To see how it works, you need to apply some
in the fluid power systems of support equipment are the
pressure to the double-acting cylinder shown in figure
direct reading type. That is, the gauge is connected
8-37, view B. The stroke of the piston and piston rod
directly into the unit or lines leading to the unit and
assembly in either direction is produced by fluid
becomes part of the container or system. The gauge is
pressure. Assume that the cross-sectional area of the
able to sample the existing pressure at its location in the
piston is 3 square inches and the cross-sectional area of
system.
the rod is 1 square inch. In a 2,000-psi system, pressure
acting against the blank side of the piston creates a
The Bourdon Tube
force of 6,000 pounds (2,000 psi 3 in.2). When the
The main part of a pressure gauge is the Bourdon
pressure is applied to the rod side of the piston, the
tube (fig. 8-38). It is a curved metal tube that is oval in
2,000 psi acts on 2 square inches (the cross-sectional
cross-sectional shape. One end of the tube is closed;
area of the piston less the cross-sectional area of the
the other end has a fitting for connecting it to a pressure
rod) and creates a force of 4,000 pounds (2,000 psi 2
in.2). As noted earlier, double-acting cylinders are
source. The fitting end is fastened to the frame of the
2
normally used in such a way that the blank side of the
gauge frame, while the other end is free to move so it
piston carries the greater load. That is, the cylinder
can operate the mechanical linkage.
carries the greater load during the piston rod extension
Assume that fluid pressure enters the Bourdon
stroke, which may be required to lift a heavy load, but
tube. Since fluid pressure will be transmitted equally in
when the piston rod is retracted, gravity helps make up
all directions and the area on the outside radius of the
loss of force caused by a smaller surface area. This
tube is greater than that of its inside, the force will also
same cylinder can be used to operate the side shift on a
be greater on the outside radius, which tends to
forklift where the working load is not as important as
straighten out the tube. As the movable end of the tube
the ability to move the load from side to side.
tries to turn outward, it turns the pivot segment gear.
A four-way directional control valve is normally
This gear meshes with a smaller rotary gear to which a
used to control the operation of this type of cylinder.
pointer is attached, and its movement causes a reading
The valve can be positioned to direct fluid under
on the pressure gauge. The gauge dial is calibrated so
pressure to either end of the cylinder, and to allow the
the needle points to a number that corresponds to the
displaced fluid to flow from the opposite end of the
exact pressure that is applied. When the pressure is
cylinder through the control valve to return the
removed, the Bourdon tube acts as a spring and returns
hydraulic fluid to the reservoir.
to its normal position.
Pressure gauges are designed to indicate different
PRESSURE GAUGES
ranges of pressure in pounds per square inch. In
support equipment, these gauges may range from 0 to
Pressure gauges installed in hydraulic systems are
used to indicate existing pressures. Most gauges used
100 psi, or up to 6,000 psi or greater. Figure 8-39 shows
ROTARY
GEAR
POINTER
PIVOT
SEGMENT
GEAR
BOURDON
TUBE
ASf08038
SECTION
A
A
Figure 8-38.--Bourdon tube.
8-31
