HU 121 History of Engineering and Technology

Fall 2012/13

Pharos University in AlexandriaFaculty of Engineering

Machines

Lect5

HU121_Lect5_Machines 2

ILOs

• Review the CDIO concept: Products and Systems

• Simple machines and compound machines

• Illustrative examples within time-line perspective.

• Perpetual motion machines.

HU121_Lect5_Machines 3

Engineer

“Engineers Conceive, Design,

Implement and Operate complex products and systems in a team-based

environment”

http://www.cdio.org/MIT and KTH, 2000Present 30 countries

HU121_Lect5_Machines 4

Machine Design

What is Machine Design?

Creation of new and better machines AND

Improving existing ones

So that it is economical in the cost of production and

operation.

HU121_Lect5_Machines 5

Machines and Engines

• Simple machines are prehistoric.

• More complex engines were introduced in the middle ages using – human power, – animal power, – water power, – wind power – steam power.

Still in use today

HU121_Lect5_Machines 6

Machines and Engines

• Simple machines are simple tools used to make work easier.

Compound machines have two or more simple machines working together to make work easier.

• Engines convert energy into useful mechanical motion.

HU121_Lect5_Machines 7

Simple Machines

HU121_Lect5_Machines 8

Timeline of Simple Machines3rd century BC • The idea of a "simple machine" originated by

Archimedes around the, who studied the "Archimedean" simple machines: lever, pulley, and screw.

• He discovered the principle of mechanical advantage in the lever.

ca. 10–75 AD• Heron of Alexandria

in his book Mechanics lists five mechanisms; lever, windlass, pulley, wedge, and screw, and describes their fabrication and uses..

HU121_Lect5_Machines 9

Ideal Mechanical Advantage

• A simple machine multiplies the input force to accomplish a task

• The factor by which it multiplies the force is often called the "mechanical advantage".

• If you idealize the machine by neglecting friction, then you can state an "ideal mechanical advantage" or IMA for the machine.

HU121_Lect5_Machines 10

1. Liver

• The lever is a "simple machines" from which many more complex machines are derived.

• A lever provides a multiplication of force, resulting from the equilibrium of torques, where an input force Fe with a long lever arm Le can balance a larger resistance force Fr with a short lever arm Lr.• Therefore, Fe = (Lr/Le)Fr • and the ideal mechanical advantage is given by:

Fr/Fe = Le/Lr.

HU121_Lect5_Machines 11

2. Wheel and Axle• The wheel and axle combination

constitutes one of the so-called "simple machines" from which many more complex machines are derived.

• The principle of operation is essentially a lever, since it depends upon the effort force Fe having a longer lever arm than the resistance force Fr

• The ideal mechanical advantage is just the ratio of those lever arms R/r. • The clear advantage of the wheel and axle over a simple lever is that the distance of travel is limited only by the amount of rope or cable that you can wrap around the wheel or axle.

HU121_Lect5_Machines 12

2. The Incline• The incline is a "simple machine" from

which many more complex machines are derived.

• By pushing an object up an inclined surface, one can move the object to height h with a smaller force than the weight of the object.

• Neglecting friction, the mechanical advantage could be determined by just setting the input work (pushing the object up the incline) equal to the output work (lifting the object to height h).

FeL = Frh

• Therefore, the ideal mechanical advantage is:

Fr/Fe = L/h.

HU121_Lect5_Machines 13

4. The Wedge

• The wedge works by the same principles as the incline: A smaller force working over a longer distance can produce a larger force acting through a small distance.

• As a double incline, its ideal mechanical advantage is the ratio of the depth of penetration L to the amount of separation t.

HU121_Lect5_Machines 14

5. The Pulley• With a single fixed-axis pulley, the

ideal mechanical advantage is just N=1.

• With a suspended pulley (in the middle), the upward forces in the two ropes is equal, and therefore each supports half of the load, giving an IMA of N=2.

• With a four-pulley set as shown, you have four ropes supporting the load, so the effort force Fe that establishes the rope tension is just one-fourth of the load in the ideal case, so IMA=4.

• All these force relationships are obtained from the force equilibrium condition, which in this case just amounts to "forces up = forces down".

HU121_Lect5_Machines 15

6. The Screw• When a screw is turned once, it

advances by the distance between adjacent screw threads. This distance is commonly called the "pitch" of the thread (P).

• As shown, the handle also adds a lever. The handled is moved one circumference 2πL to lift the load by the amount P. So the ideal mechanical advantage is 2πL/P.

HU121_Lect5_Machines 16

HU121_Lect5_Machines 17

Screw Pump

HU121_Lect5_Machines 18

Screw Extruder

HU121_Lect5_Machines 19

Water Wheelsconvert the energy of free-flowing or falling water into useful forms.

A water wheel consists of a large wooden or metal wheel, with blades or buckets arranged on the outside rim forming the driving surface.

Prior uses of water wheels include milling flour and grinding wood into pulp for papermaking.

HU121_Lect5_Machines 20

Applications of Water Wheels

Mine hoist

HU121_Lect5_Machines 21

Applications of Water Wheels

Saw Mill

HU121_Lect5_Machines 22

Sakia

• In the Alexandrian War in 48 BC Caesar's enemies employed waterwheels to pour sea water from elevated places on the position of the trapped Romans

• a water wheel used primarily in irrigation. • It is a large hollow wheel, normally made of metal sheet.

HU121_Lect5_Machines 23

Water Wheels in the Muslim World

• The engineers of the Islamic world developed several solutions to achieve the maximum output from a water wheel.

• One solution was to mount them to piers of bridges to take advantage of the increased flow.

• Shipmill, a type of water mill powered by water wheels mounted on the sides of ships as they sail. This technique was employed along the Tigris and Euphrates rivers in 10th century Iraq,

• The engineers Al-Jazari in the 13th century and Taqi al-Din in the 16th century described many inventive water-raising machines in their technological treatises.

HU121_Lect5_Machines 24

Norias of Hamah• The noria wheel is up to 20 meters in diameter.

• Wooden boxes attached to the wheel raise the water from the river into an artificial channel at the summit of the wheel's rotation.

• The water is then led by gravity along a series of aqueduct channels.

HU121_Lect5_Machines 25

Persian Wheel• The first recorded perpetual motion machines were

documented by the Indian author Bhaskara (c. 1159), who made sketches of something called the “Persian Wheel”

• This device suggests flowing water (or sand) to create a “perpetual motion”

25

HU121_Lect5_Machines 26

Perpetual Motion • The first sketch of a perpetual motion machine in the

Western world was made by Villard de Honnecourt (c. 1250).

• The principle was similar tothe “Persian Wheel”, but itrelied on levers and weightsto achieve imbalance.

• It was studied extensively by Leonardo Da Vinci

in the late 1400's.

26

HU121_Lect5_Machines 27

Leonardo DaVinci’s Machine

15th Century • On one side of the wheel the hammers

extend straight out, making a longer lever arm (the distance from the center of the wheel to the hammer's head).

• On the opposite side of the wheel, the hammers lay close and make a short lever arm.

• Since the lever ratio is greater where the

hammers stick out, it is unbalanced and makes the system want to tip, and thus the wheel to rotate.

• As it rotates around, the next hammer falls

into place continuing the motion "perpetually".

HU121_Lect5_Machines 28

Animation

Would such a device work? Why?

HU121_Lect5_Machines 29

Review Questions1. (a) Distinguish machine and engine

(b) Define “simple machine”

2. (a) To which age do simple machines belong?(b) Sckitch the six types of simple machines.

3. (a) Define the ideal mechanical advantage (IMA).

(b) Derive IMA for one simple machine.

4. (a) What is a “perpetual motion machine”?(b) Is “perpetual motion” possible? Why?

5. (a) Describe a water wheel.(b) Show how water wheels was evolved into water turbines.

HU121_Lect5_Machines 30