2Gears Gears are used in machines with spinning axles Several uses To reverse the direction of rotationTo increase or decrease the speed of rotationTo move rotational motion to a different axis
3To reverse the direction of rotation To increase or decrease the speed of rotationTo move rotational motion to a different axis
4Drawing Gears Drawing the teeth would be a problem Simply draw the inner and outer diameter circles
5Spur 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)
6Gear 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?
7Gear 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
8Working out Gear Ratios 40 teeth10 teethdriverdrivenConnected to motorTeeth on drivenTeeth on driverGear Ratio =4010=41Gear Ratio =Gear Ratio = 4 : 1Driver – is powered (connected to the motor) Driven – is driven by the driveri.e. For every 4 turns of the driver the driven turns once
9Changing rotational speed Why bother?Motors tend to spin very quickly but without much torque!What’s torque?
10Torque 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.
11Wheel WrenchPush DownPressing down on the wrench creates a turning force (torque)Creates rotationThe same force is created between the wheel and the floor
12MovementTorqueIn a vehicle the rotational force is converted into forward motionHigher torque allows more weight to be movedForce
13Bicycle example Imagine riding a bike with gears uphill – low gearGives low speed but high torqueFlat ground – high gearGives high speed low torque
14Torque vs Speed What does the sumobot need? Low speed with high torque High speed with low torqueLow speed with high torqueGood speed but easy to stopNo point racing but very difficult to stopWhat does the sumobot need?
15Large Ratios What if you wanted a reduction of speed of 100:1 ? Teeth would have to be very smallWhat if you wanted a reduction of speed of 100:1 ?With simple gear train1000 teeth on one gear 10 on the otherThis is silly!
16Compound Gears Allows higher ratios to be created Put two gears on one shaftTwo gears on one shaft
17Compound 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.
18Compound Gears1 : 21 : 21 : 2Overall 1 : 8 (2 x 2 x 2)
19Sumobots Here is the gear train Connected to wheel Motor drives Compound GearTwo gears on one shaftSplit it up to work out the gear ratio