CHAPTER 5THE SCREWCHAPTER LEARNING OBJECTIVESUpon 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 micrometerThe screw is a simple machine that has many uses.The vise on a workbench makes use of the mechanicaladvantage (M.A.) of the screw. You get the sameadvantage using glued screw clamps to hold pieces offurniture together, a jack to lift an automobile, or a foodprocessor to grind meat.A screw is a modification of the inclined plane. Cuta sheet of paper in the shape of a right triangle and youhave 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 actuallyan inclined plane wrapped around a cylinder. As youturn the pencil, the paper is wound up so that itshypotenuse forms a spiral thread. The pitch of the screwand paper is the distance between identical points on thesame threads measured along the length of the screw.THE JACKTo understand how the screw works, look at figure5-2. Here you see the type of jack screw used to raise ahouse or apiece of heavy machinery. Notice that the jackhas a lever handle; the length of the handle is equal to r.Figure 5-2.-A jack screw.5-1
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