Millions of screws and bolts are used daily for the purpose of holding things together. We have already mentioned the importance of the screw in connection with micrometers (page 7). In addition, the screw is an essential feature of machines such as vices and screw jacks (Fig. 8.11, 8.12). The distance between successive threads on a screw is called its pitch. For one complete turn, therefore, a screw moves through a distance equal to its pitch. As an example of the screw as a machine, we shall consider the working of a car jack. Fig. 8.12 shows one type of jack which consists of a long screw carrying a nut. This nut is hinged to a short steel bar which fits into a slot in the car chassis. To raise the car, the screw is turned by hand using a short steel rod known as a tommy bar. Approximate data for this type of jack are,
In practice, of course, the effort must be much greater than this in order to overcome friction. Even so, the total effort required will still be very small compared with the load being lifted.