Mechanical Comprehension Review

Basic and Compound Machines 

Most mechanical machines and devices were invented  in a similar manner: people were looking for easier  ways to perform their everyday jobs. Some mechanical  devices are thousands of years old, such as the lever,  the wheel, and many hand tools. Other more complex  devices, such as pumps and valves, were invented more  recently. Often the idea of a new mechanical device  exists, but the technology to actually make it does not. 

For example, many years before the pump was  invented, people probably discussed the need for an  easier way to move water from the river to the town  on the hill. However, the technologies of the electric  motor and metal casting had not yet been discovered, so  the modern pump could not be invented. 

In general, a mechanical device is a tool that does  physical work and is governed by mechanical forces and  movements. In other words, you can usually see what  a mechanical device does and how it works—as  opposed to, say, electrical devices such as light switches  or batteries. Some tools are used to directly accomplish  a specific task, as when you use a hand saw to cut  a piece of wood. Others, such as pulleys and gears,  may be used indirectly to accomplish tasks that would  be possible without the device but are easier with it. Still  others, such as gauges, provide feedback on how well  other mechanical devices are working. You see and use  mechanical devices many times each day, so there’s little  reason to be intimidated by an exam question on a  mechanical device. 

Gears 
A gear is a toothed wheel or cylinder that meshes with  another gear to transmit motion or to change speed or  direction. Gears are usually attached to a rotating shaft  that is turned by an energy source such as an electric  motor or an internal combustion engine. Mechanical  devices that use gears include automotive transmissions,  carpenter’s hand drills, elevator lifting mechanisms,  bicycles, and carnival rides such as Ferris wheels  and merry-go-rounds. 

Gears are used in different configurations. In an  automotive transmission, for instance, two gears may  directly touch each other. As one gear spins clockwise,  the other rotates counterclockwise. Another possible  configuration is to have two gears connected by a loop  of chain, as on a bicycle. In this arrangement, the first  gear rotates in one direction, causing the chain to move. 

Since the chain is directly connected to the second gear,  the second gear will rotate in the same direction as the  first gear. 

Often a system will use two gears of different  sizes, as on a ten-speed bicycle. This allows changes in  speed of the bicycle or machine. 

Pulleys 
A pulley consists of a wheel with a grooved rim through  which a rope or cable is run. 

Pulleys are often used to change the direction of  a pulling force. For instance, a pulley could be attached  to the ceiling of a room. A rope could be run from the  floor, up through the pulley, and back down to a box  sitting on the floor. The pulley would allow you to pull  down on the rope and cause the box to go up. 

Another common use for a pulley is to connect an  electric motor to a mechanical device such as a pump. 

One pulley is placed on the shaft of the motor, and a  second pulley is placed on the shaft of the pump. A belt  connects the two pulleys.When the motor is turned on,  the first pulley rotates and causes the belt to rotate,  which in turn causes the second pulley to rotate and  turn the pump. This arrangement is very similar to  the previous example of a bicycle chain and gears. 
You may have seen pulleys used in a warehouse to  lift heavy loads. Another use for a pulley is on a large  construction crane. The cable extends from the object  being lifted up to the top of the crane boom, across a  pulley, and back down to the electric winch that is used  to pull on the cable. In this situation the pulley again  causes a change in direction of the pulling force, from  the downward force of the winch that pulls the cable to  the upward movement of the object being lifted. 

Levers 
The lever is a very old mechanical device. A lever typically  consists of a metal or wooden bar that pivots on  a fixed point. The point of using a lever is to gain a  mechanical advantage. Mechanical advantage results  when you use a mechanical device in order to make a  task easier; that is, you gain an advantage by using a  mechanical device. A lever allows you to complete a  task, typically lifting, that would be more difficult or  even impossible without the lever. 
The most common example of a lever is a playground  seesaw. A force—a person’s weight—is applied  to one side of the lever and causes the weight on the  other side—the other person—to be lifted. However,  since the pivot point on a seesaw is in the center, each  person must weigh the same or the seesaw won’t work  well. A seesaw is a lever with no mechanical advantage. 
If you push down on one side with a weight of ten  pounds, you can only lift a maximum of ten pounds on  the other side. This is no great advantage. 

This brings us to the secret of the lever: in order  to lift an object that is heavier than the force you want  to apply to the other side of the lever, you must locate  the pivot point closer to the object you want to lift. If  two 50-pound children sit close to the center of the seesaw,  one 50-pound child close to the end of the board  on the other side will be able to lift them both. 
Test questions about levers may require a bit of  math—simple multiplication and division. Lever problems  rely on one simple concept: the product of the  weight to be lifted times the distance from the weight  to the pivot point must be equal to the product of the  lifting force times the distance from the force to the  pivot point. Stated as an equation, w × d1 = f × d2. 

Fasteners 
A mechanical fastener is any mechanical device or  process used to connect two or more items together. 

Typical examples of fastening devices are bolts, screws,  nails, and rivets. Processes used to join items together  mechanically include gluing and welding. The “hook  and loop” is a unique mechanical fastener consisting of  two tapes of material with many small plastic hooks  and loops that stick together. Children’s sneakers often  use such fastening tape instead of laces. 

Springs 
A spring is an elastic mechanical device, normally a coil  of wire, that returns to its original shape after being  compressed or extended. There are many types of  springs including the compression coil, spiral coil, flat  spiral, extension coil, leaf spring, and torsional spring. 

Springs are used for many applications such as car  suspensions (compression coil and leaf springs), garage  doors (extension coil and torsion springs), wind-up  clocks (flat spiral and torsion springs), and some styles  of retractable pens (compression coil). 

In most exam questions, springs behave linearly. That is, if an extension  spring stretches one inch under a pull of ten pounds,  then it will stretch two inches under a pull of twenty  pounds. In real life, if you pull too hard on a spring, it  will not return to its original shape. This is called  exceeding the spring’s elastic limit. 

If several springs are used for one application,  they can be arranged in one of two ways: in series or in  parallel. The easiest way to remember the difference is  that if the springs are all hooked together, end to end,  then you have a series of springs. The other option is  for the springs not to be hooked together but to be lined  up side by side, parallel to each other. If two springs are  arranged in series, they will stretch much farther than  the same two springs arranged in parallel under the  same pulling force. This is because in series, the total pulling force passes through both springs. If the same  springs are arranged in parallel, the pulling force is  divided equally with half going through each spring. 

Valves 
A valve is a mechanical device that controls the flow of  liquids, gases, or loose material through piping systems. 
There are many types of valves including butterfly  valves, gate valves, plug valves, ball valves, and  check valves. 
A valve is basically a gate that can be closed or  opened in order to permit a fluid or gas to travel in a  particular direction. Exam questions on valves typically  require you to follow a piping flow diagram through  several sets of valves. The best way to approach these  problems is to methodically follow each branch of the  piping system from start to finish. 

Gauges and Pumps 
Gauges and pumps may appear in the Mechanical Comprehension subtest. These devices are discussed in Chapter 10, “Auto and Shop Information.” 

Linkages 
A linkage is a way of connecting objects in order to  transfer energy. Belts and chains are commonly used in  conjunction with gears and pulleys for this purpose. 
Chains are typically made of steel or some other metal,  while belts are typically made of fiber-reinforced rubber. 
An example of the use of a belt is the fan belt on the  engine of an automobile, which helps transfer the  energy from the engine camshaft to the fan. A bicycle  uses a chain to transfer the energy from the pedals to  the wheel. Another mechanical linkage is the tie rod  that connects the piston and crankshaft in an internal  combustion engine. 

Mechanical Motion 
Motion simply means a change of position. The  parameters that describe mechanical motion include  velocity, direction, acceleration, and friction. 

Velocity 
Velocity means the rate at which an object is moving in  such units as miles per hour or feet per minute. Exam  questions on velocity might ask you to use velocity and  time to determine the distance traveled. For instance, if  a car travels at a constant velocity of 60 miles per hour  for two hours, how far does it travel? The answer is  velocity multiplied by time, or 60 mph times 2 hours for  a total of 120 miles. You might also be asked about relative  velocity in a question in which two objects travel  at different speeds for different lengths of time. 

Direction 
If you want to travel quickly from Boston to Chicago,  your velocity is unimportant if you’re not traveling  in the right direction. 

Acceleration 
Acceleration is the rate of change of velocity or, in  other words, how much faster you are going from one  minute to the next. This is simpler than it sounds. If  you are sitting in your car at a stop sign and then you  press hard on the gas pedal, you get pushed back into  the seat a bit. If you are traveling along the highway at  a constant 50 mph, you don’t have this feeling.However,  if you hit the gas and accelerate to 65 mph, you are  again pushed back into your seat. You have the same  sensation when your airplane takes off on the runway. 
This sensation is the result of acceleration, an increase  in how fast an object is traveling. The opposite of acceleration  is deceleration, slowing down. 

Friction 
Friction is the naturally occurring force that acts to  hold back an object in motion. If you slide a block of  wood across a floor, the friction between the floor and  the block causes a drag on the movement of the block. 
There are two things you should remember if you  encounter an exam question about friction: 
- Friction always slows down movement. 
- All movement experiences frictional force to  some degree. 

The drag force of friction varies depending on the  materials involved. If you’ve ever tried to drag a piece  of furniture from a room with a carpeted floor to  another room with a wood floor, you found that the  piece of furniture was much easier to drag on the wood  floor than on the carpet. The carpet has a higher coefficient  of friction than wood. Materials with a high  coefficient of friction include such things as sandpaper  and brick. Examples of materials with a low coefficient  of friction include non-stick cooking surfaces and ice. 

The differing coefficients of fiction explain why it’s  more difficult to pull a wooden block across a rough  surface such as sandpaper than across a slick surface  such as ice. 

Fluid Statics and Dynamics 
The Mechanical Comprehension subtest includes questions  on the behavior of fluids, including questions on  pressure, density, and buoyancy. 

Pressure 
As a solid object is submerged below the surface of a  fluid, the fluid exerts a pressure on it. Have you ever  noticed that when diving in a swimming pool you feel  more and more pressure on your ears as you go deeper? 
This is the effect of the pressure of the fluid, water in  this case, on your body. The fluid has weight. As you go  deeper, more of this weight presses on your body. All  fluids behave this way. The deeper a solid object is submerged,  the higher the pressure. This behavior of fluids  affects the design of machines such as submarines. 

The formula for pressure is: P = F/A. Because pressure is force divided by area.

Density 
Density is a proportion of weight to volume. If you are  comparing two fluids, for example, a gallon of the one  with the higher density weighs more than the same  volume (a gallon) of another liquid. The density of a  solid object or other fluid is usually compared to the  density of water, 62.4 pounds per cubic foot. Density  controls whether an a solid object or another fluid will  sink or float in a given fluid. If a solid object sinks  when placed in water, then its density is more than  that of water. Conversely, if an object floats, then it is  less dense than water. Some liquids, such as mercury,  are more dense than water. If mercury and water are  combined in a jar, the water will float on top of the mercury. Other fluids, such as gasoline or motor oil, are  slightly less dense than water. That is why when an oil  tanker has a spill, it leaves an oil slick—the oil is floating  on the surface of the water. 
Density influences the amount of pressure a fluid  exerts on an object. The denser the fluid, the faster the  pressure increases on an object as it is submerged. 

Buoyancy 
Buoyancy is the force that acts to push an object submerged  in a fluid to the surface. When you force a  beach ball under water and then let it go, it springs to  the surface. That’s the effect of buoyancy. 

Here’s an example that shows how buoyancy  works for an object that is denser than water. Let’s say  you have a glass that is completely full of water, and the  water in the glass weighs one pound. Now put in a  eight-pound steel ball, which occupies half of the volume  of the glass. When the ball sinks, what happens? 
Half of the water in the glass, a half-pound worth,  spills over the edge of the glass because the ball occupies  half the volume of the glass. Now, here’s a definition:  the uplifting buoyant force acting on this ball is  equal to the weight of the water displaced out of the  glass by the ball. By definition, therefore, this ball  weighs half a pound less when submerged in water  than it does just sitting on the table. 
The ball weighs less underwater, but it still sinks. 
Why? Because the ball weighs more than the water it  displaces. How, then, is it possible to make a ship that  floats in water out of steel, when steel is more dense  than water? Simple. Take a thin sheet of steel and form  it into a kind of bowl shape. As this thin shell is lowered  into the water, it will displace enough water to  make it float. 

Properties of Materials 
Mechanical components and systems can be fabricated  using many different materials such as steel,  wood, concrete, and plastic. All of these materials  react differently to stress, temperature changes, and  other external factors. You must understand the properties  of materials—weight, strength, density, and  thermal properties—in order to answer test questions  about them. 

Weight 
The weight of an object is simply a measure of its  heaviness. 

Strength 
The loads and stresses placed on a material must be  less than the strength of the material in order to prevent  failure. A material’s strength can be measured in  several ways. A concrete building foundation has lots  of weight compressing on it and must have high compressive  strength. A steel construction girder has a  large pulling force acting on it and must therefore  have a high tensile strength. The materials selected for  a given project depend in part on the loads the structure  will have to bear. 

Density 
Think of a one-gallon bottle full of feathers and another  full of steel.Which bottle would be heavier? Both bottles  have the same volume, but the one full of steel  would obviously weigh more, because steel has a higher  density (weight per unit volume) than feathers. Feathers  have a low density; it would take a large volume—  a big stack of them—to amount to any significant  weight. On the other hand, a small volume of steel,  which has a fairly high density, is reasonably heavy. 
Just remember that a material with a higher density will  hurt more if you drop it on your toe! 
In the English system of units, density is typically  measured in pounds per cubic foot or pounds per  cubic inch. 

Thermal Properties 
The thermal properties of materials—how they  respond to changes in termperature—affect their suitability  for various applications.Most materials expand  slightly as the temperature increases and contract as the  temperature decreases. This amount of expansion and  contraction varies for each material but is typically  very small; you could not see it with your eyes. 
The effect of even this small amount of expansion  or contraction can be significant on some mechanical  systems. For instance, the internal combustion engine  of a vehicle generates heat as it operates. All of the  parts of the engine must be manufactured so that they  fit together properly at both high and low temperatures. 
Likewise, an airplane experiences very low temperatures  when flying at high altitudes, so that the metal of  its body contracts a bit. The designers of the airplane  must take this effect into account. 
The strength of some materials is also affected by  changes in temperature. Most materials get weaker as  the temperature increases because the bonds between  the individual molecules that make up the material get  weaker as the molecules move more rapidly. This is why  some building materials, such as steel, are coated with  insulation during construction. If the building catches  fire, the insulation will help maintain the strength of the  steel girders. 

Choosing Materials for a Given Application 
In deciding what materials to use for a given application,  weight, strength, density, and thermal properties  must all be taken into consideration. For instance, if  you wanted to build an airplane wing, you might consider  using either steel or aluminum. Steel is stronger  than aluminum.However, aluminum has a lower density;  that is, an aluminum wing would be lighter than  a steel wing of the same size. Therefore, you could use  more aluminum to provide adequate strength and still  have a lighter total weight. 
Other factors, such as cost and how easy the materials  are to work with, are also taken into account when  selecting materials for a project. 

Structural Support 
Mechanical systems such as buildings and bridges  require proper structural support in so they can hold up  heavy loads. An object’s center of gravity and its weight  distribution affect the design of structural support. 

Center of Gravity 
The center of gravity of an object is the point at which  all of the object’s weight appears to act. For instance,  you can balance a pencil on your finger by placing your  finger under the pencil at the middle of its length. The  center of gravity of that pencil is halfway along its  length. Likewise, a round ball has its center of gravity  at its center. Other objects that are not so symmetrical  also have a center of gravity, which can be located  through calculations. 
Exam questions on center of gravity usually  involve symmetrical objects so that the math does not  become complicated. Take your time, draw a sketch of  the object, and use common sense. 

Weight Distribution 
The distribution of weight on a structure such as on  a bridge is also important to understand. If there are  three trucks uniformly spaced across the length of a  bridge that is supported only at its ends, then each  support bears an equal amount of the load. However,  if the trucks are all located close to one end of the  bridge, then the support on that end will be holding up  a higher load than the support on the opposite end. 
Bridges and buildings have highly variable loads. 
The worst-case weight distribution must be accounted  for—for instance, trucks standing nose to tail for the  whole length of the bridge—even if that isn’t very likely  to happen 
As with most Mechanical Comprehension questions,  using the picture given, or drawing one if it’s not  provided, will help you see the location and distribution  of the objects. 

Things To Know To increase your knowledge of  math and science and your general mechanical  comprehension. 

Math 
The math you may need for Mechanical Comprehension  questions are simple arithmetic (addition, subtraction,  multiplication, and division) and geometry  (angles and shapes). If you had trouble with arithmetic  or geometry in your past schooling, you can brush up  by reading the math chapter of this book. If you still  want more help, pull out your old high school math  book or check out a math book from the library. 

Science 
Science subjects such as physics, materials science,  thermodynamics, and chemistry. Science is real,  everyday life. You see science in action dozens of times  every day. A car is stopped by brakes, which use friction— that’s physics. A magnet adheres to the refrigerator  door due to the properties of the magnet and  carbon steel of which the door is made—that’s materials  science. A pot of water boils when you set it on  the stove and turn on the burner—that’s thermodynamics. 
A cake rises through the release of carbon  dioxide from the baking powder or baking soda reacting  with heat or an acid—that’s chemistry. This chapter  has reviewed many of the scientific concepts that  are involved in mechanical devices. Again, as with  math, you may have science books from previous  schooling that you can use to help you solidify your  scientific knowledge. If not, the library is full of scientific  resources. 

How To Improve Your General Mechanical Comprehension 
Mechanical devices are such an integral part of everyday  life that there are many real-life sources you can  investigate to gain more knowledge of their design and  use. A construction site is a  great place to visit for a day to learn more about hand  tools, cranes, pumps, and other devices. Visit an auto  repair shop. Internal combustion engines, lifts, levers,  and hand tools are only a few of the types of tools and  systems you could see in use. Visit a local  manufacturer in your town. Examples include a  foundry, a sheet metal fabricator, an automotive manufacturer,  or a pump manufacturer.