Gears

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

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

Drawing Gears

Drawing the teeth would be a problem Simply draw the inner and outer

diameter circles

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)

Watch the gears

Gear Ratios

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?

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

Working out Gear Ratios

Teeth on driven

Teeth on driverGear Ratio =

Driver – is powered (connected to the motor)Driven – is driven by the driver

40

10Gear Ratio = =

4

1

Gear Ratio = 4 : 1

40 teeth

10 teethdriver

driven

Connectedto motor

i.e. For every 4 turns of the driver the driven turns once

Changing rotational speed

Why bother? Motors tend to spin very quickly but

without much torque! What’s torque?

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.

Wheel Wrench

Pressing down on the wrench creates a turning force (torque)

Push Down Creates

rotation

The same force is created between the wheel and the floor

Torque

In a vehicle the rotational force is converted into forward motion

Movement

Higher torque allows more weight to be moved

Force

Bicycle example

Imagine riding a bike with gears

Flat ground – high gear

Gives high speed low torque uphill – low gear

Gives low speed but high torque

Torque vs Speed

Low speed with high torque

High speed with low torque

Good speed but easy to stop

No point racing but very difficult to stopWhat does the

sumobot need?

Large Ratios

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!

Teeth would have to be very small

Compound Gears

Allows higher ratios to be created Put two gears on one shaft

Two gears on one shaft

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.

1 : 2 1 : 2 1 : 2

Overall 1 : 8 (2 x 2 x 2)

Compound Gears

Sumobots

Here is the gear train

Motor drives

Connected to wheel

Split it up to work out the gear ratio

Compound Gear

Two gears on one shaft

Sumobots

10 teeth

40 teeth

driver

driven

50 teeth

20 teethdriver

driven

Sumobots

10 teeth

40 teeth

driver

driven

Teeth on driven

Teeth on driverGear Ratio =

40

10Gear Ratio = =

4

1

Gear Ratio = 4 : 1

Sumobots

50 teeth

20 teethdriver

drivenTeeth on driven

Teeth on driverGear Ratio =

50

20Gear Ratio = =

5

2

Gear Ratio = 5 : 2

Gear Ratio 2 = 5 : 2

Gear Ratio 1 = 4 : 1

4

1 2

5xGear Ratio =

20

2= =

10

1

Gear Ratio = 10 : 1

Every 10 turns of the motor the wheel will turn once (driver) (driven)

Sumobot Gear Ratio

Sumo Chassis

Sumo Chassis

10 teeth(connected to motor)

40 teeth(compound gear) 20 teeth

(compound gear)

50 teeth(connected to wheel)

Gear Mesh

The centre distance is crucial

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

Sumo Chassis

35mm

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

Any Questions