1. A Lever,
2. A Pump,
3. and a Ram working in a
4. Cylinder.
Bramah in the year 1796 brought out a very interesting apparatus which
illustrates the law of the equality of pressure which has been widely
adopted in the practical use of the hydraulic press. The principle upon
which this press works is due to Pascal but it remained for Bramah to
put it to practical use. Enormous pressures are developed by operating
the hand lever shown at _M_ in Fig. 145, which is connected with pump
plunger P. The pump barrel A is very thick and receives its supply from
the cistern H through the suction pipe _a_.
[Illustration: FIG. 146.]
Water is delivered from the pump A through a heavy lead pipe into the
cylinder _B_ of the hydraulic press. The ram P is made tight by the
leather packing _n_ and has a table or platform attached to its upper
end as shown. The stationary part _Q_ consists of a heavy cast-iron
plate supported by four wrought-iron or machinery steel columns. By
operating the handle _M_ of the pump any substances placed between the
table on the ram P and the plate Q may be compressed to any reasonable
extent.
The pressure which can be obtained by this press depends on the
relation of the ram P to that of the plunger P. If the former has a
transverse section fifty or a hundred times as large as the latter,
the upward pressure on the ram will be fifty or a hundred times that
exerted upon the pump plunger. By means of the lever M an additional
advantage is obtained.
If the distance from the fulcrum to the point where the power is
applied is five times the distance from the fulcrum to the plunger P
the pressure on it will be five times the power. Thus, if a man acts on
M with a force of sixty pounds, the force transmitted by the plunger P
will be 300 pounds, and the force which tends to raise the ram will be
3,000, supposing the section of ram is a hundred times that of the pump
plunger.
Over-pressure, is prevented by safety-valve shown in front of the pump
A. Fig. 146 shows an enlarged section of the pump used in connection
with this press. When the plunger _P_ rises a partial vacuum is formed
below it and the suction valve _O_ rises allowing the pump barrel to
fill with water through the strainer and suction pipe in the cistern.
When the plunger descends the valve _O_ closes and the water passes
through the discharge valve _h_ into the pipe _K_, thence into the
cylinder _B_ of the press where it acts upon the ram. When the press
has done its work the ram may be lowered by opening the relief valve
_r_. The safety valve is shown at _i_. By removing the plug _h_ the
discharge valve can be reached to grind it in when necessary.
NOTE.—_Hydraulic Pressure Transmission._ Water under high
pressure—500 to 3000 lbs. per square inch and upwards—affords a very
satisfactory method of transmitting power to a distance, especially
for the movement of heavy loads at small velocities, as by cranes and
elevators. The system consists usually of one or more pumps capable
of developing the required pressure; 2, accumulators, described
on the next page; 3, the distributing pipes, and 4, the presses,
cranes, or other machinery to be operated. This property of fluids
invests us with a power of increasing the intensity of a pressure
exerted by a comparatively small force, without any other limit than
that of the strength of the materials of which the engine itself is
constructed. It also enables us with great facility to transmit the
motion and force of one machine to another, in cases where local
circumstances preclude the possibility of instituting any ordinary
mechanical connection between the two machines. Thus, merely by
means of water-pipes, the force of a machine may be transmitted to
any distance, and over inequalities of ground, or through any other
obstructions.