D&T upper Sec Mechanisms_Regent Sec Singapore


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D&T upper Sec Mechanisms_Regent Sec Singapore

  3. 3. MECHANISMS What are mechanisms?Mechanisms are used to make simple machinesthat make work easy.
  4. 4. MECHANISMS EVERYWHERECan you identify the mechanisms?
  5. 5. How did the Egyptians build the pyramid?
  6. 6. ANCIENT & MODERN MECHANICAL SYSTEMS Well Water pump Trebuchet Missile launcher Back hoe ExcavatorCompare between the ancient and modernmechanical systems?
  8. 8. MECHANICAL SYSTEMSMechanisms are mechanical systems.A mechanical systems is used to changeone kind of movement into another. Input motion Output motion MECHANISM ProcessA mechanical system has an INPUT, a PROCESS and an OUTPUT
  9. 9. MECHANICAL SYSTEMSOPEN AND CLOSED LOOP SYSTEMSOpen-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.
  10. 10. MECHANICAL SYSTEMSOPEN AND CLOSED LOOP SYSTEMSClosed-Loop control Feedback Input float is low Output PROCESS Fills water Lever Releases mechanism water Toilet Cistern An closed-loop control system can respond to changes in situation using feedback.
  11. 11. MECHANICAL SYSTEMSClosed-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
  12. 12. TYPES OF MOVEMENTFOUR BASIC MOTIONS Circular motion Oscillating motion Linear motion Reciprocating motion
  13. 13. QUIZBoth the oscillating and reciprocating motion aremoving to and fro. What’s the difference betweenthe 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.
  14. 14. 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
  15. 15. FUNCTIONS OF MECHANISMSConversion of MotionMechanisms change one type of movement into another. Screw mechanisms – rotary to linear motion.
  16. 16. FUNCTIONS OF MECHANISMSTransmission of MotionMechanisms change the place of movement. linkage lever Lever-linkage mechanisms
  17. 17. FUNCTIONS OF MECHANISMSControl of Motion Clock Mechanisms can change the speed of movement. Gear mechanisms Hand drill Mechanisms can change the direction of movement. Gear mechanisms
  18. 18. FUNCTIONS OF MECHANISMSControl 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 – rotary to linear motion
  19. 19. MECHANICAL SYSTEMSMechanisms Activity 1Construct a pantograph.Challenge :You are to construct a pantograph using the ice-creamsticks.Discussion :Do the lengths of each part affectthe enlargement/reduction of thedrawing?Duration :10 mins
  22. 22. MECHANICAL CONTROLLEVERS L E E L F FA 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 therelationship between the effort, load and fulcrum determine its class.
  24. 24. MECHANICAL CONTROLClasses of Levers fulcrum fulcrum fulcrum Fulcrum Force you apply Force you produce L E E L E F F L F
  25. 25. MECHANICAL CONTROLExamples of Levers L Scissor E 1st class F LWall nut cracker Tweezer2nd class F 3rd class L F E E E L F E F Rowing using an oar Crowbar L 1st class 2nd class
  26. 26. 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 ofsnips. The shorter distance of the load to the fulcrum willcreate a smaller moment. Thus less effort needed. L E E F L F
  27. 27. QUIZIs 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.
  28. 28. MECHANICAL CONTROLLEVERS- Wheels and axles A wheel and axle is a form of a lever. axle wheel
  30. 30. MECHANICAL CONTROLLINKAGES Levers are sometimes connected in different ways to create linkage.
  31. 31. MECHANICAL CONTROLLINKAGES- Lever-linkage Mechanism enlarges or reduces sketches. Pantograph Mechanism open and close toolbox. Mechanism turn small effort into large pushing Tool box force. Scissor platform
  32. 32. MECHANICAL CONTROLLINKAGES- 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
  33. 33. MECHANICAL CONTROLExamples of Four-bar linkage
  34. 34. MECHANICAL CONTROLLINKAGES- Types of linkage
  35. 35. MECHANISMS Mechanisms Activity 2 Construct a four-bar linkage.Procedure :Make up a four bar linkage as shown using the ice cream sticksprovided. Used the paper fasteners for the moving pivots anddrawing pins for the fixed pivots.Discussion :Investigate how strips of differentlength affect the movement.Duration :10 mins
  37. 37. MECHANICAL CONTROLPULLEYSA pulley is used to transmit motion using TWO types of pulley system : Lifting Pulley System : Driving Pulley System : To lift heavy loads using rope To transfer rotary motion from one or chain. shaft to another using belt.
  39. 39. MECHANICAL CONTROLPulleys Pulley Force- lifting pulleys Rope Fixed Pulley • Object moves Weight • Pulley stays in the same spot • Force applied only on one end of the rope
  40. 40. MECHANICAL CONTROLPulleys Reaction Force- lifting pulleys Rope Force Movable Pulley • Pulley is attached to object Pulley • Pulley and object move together • Rope is attached to something that does not move Weight • Force applied to other end of rope
  41. 41. QUIZWhy 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.
  42. 42. MECHANICAL CONTROLPulleys 2 pulleys 3 pulleys 4 pulleys- lifting pulleys Fixed pulley Movable pulleyA fixed and movable pulleys can be combined to form a compound pulley.Advantage:More pulleys, less effort to lift up weight.
  43. 43. QUIZWhat are the mechanisms used by the mobilecrane to lift up heavy objects? Do you think thecrane can lift twice its own weight?The mechanism is a pulley system.By using compound pulley system,the crane can lift up weight twice itsown. But the mobile crane needed theSupport of the outrigger to anchor it tothe ground to transmit the reaction loadto the ground for stability.
  44. 44. MECHANICAL CONTROLPulleys-Belt pulleys Flat Belt Vee Belt Toothed Belt Advantages : Advantages : Advantages :  quickly and easily slid into  Better grip than flat belt  very little noise is produced position over the edge of (more efficient),  transmit high power load the pulleys  Can transmit larger load  no slipping  can be driven at high than flat belt speeds Disadvantages : Disadvantages : Disadvantages :  relatively difficult to fit  difficult to manufacture  can not transmit large load  slipping when overloaded  slipping when overloaded
  45. 45. MECHANICAL CONTROLPulleys-Belt pulleys Flat belt pulley Vee belt pulley Toothed belt pulley Eg. Conveyor belt Eg. Bench drilling Eg. Motorcycle gears machine
  46. 46. MECHANICAL CONTROLPulleys-Open and crossed drive belt pulleysOpen drive : Crossed drive :pulley rotate same direction. pulley rotate in opposite direction.
  47. 47. QUIZHow can you overcome the problem of ‘slip’ forthe flat belt pulley without using other type of beltpulleys?Use a pulley with groove that will holdthe pulley in placed.
  48. 48. MECHANICAL CONTROLPulleys-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 pulleyDriven pulley turns than Driver pulley.
  49. 49. MECHANICAL CONTROLPulleys- Applications of pulleys
  50. 50. MECHANICAL CONTROLPulleys Pulleys transmit movement from- Applications of pulleys Motor to drum Gantry crane Pulleys raise and Washing Lower sails and machine blinds.Pulleys lift and lower Blindsweights for crane.
  51. 51. MECHANICAL CONTROLMechanical 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.
  52. 52. MECHANICAL CONTROLMechanical 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
  53. 53. MECHANICAL CONTROLMechanical 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
  54. 54. MECHANICAL CONTROLMechanical 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
  55. 55. MECHANICAL CONTROLMechanical Advantage(MA) & Velocity Ratio(VR)- Mechanical Advantage for Lifting Pulley System Mechanical Advantage (MA) = No. of Pulleys No. of Pulley =2 No. of Pulley =3 No. of Pulley =4 MA = 2 MA = 3 MA = 4
  56. 56. QUIZ How many pulleys are used to raise the shelter and its mechanical advantage? 3 pulleys are used. M.A. = 31st You can also count the no. ofpulley rope lifting the load equal to no. of pulleys.2ndpulley3rdpulley
  57. 57. MECHANICAL CONTROLMechanical Advantage(MA) & Velocity Ratio(VR)- Mechanical Advantage for Lifting Pulley System Mechanical Advantage (M.A) = No of pulleys =4 Mechanical Advantage = Output = LoadEffort Input Effort M.A = Output = Load Input Effort 4 = 200 Effort Effort = 200 = 50 N 200N 4 Load
  58. 58. MECHANICAL CONTROLMechanical 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 F + F + F + F = 200 4 F = 200 F = 200 4 200 N = 50 N 200N Effort = 50 N Load
  59. 59. MECHANICAL CONTROLMechanical Advantage(MA) & Velocity Ratio(VR)- Mechanical Advantage for Lifting Pulley System
  60. 60. MECHANICAL CONTROLMechanical Advantage(MA) & Velocity Ratio(VR)- Velocity Ratio for Lifting Pulley System Velocity Ratio = Input = Distance moved by Effort Output Distance moved by LoadEffort VR = 40 = 4 10 40cm 40cm In order to lift the 200 N load with a small 10cm effort of 50 N, the effort of pulling the rope 10cm will have to move 4 times longer than the distance lifted for the load. 200N Load
  61. 61. MECHANICAL CONTROLMechanical Advantage(MA) & Velocity Ratio(VR)- Velocity Ratio for Lifting Pulley System
  62. 62. MECHANICAL CONTROLMechanical Advantage(MA) & Velocity Ratio(VR)- Efficiency for Lifting Pulley System Efficiency = Mechanical Advantage X 100% Velocity RatioEffort Efficiency = MA x 100% MA = 4 VR = 4 x 100 VR = 4 4 = 100 % 200N This is a perfect condition but in reality the efficiency will be less than perfect due to friction. Load
  63. 63. 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?Effort60N VR = No of pulleys Efficiency = MA x 100 =4 VR = 3.33 x 100 MA = Load 4 Effort = 83.3 % = 200 200N 60 A higher effort is needed to overcome the = 3.33 frictional force between the rope and the Load pulley during lifting.
  64. 64. MECHANICAL CONTROLVelocity 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). Input movement Velocity Ratio = Output movement The ratio can be used to compare distances, angles or number of revolutions.
  65. 65. MECHANICAL CONTROLVelocity Ratio(VR)- Velocity Ratio for Belt Pulley System DriveR pulley DriveN pulley motor Input Speed dia. of DriveN pulley Velocity Ratio = = Output Speed dia. of DriveR pulley
  66. 66. MECHANICAL CONTROLVelocity Ratio(VR)- Velocity Ratio for Belt Pulley System Drum dia. 200 mm dia. of DriveN pulley Velocity Ratio = dia. of DriveR pulley 200 5 = = 40 1 Input speed 5 = Output speed 1 Motor, 1500 Output speed = 1500 rpm dia. 40 mm 5 = 300 rpm
  67. 67. 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 VR = Input movement Input force Output movementØ input Ø output Ø input Ø outputInput speed Output speed Input speed Output speed(Driver) (Driven) (Driver) (Driven) VR is inversely proportional to Ø input / Ø output VR = Ø Output / Ø Input Input speed/output speed = Ø Output / Ø Input
  69. 69. MECHANICAL CONTROLCAMSA cam is a specially designed and shaped piece of material that rotates,causing a lever or rod to move.
  70. 70. MECHANICAL CONTROLCAMS The THREE common type of cams :- Rotary cams Pear Snail Eccentric The cam-and-follower converts circular movement to a kind of oscillatory motion. It cannot work the other round.
  71. 71. MECHANICAL CONTROLCAMS- Linear cams
  72. 72. MECHANICAL CONTROLCAMS- 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.
  73. 73. MECHANICAL CONTROLCAMS- Pear shaped cam DwellDwell is the periodwhen the followerdoes not move
  74. 74. MECHANICAL CONTROLCAMS- 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.
  75. 75. MECHANICAL CONTROLCAMS- Snail shaped cam
  76. 76. MECHANICAL CONTROLCAMS- 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.
  77. 77. MECHANICAL CONTROLCams- Eccentric cam
  78. 78. MECHANICAL CONTROLCams- Distance and rotation graphs
  79. 79. MECHANICAL CONTROLCAMS-Applications of cams Cam key lock
  80. 80. MECHANICAL CONTROLCAMS-Applications of cams Cam operated pushchair brake Cam timer Internal combustion engine
  81. 81. 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.
  84. 84. MECHANICAL CONTROLGEARS A gear is a wheel with teeth around its edges. Gears can be combined indifferent 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.
  85. 85. MECHANICAL CONTROLGEARS- 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
  87. 87. MECHANICAL CONTROLGEARS- 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.
  89. 89. MECHANICAL CONTROLGEARS- 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)
  90. 90. MECHANICAL CONTROLGEARS- Gear train and Compound gear train• make large speed change• increase or decrease the torque (turning force)
  91. 91. QUIZHow do you increase the gear ratio of a spurgear without replacing any of the gears? You can increase or decrease the gear ratio by adding gears to form a compound gear train.
  92. 92. MECHANICAL CONTROLGEARS- Bevel gears • to transmit motion through 90⁰
  93. 93. MECHANICAL CONTROLGEARS- 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 systemIf worm wheel has 50 teeth, worm gearmust rotate 50 times.
  94. 94. MECHANICAL CONTROLGEARS- Applications of worm gear
  95. 95. MECHANICAL CONTROLGEARS- Applications of gears
  97. 97. MECHANICAL CONTROLCRANKSA cam is an arm that has one end connected to a shaft.
  98. 98. MECHANICAL CONTROLCRANKS- Cranks and slider Cranks and slider converts movement from circular to reciprocating motion or the other way round.
  99. 99. MECHANICAL CONTROLCRANKS- Applications of cranks Sheet metal roller Piston engine
  101. 101. MECHANICAL CONTROLRACK & PINION MECHANISMSThe rack-and –pinion mechanism consists of a straight toothed ‘rack’ thatmeshes with a toothed wheel called a ‘pinion’. Rack-and-pinion mechanisms – rotary to linear motion. Rack Pinion Rack
  102. 102. MECHANICAL CONTROLRACK-AND-PINION-Applications of rack-and-pinion Railway track Car steering wheel Bench drilling machine
  103. 103. 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.
  105. 105. MECHANICAL CONTROLRACHET & PAWL MECHANISMSThe ratchet & pawl mechanism allows movement in one direction but notthe other. Spring-loaded The pawl allows the teeth to move one way but the other.
  106. 106. MECHANICAL CONTROLRATCHET & PAWL MECHANISMS- Applications of ratchet & pawl Ratchet Fishing reel
  108. 108. MECHANICAL CONTROLSPRING-LOADED MECHANISMSSprings store elastic energy that can be released to provide a returnmovement in mechanisms. The spring can be used in tension or compression. Spring in compression Clockwork motor Spring in tension Air pump
  109. 109. MECHANICAL CONTROLSPRING 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.
  110. 110. MECHANICAL CONTROLSPRING 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.
  112. 112. MECHANICAL CONTROLSCREWA screw is a ramp wrapped around a cylinder. It can produced a very largeforce. Screw mechanisms – rotary to linear motion
  114. 114. MECHANICAL CONTROLSCREW- Applications of screw
  116. 116. MECHANICAL CONTROLCABLE 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.
  117. 117. MECHANICAL CONTROLCABLE CONTROL- Applications of cable control
  118. 118. QUIZ Why do concrete structures have metal bars (reinforcements) inside them?Concrete has HIGH compressive strength but LOW tensilestrength. When a heavy load is placed on a concrete beam, thebottom is likely to crack as it is in tension. Therefore Engineer’sovercome the low tensile strength of the concrete by addingreinforcement steel bars along the tensile stress area.
  119. 119. MECHANICAL CONTROLActivityBuild and test different shape of beams using papersby folding. Find out which shape is the strongest.Try the activity worksheet to find the answer!
  120. 120. 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.