The hydraulic press When a fluid completely fills a vessel, and a pressure is applied to it at any part of the surface, that pressure is transmitted equally throughout the whole of the enclosed fluid. This is known as Pascal’s.principle (see also page 122). This principle finds an important industrial application in the hydraulic press (Fig. 8.16). This […]
Wheel and axle principle. Gears Fig. 8.14 opposite shows some examples of devices using the wheel and axle principle. The steering-wheel of a car is another obvious example, but on” of the main applications of the principle in modern engineering is found in gear-boxes where toothed wheels of different diameters engage to give turning forces at low […]
The screw 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 […]
The inclined plane A heavy load may be raised more easily by pulling it along a sloping surface than by lifting it vertically. Heavy packing cases are often loaded into vans by hauling them up an incline formed by two stout planks held apart by iron stays (Fig. 8.9). It is believed that the large blocks of stone used […]
To study the variation of the mechanical advantage of a pulley system with load We shall use a block and tackle with two pulleys in each block for this experiment, but the same principle may equally well be applied to other types of machine. The pulleys are set up as in Fig. 8.7, scale-pans being provided for the […]
Work done by a machine. Efficiency If the pulley system shown in Fig. 8.7 were a “perfect machine”, i.e., composed of weightless and frictionless strings and pulleys, then a load of 40 N would be raised through a distance of 1 m by an effort of ION moving a distance of 4 m. The work done by the […]
Velocity ratio (or speed ratio) (V.R.) In the pulley systems we have already considered where the mechanical advantage is greater than 1, it might appear at first sight that we are getting more out of the machine than we are putting into it. But while in such cases the load is greater than the effort, it must be […]
The block and tackle This is by far the most important pulley system of all, being commonly used for lifts and cranes. Two blocks are employed containing from two to eight pulleys in each, according to the mechanical advantage required. To illustrate the principle, Fig. 8.6 has been drawn to show two pulleys in each block. For simplicity, the […]
Direction of a tension in a string In all pulley diagrams some consideration must be given to the direction in which force arrows are drawn on the strings. Take the case of a 2 kg mass supported by a string held in the hand (Fig. 8.5). Assuming g = 10m/s2 the tension in the string is 20 […]
The single moving pulley This is shown in Fig. 8.4. Here the tension in the string or rope is equal to the effort applied, so that the total upward pull on the pulley is twice the effort E. Suppose a load of 4 N is supported by the pulley and that the weight of the pulley block and […]