1. Gears

2. Gears Gears are used in machines with spinning axles Several uses
To reverse the direction of rotation
To increase or decrease the speed of rotation
To move rotational motion to a different axis

3. To reverse the direction of rotation
To increase or decrease the speed of rotation
To move rotational motion to a different axis

4. Drawing Gears Drawing the teeth would be a problem
Simply draw the inner and outer diameter circles

5. Spur Gears Spur gears are the most common type of gears.
They have straight teeth, and are mounted on parallel shafts.
Sometimes, many spur gears are used at once to create very large gear reductions.
Spur gears are used in many devices like:-
the electric screwdriver, windup alarm clock, washing machine and clothes dryer.
Very noisy so not used in cars (helical gears are used)

6. Gear Ratios Watch the gears
Every time the larger gear goes around once, the smaller gear goes around twice.
The diameter of the gear on the left is twice that of the gear on the right.
The gear ratio is therefore 2:1 (pronounced "two to one").
What would happen if both gears were the same size?

7. Gear Ratios Most gears that you see in real life have teeth.
They make it possible to determine exact gear ratios.
e.g. one gear has 40 teeth and another has 10 teeth

8. Working out Gear Ratios
40 teeth
10 teeth
driver
driven
Connected to motor
Teeth on driven
Teeth on driver
Gear Ratio = 40 10 = 4 1
Gear Ratio =
Gear Ratio = 4 : 1
Driver – is powered (connected to the motor) Driven – is driven by the driver
i.e. For every 4 turns of the driver the driven turns once

9. Changing rotational speed
Why bother?
Motors tend to spin very quickly but without much torque!
What’s torque?

10. Torque Torque is a force that tends to rotate or turn things.
You generate a torque any time you apply a force using a wrench.
Tightening the lug nuts on your wheels is a good example.

11. Wheel Wrench
Push Down
Pressing down on the wrench creates a turning force (torque)
Creates rotation
The same force is created between the wheel and the floor

12. Movement
Torque
In a vehicle the rotational force is converted into forward motion
Higher torque allows more weight to be moved
Force

13. Bicycle example Imagine riding a bike with gears
uphill – low gear
Gives low speed but high torque
Flat ground – high gear
Gives high speed low torque

14. Torque vs Speed What does the sumobot need? Low speed with high torque
High speed with low torque
Low speed with high torque
Good speed but easy to stop
No point racing but very difficult to stop
What does the sumobot need?

15. Large Ratios What if you wanted a reduction of speed of 100:1 ?
Teeth would have to be very small
What if you wanted a reduction of speed of 100:1 ?
With simple gear train
1000 teeth on one gear 10 on the other
This is silly!

16. Compound Gears Allows higher ratios to be created
Put two gears on one shaft
Two gears on one shaft

17. Compound Gears Several compound gears in a line
the purple gear turns at a rate twice that of the blue gear.
the green gear turns at twice the rate of the purple gear.
the red gear turns at twice the rate as the green gear.

18. Compound Gears
1 : 2
1 : 2
1 : 2
Overall 1 : 8 (2 x 2 x 2)

19. Sumobots Here is the gear train Connected to wheel Motor drives
Compound Gear
Two gears on one shaft
Split it up to work out the gear ratio

20. Sumobots driven driver 50 teeth 10 teeth driver 20 teeth driven

21. Sumobots driver driven Teeth on driven Teeth on driver Gear Ratio =40 10 = 4 1
Gear Ratio =
Gear Ratio = 4 : 1
40 teeth

22. Sumobots Teeth on driven Teeth on driver Gear Ratio = 50 teeth20 = 5 2
Gear Ratio =
Gear Ratio = 5 : 2

23. Sumobot Gear Ratio Gear Ratio 1 = 4 : 1 Gear Ratio 2 = 5 : 2 4 1 2 5 x20 2 = = 10 1
Gear Ratio =
Gear Ratio = 10 : 1
Every 10 turns of the motor the wheel will turn once (driver) (driven)

24. Sumo Chassis

25. Sumo Chassis 50 teeth (connected to wheel)
10 teeth (connected to motor)
40 teeth (compound gear)
20 teeth (compound gear)

26. If you get the centre distance wrong your gears will not turn smoothly
Gear Mesh
The centre distance is crucial

27. Sumo Chassis
35mm
If you get the 35mm wrong your gears will not turn smoothly

28. Any Questions