Gears
Gears are used to change speed in rotational movement. In the example above the blue gear has eleven teeth and the orange gear has twenty five. To turn the orange gear one full turn the blue gear must turn 25/11 or 2.2727r turns.
Notice that as the blue gear turns clockwise the orange gear turns anti-clockwise.
In the above example the number of teeth on the orange gear is not divisible by the number of teeth on the blue gear. This is del
iberate. If the orange gear had thirty three teeth then every three turns of the blue gear the same teeth would mesh together which could cause excessive wear. By using none divisible numbers the same teeth mesh only every seventeen turns of the blue gear.
Rings of Gears and the Two Pound Coin
Gears turn in alternate directions. In the example, left the blue gear turns anti-clockwise, the two green gears on either side of it are then driven clockwise, these in turn drive the yellow and purple gears anti-clockwise and so on...
A ring of gears containing an even number of gears turns freely, as you can see, but imagine a ring with an odd number of gears , image that the blue gear is removed, you will see that the two green gears be turning the same direction and so would not be able to turn, the gears would lock together.
Now, look at the ring of gears around the centre
of the British two pound coin. Nineteen gears...!
Check out the gears model to download and make in the shop!
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Rack and Pinion
The rack and pinion is used to convert between rotary and linear motion. The rack is the flat, toothed part, the pinion is the gear. Rack and pinion can convert from rotary to linear of from linear to rotary.
The diameter of the gear determines the speed that the rack moves as the pinion turns. Rack and pinions are commonly used in the steering system of cars to convert the rotary motion of the steering wheel to the side to side motion in the wheels.
Rack and pinion gears give a positive motion especially compared to the friction drive of a wheel in tarmac. In the rack and pinion railway a central rack between the two rails engages with a pinion on the engine allowing the train to be pulled up very steep slopes.
Download and make your own Rack and Pinion model in the shop
Bell Crank
The bell crank is used to convert the direction of reciprocating movement. By varying the angle of the crank piece it can be used to change the angle of movement from 1 degree to 180 degrees. The bell crank was originally used in large house to operate the servant’s bell, hence the name.
“Jeeves, where’s my tea?!”
Download and make your own bell crank mechanism here.
There's an updated animation here:
