Mechanical Comprehension Review for Armed Services Vocational Aptitude Battery (ASVAB) Study Guide (page 2)
This article will help you prepare for the Mechanical Comprehension subtest of the ASVAB. It presents the most commonly tested concepts: basic and compound mechanical machines and devices, mechanical motion, fluid dynamics, properties of materials, and structural support.
Every day, often without even thinking about it, you use mechanical devices. These could be simple machines such as levers and pulleys, or more complex compound machines such as linkages and gears. The ability to understand and use mechanical concepts is important both for many tasks required in the armed services and in everyday life.
The Mechanical Comprehension subtest of ASVAB may cover topics you are familiar with, as well as some that are new. Understanding the concepts in this chapter will benefit you both for the exam and in your career in the armed services. After an introduction to the Mechanical Comprehension subtest, this chapter summarizes definitions and the most commonly tested mechanical concepts. It also suggests how you can add to your knowledge of mechanical concepts and related scientific and mathematical knowledge. At the end of the chapter, you get an opportunity to review what you've learned by answering sample Mechanical Comprehension questions like those found on the ASVAB.
What Mechanical Comprehension Questions are Like
The Mechanical Comprehension subtest covers a wide range of topics. It consists of 25 multiple-choice questions, which you will have 19 minutes to answer. Most questions require previous knowledge of the topic, while some questions will themselves provide all of the information you need to figure out the answer.
Some questions require you to identify various mechanical machines or devices. Some of the mechanical devices that may appear on the exam—and are covered in this chapter—include gears, pulleys, levers, fasteners, springs, gauges, hinges, and linkages.
Other questions require knowledge of mechanical motion such as velocity, acceleration, direction, and friction for both solid bodies and fluids. These questions test concepts such as the motion and acceleration of automobiles or the buoyancy and pressure of fluids.
The Mechanical Comprehension subtest also covers the properties of materials and the concept of structural support. The material properties include weight, strength, density, thermal properties, and center of gravity. Structural support includes concepts such as weight distribution.
A typical mechanical comprehension question will look something like this:
- What is the main function of a pulley?
- to increase the strength of a construction crane
- to override the power of an electric motor
- to add energy to a system
- to change the direction of a pulling force
The correct answer is d, to change the direction of a pulling force. Pulleys are used to change not the strength of a force but its direction.
Review of Mechanical Comprehension Concepts
As aforementioned, some of the mechanical concepts most likely to appear on the ASVAB include basic and compound machines, mechanical motion, the behavior of fluids, the properties of materials, and structural support.
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.
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.
Test questions about gears will always involve rotation, or spinning. The easiest way to approach questions about gears is to use the picture given or to draw one, if it's not already provided. Draw an arrow next to each gear to indicate which direction (clockwise or counterclockwise) it is rotating.
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.
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. Here's an example of a test question using this concept:
- Bill has a 15-foot long lever and he wants to lift a 100-pound box. If he locates the pivot point 5 feet from the box, leaving 10 feet between the pivot point and the other end of the lever where he will apply the lifting force, how hard must he press on the lever to lift the box?
To solve this problem, use the lever formula, w × d1 = f × d2. The weight of 100 pounds times 5 feet must equal 10 feet times the force: 100 × 5 = 10 × f. Multiply 100 by 5 to get 500, and then divide by 10 to get 50 pounds of force, which Bill must apply to the lever to raise the box.
For the ASVAB, it is important to know the three types of levers. They are called first, second, and third class levers. First class levers have the fulcrum, or pivot point, in the middle, as in a see-saw. Second class levers have the fulcrum at one end, with the effort (force) at the other end and the load in the middle. An ideal example of a second class lever is a wheelbarrow. Third class levers have the fulcrum at one end and the load at the other end, with the effort (force) in the middle. A pair of bar-b-que tongs is an example of a third class lever.
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.
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 questions on the ASVAB, you can assume that 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.
The key to solving spring problems is to draw a diagram of the arrangement, if one isn't already provided, and follow the pulling force through the system.
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."
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.
Motion simply means a change of position. The parameters that describe mechanical motion include velocity, direction, acceleration, and friction.
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