult
to lift. The moveable pulley will make the toolbox easier to lift. The
compound pulley system will make the toolbox easiest to lift.
Mechanical Advantage
Movable pulleys give you a mechanical advantage, which is measure of
performance for machines. Having a mechanical advantage means the
force applied by the system is greater than the force you put in. In a
pulley system, this means the pulley can lift the object with a greater
amount of force than you apply to the rope. This allows you to lift heavy
things more easily.
17. Block And Tackle:
A Good way To Pull Heavy Loads.
DEFINITION:- A block is a set of pulleys or "sheaves" mounted on a
single axle. The assembly of two blocks with a rope threaded through
the pulleys is called tackle.
Block And Tackle System are among the simplest and most useful
machines that man has ever invented. Both are used for lifting loads
and for controlling the motion of a rope or cable. A block is a set of
pulleys designed to rotate separately but along the same axis; such a
construction is often referred to as a block and tackle, and may be
rigged in different ways, for many purposes.
Gun tackle
Gyn tackle
Double tackle
And others
18. Working:
A block and tackle is a system of two or more pulleys
with a rope or cable threaded between them, usually
used to lift or pull heavy loads.The pulleys are
assembled together to form blocks and then blocks are
paired so that one is fixed and one moves with the load.
Generally, Block And Tackle are used to change the
direction of an applied force.
19. Example:
This arrangement actually does change things in an important way. You can
see that the weight is now suspended by two pulleys rather than one. That
means the weight is split equally between the two pulleys, so each one holds
only half the weight, or 50 pounds (22.7 kilograms). That means that if you
want to hold the weight suspended in the air, you only have to apply 50
pounds of force (the ceiling exerts the other 50 pounds of force on the other
end of the rope). If you want to lift the weight 100 feet higher, then you have to
reel in twice as much rope 0- 200 feet of rope must be pulled in. This
demonstrates a force-distance tradeoff. The force has been cut in half but the
distance the rope must be pulled has doubled.
20. Applications:
Single Fixed Pulley:
Wells use the pulley system to hoist the
bucket out of the well.
Flagpoles use pulleys in order to hoist the
flag up or to bring it down. You pull the
string on the pulley and the flag runs up
or down the pole.
Rock climbers use pulleys to help them
to climb
This kind of pulley system is used in some
elevators. The elevator has cable attached
to it that goes up, around a pulley, then
comes down
21. Movable Pulley:
ESCELATORS Are example of moveable pulley used for
transporting people from one side to other.
Modern cranes are also examples of moveable pulley.
The pedaling system of a bicycle is a moveable pulley.
Some elevators and cable cars use moveable pulleys.
