Regardless of their classification, check valves are
probably the most widely used valves in fluid power
systems. The check valve may be installed inde-
pendently in a line to allow flow in one direction only.
This is indicated in the simple system described earlier
with the hand pumps. Check valves are also incorpo-
rated as an integral part of some other valve, such as the
sequence valve, the counterbalance valve, and the
pressure regulator valve, also described earlier. A
modification of the check valve-the orifice check
valve-allows free flow in one direction and a limited or
restricted flow in the opposite direction.
Check valves are available in various designs. As
stated previously, the ball and the cone, or sleeve, are
commonly used as the valving elements. The poppet,
piston, spool, or disk are also used as valving elements
in some types of check valves. The force of the fluid in
motion opens a check valve, while it is closed by fluid
attempting to flow in the opposite direction aided by the
action of a spring or by gravity.
As stated previously, an adequate supply of the
recommended fluid is an important requirement for the
efficient operation of a fluid power system. The
reservoir, which provides a storage space for fluid in
hydraulic systems, differs to a great extent from the
receivers used for this purpose in pneumatic systems.
For this reason, the two components are covered
separately in the paragraphs below.
The reservoir is the fluid storehouse for the
hydraulic system. It contains enough fluid to supply the
normal operating needs of the system and an additional
supply to replace fluid lost through minor leakage.
Although the function of a reservoir is to supply an
adequate amount of fluid to the entire hydraulic system,
it is more than just a vessel containing fluid. It is here
that the fluid has the greatest potential danger of
becoming contaminated. It is in the reservoir where any
air entering the fluid system has the opportunity of
escaping; dirt, water, and other matter settle to the
bottom. Reservoirs are designed in a way that permit just
clean hydraulic fluid to come to the top.
The construction features of a typical reservoir are
shown in figure 10-23. These reservoirs have a space
above the fluid, even when they are full. This space
allows the fluid to foam, and thus purge itself of air
bubbles that normally occur as the fluid makes its way
Figure 10-23.-Typical hydraulic reservoir.