Mechanisms by hokokwah

DESIGN & TECHNOLOGY 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

Water pump

Well

Trebuchet

Missile launcher

Back hoe

Compare between the ancient and modern mechanical systems?

Excavator

ANCIENT & MODERN MECHANICAL SYSTEMS

TREBUCHET

DIGGING

MISSILE LAUNCHER

EXCAVATOR

MECHANICAL SYSTEMS Mechanisms are mechanical systems. A mechanical systems is used to change one kind of movement into another. Input motion

Output motion

MECHANISM Process

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

MECHANICAL SYSTEMS OPEN AND CLOSED LOOP SYSTEMS Open-Loop control Input

Screw mechanism

Output

PROCESS Turns tap

Releases water

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 Closed-Loop control Input

Feedback float is low

Output

PROCESS Fills water

Lever mechanism

Releases water Toilet Cistern

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

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.

Toilet cistern : 1.Cistern 2.Flapper 3.Rim 4.S-bend (S- trap) 5.Main drain 6.Float 7.Ballcock lever 8.Inlet valve

TYPES OF MOVEMENT FOUR BASIC MOTIONS Circular motion

Linear motion

Oscillating motion

Reciprocating motion

QUIZ Both the oscillating and reciprocating motion are moving to and fro. Whatâ&#x20AC;&#x2122;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 â&#x20AC;&#x201C; rotary to linear motion.

FUNCTIONS OF MECHANISMS Transmission of Motion Mechanisms change the place of movement.

lever

linkage

Lever-linkage mechanisms

FUNCTIONS OF MECHANISMS Control of Motion Clock

Mechanisms can change the speed of movement. Gear mechanisms Hand drill

Mechanisms can change the direction of movement.

Gear mechanisms

FUNCTIONS OF MECHANISMS Control of Motion Pantograph

Mechanisms can change the distance of movement. Linkage mechanisms Lifting jack

Mechanisms can change the amount of force produced. Screw-linkage mechanisms â&#x20AC;&#x201C; rotary to linear motion

MECHANICAL SYSTEMS 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

MECHANISMS

MECHANICAL CONTROL

MECHANISMS

LEVERS

MECHANICAL CONTROL LEVERS

L E

E L

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.

MECHANICAL CONTROL LEVERS

MECHANICAL CONTROL Classes of Levers

fulcrum

Force you apply

Fulcrum

Force you produce

E E E F

L F

MECHANICAL CONTROL Examples of Levers Scissor 1st class

E F

L Wall nut cracker 2nd class

F E

Tweezer 3rd class

F E F Rowing using an oar 1st class

Crowbar 2nd class

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. L E

E 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.

wheel

axle

MECHANISMS

LINKAGES

MECHANICAL CONTROL LINKAGES

Levers are sometimes connected in different ways to create linkage.

MECHANICAL CONTROL LINKAGES - Lever-linkage Mechanism enlarges or reduces sketches.

Pantograph Mechanism open and close toolbox.

Tool box

Mechanism turn small effort into large pushing force.

Scissor platform

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 Mechanisms Activity 2 Construct a four-bar linkage. 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 PULLEYS A pulley is used to transmit motion using TWO types of pulley system :

Lifting Pulley System :

Driving Pulley System :

To lift heavy loads using rope or chain.

To transfer rotary motion from one shaft to another using belt.

MECHANICAL CONTROL PULLEYS

MECHANICAL CONTROL Pulleys - lifting pulleys

Pulley Force

Rope

Fixed Pulley Weight

• Object moves • Pulley stays in the same spot • Force applied only on one end of the rope

MECHANICAL CONTROL Pulleys - lifting pulleys

Reaction Force Rope

Force

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

Pulley

Weight

QUIZ Why do we need a fixed and movable pulley? Whatâ&#x20AC;&#x2122;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

2 pulleys

3 pulleys

4 pulleys

Fixed pulley

Movable pulley

A fixed and movable pulleys can be combined to form a compound pulley. Advantage: More pulleys, less effort to lift up weight.

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.

MECHANICAL CONTROL Pulleys -Belt pulleys

Flat Belt 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

Vee Belt Advantages :  Better grip than flat belt (more efficient),  Can transmit larger load than flat belt Disadvantages :  relatively difficult to fit  slipping when overloaded

Toothed Belt 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 â&#x20AC;&#x2DC;slipâ&#x20AC;&#x2122; 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

Gantry crane

Pulleys lift and lower weights for crane.

Pulleys transmit movement from Motor to drum

Pulleys raise and Lower sails and blinds.

Washing machine

Blinds

MECHANICAL CONTROL Mechanical Advantage(MA) & Velocity Ratio(VR)

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

When one pulley is larger than another, then mechanical advantage and velocity ratio are introduced.

MECHANICAL CONTROL Mechanical Advantage(MA) & Velocity Ratio(VR) - Mechanical Advantage Mechanisms are often used to allow a small effort to move a large load. This property is called Mechanical Advantage (MA). Mechanical advantage is calculated by dividing the load by the effort.

Mechanical Advantage = Output = Load Input Effort

MECHANICAL CONTROL Mechanical Advantage(MA) & Velocity Ratio(VR) - Velocity Ratio 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. Velocity Ratio = Input = Distance moved by Effort Output Distance moved by Load

MECHANICAL CONTROL Mechanical Advantage(MA) & Velocity Ratio(VR) - Efficiency In 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 Mechanical Advantage (MA) = No. of Pulleys

2 MA = 2

No. of Pulley =

3 MA = 3

No. of Pulley =

4 MA = 4

No. of Pulley =

QUIZ How many pulleys are used to raise the shelter and its mechanical advantage? 1st pulley 2nd pulley 3rd pulley

3 pulleys are used. M.A. = 3 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 Mechanical Advantage = Output = Load Input Effort

Effort

M.A = Output = Load Input Effort 4 = 200 Effort 200N Load

Effort = 200 = 50 N 4

MECHANICAL CONTROL Mechanical Advantage(MA) & Velocity Ratio(VR) - Mechanical Advantage for Lifting Pulley System Mechanical Advantage (M.A) = No of ropes supporting load =4 Each rope will exert an effort of F Newton.

Effort

200 N

200N Load

F + F + F + F = 200 4 F = 200 F = 200 4 = 50 N

Effort = 50 N

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 Effort

VR = 40 = 4 10 40cm

40cm 10cm 10cm

200N Load

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.

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 Effort

MA = 4 VR = 4 200N Load

Efficiency = MA x 100% VR = 4 x 100 4 = 100 %

This is a perfect condition but in reality the efficiency will be less than perfect due to friction.

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. Whatâ&#x20AC;&#x2122;s the efficiency and why is the effort higher than calculated? Effort 60N

VR = No of pulleys =4

200N

Load

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.

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).

Velocity Ratio =

Input movement Output movement

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

MECHANICAL CONTROL Velocity Ratio(VR) - Velocity Ratio for Belt Pulley System DriveR pulley

DriveN pulley

Velocity Ratio =

motor

Input Speed Output Speed

dia. of DriveN pulley dia. of DriveR pulley

MECHANICAL CONTROL Velocity Ratio(VR) - Velocity Ratio for Belt Pulley System Drum

dia. 200 mm dia. of DriveN pulley Velocity Ratio =

dia. of DriveR pulley 200

5 =

Input speed

5 =

Output speed

Motor, 1500 rpm

1500 dia. 40 mm

Output speed = =

5 300 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 force Ø input

Input speed (Driver)

Ø output

Output speed (Driven)

VR =

Input movement Output movement

Ø input

Input speed (Driver)

VR is inversely proportional to Ø input / Ø output

VR = Ø Output / Ø Input Input speed/output speed = Ø Output / Ø Input

Ø output

Output speed (Driven)

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 cams

Pear

The THREE common type of cams :

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 slide follower 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.

MECHANICAL CONTROL CAMS - Pear shaped cam

Dwell Dwell is the period when the follower does not move

MECHANICAL CONTROL CAMS - Snail shaped cam

A snail drop cam is used where the drop or fall of the follower must be sudden. Disadvantage : Rotating in a clockwise direction would probably lead to the entire mechanism jamming.

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 â&#x20AC;&#x2DC;spinning topâ&#x20AC;&#x2122;. The followers move the arms of the model up and down as if it is waving.

MECHANICAL CONTROL

GEARS

MECHANICAL CONTROL GEARS

MECHANICAL CONTROL GEARS A 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 â&#x20AC;˘ 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 DriveN gear Gear Ratio = No of teeth on DriveR gear

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

MECHANICAL CONTROL GEARS - Worm gear • 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 CRANKS A 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 MECHANISMS The 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 MECHANISMS The 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 MECHANISMS Springs store elastic energy that can be released to provide a return movement in mechanisms.

The spring can be used in tension or compression. Spring in compression

Clockwork motor Spring in tension

Air pump

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 SCREW A screw is a ramp wrapped around a cylinder. It can produced a very large force.

Screw mechanisms â&#x20AC;&#x201C; rotary to linear motion

MECHANICAL CONTROL SCREW

MECHANICAL CONTROL SCREW - Applications of screw

MECHANISMS

CABLE CONTROL

MECHANICAL CONTROL CABLE CONTROL Cable 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

QUIZ Why do concrete structures have metal bars (reinforcements) inside them?

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 Engineerâ&#x20AC;&#x2122;s overcome the low tensile strength of the concrete by adding reinforcement steel bars along the tensile stress area.

MECHANICAL CONTROL 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!

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