AQA DT

• What are the two main types of linkage?


  Shown below:

  
• Gears are toothed wheels, that mesh to transfer motion from one part of a machine to another. Gears are found in many things from cars and bicycles, to clocks and can openers.
• A gear train transmits rotary motion and torque. Torque is a force that causes rotation. Different sized gears connect to increase or decrease the speed of rotation. They also increase or decrease the torque.
• Drive gear:

  • The drive gear (input) turns the driven gear (output)
  • Gears turn in opposite directions
  • Toothed wheels interlock to prevent slipping
• Idle Gear:

  • Used to change the direction of rotation
  • Transfers movement from drive gear to driven gear
  • Size does not affect speed
• Gear ratio

  The ratio between the drive gear and the driven gear
• Gear ratio

  • The relative sizes of two gear wheels determines how fast each will turn
  • A small gear with fewer teeth, will turn faster than a large gear with many teeth
• Calculating gear ratio

  1. Count the number of teeth on each gear

  2. Divide the number of teeth on the driven gear, by the number of teeth on the drive gear
• For every rotation of the drive gear

  The driven gear rotates twice
• Gearing up

  When the driven gear rotates faster than the drive gear
• Velocity Ratio

  To achieve maximum speed, have a larger driver gear paired with a smaller driven gear. The drive gear will be connected to the power source, for instance the pedals on a bicycle. Velocity ratiOS calculated by dividing the drive gear by the driven gear
• Pulleys can help to lift a load providing a mechanical advantage. In machines, pulleys transmit rotary motion and force from the input or drive shaft to the output or driven shaft. For the movement to be transferred it's important the belt does not slip or stretch. Pulleys are not ideal for transmitting high torque due to their tendency to slip. Belts are often made of reinforced rubber or high strength materials such as Kevlar®
• Fixed pulley

  • Uses a wheel with a groove in it and a rope that sits in the groove
  • Changes direction of the force needed
  • Makes lifting easier, but the weight will feel the same

  
• Block and Tackle Pulley:

  • Uses two or more pulleys, one fixed and one moveable
  • Reduces the amount of effort needed to lift a load
  • Provides a mechanical advantage

  
• Belts

  A drive belt (loop of flexible rubber) connects two or more pulleys together. The belt transfers power from the pulley to the receiving system. In the instance of a car, a belt transfers engine power to the alternator to recharge the battery.
• Physical properties


  Important to know the physical and working properties of a range of materials
• Physical properties


  • Absorbency
  • Density
  • Electrical conductivity
  • Fusibility
  • Thermal conductivity
• Absorbency


  The ability of a material to absorb or soak up liquids
• Density


  The mass per unit volume of a material
• Electrical conductivity


  The ability of a material to transport electricity
• Fusibility


  The ability of a material to be melted and fused together
• Thermal conductivity


  The measure of a material's ability to conduct heat
• Toughness


  The ability of a material to absorb energy and deform without fracturing
• Hardness


  The ability of a material to resist indentation, scratching or cutting
• Working properties

  • Ductility
  • Elasticity
  • Hardness
  • Malleability
  • Strength
  • Toughness
• Ductility


  The ability of a material to be drawn out into a wire without breaking
• Elasticity


  The ability of a material to return to its original shape after being stretched or compressed
• Hardness


  The ability of a material to withstand impact, wear, abrasion and indentation
• Malleability


  The ability of a material to be bent and shaped without cracking or spiking
• Strength


  The ability of a material to withstand a force such as pressure, compression, tension or shear
• Toughness


  The ability of a material to absorb shock without fracturing
• Paper is measured by weight in grams per square metre (GSM)
• Common paper weights


  • Tracing paper 60-90 GSM
  • Cartridge paper 120-150 GSM
  • Copier paper 80-100 GSM
• Card and board weights

  • Card 200-350 GSM
  • Board measured in microns
• Corrugated card


  • Two layers of card with a wavy layer in between for strength and impact protection
  • Recyclable and biodegradable
• Duplex card


  • Coated card containing a printed layer
• Foam core


  • Inner foam core with a paper face
  • Rigid and used for mounting artworks
• Solid white card


  • Coated card with a smooth finish and bleed proof printable surface
  • Used for greeting cards, packaging and mounting
• Hardwoods come from deciduous trees and are usually a broad-leaved variety that does not shed its leaves in the winter
• Softwoods come from coniferous trees, which are usually evergreen and have needles