CHAPTER 5
THE SCREW
CHAPTER LEARNING OBJECTIVES
Upon completion of this chapter, you should be able to do the following:
State the uses of the screw.
Explain the use of the jack.
Discuss the use of the micrometer
The screw is a simple machine that has many uses.
The vise on a workbench makes use of the mechanical
advantage (M.A.) of the screw. You get the same
advantage using glued screw clamps to hold pieces of
furniture together, a jack to lift an automobile, or a food
processor to grind meat.
A screw is a modification of the inclined plane. Cut
a sheet of paper in the shape of a right triangle and you
have an inclined plane. Wind this paper around a pencil,
Figure 5-1.—A screw is an inclined plane in spiral form.
as in figure 5-1, and you can see that the screw is actually
an inclined plane wrapped around a cylinder. As you
turn the pencil, the paper is wound up so that its
hypotenuse forms a spiral thread. The pitch of the screw
and paper is the distance between identical points on the
same threads measured along the length of the screw.
THE JACK
To understand how the screw works, look at figure
5-2. Here you see the type of jack screw used to raise a
house or apiece of heavy machinery. Notice that the jack
has a lever handle; the length of the handle is equal to r.
Figure 5-2.-A jack screw.
5-1
