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Gears

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**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

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**To reverse the direction of rotation**

To increase or decrease the speed of rotation To move rotational motion to a different axis

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**Drawing Gears Drawing the teeth would be a problem**

Simply draw the inner and outer diameter circles

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**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)

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**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?

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**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

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**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

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**Changing rotational speed**

Why bother? Motors tend to spin very quickly but without much torque! What’s torque?

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**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.

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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

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

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**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

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**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?

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**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!

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**Compound Gears Allows higher ratios to be created**

Put two gears on one shaft Two gears on one shaft

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**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.

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

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**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

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**Sumobots driven driver 50 teeth 10 teeth driver 20 teeth driven**

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**Sumobots driver driven Teeth on driven Teeth on driver Gear Ratio =**

40 10 = 4 1 Gear Ratio = Gear Ratio = 4 : 1 40 teeth

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**Sumobots Teeth on driven Teeth on driver Gear Ratio = 50 teeth**

20 = 5 2 Gear Ratio = Gear Ratio = 5 : 2

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**Sumobot Gear Ratio Gear Ratio 1 = 4 : 1 Gear Ratio 2 = 5 : 2 4 1 2 5 x**

20 2 = = 10 1 Gear Ratio = Gear Ratio = 10 : 1 Every 10 turns of the motor the wheel will turn once (driver) (driven)

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Sumo Chassis

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**Sumo Chassis 50 teeth (connected to wheel)**

10 teeth (connected to motor) 40 teeth (compound gear) 20 teeth (compound gear)

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**If you get the centre distance wrong your gears will not turn smoothly**

Gear Mesh The centre distance is crucial

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

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Any Questions

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