TERM 3 Unit 1 Topic 5: Mechanical systems and control
• Revise: levers – single levers and levers linked in pairs. • Single first-class lever – mechanical advantage depends on the position of the fulcrum. • Linked first-class levers – consider various samples, e.g.:
– Paper scissors (if equal length blade and handle) – no mechanical advantage. – Secateurs (long handle and short, strong blades) – mechanical advantage > 1.
• Single second-class lever – always gives some mechanical advantage. • Linked second-lass levers – consider various samples, e.g.:
– Office punch – mechanical advantage > 1. – Heavy duty stapler – mechanical advantage > 1.
• Single third-class lever – never gives any mechanical advantage. • Linked third-class levers – consider various samples, e.g.:
– Office light-duty stapler – mechanical advantage < 1. – Pair of tweezers – mechanical advantage < 1.
• Gear systems – concepts (counter rotation, idler, velocity ratio, force multiplication). – Two spur gears of unequal size – note counter rotation and velocity ratio. – Two spur gears of unequal size – note velocity ratio and force ratio (mechanical advantage < or > 1). – Two spur gears connected via an idler – note synchronised rotational direction. – Suitable materials – the idler needs to be of a harder material than the other gears. – Two bevel gears linked to transfer the axis of rotation through 90°.
2
• Calculate mechanical advantage (ma) levers: mechanical advantage calculations for levers using ratios. • Calculations using LOAD/EFFORT; load ARM/effort ARM; etc. • Gears: mechanical advantage calculations for gears using ratios. • Calculations using tooth ratios; gear wheel diameters; velocity ratios.
108–112 2
Learner’s Book page
100–107 Time
allocation (2 hours)
2
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