TEXT: Machine Components

Essentially all machines are variations or combinations of the six basic types described in the previous unit. There are a number of different kinds of mechanisms or components that transmit motion or change it in one way or another. Modern machines and their components have become so complex that a branch of the science of mechanics called kinematics evolved in order to study mechanisms and their actions. Regardless of the original input and final output of most modern machines, it is their mechanisms that give them their great versatility and flexibility.

Gears play such an important part in machines that they have become the symbol for machinery. They are wheels with teeth that engage or mesh with each other so that they work in pairs to transmit or change motion. They are frequently used to reduce or increase the speed of a motion and they can also change the direc­tion of motion. The line around which a wheel rotates is its axis; gears can change axial motion.

By classifying gears according to the shape and arrangement of their teeth we discover four basic types. The simplest and most common is the spur gear. Spur gears have teeth that are straight and parallel to the axis. One member of a pair or series of gears receives input motion, usually from a shaft. The teeth of the first gear mesh (engage) with the teeth of the next one, passing the motion (energy) along. If the two gears have the same number of teeth, the velocities will be inversely proportional to the number of teeth. That is, if the first gear has sixty teeth and the second gear has twenty, the second gear will turn three times as fast as the first. Spur gears are used for transmitting motion along parallel axes rather than for changing direction.

In helical gears the teeth are at such an angle to the wheel that they form helices. There are often two sets of teeth on each gear with the teeth at equal but opposite angles; this variation is called a herringbone gear. These gears are particularly useful for transmitting power at high speeds. They are also used to change the direction of motion, most frequently when the axes are crossed through a 90° angle.

A bevel is a surface that is slanted at an angle in relation to another surface. In a bevel gear the teeth are slanted in rela­tion to the plane of the wheel. Bevel gears are useful in changing the direction of motion, the change being in proportion to the angle of the beveled surface. One variation is the spiral bevel gear which has the same relationship to bevel gears as helical gears have to spur gears. With a bevel gear one tooth at a time has to bear the entire load but in the spiral configuration more than one tooth always remains in contact.

The fourth basic type is called the worm gear. Basically a pair consists of the gear itself, a wheel with teeth which meshes with a worm —a screw which is a helix wrapped around a cylinder. A variation is a worm shaped in an hour-glass figure. Worm gears are used primarily for changing the direction of axial motion.

Another type of gear mechanism is rack and pinion. The rack is a straight bar with teeth and the pinion is a small spur gear.

Gear devices can be used in a variety of shapes and combina­tions. They are essential elements in tiny devices like watches and in large ones like automobiles. Without the gears that transmit motion to the driving wheels we could not have the kind of transportation that exists today. In their variations on the basic machines gear mechanisms are key elements that produce enormous versatility.

Another kind of mechanism is the cam. Like the gear, it consists of a pair of components; the cam itself is the input member and the follower is the output mem­ber. The cam is attached to a rotating shaft; it transmits motion to the follower. Cams come in many different shapes — there are heart-shaped cams, clover-leafed cams, elliptical cams and others. By means of these different shapes cams can change rota­ting into reciprocating (back and forth or up and down) motion or into oscillating or vibrating motion. The follower is usually a rod or shaft. Cams can transmit exact motions at specific times in a cycle. They are therefore use­ful where the timing of com­plex motions is important. They are in automobile engines to raise and lower the valves and in sewing machines to control the movements of the needle.

Another kind of mechanism is known as a linkage; it is a series of at least three rods or solid links that are connected by joints that permit the links to pivot. When one link is fixed the other links can move only in paths that are predetermined. Like cams, link­ages are used to change the di­rection of motion, to transmit different kinds of motion, or to provide variations in timing in different parts of a cycle by varying the lengths of the links in relation to each other.

The spring is a mechanism that is used in a wide variety of machines; it is frequently an elastic helical coil that returns to its original shape after being dis­torted. Springs are essential components in watches; in some cam mechanisms they hold the follower in place; they are found in scales and they help to cushion an automobile ride. There are many variations on the basic coiled or spiral spring, including the leaf spring which is made of strips of elastic material and springs that depend on the compression and expansion of air.

A ratchet is another paired mechanism consisting of a wheel with teeth and a pawl which drops into the spaces between the teeth. The ratchet mechanism is used to prevent a motion from be­ing reversed or to change recipro­cating into rotary motion.

This is a brief introduction to the complex world of machine components. The infinite number of combinations and variations in which these mechanisms can be combined is the heart of the work of a mechanical engineer.

Discussion:

1. On what do the variations in machines depend?

2. What science has developed to study mechanisms?

3. Explain why gears have become the symbol for machinery.

4. How do gears work?

5. For what purposes are gears used?

6. What determines the four basic types of gears?

7. What is the simplest and most common kind of gear? How are its teeth arranged?

8. What is the difference between the gear and the pinion?

9. Why is it necessary to calculate the ratio of teeth in gears that mesh together?

10. How are the teeth arranged in helical gears? What is a variation of the helical gear?

11. For what purpose are helical gears particularly useful?

12. How are the teeth arranged in a bevel gear?

13. What are bevel gears especially useful for?

14. What is a variation of the bevel gear? Discuss its advantage.

15. Describe the worm gear and its uses.

16. What is the name for a gear mechanism with a straight bar and a small spur gear used as a pair?

17. What are some of the uses of gears? Give examples in addition to those in the reading.

18. In a cam mechanism, which is the input member and which is the output member?

19. Are all cams the same shape?

20. What are some of the things that cams do?

21. Name some machines which use cams.

22. What is a linkage and how does it work?

23. How are linkages like cams?

24. What is an everyday example of a linkage? Can you think of another one?

25. Discuss the spring mechanism.

26. What are some of its common uses? Give examples in addition to those in the reading.

27. Name some different types of springs.

28. How does a ratchet and pawl mechanism work?

29. Mechanisms are the heart of the work of a mechanical engineer. Discuss this statement.

Review:

A. Match the terms on the left with the statements on the right.

1. Mechanism ____ A spring made of strips rather than a spiral coil.

2. Kinematics ____ A mechanism consisting of rods connected to each other by joints that permit motion.

3. Gear ____ A device of elastic material, usually metal, that returns to its original shape after being distorted.

4. Axial Motion ____ A branch of the science of mechanics that deals with aspects of motion apart from mass and force.

5. Pinion ____ A gear with teeth slanted at an angle to the plane of the wheel itself.

6. Spur Gear ____ A component of a machine that transmits or changes motion.

7. Helical Gear ____ The part of a cam mechanism, usually a rod or shaft that is the output member of the pair.

8. Bevel Gear ____ Motion in the line around which a wheel rotates.

9. Worm Gear ____ A piece of metal that rotates or slides to transmit or change motion.

10. Rack and Pinion ____ A wheel with teeth cut at an angle to its axis; paired with a pawl it governs or prevents motion.

11. Cam _____ The smaller member of a pair of gears.

12. Follower _____ A wheel with teeth that can engage or mesh with another wheel with teeth.

13. Linkage _____ A gear with teeth straight and parallel to the axis.

14. Spring _____ A gear that meshes with a screw-like cylinder that has teeth cut in helices.

15. Leaf Spring ______ A gear with teeth cut in the shape of a helix.

16. Ratchet ______ A gear mechanism in which a spur gear (pinion) meshes with teeth in a straight bar (rack).