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CHAPTER 5 THE SCREW CHAPTER LEARNING OBJECTIVES Upon completion of this chapter, you should be able to do the following:
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. If you pull the lever handle around one turn, its outer end has described a circle. The circumference of this circle is equal to 2p. (Remember that p equals 3.14, or 22/7). That is the distance you must apply the effort of the lever arm. At the same time, the screw has made one revolution, raising its height to equal its pitch (y). You might say that one full thread has come up out of the base. At any rate, the load has risen a distance p.Remember that the theoretical mechanical advantage (T.M.A.) is equal to the distance through which you apply the effort or pull, divided by the distance and resistance the load is moved. Assuming a 2-foot, or 24-inch, length for the lever arm and a 1/4-inch pitch for the thread, you can find the theoretical mechanical advantage by the formula T.M.A. =
in that r = length of handle = 24 inchesp = pitch, or distance between corresponding points on successive threads = 1/4 inch.Substituting, A 50-pound pull on the handle would result in a theoretical lift of 50 x 602 or about 30,000 pounds15 tons for 50 pounds. However, jacks have considerable friction loss. The threads are cut so that the force used to overcome friction is greater than the force used to do useful work. If the threads were not cut this way and no friction were present, the weight of the load would cause the jack to spin right back down to the bottom as soon as you released the handle. |
 
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