Gear Ratio Explained
A gear is used in mechanical devices whenever you need to transfer rotational motion somewhere else. Gears have some unique properties that are all impacted by a somewhat mysterious number, the gear ratio. Fortunately, it's pretty simple to understand when you know a little more about how gears work and how they work together.
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Types
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A typical gear is a wheel-shaped device, usually made of metal--although in toys and some devices, you'll find plastic gears. What makes a gear useful is the teeth: the ridges along the outer edge of the wheel that are designed to mesh with similar teeth on other gears. There are also elliptical gears, face gears that have the teeth along the front of the wheel instead of along the outer edge; bevel gears with an angled set of teeth; and worm gears, which are shaped like the shaft of a screw. For understanding gear ratios, however, it's best to start simple, with a pair of simple, standard gears.
Gear Ratios
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The gear ratio is a number calculated from the number of teeth on two gears that are working together. This doesn't necessarily have to imply that they're directly attached to each other: the ratio is between the gear that's attached to the power source (the input gear), and the last gear in the sequence (the output gear). So, the ratio is input gear teeth:output gear teeth. From there, you can deduce the number: a ratio of 50 to 25 teeth would produce a ratio of 2:1.
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Idler Gears
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Gears are often used in chains--two or more gears meshing with each other. You may have noticed that there were some gears skipped when calculating the ratio. These are the gears in the middle, and are referred to as "idler gears." They're never used in calculating gear ratios because they actually have no effect on them. The first and last gears will move at the same speed with idler gears in between, as if they were meshing directly. They do, however, impact the direction: If the first gear is rotating clockwise, and there are an odd number of gears in the chain, the last will rotate clockwise; if the number is even, the last will run counter-clockwise.
Speed
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A ratio is a number that gives you information about a relationship between two other numbers. This information can be used to calculate the gear speed in a chain. The simple equation is: speed of input gear x teeth on input gear = speed on output gear x teeth on output gear. The two teeth numbers come from the ratio: plug the first number into the input gear side and the second into the output gear side.
Torque
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Torque is the twisting force on something. It's not direct work on that object, but is a way to transfer that force. Torque has a strange relationship with the speed of a gear chain--the more speed, the less torque and vice-versa. Or, to put it mathematically: motor torque x (input gear teeth/output gear teeth) = output torque. Again, the ratio is plugged into the equation.
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References
- Photo Credit gears image by Stanisa Martinovic from Fotolia.com
