TEXT: The Basic Machines

When a prehistoric man or woman used a stick to pry up a stone, the lever was invented. It is one of the six basic machines in the system of classification we will follow in this book. A lever is a rigid piece or bar, like the early person's stick, which turns on a point called the fulcrum. When force is applied at a second point, that force is transmitted to a third point where it can perform work. A children's seesaw is an excellent example of a lever. The point of balance on which the seesaw rests is the fulcrum; when downward force is applied to one end, the other end rises.

The organized use of levers goes hack beyond the beginning (recorded history. Levers were probably used to raise the hug blocks of stone from which Stonehenge was constructed. Perhaps the stones were raised by using tree trunks as levers until the stone toppled into place.

There are three classes of levers. The seesaw is a lever of the first class, with the fulcrum be­tween the point where force is applied - the effort end - and the point where there is resisting force - the load end. The wheelbarrow is a lever of the sec­ond class, with the load between the fulcrum and the effort. The fulcrum is in front, the load is in the wheelbarrow itself and the effort is applied behind the load. A foot-treadle is a lever of the third class with the effort between the fulcrum and the load. The fulcrum is at one end, force in the form of pressure from the foot is applied behind the fulcrum, and the load is still farther beyond the point where the foot presses down.

We can observe that a seesaw will balance when a heavier person at the effort end is a short distance from the fulcrum and a lighter person at the load end is farther from the fulcrum. This is an illustration of the law of the lever: the effort force times its distance from the fulcrum is equal to the resisting force times its distance from the fulcrum when the lever is balanced. To gain more mechanical advantage, the distance between the point of effort and the fulcrum can be lengthened so that the effort is exerted through a greater distance. The fulcrum must exert an upward force equal to the two downward forces exerted on it—the downward force required to lower the effort arm and the downward force of the load.

The wheel and axle is the second basic type of machine. Like the lever, the wheel goes back to prehistoric times when someone probably discovered that it was easier to move heavy weights by sliding them on logs than by carrying them. The axle is a shaft on which a wheel can turn and the wheel and axle combination may have first been used sometime around 3,000 B.C. for water-raising devices. War chariots were the tanks of ancient times and wagons were the trucks.

In addition to its uses for transportation the wheel has endless applications. An early and important one was for the potter's wheel which permitted craftspeople to shape clay into controlled thickness for greater variety of forms and uses. Wheels were also put to work early for irrigation by raising water from streams or wells to divert it into artificial channels. Other early uses were for millstones to grind grain and for waterwheels that could transmit energy for many purposes.

The potential of the wheel was increased by the development of the crank. The crank is a device which can transmit motion or can change rotary motion into reciprocating motion and the reverse. With the development of the crank, waterwheels could be put to work for essential purposes such as crushing rock or sawing wood.

The third basic machine is the pulley. In its simplest form it consists of a wheel with a groove around its outer surface through which a rope, wire, or chain can be passed. This simple device was used in ancient times for tasks such as raising water from wells or streams and hoisting sails onto ships. A pulley con­tained in housing is called a block. When a fixed block is used with a movable block towhich a weight is attached, down­ward pull on the rope will raise the weight. This device is called a block and tackle.

The block and tackle illustrated has a mechanical advantage of two. The mechanical advantage can be increased by different ar­rangements and combinations of blocks.

The movable pulley acts on the leverage principle; it forms a lever of the second class (like the wheelbarrow) with the fulcrum at the downward point of contact of rope and wheel, the load suspended from the axle, and the effort at the upward point of contact of rope and wheel. Increasing the number of fixed and movable pulleys increases the mechanical advantage.

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 rope very early times for such purposes as quarrying rock, plough 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.

We have already mentioned the inclined plane as the probable methodemployed 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 effortthan 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 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. When a prehistoric man or woman pried up a stone with a stick, what machine was in use? What is the principle on which the lever works?

2. Why is a seesaw an excellent example of a lever?

3. How were the rocks for Stonehenge moved probably?

4. What is a lever of the first class?

5. What is a lever of the second class?

6. What is a lever of the third class?

7. What is the law of the lever? How does the seesaw illustrate this?

8. How can the mechanical advantage of the lever be increased?

9. What kind of force must the fulcrum exert?

10. What is the difference between a wheel and a wheel and axle?

11. When was the wheel probably discovered? When was the wheel mid axle combination probably first used? For what purpose?

12. What animal made possible the early use of the wheel and axle for transportation?

13. Name some other early uses of the wheel.

14. What can a crank do? What did it make possible?

15. What is the third basic machine? Describe its simplest form. What was this device used for in ancient times?

16. What is a block and tackle and what can it do?

17. What is the mechanical advantage of a simple block and tackle? How can this be increased?

18. Explain how a movable pulley acts as a lever of the second class.

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

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

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

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

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

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

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

26. What were some early uses of the screw?

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

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

Review:

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

1. Lever ____ A pulley contained in housing.

2. Fulcrum ____ A corkscrew-shaped figure.

3. Effort End ____ A triangle of material tapering to a thin edge.

4. Load End ____ The resistance to motion produced by two bodies moving in contact with each other.

5. Axle ____ A machine consisting of a rigid bar that turns on a point.

6. Crank ____ A helical inclined plane often used as a fastening device.

7. Pulley ____ The point on which a lever turns.

8. Block _____ A surface like a ramp that is set at an angle to the horizontal.

9. Block and Tackle _____ The point where there is resisting force on a lever.

10. Wedge _____ A device used to raise heavy weights for short distances.

11. Inclined Plane _____ A bent shaft or arm for transmitting motion, changing reciprocating to rotary motion or the opposite.

12. Screw ____ The point where force is applied to a lever.

13. Helix ____ A shaft on which a wheel rotates.

14. Jack ____ A wedge which can be used as a fastening device because of friction.

15. Friction _____ 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 V