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BELLOWS ELASTIC ELEMENTS
A bellows elastic element is a convoluted unit that
expands and contracts axially with changes in
pressure. The pressure to be measured can be applied
to either the outside or the inside of the bellows;
in practice, most bellows measuring
Figure 8-5.—Helical Bourdon tube.
Figure 8-6.—Simple bellows gauge.
devices have the pressure applied to the outside of
the bellows (fig. 8-6).
Simple Bellows Elements
Bellows elastic elements are made of brass, phosphor
bronze, stainless steel, beryllium-copper, or
other metal suitable for the intended service
of the gauge. Motion of the element (bellows)
is transmitted by suitable linkage and gears
to a dial pointer. Most bellows gauges are spring-loaded—that
is, a spring opposes the bellows
and thus prevents full expansion of the bellows.
Limiting the expansion of the bellows in this
way protects the bellows and prolongs its life. Because
of the elasticity in both the bellows and the
spring in a spring-loaded bellows element, the relationship
between the applied pressure and bellows
movement is linear.
Dual Bellows Indicators
Another type of bellows element is the dual-bellows element.
Figure 8-7 is a schematic diagram of
this indicator. Dual-bellows element pressure indicators
are used throughout the Navy as flow-measuring, level-indicating,
or pressure-indicating devices.
Figure 8-7.–Differential pressure sensor dual bellows.
Figure 8-8.–Dual bellows assembly.
When in operation, the bellows will move in proportion
to the difference in pressure applied across
the bellows unit assembly. The linear motion
of the bellows is picked up by a drive arm and
transmitted as a rotary motion through a torque
tube assembly (fig. 8-8). The indicating mechanism
multiplies rotation of the torque tube through
a gear and pinion to the indicating pointer.
Bellows elements are used in various applications where
the pressure-sensitive device must be
powerful enough to operate not only the indicating
pointer but also some type of recording device.
PRESSURE SWITCHES
Often when a measured pressure reaches a certain
maximum or minimum value, it is desirable to
have an alarm sound a warning, a light to
give a signal, or an auxiliary control system to energize
or de-energize. A pressure switch is the device
commonly used for this purpose. One
of the simplest pressure switches is the single-pole,
single-throw, quick-acting type shown in
figure 8-9. This switch is contained in a metal
Figure 8-9.—Typical pressure
switch.
case that has a removable cover, an electrical connection,
and a pressure-sensing connection. The
switch contains a seamless metallic bellows located
in its housing. Changes in the measured pressure
causes the bellows to work against an adjustable
spring. This spring determines the pressure
required to actuate the switch. Through suitable
linkage, the spring causes the contacts to open
or close the electrical circuit automatically when
the operating pressure falls below or rises above
a specified value. A permanent magnet in the
switch mechanism provides a positive snap on both
the opening and closing of the contacts. The switch
is constantly energized. However, it is the closing
of the contacts that energizes the entire electrical
circuit.
Another pressure switch is an electric-hydraulic assembly
that is used for shutting off the
pump’s motor whenever the system pressure exceeds
a pre-determined maximum value (fig. 8-10).
The switch is mounted on the pump housing so
that the former’s low pressure ports drain directly
into the pump housing. This
pressure switch principally consists of a flange-mounted
hydraulic valve to which is fixed a
normally closed electrical limit switch. The
valve consists of two hydraulically interconnected
components, the pilot valve sub-assembly, which
bolts on the bottom of the body
(l), functions to sense system pressure continuously
and initiates pressure switch action whenever
this pressure exceeds the adjusted setting of
the pilot adjustment. System pressure is directed
into the bottom port and is applied against
the exposed tip of the pilot piston (5). This piston
is held on its seat by compression from the piston
spring (6) which is dependent on the position
of the adjusting screw (8). Whenever the pressure
causes a force sufficiently large enough to
raise the pilot piston from its seat, fluid flows
through an interconnecting passage to the actuating
piston (2) chamber. The accompanying fluid
force raises the actuating piston against the force
of spring 3 and causes depression of the
extended switch plunger. This, in turn, disconnects
the contained electrical switch, which may
be connected into the pump motor’s electric supply
system.
Pressure switches come in many sizes and configurations
depending on how they will be used.
Figure 8-10.—Electric-hydraulic pressure switch.
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