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.

RESERVOIRS 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.