Mechanisms

A gear ratio trades speed for turning force: a small gear driving a big gear turns the output slower but stronger, by the same factor. Students test gears on a winch, work out the ratio, speed, strength and lift time, then choose a pair that meets a brief, and find a brief where the constraints conflict.

1. Small gear, big gear
2. Try: turn it and load it › Count the turns
3. Try: turn it and load it › Find the stall
4. Try: turn it and load it › The heaviest lift
5. Worked example: does 8 → 24 meet the brief? › Check 8 → 24, then 16 → 40
6. Worked example: does 8 → 24 meet the brief? › Strength
7. Worked example: does 8 → 24 meet the brief? › Time and criteria
8. Worked example: does 8 → 24 meet the brief? › Find the mistake
9. Vary: change one thing › A lighter bucket
10. Vary: change one thing › A faster motor
11. Vary: change one thing › Too heavy
12. Vary: change one thing › When nothing passes
13. Practise: ratios › Ratio
14. Practise: ratios › Speed
15. Practise: ratios › Strength
16. Practise: ratios › A bike
17. Write a brief › Exactly one pair
18. Write a brief › Evaluate

More than two gears. A gear between two others (an idler) changes the direction but not the ratio; two gears on one axle make a compound train, whose stage ratios multiply. Students test an idler, work out compound trains, find one for a heavy brief, and see why big ratios come in stages.

1. A gear in the middle
2. Try: the idler › Which way?
3. Try: the idler › Find the stall
4. Try: the idler › Change the idler
5. Try: the idler › The heaviest lift
6. Worked example: two gears on one axle › Check 8 → 24 = 8 → 40, then 16 → 40 = 8 → 24
7. Worked example: two gears on one axle › Strength
8. Worked example: two gears on one axle › Time and criteria
9. Worked example: two gears on one axle › Find the mistake
10. Vary: choose your train › Meet the brief
11. Vary: choose your train › Heavier still
12. Vary: choose your train › Why stages?
13. Practise: trains › Compound ratio
14. Practise: trains › Idler
15. Practise: trains › Direction
16. Practise: trains › Speed
17. Practise: trains › A clock
18. A brief only a train can meet › A brief for a train
19. A brief only a train can meet › Evaluate

A lever trades force for distance: a small push far from the pivot lifts a big load near it, and your hand moves farther than the load. Students find where to put a pivot by testing, work out the push from turning effects, meet the three classes of lever (including the one in their arm), and design a lever to a brief.

1. Move the pivot
2. Try: find the spot › Middle
3. Try: find the spot › Find the spot
4. Try: find the spot › How far?
5. Worked example: turning effects balance › Balance 600 N with the pivot at 0.5 m, then 0.4 m
6. Worked example: turning effects balance › Distance
7. Worked example: turning effects balance › Find the mistake
8. Vary: three kinds of lever › A wheelbarrow
9. Vary: three kinds of lever › Your arm
10. Vary: three kinds of lever › Muscle force
11. Vary: three kinds of lever › Why?
12. Practise: levers › Seesaw
13. Practise: levers › Crowbar
14. Practise: levers › Three kinds
15. Practise: levers › Distance
16. A lever to a brief › Two criteria
17. A lever to a brief › Evaluate

One pulley changes the direction of a pull, not its size. Ropes that hold up a moving block share the load, so the pull shrinks, but you pull more rope by the same factor. Students test pulleys, work out pulls and rope lengths, see that work in equals work out, and choose a pulley system for a brief.

1. One pulley
2. Try: add ropes › One rope
3. Try: add ropes › Add ropes
4. Try: add ropes › Rope pulled
5. Worked example: count the ropes › Four ropes, then six
6. Worked example: count the ropes › Rope
7. Worked example: count the ropes › Find the mistake
8. Vary: choose a system › Fewest ropes
9. Vary: choose a system › Why one pulley?
10. Practise: pulleys › Pull
11. Practise: pulleys › Rope
12. Practise: pulleys › Trade-off
13. Practise: pulleys › Same idea everywhere
14. A brief that needs four ropes › Four ropes
15. A brief that needs four ropes › Evaluate

Every real gear mesh and pulley wheel turns a little of the work into heat and sound. Students find a gear train lifts less than the ideal, that adding pulleys stops helping, work out overall efficiency by multiplying, and redesign a brief with friction in it.

1. More and more pulleys
2. Try: friction in the winch and the pulleys › The winch
3. Try: friction in the winch and the pulleys › Its heaviest lift
4. Try: friction in the winch and the pulleys › Fewest ropes
5. Try: friction in the winch and the pulleys › 9 or 10?
6. Worked example: multiply the losses › Four pulleys, step by step
7. Worked example: multiply the losses › The work in
8. Worked example: multiply the losses › Where it goes
9. Vary: a brief with friction
10. Practise: efficiency › Two stages
11. Practise: efficiency › Work in
12. Practise: efficiency › Better
13. The winch again, with friction › With friction
14. The winch again, with friction › Evaluate