A mechanism is a device which takes an input motion of force and converts it to a different output motion of force.
Terminologies:
- load – the object you are trying to move/exerting force upon
- effort – the force applied to move the load
- fulcrum (pivot) – the pivot point where the lever turns
- mechanical advantage (load ÷ effort) – using less effort to move a larger output load
- to increase MA: move fulcrum closer to the load/increase length of arm away from load
- velocity ratio – distance moved by effort ÷ distance moved by load
- efficiency
The position of fulcrum relative to the load and effort determines the type of lever.
First Class Lever
Second Class Lever
Third Class Lever
Gears and motion transmission
- Motion conversion that takes place when using a hand drill:
- rotary —> rotary, motion is converted at an 90˚ angle at a faster speed (less teeth on driven than driver)
- Sprocket and chain (Chain drive): bicycle transmission; positive drive (no slip)
- Ratchet and pawl: rotary —> oscillating
- Makes a shaft rotate; in one direction only
- Winch, fishing reel
- Crank and slider: rotary to reciprocating
- e.g. water pump
- Screw-thread: rotary to linear
- wood-working vice (buttress thread so that force is applied in one direction only)
- Car jacks (as handle is turned, linkage pushes apart in a linear motion to provide enough force to lift a car)
- G clamps (as handle is turned, it pulls two parts of the clamp together and squeezes what’s in between)
Belt
- toothed belt:
- positive drive system, little slip (slip must be eliminated)
- teeth located accurately into similar shaped grooves in the pulley wheel
- over time the belt will stretch and lose its elasticity, thus efficiency will suffer
- example: Timing belt system in car engine, plotter drive, printer head
- Flat belt:
- transmit torque and motion
- may slip on the pulley wheel
- record player turntable, steam powered traction engines
- Vee belt:
- wedges tightly into the pulley wheels to avoid slipping
- more surface contact of the belt with the groove
- drilling machine pulley system, pillar drilling machine with a stepped cone pulley to adjust drill speed
- Round belt:
- used when the belt has to twist/small forces are involved; likely to slip
- fit int o a circular groove in the pulley itself
- Mecanno toy pulley belt, vacuum cleaners to connect motor to fan
Friction and Energy
- Give two energy losses for the battery-powered vehicle
- heat energy/friction
- badly made/inaccurately fitted components
- Two ways to improve energy efficiency
- lubrication or use low friction materials
- greater accuracy in manufacturing
- Give two reasons for lubricating mechanical systems.
- smooth running/reduce friction/reduce wear and tear/increase machinery life span/cooling/increase efficiency
- Oil: motor car engine; Grease: wheel bearings
Mechanical advantage and velocity ratio
- Using the example of the can-crushing press, explain what is meant by the term Mechanical Advantage.
- The ratio between the effort and load/using a smaller effort to lift a greater load
- makes it easier for the operator to crush the can
- Gear ratio: driver/driven, if output is faster, than greater gear ratio, state the ratio e.g. 32:1 NOT just one number
- Explain why the handle is cranked.
- Greater mechanical advantage
- thus makes turning the drill bit easier
- Explain why the driver gear wheel is very much larger than the driven gear wheels.
- gives greater velocity ratio
- and increases the speed of the drill chuck
Bearings
- Ball bearing: bicycle
- Roller bearing: Heavy loading, vehicles, printing press
- Explain the need for bearings in mechanical systems.
- reduce friction / make operation smoother
- reduce wear and tear
Cam – rotary—>reciprocating
- Pear cam and flat follower
- long dwell (follower does not move since half of the cam is circular)
- rise and fall times are equal because shape is symmetrical
- use: valve operation in car engines
- Eccentric cam – centre of rotation is offset from the geometric centre of circle
- produces a smooth form of motion
- Heart shaped cam (used with a knife follower to get into the groove) knife follower wears easily
- follower move with a uniform velocity
- use: sewing machine
- Snail cam
- sudden fall
- rotate in anti clockwise direction
- stays level for approximately the first 120˚ and slowly rises, suddenly drops after reaching the peak
Useful resource for more basic concepts!
IGCSE REVISION 