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 addition of ‘eights to represent load and effort. Since the scale-pans are more or less an essential convenience, they are best treated as part of the machine itself. Some people, however prefer to include the weights of the pans as part of the load and effort respectively. Whichever course is adopted should be mentioned in the account of the experiment.
An initial mass of, say, 0.05 kg is added to the load pan, and further masses are then added to the effort pan until the load just rises slowly with a steady velocity. The load and effort, in newtons, are given by mg, where m is the mass in kg and
= 10 m/s2. The experiment is repeated for a series of increasing loads and the re ults recorded in a table. The velocity ratio of this machine may be found by measuring a pair of corresponding distances moved by effort and load. For each pair of readings of effort and load obtained, the mechanical advantage and the efficiency should be calculated from the appropriate formula and entered in e table.
In connection with this experiment, the following points should be noticed:
(1) The useless load consists of the weight of the lower pulley bloek and the string and friction in the string and bearings. The weight of string lifted depends on the distance between the pulley blocks, but the weight of the lower block is constant. The friction varies with the load, but is small in most cases. Thus, although the useless load varies somewhat, it becomes a smaller proportion of the total load as the total load increases. Consequently, the mechanical advantage increases with load.
(2) The efficiency also increases with load for the same reasons. (3) Owing to the work wasted in overcoming friction and raising moving parts, the efficiency is less than 100 per cent. Also, since there are only four pulleys altogether, the mechanical advantage cannot exceed 4.
Graphs should be plotted of mechanical advantage against load, and efficiency against load. The shape of the curves obtained will illustrate the above remarks. Typical graphs are shown in Fig. 8.8.