223. We shall conclude this lecture with some experiments upon a
useful mechanical power introduced by Mr. Eade under the name of the
epicycloidal pulley-block. It is shown in Fig. 33, and also in Fig. 49.
In this machine there are two chains: one a slight endless chain to
which the power is applied; the other a stout chain which has a hook at
each end, from either of which the load may be suspended. Each of these
chains passes over a sheave in the block: these sheaves are connected
by an ingenious piece of mechanism which we need not here describe.
This mechanism is so contrived that, when the power causes the sheave
to revolve over which the slight chain passes, the sheave which carries
the large chain is also made to revolve, but very slowly.
224. By actual trial it is ascertained that the power must be exerted
through twelve feet and a half in order to raise the load one foot; the
velocity ratio of the machine is therefore 12·5.
225. If the machine were frictionless, its mechanical efficiency would
be of course equal to its velocity ratio; owing to the presence of
friction the mechanical efficiency is less than the velocity ratio, and
it will be necessary to make experiments to determine the exact value.
I attach to the load hook a weight of 280 lbs., and insert a few small
hooks into the links of the power chain in order to receive weights: 56
lbs. is sufficient to produce motion, hence the mechanical efficiency
is 5. Had there been no friction a power of 56 lbs. would have been
capable of overcoming a load of 12·5 × 56 = 700 lbs. Thus 700 units of
energy must be applied to the machine in order to perform 280 units
of work. In other words, only 40 per cent. of the applied energy is
utilized.
226. An extended series of experiments upon the epicycloidal
pulley-block is recorded in Table XII.