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# Mechanical Comprehension Study Guide 2 for McGraw-Hill's ASVAB (page 2)

By Dr. Janet E. Wall
McGraw-Hill Professional

### Systems of Gears

Another way to transfer power between shafts is through systems of gears. The gears in a system typically have different diameters and different numbers of teeth per gear. The teeth of one gear mesh with the teeth of another, and as one gear (the drivinggear) turns, it turns the other gear (the drivengear). When interlocking gears have different numbers of teeth, the gear with fewer teeth will rotate more times in a given period than the gear with more teeth. To see how this works, look at the following example.

Example

Gear A and gear B make up a system of gears. If gear A makes 6 revolutions, how many revolutions will gear B make?

To solve this problem, use the picture and your common sense. Count the teeth on each gear. Gear A has 9 teeth. Gear B has 27 teeth. The ratio of the teeth on the two gears is 9:27 or 1:3. Common sense should tell you that gear A must rotate 3 times to make gear B rotate once. So if gear A rotates 6 times, gear B will rotate twice. Always keep in mind that in this kind of system, the gear with more teeth makes fewer rotations in the same period than the gear with fewer teeth.

Notice, too, that gears change the rotation direction, while pulleys usually do not. To rotate a gear in the same direction as the driving gear, you need a third gear, called an idlergear.

Example

In this system, gear A (the driving gear) is rotating clockwise. Gear B is the idler gear. Gear A makes gear B rotate counterclockwise. Gear B then makes gear C rotate clockwise, the same direction as gear A.

### Rotating Wheels and Disks

Another way to drive shafts is to use what is called a pin and slot arrangement. In this arrangement, a pin is attached to a driving shaft, and a slotted disk is attached to a driven shaft. When the driving shaft rotates, the pin enters a slot on the disk and turns the driven shaft.

Example

In this pin and slot arrangement, each time the driving shaft turns one full revolution, the disk on the driven shaft will make 1/4 revolution. How far will the disk rotate when the pin turns three complete revolutions?

3 × 1/4 = 3/4 turn.

You may also be asked simple questions about what happens to points on a wheel when the wheel turns.

Example

How many rotations will point A make when point C makes 5 rotations? Point A will make 5 rotations because Point A and Point C are both fixed on the same wheel.

Example

Which point will travel farthest as the wheel makes 10 rotations? Point B will travel farthest because it is farthest from the center. The distance it travels in each rotation is greater than the distance traveled by the other points.

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