d&t upper sec mechanisms_regent sec singapore

DESIGN & TECHNOLOGY

MECHANISMS

UPPER SECONDARY

MECHANISMS

INTRODUCTION TO

MECHANISMS

MECHANISMS

What are mechanisms?

Mechanisms are used to make simple machines

that make work easy.

MECHANISMS EVERYWHERE

Can you identify the mechanisms?

How did the Egyptians build the pyramid?

ANCIENT & MODERN MECHANICAL SYSTEMS

Compare between the ancient and modern

mechanical systems?

Well Water pump

Trebuchet Missile launcher Back hoe Excavator

ANCIENT & MODERN MECHANICAL SYSTEMS

TREBUCHET MISSILE LAUNCHER

DIGGING EXCAVATOR

MECHANICAL SYSTEMS

Mechanisms are mechanical systems.

A mechanical systems is used to change

one kind of movement into another.

A mechanical system has an INPUT, a PROCESS and an OUTPUT

MECHANISM

Input motion Output motion

Process

An open loop system is a one way process. The water will continue

flowing until someone close the tap.

MECHANICAL SYSTEMS

OPEN AND CLOSED LOOP SYSTEMS

Open-Loop control

PROCESS

Input Output

Turns tap Releases

water

Screw mechanism

MECHANICAL SYSTEMS

OPEN AND CLOSED LOOP SYSTEMS

Closed-Loop control

PROCESS

Input Output

Fills water Releases

waterLever

mechanism

Feedback

float is low

An closed-loop control system can respond to changes in situation

using feedback.

Toilet Cistern

Toilet cistern :

1.Cistern 2.Flapper 3.Rim 4.S-bend (S- trap)

5.Main drain 6.Float 7.Ballcock lever 8.Inlet valve

MECHANICAL SYSTEMS

Closed-Loop control(1)Cistern lever push down.

(2)Flapper lift up and release

water into (3)rim.

(6)Float will lowered down until all

water released and lift up (7)ballcock.

(7)Ballcock will let in water to fill

up the tank.

(7)Ballcock will rise as water to fill

up the tank and close again the inlet.

TYPES OF MOVEMENT

FOUR BASIC MOTIONSCircular motion Oscillating motion

Linear motion Reciprocating motion

QUIZ

Both the oscillating and reciprocating motion are

moving to and fro. What’s the difference between

the two movements that distinct them?

Oscillating is moving to and fro but at an

angular movement

Reciprocating is moving to and fro but in

linear motion.

FUNCTIONS OF MECHANISMS

The functions of mechanisms can be grouped into :

Conversion of motion Transmission of motion

Control of motion

•The speed of movement

•Transmission of motion

•Control of motion

FUNCTIONS OF MECHANISMS

Conversion of Motion

Mechanisms change one type of movement into another.

Screw mechanisms – rotary to linear motion.

FUNCTIONS OF MECHANISMS

Transmission of Motion

Mechanisms change the place of movement.

Lever-linkage mechanisms

linkage lever

Control of Motion

Mechanisms can

change the speed of movement.

FUNCTIONS OF MECHANISMS

Mechanisms can

change the direction of movement.

Gear mechanisms

Gear mechanisms

Clock

Hand drill

FUNCTIONS OF MECHANISMS

Control of Motion

Mechanisms can

change the distance of movement.

Mechanisms can

change the amount of force produced.

Screw-linkage mechanisms –

rotary to linear motion

Linkage mechanisms

Pantograph

Lifting jack

Mechanisms Activity 1

Construct a pantograph.

Challenge :

You are to construct a pantograph using the ice-cream

sticks.

Discussion :

Do the lengths of each part affect

the enlargement/reduction of the

drawing?

Duration :

10 mins

MECHANICAL SYSTEMS

MECHANISMS

MECHANICAL

CONTROL

MECHANISMS

LEVERS

MECHANICAL CONTROL

LEVERS

A lever helps us to do work. With a lever, a small input force (effort)

can create a large output force. There are 3 classes of levers and the

relationship between the effort, load and fulcrum determine its class.

E

FL

L

E

F

MECHANICAL CONTROL

LEVERS

MECHANICAL CONTROL

Classes of Levers

fulcrum fulcrum fulcrum

Force you

apply

Force you

produce

Fulcrum

L E

F LE

F

LE

F

MECHANICAL CONTROL

Examples of Levers

Wall nut cracker

2nd class

Scissor

1st class

Tweezer

3rd class

Rowing using an oar

1st class

Crowbar

2nd class

E

F

L

EF

L

EFL

E

F

LE

F

L

QUIZ

A pair of scissors is actually made up of two first

class levers. It is easy to cut a paper but not a

thick cardboard. How will you modify the scissors

so that cutting a thick cardboard is easy? Why?

Move the load closer to the fulcrum just like a pair of

snips. The shorter distance of the load to the fulcrum will

create a smaller moment. Thus less effort needed.

E F

LE

F

L

QUIZ

Is a wheel a group of 1st class levers?

Yes it is ! The 1st class levers are connected at their

common pivot and as they turn they form a wheel.

MECHANICAL CONTROL

LEVERS- Wheels and axles

A wheel and axle is a

form of a lever.

wheelaxle

MECHANISMS

LINKAGES

MECHANICAL CONTROL

LINKAGES

Levers are sometimes connected in different ways to create linkage.

LINKAGES

- Lever-linkage

MECHANICAL CONTROL

Pantograph

Tool box Scissor platform

Mechanism turn small

effort into large pushing

force.

Mechanism enlarges or reduces

sketches.

Mechanism open and close toolbox.

MECHANICAL CONTROL

LINKAGES

- Four-bar linkage

Four-bar linkage has four

connected parts.

Convert motion from :

•One type to another

•One speed to another

•One size to another

•One axis to another

MECHANICAL CONTROL

Examples of Four-bar linkage

MECHANICAL CONTROL

LINKAGES

- Types of linkage

Mechanisms Activity 2Construct a four-bar linkage.

MECHANISMS

Procedure :

Make up a four bar linkage as shown using the ice cream sticks

provided. Used the paper fasteners for the moving pivots and

drawing pins for the fixed pivots.

Discussion :

Investigate how strips of different

length affect the movement.

Duration :

10 mins

MECHANISMS

PULLEYS

MECHANICAL CONTROL

A pulley is used to transmit motion using TWO types of pulley system :

PULLEYS

Lifting Pulley System :

To lift heavy loads using rope

or chain.

Driving Pulley System :

To transfer rotary motion from one

shaft to another using belt.

MECHANICAL CONTROL

PULLEYS

Pulley

Rope

Force

Weight

Fixed Pulley• Object moves

• Pulley stays in the same spot

• Force applied only on one end of the rope

MECHANICAL CONTROL

Pulleys

- lifting pulleys

Pulley

Rope

Force

Weight

Movable Pulley• Pulley is attached to object

• Pulley and object move together

• Rope is attached to something

that does not move

• Force applied to other end of rope

Reaction

Force

MECHANICAL CONTROL

Pulleys

- lifting pulleys

QUIZ

Why do we need a fixed and movable pulley?

What’s their uses?

Pulleys help us to

do work easily .

A movable pulley

has a mechanical

advantage over a

fixed pulley.

MECHANICAL CONTROL

Pulleys

- lifting pulleys

A fixed and movable pulleys can be combined to form a compound pulley.

Advantage:

More pulleys, less effort to lift up weight.

Fixed pulley

Movable pulley

2 pulleys 3 pulleys 4 pulleys

QUIZ

What are the mechanisms used by the mobile

crane to lift up heavy objects? Do you think the

crane can lift twice its own weight?

The mechanism is a pulley system.

By using compound pulley system,

the crane can lift up weight twice its

own. But the mobile crane needed the

Support of the outrigger to anchor it to

the ground to transmit the reaction load

to the ground for stability.

Flat Belt Toothed BeltVee Belt

MECHANICAL CONTROL

Pulleys

-Belt pulleys

Advantages :

quickly and easily slid into

position over the edge of

the pulleys

can be driven at high

speeds

Disadvantages :

can not transmit large load

slipping when overloaded

Advantages :

Better grip than flat belt

(more efficient),

Can transmit larger load

than flat belt

Disadvantages :

relatively difficult to fit

slipping when overloaded

Advantages :

very little noise is produced

transmit high power load

no slipping

Disadvantages :

difficult to manufacture

MECHANICAL CONTROL

Pulleys

-Belt pulleys

Flat belt pulley

Eg. Conveyor belt

Vee belt pulley

Eg. Bench drilling

machine

Toothed belt pulley

Eg. Motorcycle gears

MECHANICAL CONTROL

Pulleys

-Open and crossed drive belt pulleys

Open drive :

pulley rotate same direction.

Crossed drive :

pulley rotate in opposite direction.

QUIZ

How can you overcome the problem of ‘slip’ for

the flat belt pulley without using other type of belt

pulleys?

Use a pulley with groove that will hold

the pulley in placed.

MECHANICAL CONTROL

Pulleys

-Driver and driven pulleys

Diameter of Driven pulley > Diameter of Driver pulley

Driven pulley turns than Driver pulley.

Diameter of Driven pulley < Diameter of Driver pulley

Driven pulley turns than Driver pulley.

MECHANICAL CONTROL

Pulleys

- Applications of pulleys

MECHANICAL CONTROL

Pulleys

- Applications of pulleys

Pulleys transmit

movement from

Motor to drum

Washing

machine

Pulleys lift and lower

weights for crane.

Gantry crane

Blinds

Pulleys raise and

Lower sails and

blinds.

If both pulleys are of the same diameter,then they will rotate at the same speed.

When one pulley is larger than another, thenmechanical advantage and velocity ratio

are introduced.

MECHANICAL CONTROL

Mechanical Advantage(MA) & Velocity Ratio(VR)

MECHANICAL CONTROL

Mechanisms are often used to allow a small effort to move a

large load. This property is called Mechanical Advantage (MA).

Mechanical advantageis calculated by dividingthe load by the effort.

Mechanical Advantage(MA) & Velocity Ratio(VR)- Mechanical Advantage

Mechanical Advantage = Output = Load

Input Effort

MECHANICAL CONTROL

Mechanisms are used to translate a small amount of movement into

a larger amount. This property is known as Velocity Ratio (VR).

It can be calculated by dividing

the movement of the effort by

the movement of the load.

Mechanical Advantage(MA) & Velocity Ratio(VR)

- Velocity Ratio

Velocity Ratio = Input = Distance moved by Effort

Output Distance moved by Load

MECHANICAL CONTROL

Mechanical Advantage(MA) & Velocity Ratio(VR)

- EfficiencyIn an ideal world, mechanical advantage and velocity ratio would always

be equal to each other. In reality, because of friction, air resistance, this

ideal situation would never be achieved.

We said the system is not 100% efficient.

Efficiency = Mechanical Advantage X 100%

Velocity Ratio

MECHANICAL CONTROL

Mechanical Advantage(MA) & Velocity Ratio(VR)- Mechanical Advantage for Lifting Pulley System

No. of Pulley = 2 No. of Pulley = 4No. of Pulley = 3

MA = 2 MA = 3 MA = 4

Mechanical Advantage (MA) = No. of Pulleys

QUIZ

How many pulleys are used to raise the shelter

and its mechanical advantage?

3 pulleys are used. M.A. = 31st

pulley

2nd

pulley

3rd

pulley

You can also count the no. of

rope lifting the load equal to no.

of pulleys.

MECHANICAL CONTROL

Mechanical Advantage(MA) & Velocity Ratio(VR)- Mechanical Advantage for Lifting Pulley System

Mechanical Advantage (M.A) = No of pulleys

= 4

M.A = Output = Load

Input Effort

4 = 200

Effort

Effort = 200 = 50 N

4

Mechanical Advantage = Output = Load

Input EffortEffort

Load

200N

MECHANICAL CONTROL

Mechanical Advantage(MA) & Velocity Ratio(VR)- Mechanical Advantage for Lifting Pulley System

Effort

Load

Mechanical Advantage (M.A) = No of ropes supporting load

= 4

Effort = 50 N

200 N

Each rope will exert an effort of F Newton.

F + F + F + F = 200

4 F = 200

F = 200

4

= 50 N

200N

MECHANICAL CONTROL

Mechanical Advantage(MA) & Velocity Ratio(VR)- Mechanical Advantage for Lifting Pulley System

MECHANICAL CONTROL

Mechanical Advantage(MA) & Velocity Ratio(VR)- Velocity Ratio for Lifting Pulley System

Velocity Ratio = Input = Distance moved by Effort

Output Distance moved by Load

10cm

40cm

VR = 40 = 4

10

In order to lift the 200 N load with a small

effort of 50 N, the effort of pulling the rope

will have to move 4 times longer than the

distance lifted for the load.

10cm

40cm

Effort

Load

200N

MECHANICAL CONTROL

Mechanical Advantage(MA) & Velocity Ratio(VR)- Velocity Ratio for Lifting Pulley System

MECHANICAL CONTROL

Mechanical Advantage(MA) & Velocity Ratio(VR)- Efficiency for Lifting Pulley System

Efficiency = Mechanical Advantage X 100%

Velocity Ratio

Efficiency = MA x 100%

VR

= 4 x 100

4

= 100 %

MA = 4

VR = 4

This is a perfect condition but in reality the efficiency will be less

than perfect due to friction.

Effort

Load

200N

QUIZ

The same 4 pulley system is used to lift up 200 N of

load. But when the effort was measured it showed 60

kg more than in ideal condition. What’s the efficiency

and why is the effort higher than calculated?

VR = No of pulleys

= 4

MA = Load

Effort

= 200

60

= 3.33

Efficiency = MA x 100

VR

= 3.33 x 100

4

= 83.3 %

A higher effort is needed to overcome the

frictional force between the rope and the

pulley during lifting.

Effort

60N

Load

200N

MECHANICAL CONTROL

Velocity Ratio(VR)

Velocity Ratio (VR) is the relationship between the input and

output movements in a mechanical system. It is also known as

Transmission Ratio (TR).

Output movement

Input movementVelocity Ratio =

The ratio can be used to compare distances, angles or number of revolutions.

MECHANICAL CONTROL

Velocity Ratio(VR)- Velocity Ratio for Belt Pulley System

motor

DriveR pulley

DriveN pulley

dia. of DriveR pulley

dia. of DriveN pulley

Output Speed

Input SpeedVelocity Ratio = =

MECHANICAL CONTROL

dia. 200 mm

dia. 40 mm

Motor,

1500 rpm

Drum

Velocity Ratio(VR)- Velocity Ratio for Belt Pulley System

Velocity Ratio = dia. of DriveR pulley

dia. of DriveN pulley

1

5

Output speed

Input speed=

= =40

200

1

5

5

1500Output speed =

= 300 rpm

QUIZ

Both MA and VR are ratio of their output to input. But

for a belt pulley, why the VR in terms of pulley diameter

is input diameter to output diameter and sometimes

the opposite?

MA = Output force

Input forceVR = Input movement

Output movement

Input speed

(Driver)

Output speed

(Driven)Output speed

(Driven)

Input speed

(Driver)

Ø input Ø output Ø input Ø output

VR is inversely proportional to Ø input / Ø output

VR = Ø Output / Ø Input

Input speed/output speed = Ø Output / Ø Input

MECHANISMS

CAMS

MECHANICAL CONTROL

CAMS

A cam is a specially designed and shaped piece of material that rotates,

causing a lever or rod to move.

MECHANICAL CONTROL

CAMS

- Rotary camsThe THREE common type of cams :

Pear Snail Eccentric

The cam-and-follower converts circular movement to a kind of oscillatory

motion. It cannot work the other round.

MECHANICAL CONTROL

CAMS

- Linear cams

MECHANICAL CONTROL

CAMS

- Pear shaped cam

The follower remains motionless for about

half of the cycle of the cam and during the

second half it rises and falls.

follower

pear shaped cam

slide

CAMS

- Pear shaped cam

MECHANICAL CONTROL

Dwell

Dwell is the period when the follower does not move

MECHANICAL CONTROL

CAMS

- Snail shaped cam

Disadvantage : Rotating in a clockwise

direction would probably lead to the entire

mechanism jamming.

A snail drop cam is used where the drop

or fall of the follower must be sudden.

MECHANICAL CONTROL

CAMS

- Snail shaped cam

MECHANICAL CONTROL

CAMS

- Eccentric cam

An eccentric cam is a disc with its centre of

rotation positioned ‘off centre’. This means

as the cam rotates the flat follower rises

and falls at a constant rate.

MECHANICAL CONTROL

Cams

- Eccentric cam

MECHANICAL CONTROL

Cams

- Distance and rotation graphs

MECHANICAL CONTROL

CAMS

-Applications of cams

Cam key lock

MECHANICAL CONTROL

CAMS

-Applications of cams

Cam operated

pushchair brake

Cam timer

Internal combustion engine

QUIZ

A local toy shop has asked you to design a

model to encourage parents to buy their young

children mechanical toys. The partially made

model is seen opposite. Add a suitable cam that

controls two followers so that they rise and fall.

As the swash cam rotates the

Followers move up and down

alternately. The swash cam

operates like a ‘spinning top’.

The followers move the arms

of the model up and down as

if it is waving.

MECHANICAL CONTROL

GEARS

MECHANICAL CONTROL

GEARS

MECHANICAL CONTROL

GEARSA gear is a wheel with teeth around its edges. Gears can be combined in

different ways to :

- Control speed

- Increasing turning force

- Changing direction of motion

Spur gear Worm gear Bevel gear

Gears are used to transmit power and motion.

MECHANICAL CONTROL

GEARS

- Spur gears• Two gears meshes together

• Both gears rotate in opposite directions

to each other

How do you make the spur gears rotate

in the same direction?

Add idler gear

MECHANICAL CONTROL

GEARS

- Spur gears

MECHANICAL CONTROL

GEARS

- Spur gears• Several spur gear meshes together form GEAR TRAIN.

GEAR TRAIN

Gear train has a driver and driven gear.

Driver gear is connected to a motor to drive the driven gear.

MECHANICAL CONTROL

GEARS

- Gear ratio

MECHANICAL CONTROL

GEARS

- Gear ratio

No of teeth on DriveR gear

No of teeth on DriveN gearGear Ratio =

Note : Gear ratio is also known as Velocity Ratio (VR)

MECHANICAL CONTROL

GEARS

- Gear train and Compound gear train• make large speed change

• increase or decrease the torque (turning force)

QUIZ

How do you increase the gear ratio of a spur

gear without replacing any of the gears?

You can increase or decrease

the gear ratio by adding gears

to form a compound gear train.

MECHANICAL CONTROL

GEARS

- Bevel gears• to transmit motion through 90⁰

GEARS

- Worm gear

MECHANICAL CONTROL

• to turn a worm wheel

•reduce speed considerably but increase turning force

•worm gear has only ONE tooth.

•Worm gear is a ONE-WAY drive system

If worm wheel has 50 teeth, worm gear

must rotate 50 times.

MECHANICAL CONTROL

GEARS

- Applications of worm gear

MECHANICAL CONTROL

GEARS

- Applications of gears

MECHANICAL CONTROL

CRANKS

MECHANICAL CONTROL

CRANKSA cam is an arm that has one end connected to a shaft.

MECHANICAL CONTROL

CRANKS

- Cranks and slider

Cranks and slider converts

movement from circular to

reciprocating motion or the

other way round.

MECHANICAL CONTROL

CRANKS

- Applications of cranks

Sheet metal roller Piston engine

MECHANISMS

RACK-AND-PINION

MECHANICAL CONTROL

RACK & PINION MECHANISMSThe rack-and –pinion mechanism consists of a straight toothed ‘rack’ that

meshes with a toothed wheel called a ‘pinion’.

Rack-and-pinion mechanisms – rotary to linear motion.

Rack

Pinion

Rack

MECHANICAL CONTROL

RACK-AND-PINION

-Applications of rack-and-pinion

Railway track Car steering wheel Bench drilling machine

QUIZ

How does a train manage to climb up a steep

slope and a forklift able to lift heavy things?

By using the rack and pinion.

MECHANISMS

RATCHET-AND-PAWL

MECHANICAL CONTROL

RACHET & PAWL MECHANISMSThe ratchet & pawl mechanism allows movement in one direction but not

the other.

Spring-loaded

The pawl allows the teeth to move one way but the other.

MECHANICAL CONTROL

RATCHET & PAWL MECHANISMS

- Applications of ratchet & pawl

Ratchet Fishing reel

MECHANISMS

SPRING-LOADED

MECHANISMS

MECHANICAL CONTROL

SPRING-LOADED MECHANISMSSprings store elastic energy that can be released to provide a return

movement in mechanisms.

Air pumpClockwork motor

The spring can be used in tension or compression.

Spring in compression

Spring in tension

MECHANICAL CONTROL

SPRING LOADED MECHANISMS

- Compression and Tension Spring

A compression spring is used to resist a squashing force

or compressive force.

A tension spring is used to resist a stretching force

or tensile force.

MECHANICAL CONTROL

SPRING LOADED MECHANISMS

- Torsion and Flat Spring

A torsion spring is used to resist

a turning force or torque force.

A flat spring is a piece of material

that returns to its original shape

when bent.

MECHANISMS

SCREWS

MECHANICAL CONTROL

SCREWA screw is a ramp wrapped around a cylinder. It can produced a very large

force.

Screw mechanisms – rotary to linear motion

MECHANICAL CONTROL

SCREW

MECHANICAL CONTROL

SCREW

- Applications of screw

CABLE CONTROL

MECHANISMS

MECHANICAL CONTROL

CABLE CONTROLCable allow things to be controlled from some distance away.

- operated with pedal or lever

- wound on rotating drum

Cable work well in tension for pulling things.

MECHANICAL CONTROL

CABLE CONTROL

- Applications of cable control

Concrete has HIGH compressive strength but LOW tensile

strength. When a heavy load is placed on a concrete beam, the

bottom is likely to crack as it is in tension. Therefore Engineer’s

overcome the low tensile strength of the concrete by adding

reinforcement steel bars along the tensile stress area.

QUIZ

Why do concrete structures have metal bars

(reinforcements) inside them?

Activity

Build and test different shape of beams using papers

by folding. Find out which shape is the strongest.

Try the activity worksheet to find the answer!

MECHANICAL CONTROL

QUIZ

A thin sheet of wood usually breaks easily

along its grain. However, if a few sheets of

wood are glued together with their grains at

900 to one another, this will caused the

completed structure to be stronger.

The layers of ply with the grains at 900 to one another created

an interlocking system that make it strong.

MECHANICAL CONTROL