288. The importance of the screw as a mechanical power justifies us in
examining another of its useful forms, the screw-jack. This machine is
used for exerting great pressures, such for example as starting a ship
which is reluctant to be launched, or replacing a locomotive upon the
line from which its wheels have slipped. These machines vary in form,
as well as in the weights for which they are adapted; one of them is
shown at D in Fig. 44, and a description of its details is given in
Table XVII. We shall determine the powers to be applied to this machine
for overcoming resistances not exceeding half a ton.
289. To employ weights so large as half a ton would be inconvenient if
not actually impossible in the lecture room, but the required pressures
can be produced by means of a lever. In Fig. 44 is shown a stout wooden
bar 16' long. It is prevented from bending by means of a chain; at E
the lever is attached to a hinge, about which it turns freely; at A a
tray is placed for the purpose of receiving weights. The screw-jack is
2' distant from E, consequently the bar is a lever of the second order,
and any weight placed in the tray exerts a pressure eightfold greater
upon the top of the screw-jack. Thus each stone in the tray produces
a pressure of 1 cwt. at the point D. The weight of the lever and the
tray is counterpoised by the weight C, so that until the tray receives
a load there is no pressure upon the top of the screw-jack, and thus
we may omit the lever itself from consideration. The screw-jack is
furnished with an arm D G; at the extremity G of this arm a rope is
attached, which passes over a pulley and supports the power B.
290. The velocity ratio for this screw-jack with an arm of 33", is
found to be 414, by the method already described (Art. 283).
291. To determine its mechanical efficiency we must resort to
experiment. The result is given in Table XVII.