Vocabulary Practice

1. How does a mechanism differ from a machine?

2. What is studied in kinematics?

3. Describe a gear.

4. What is axial motion?

5. How are the teeth cut in a spur gear?

6. What is a pinion?

7. How are the teeth cut in a helical gear?

8. Describe a herringbone gear.

Machine Components: Gears (Part I)

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 gear of a pair or series of gears receives 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.

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?

MACHINE COMPONENTS: GEARS (PART II)

Special Terms

Bevel Gear: A gear with teeth slanted at an angle to the plane of the wheel itself.

Worm Gear: A mechanism consisting of a gear that meshes with a worm, a screw with helical teeth.

Rack and Pinion: A gear mechanism composed of a rack (a straight bar with teeth) and a pinion (a spur gear).

Cam: A rotating or sliding piece of machinery that acts as part of a pair to impart or receive motion.

Follower: The other part of a cam mechanism, usually a rod and shaft that receives and transmits motion from the cam.