Vocabulary Practice

1. What is a linkage?

2. Describe a spring.

3. How does a leaf spring differ from the usual type of spring?

4. Discuss a ratchet and pawl.

Machine Components: Linkage, Spring, Ratchet

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. What is a linkage and how does it work?

2. How are linkages like cams?

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

4. Discuss the spring mechanism.

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

6. Name some different types of springs.

7. How does a ratchet and pawl mechanism work?

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

Review of Unit Four

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 cairrmechanism, 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).

UNIT FIVE

FRICTION

Special Terms

Friction: The resistance to relative motion of two bodies moving in contact with each other.

Coefficient of Friction: The constant ratio of the friction to the force pressing the surfaces together.

Static Friction: The resistance to motion between two bodies in contact with each other but at rest.

Sliding Friction: The resistance to continued motion when one body has started to move.

Rolling Friction: The resistance to motion when one or more bodies is rotating or rolling, but not on the same axis.

Babbitt Metal: An alloy or mixture of tin, copper, and antimony with a low coefficient of friction.

Teflon: A plastic containing fluorine with a low coefficient of friction. Antimony: A metalloid element, silvery-white, brittle yet soft with the unusual quality of expanding when solidified; used as a constituent of alloys.

Lubrication: Application of a substance, usually oil or grease (a lubricant) to reduce friction.

Polymers: Long, heavy, complex molecules forming repeating structural units that occur in many plastics.