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

By — McGraw-Hill Professional
Updated on Jun 26, 2011

### 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.

### Cams and Cam Followers

Camsare lobes attached to rotating shafts to push separate pieces, called cam followers. Cams are often found in engines, where they push intake and exhaust valves open when the engine turns. For every complete rotation of the camshaft, the cam follower will move away from and then back to its original position. A spring pushes the follower tight to the cam.

### Cranks and Pistons

Cranksare used to change motion in a straight line to motion in a circle. You'll find cranks connected to pedals on a bike, and to pistons in a car engine. When a crank makes one complete revolution, the piston must go up and down and return to its original position.

### Fluid Dynamics

Fluidsare substances that take the shape of their container. Gases and liquids are both fluids. The behavior of fluids is called fluid dynamics, and it can get rather complicated, but not on the ASVAB. Let's start with air, and move on to hydraulics—the engineering of liquids.

### Air Pressure

Air pressure is measured in pounds per square inch. Atmospheric pressure at sea level is 14.7 lb/in2, which is actually quite a bit of pressure. Since it's present all around us, we don't notice it. However, if you create a vacuum inside a weak container, the container will be crushed by all that pressure.

Pneumatics and the Gas LawsSystems that use compressed air to do work are called pneumatic systems. Air is easily compressed, and the calculations are more complicated than they are with liquids, which usually can't be compressed. The larger the driven cylinder, the more air pressure it is exposed to, and the greater the force it can exert.

The "gas laws" apply to air as it is compressed and expanded.

• When a gas is compressed, it gains thermal energy—it warms up. The gas also gains potential energy, which is why compressed air can be used to drive nail guns and pneumatic hammers.
• When a given amount of gas expands, its pressure drops and the gas cools.
• When a gas cools without a change in outside pressure, it loses volume.

What happens when you increase the air pressure outside a balloon? The balloon shrinks until the pressure inside becomes great enough to balance the pressure outside.

Air pressure is also what keeps airplanes aloft. The bulge on the top of an airplane wing increases the speed of air passing over the wing, and that causes a reduction in pressure. Because air pressure does not change below the wing, the result is an unbalanced upward force. This force lifts the airplane.

RefrigeratorsRefrigerators and air conditioners provide interesting applications of the gas laws. A compressor compresses a fluid, called a refrigerant. The refrigerant warms up, as predicted by the gas laws. Then the refrigerant loses heat (but not pressure) in the condenser. The refrigerant is piped into an evaporator, where it goes through a small hole and evaporates under reduced pressure. Expansion causes the temperature to drop, and the cold refrigerant can pick up heat from the surroundings. This is why the evaporator is placed in the area to be cooled. The condenser is placed where it's easy to get rid of excess heat—in the backyard for an air conditioner, or in back for a refrigerator.