Tubing, Piping, and Hose
Figure 3-34.Spring-loaded accumulator.
similar to the weight-loaded accumulator except that
springs do the loading. In operation, pressure oil loads
the piston by compressing the spring. When pressure
drops, the spring forces oil into the system.
A hydraulic motor is a device that converts fluid
power energy to rotary motion and force. The function
of a motor is opposite that of a pump. However, the
design and operation of motors are very similar to
Motors have many uses in fluid power systems. In
hydraulic power drives, pumps and motors are
combined with suitable lines and valves to form
Fluid motors may either be fixed- or variable-
displacement. Fixed-displacement motors provide
constant torque and variable speed. Controlling the
amount of input flow varies the speed. Variable-
displacement motors are constructed so that the
working relationship of the internal parts can be varied
to change displacement. The majority of the motors
used in fluid power systems are the fixed-displacement
Fluid motors are usually classified according to the
type of internal element that is directly actuated by the
flow. The most common element are the gear (fig. 3-7),
the vane (fig. 3-12), and the piston (fig. 3-15).
The three types of lines used in fluid power
systems are tubing (semirigid), pipe (rigid), and hose
(flexible). A number of factors are considered when
the type of line is selected for a particular system.
These factors include the type of fluid, the required
system pressure, and the location of the system. For
example, heavy pipe might be used for a large
stationary system, but comparatively lightweight steel
tubing is used in the automotive brake system. Flexible
hose is requires in installations where units must be
free to move relative to each other.
PIPING AND TUBING.The choice between
pipe and tubing depends on system pressure and flow.
The advantages of tubing include easier bending and
flaring, fewer fittings, better appearance, better
reusability, and less leakage. However, pipe is cheaper
and will handle large volumes under high pressures.
Pipe is also used where straight-line hookups are
required and for more permanent installations.
In either case, the hydraulic lines must be
compatible with the entire system. Pressure loss in the
line must be kept to a minimum for an efficient system.
Pipes for hydraulic systems should be made of
seamless cold-drawn mild steel. Galvanized pipe
should NOT be used because the zinc coating could
flake or scale, causing damage to the valves and
Tubing used in fluid power systems is commonly
made from steel, copper, aluminum, and, in some
instances, plastic. Each of these materials has its own
distinct advantages or disadvantages in certain
Copper. The use of copper is limited to low-
pressure hydraulic systems where vibration is limited.
Copper has high resistance to corrosion and is easily
drawn or bent. However, it is unsatisfactory for high
temperatures and has a tendency to harden and break
due to stress and vibration.
Steel.Tubing constructed of cold-drawn steel is
the accepted standard in hydraulics where high
pressures are encountered. Steel is used because of its
strength, stability for bending and flanging, and
adaptability to high pressures and temperatures. Its
chief disadvantage is its comparatively low resistance
to corrosion. There are two types of steel tubing
seamless and electric welded.