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Technology: Engineering Our World
The Screw
A screw is an inclined plane wrapped in the form of a cylinder. To
illustrate how this works, take a rectangular piece of paper and cut it
along a diagonal. This triangle will remind you of an inclined plane. Now
wrap it around a pencil. Roll from the edge of the triangle toward the
point to shape it like a screw. See Figure 8-31.
When we examine the inclined plane, we find that the time taken for
the load to be pushed up a longer inclined plane increases, but the effort
decreases. The same is true in the case of the screw threads on a nut and
bolt. The greater the number of threads, the shallower the slope, and the
longer it takes to move the nut to the head of the bolt. However, a greater
number of threads makes it easier to move the nut against a resistance.
The wedge-shaped section of a tapering wood screw reveals another
application of the wedge. See Figure 8-32. It allows the screw to force its
way into the wood.
Screw threads may be used in two quite different ways. They may be
used to fasten, as with wood screws, machine screws, and light bulbs.
They may also transmit motion and force. Examples are C-clamps, vises,
and car jacks. See Figure 8-33. Note also that a screw converts rotary
motion into straight-line motion. See Figure 8-34.
gure 8-31.
wrapped into a cylinder.
gure 8-32. A wood screw also has a
wedge shape to push aside the wood fibers
as it enters the wood.
A B
gure 8-33. A—A scissor jack lifts heavy loads with less effort applied. B—As the screw
of the jack is turned, the two ends of the jack are forced to move toward each other, and the
car is raised.