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CHAPTER 8
For safe and efficient operation, fluid power systems
are designed to operate at a specific pressure
and/or temperature, or within a pressure and/or
temperature range. You have learned
that the lubricating power of
hydraulic fluids varies with temperature and that
excessively high temperatures reduce the life of
hydraulic fluids. Additionally, you have learned
that the materials, dimensions, and method
of fabrication of fluid power components limit
the pressure and temperature at which a system
operates. You have also learned of means of
automatically controlling pressure in both hydraulic
and pneumatic systems. Most fluid
power systems are provided with pressure
gauges and thermometers for measuring and
indicating the pressure and/or the temperature in
the system. Additionally, various temperature and
pressure switches are used to warn of an adverse
pressure or temperature condition. Some switches
will even shut the system off when an adverse
condition occurs. These devices will be discussed
in this chapter.
PRESSURE GAUGES
Many pressure-measuring instruments are called
gauges. However, this section will be restricted
to two mechanical instruments that contain
elastic elements that respond to pressures found
in fluid power systems—the Bourdon-tube and
bellows gauges.
BOURDON TUBE GAUGES
The majority of pressure gauges in use have a
Bourdon-tube as a measuring element. (The gauge
is named for its inventor, Eugene Bourdon, a
French engineer.) The Bourdon tube is a device that
senses pressure and converts the pressure to displacement.
Since the Bourdon-tube displacement is
a function of the pressure applied, it may be
mechanically amplified and indicated by a pointer.
Thus, the pointer position indirectly indicates
pressure.
The Bourdon-tube gauge is available in various
tube shapes: curved or C-shaped, helical, and
spiral. The size, shape, and material of the tube
depend on the pressure range and the type of
gauge desired. Low-pressure Bourdon tubes (pressures
up to 2000 psi) are often made of phosphor
bronze. High-pressure Bourdon tubes (pressures
above 2000 psi) are made of stainless steel
or other high-strength materials. High-pressure Bourdon
tubes tend to have more circular cross
sections than their lower-range counterparts, which
tend to have oval cross sections. The Bourdon
tube most commonly used is the C-shaped
metal tube that is sealed at one end and open
at the other (fig. 8-1).
Figure 8-1.—Simplex Bourdon-tube pressure gauge.
C-shaped Bourdon Tube
The C-shaped Bourdon tube has a hollow, elliptical
cross section. It is closed at one end and is
connected to the fluid pressure at the other end. When
pressure is applied, its cross section becomes
more circular, causing the tube to straighten
out, like a garden hose when the water is
first turned on, until the force of the fluid pressure
is balanced by the elastic resistance of the
tube material. Since the open end of the tube is
anchored in a fixed position, changes in pressure move
the closed end. A pointer is attached to the closed
end of the tube through a linkage arm and a
gear and pinion assembly, which rotates the pointer
around a graduated scale.
Bourdon-tube pressure gauges are often classified
as simplex or duplex, depending upon whether
they measure one pressure or two pressures.
A simplex gauge has only one Bourdon tube
and measures only one pressure. The pressure gauge
shown in figure 8-1 is a simplex gauge. A red
hand is available on some gauges. This hand is
manually positioned at the maximum operating pressure
of the system or portion of the system in
which the gauge is installed.
When two Bourdon tubes are mounted in a
single case, with each mechanism acting independently
but with the two pointers mounted on
a common dial, the assembly is called a duplex gauge.
Figure 8-2 shows a duplex gauge with views of
the dial and the operating mechanism. Note that
each Bourdon tube has its own pressure connection
and its own pointer. Duplex gauges are
used to give a simultaneous indication of the pressure
from two different locations. For example,
it may be used to measure the inlet and outlet
pressures of a strainer to obtain the differential
pressure across it.
Differential pressure may also be measured with
Bourdon-tube gauges. One kind of Bourdon-tube differential
pressure gauge is shown in figure
8-3. This gauge has two Bourdon tubes but
only one pointer. The Bourdon tubes are connected
in such a way that they indicate the pressure
difference, rather than either of two actual
pressures.
As mentioned earlier, Bourdon-tube pressure gauges
are used in many hydraulic systems. In this application
they are usually referred to as hydraulic
gauges. Bourdon-tube hydraulic gauges are
not particularly different from other types of Bourdon-tube
gauges in how they operate; however,
they do sometimes have special design features
because of the extremely high system pressures
to which they may be exposed. For
Figure 8-2.—Duplex Bourdon-tube pressure gauge.
Figure 8-3.—Bourdon-tube differential pressure gauge.
example, some hydraulic gauges have a special type
of spring-loaded linkage that is capable of taking
overpressure and underpressure without damage
to the movement and that keeps the pointer
from slamming back to zero when the pressure
is suddenly changed. A hydraulic gauge that
does not have such a device must be protected by
a suitable check valve. Some hydraulic gauges may
also have special dials that indicate both the pressure
(in psi) and the corresponding total force being
applied, for example tons of force produced by
a hydraulic press.
Spiral and Helical Bourdon Tubes
Spiral and helical Bourdon tubes (figs. 8-4 and 8-5)
are made from tubing with a flattened cross
Figure 8-4.—Spiral Bourdon tube.
section. Both were designed to provide more travel of
the tube tip, primarily for moving the recording pen
of pressure recorders.
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