Vocabulary Practice

8. What is a wedge?

9. What is an inclined plane?

10. Describe a screw.

11. What is the shape of the helix?

12. What is a jack?

The Basic Machines: Wedge, Inclined Plane, Screw

The three remaining basic machines are so related to one another that they are sometimes grouped together. They are the wedge, the inclined plane, and the screw.

The wedge is a triangle with two chief surfaces that meet in a sharp angle or taper to a thin edge. Wedges are used for splitting open or pushing apart. They were used from very early times for such purposes as quarrying rock, plowing fields or cutting wood, as with an axe. A nail is a familiar form of the wedge.

The mechanical advantage of a wedge can be computed by dividing the length of the surface by the breadth of the wedge. A wedge twelve inches long with a breadth of three inches would have a theoretical mechanical advantage of four. However, friction is an important consideration in use of the wedge; in reality much of the advantage is lost. In fact, it is friction that holds a nail in place.

We have already mentioned the inclined plane as the probable method employed by the Egyptians for manipulating into place the huge blocks of stone in the pyramids. Early men and women knew that a weight could be pushed up a hill or a ramp of earth with less effort than would be required to move the same weight vertically. Many centuries had to pass before it was discovered that this mechanical device could be explained mathematically. The effort put forth in moving a load up an inclined plane is the same as the proportion between the height of the rise and the length of the inclined plane. If the rise is five feet and the length of the plane is twenty feet, the ratio is one to four. Therefore an effort equal to one-quarter of the load would theoretically be needed to raise the load. In actual practice, friction has to be overcome so a greater effort is required.

The inclined plane is an important factor that concerns civil engineers when designing highways or railroads. The mechanical engineer more frequently uses the screw, a spiral form of the in­clined plane. The figure that results from wrapping the line of an inclined plane around a cylinder is called a helix.

The screw was used in ancient times to press grapes for wine or olives for oil. In the Middle Ages it was important in the develop­ment of printing. Today we are most familiar with the screw as a fastener but it has numerous other uses. It is one of the most important devices for amplifying or increasing force. A familiar adapta­tion is the screw jack used to lift automobiles or any great weight through a short distance. The screw is also a major means of changing the direction of motion.

Both the screw and the helix have so many adaptations in modern machines that it is impossible to list them but one in particular is extremely interesting: the helical motion of a propeller on a boat or an airplane moves the vessel or plane ahead as though it were screwing its way through the water or air!

Discussion

1. What are the remaining basic machines that are sometimes grouped together?

2. What are wedges used for? What were some practical applications in very early times? Name some we use today.

3. How can you compute the mechanical advantage of a wedge?

4. Why is much of this mechanical advantage lost in reality? How does this relate to the nail?

5. What did ancient builders know about moving weights up hills or ramps?

6. How can you compute the mechanical advantage of an inclined plane? Why would a greater effort than this be required in practice?

7. How is the inclined plane related to the screw? Describe a helix.

8. What were some early uses of the screw?

9. What are some examples of its modern use? Why is it particularly important?

10. What kind of motion does the propeller of a ship or an airplane have? What is the result of this motion?

Review of the Basic Machines

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

1. Lever	a. A pulley contained in a housing.
2. Fulcrum	b. A corkscrew-shaped figure.
3. Effort End	c. A triangle of material tapering to a thin edge.
4. Load End	d. The resistance to motion produced by two bodies moving in contact with each other.
5. Axle	e. A machine consisting of a rigid bar that turns on a point.
6. Crank	f. A helical inclined plane often used as a fastening device.
7. Pulley	g. The point on which a lever turns.
8. Block	h. A surface like a ramp that is set at an angle to the horizontal.
9. Block and Tackle	i. The point where there is resisting force on a lever.
10. Wedge	j. A device used to raise heavy weights for short distances.
11. Inclined Plane	k. A bent shaft or arm for transmitting motion, changing reciprocating to rotary motion or the opposite.
12. Screw	l. The point where force is applied to a lever.
13. Helix	m. A shaft on which a wheel rotates.
14. Jack	n. A wedge which can be used as a fastening device because of friction.
15. Friction	o. A wheel with a grooved surface through which a rope, wire, or chain is passed.
16. Nail	Combination of a fixed and a movable block.

UNIT FOUR

MACHINE COMPONENTS: GEARS (PART I)

Special Terms

Mechanism: A component of a machine that transmits or changes motion.

Kinematics: A branch of the science of mechanics that deals with aspects of motion apart from considerations of mass and force.

Gear: A wheel with teeth that can engage another wheel with teeth; gears work in pairs to transmit or change motion.

Axial Motion: Motion around an axis, the line around which a wheel rotates.

Spur Gear: A gear with straight teeth parallel to the axis.

Pinion: The smaller member of a pair of gears or the smallest gear of a series; gear is used to designate the larger member.

Helical Gear: A gear with teeth cut in the shape of a helix.

Herringbone Gear: A helical gear with two sets of teeth at equal but opposite angles to each other.