Mechanisms

• Lever


  A rod or bar that turns around a point called a fulcrum
• Classes of single levers

  • Determined by the position of the fulcrum, the load, and the effort
• Single first-class lever

  1. Fulcrum is between the load (L) and the effort (E)

  2. If you push down on one side, the other side goes up

  3. Mechanical advantage is only achieved if the effort is further away from the fulcrum than the load
• Single second-class lever


  1. Fulcrum is at the end of the lever and the load is between the fulcrum and the effort

  2. Provides mechanical advantage
• Single third-class lever


  1. Fulcrum is at the end of the lever and the effort is between the fulcrum and the load

  2. Does not provide mechanical advantage, but provides an increase in movement at the end of the lever
• Lever

  • A rod or bar that turns around a point called a fulcrum
  • A load and the effort that is applied work on levers
  • There are three classes of lever determined by the position of the fulcrum, the load and the effort
  • The mechanical advantage of a lever is determined by the position of the fulcrum, the load and the effort
  • Linked levers are used in many devices
  • Levers and linked levers can transfer motion in different directions
• In Grade 7, you learnt about levers and linkages
• A lever is a simple machine that consists of a rod or a bar that can turn around a fixed point called a pivot or fulcrum
• When an effort (a push or pull force) is applied to one part of a lever, it can move a load that is at another part of the lever
• When you connect levers, you make a form of mechanism called a linkage
• Crank system that changes rotary motion into reciprocating motion


  1. A connecting rod is connected to the crank

  2. The connecting rod goes up and down as the wheels rotate

  3. The crankshaft is the horizontal part around which the crank turns
• Crank systems used to create movement


  • Crank system used to create moving wings for a toy penguin
• Crank


  A part of a crank system that consists of two basic parts: a crank and a crankshaft
• The crank system that is used for the handle of a beater transmits the rotary motion from the handle above the bowl, to the rotary motion of the whisks in the bowl
• The rotary motion of a bicycle's pedals is transmitted and the wheels turn
• Crank pedals are also used for pedal boats and go-karts
• Cam


  • Similar to a wheel, it turns around an axle but is not round, can be shaped in many different forms
  • Always operates with a cam follower that rests on the cam and is kept down by gravity or a spring
• Cams change rotary motion into reciprocating motion

  1. As a cam turns, the cam follower goes up and down or to and fro

  2. The irregular shape of the cam pushes the cam follower up and down, creating reciprocating motion
• Cam shapes


  • Eccentric cam, snail-shaped cam, pear cam, heart-shaped cam
• Force ratio


  Shows how much the turning force of the input (driver) gear is increased or decreased
• Velocity ratio (gear ratio)

  Shows by how much the velocity is increased or decreased, determined by the number of teeth on the driver and driven gears
• When the driver gear is large and the driven gear is small, the gear system is used to increase velocity
• When the driver gear is small and the driven gear is large, the gear system is used to decrease velocity
• Small electric motors used in toy cars and trains need to have their velocity decreased to be used successfully
• Velocity ratio (or gear ratio)

  Shows by how much velocity is increased or decreased
• Velocity ratio


  1. Determined by the number of teeth on the driver and driven gears

  2. When the driven gear has fewer teeth than the driver gear, the velocity is increased

  3. When the driven gear has more teeth than the driver gear, the velocity is decreased
• Velocity ratio


  • If the driven gear has 12 teeth and the driver gear has 24 teeth, the velocity ratio (VR) is 24/12-2. The driven gear will turn twice as fast as the driver gear
  • If the driven gear has 24 teeth and the driver gear has 12 teeth, the velocity ratio is 12/24 or 0.5. The driver gear will turn twice as fast as the driven gear
• Turning force


  Force applied while applying circular motion
• It is easier to turn a tight-fitting bolt if you use a spanner than it is to turn it with your fingers. The spanner is a lever. The longer the arm of a lever, the more turning force is applied. This makes it easier to turn the bolt
• Cranes that are used to lift heavy loads operate at a low velocity, but they need to have a high turning force
• Gears are used to apply turning force and make it easier to perform tasks. A crane that is used to lift a heavy load must have a gear system that applies a large turning force
• Idler gear


  Changes the direction of the driven gear's rotation
• The size of an idler gear does not have an effect on the velocity with which the driver or driven gears rotate
• To save space, idler gears are often very small. When an idler gear is small it rotates much faster than the other gears and it wears out much quicker than the other gears. An idler gear must, therefore, be made from a harder and stronger material than the other gears
• When the driver gear turns clockwise, its teeth push down on the driven gear's teeth. This causes the driven gear to turn in the opposite direction (anti-clockwise). We say the gears counter rotate. A pair of gears will always counter rotate
• Wheels are used everywhere around us daily. There is a wide variety of different designs for wheels
• Wheel designs


  • Tyres of bicycle, car and aeroplane wheels are filled with air
  • Tyres of shopping trolleys and some wheelchairs are solid
  • Wheels of a train run on a track
  • Small, solid, spherical wheels of office chairs that can move easily in all directions
• The size and design of a wheel must ensure that the wheel will be strong enough to support the weight of the load it must carry
• Wheels can be used to rotate around a vertical axle. In this way, a wheel can be used to move objects
• For wheels to work properly, they must be kept in an upright position, be kept from falling off, be kept from rubbing against the body of a vehicle while it moves, and turn smoothly around a fixed axle