Wind-up, grandfather and pendulum clocks contain plenty of gears, especially if they have bells or chimes. You probably have a power meter on the side of your house, and if it has a see-through cover, you can see that it contains 10 or 15 gears. Gears are everywhere where there are engines and motors producing rotational motion. In this edition of How Stuff Works , you will learn about gears, gear ratios and gear trains so that you can understand what all the different gears you see are doing.
You can see effects 1, 2 and 3 in the figure above. In this figure, you can see that the two gears are rotating in opposite directions, that the smaller gear is spinning twice as fast as the larger gear, and that the axis of rotation of the smaller gear is to the right of the axis of rotation for the larger gear.
The fact that one gear is spinning twice as fast as the other results from the ratio between the gears, or the gear ratio Check out our gear ratio chart for more info. In this figure, the diameter of the gear on the left is twice that of the gear on the right. The gear ratio is therefore pronounced "two to one". If you watch the figure you can see the ratio: Every time the larger gear goes around once, the smaller gear goes around twice. You can see that if both gears had the same diameter, they would rotate at the same speed but in opposite directions.
Understanding the concept of the gear ratio is easy if you understand the concept of the circumference of a circle. Keep in mind that the circumference of a circle is equal to the diameter of the circle multiplied by Pi Pi is equal to 3. Therefore, if you have a circle or a gear with a diameter of one inch, the circumference of that circle will be 3. The following figure shows how the circumference of a circle with a diameter of 1. You would find that, because its diameter is one half of the circle's in the figure, it has to complete two full rotations to cover the same 4 inch line.
This explains why two gears, one half as big as the other, have a gear ratio of The smaller gear has to spin twice to cover the same distance covered when the larger gear spins once. Most gears that you see in real life have teeth. The teeth have three advantages: They prevent slippage between the gears - therefore axles connected by gears are always synchronized exactly with one another. They make it possible to determine exact gear ratios - you just count the number of teeth in the two gears and divide.
So if one gear has 60 teeth and another has 20, the gear ratio when these two gears are connected together is They make it so that slight imperfections in the actual diameter and circumference of two gears don't matter. The gear ratio is controlled by the number of teeth even if the diameters are a bit off. The right-hand purple gear in the train is actually made in two parts, as shown. A small gear and a larger gear are connected together, one on top of the other.
Gear trains often consist of multiple gears in the train, as shown in the following two figures:. In the case above, the purple gear turns at a rate twice that of the blue gear.
The green gear turns at twice the rate as the purple gear. The red gear turns at twice the rate as the green. The gear train shown below has a higher gear ratio:. Planetary gears solve the following problem. Let's say you want a gear ratio of with the input turning in the same direction as the output. One way to create that ratio is with the following three-gear train:. In this train, the blue gear has six times the diameter of the yellow gear giving a ratio. The size of the red gear is not important because it is just there to reverse the direction of rotation so that the blue and yellow gears turn the same way.
However, imagine that you want the axis of the output gear to be the same as that of the input gear. A common place where this same-axis capability is needed is in an electric screwdriver. In that case, you can use a planetary gear system, as shown here:. In this gear system, the yellow gear the sun engages all three red gears the planets simultaneously. All three are attached to a plate the planet carrier , and they engage the inside of the blue gear the ring instead of the outside.
Because there are three red gears instead of one, this gear train is extremely rugged. The output shaft is attached to the blue ring gear, and the planet carrier is held stationary -- this gives the same gear ratio.
You can see a picture of a two-stage planetary gear system on the electric screwdriver page , and a three-stage planetary gear system of the sprinkler page. You also find planetary gear systems inside automatic transmissions. Another interesting thing about planetary gearsets is that they can produce different gear ratios depending on which gear you use as the input, which gear you use as the output, and which one you hold still.
For instance, if the input is the sun gear, and we hold the ring gear stationary and attach the output shaft to the planet carrier, we get a different gear ratio. In this case, the planet carrier and planets orbit the sun gear, so instead of the sun gear having to spin six times for the planet carrier to make it around once, it has to spin seven times.
This is because the planet carrier circled the sun gear once in the same direction as it was spinning, subtracting one revolution from the sun gear. So in this case, we get a reduction. You could rearrange things again, and this time hold the sun gear stationary, take the output from the planet carrier and hook the input up to the ring gear.
This would give you a 1. An automatic transmission uses planetary gearsets to create the different gear ratios, using clutches and brake bands to hold different parts of the gearset stationary and change the inputs and outputs. Imagine the following situation: You have two red gears that you want to keep synchronized, but they are some distance apart. You can place a big gear between them if you want them to have the same direction of rotation:.
However, in both of these cases the extra gears are likely to be heavy and you need to create axles for them. In these cases, the common solution is to use either a chain or a toothed belt , as shown here:.
The advantages of chains and belts are light weight, the ability to separate the two gears by some distance, and the ability to connect many gears together on the same chain or belt.
For example, in a car engine , the same toothed belt might engage the crankshaft, two camshafts and the alternator. If you had to use gears in place of the belt, it would be a lot harder.
Sign up for our Newsletter! Mobile Newsletter banner close. Mobile Newsletter chat close. Mobile Newsletter chat dots. Mobile Newsletter chat avatar. Mobile Newsletter chat subscribe. How Gear Ratios Work. Grahame Turner has worked as a freelance writer since and a freelance reporter since for Wellesley Patch and Jamaica Plain Patch in Massachusetts. He also works part-time as a bookseller at the Northeastern University bookstore. Simple Gear Ratio Explained. Sprocket Ratio Calculations.
How to Calculate Speed Ratio. Rack-and-Pinion: Gear Ratio. Uses of Spur Gears. How to Calculate Gear Pitch. How to Find Belt and Pulley Speeds. How to Use Pulleys for Speed Reduction. What Makes a Car Fast? How to Calculate Reduction Ratio.
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