Figure 3-11.Lobe pump.
Figure 3-12.Vane pump.
Figure 3-13.Hydraulic hand pump.
FLOW CONTROL VALVES.Flow control
valves are used to regulate the flow of fluid in a fluid-
power system. Control of flow in fluid-power systems
is important because the rate of movement of fluid-
powered mechanisms depends on the rate of flow of
the pressurized fluid. Some of the most commonly
used flow control valves are ball valves (fig. 3-17),
gate valves (fig. 3-18), globe valves (fig. 3-19), and
needle valves (fig. 3-20).
PRESSURE CONTROL VALVES.The safe
and efficient operation of hydraulic systems, systems
components, and related equipment requires a means
Figure 3-14.Nine-piston radial piston pump.
of controlling pressure. There are many types of
automatic pressure control valves. Some of them
merely provide an escape for pressure that exceeds a
set pressure, some only reduce the pressure to a lower
pressure system or subsystem, and some keep the
pressure in a system within a required range. The most
common pressure control valves are relief valves (fig.
3-21), pressure regulators (fig. 3-22), pressure-
reducing valves (fig. 3-23), and counterbalance valves
DIRECTIONAL CONTROL VALVES.
Directional control valves are designed to direct the
flow of fluid, at the desired time, to the point in a fluid
power system where it will do work; for example,
using a directional control valve to drive a ram back
and forth in its cylinder. Various other terms are used
to identity directional control valves, such as selector
valve, transfer valve, and control valve.
Directional control valves for hydraulic and
pneumatic systems are similar in design and operation.
However, there is one major difference. The return
port of a hydraulic valve is ported through a return line
to the reservoir, while the similar port in a pneumatic
valve, commonly referred to as an exhaust port, is
usually vented to the atmosphere.
Directional control valves may be operated by
differences in pressure acting on opposite sides of the
valving element, or they may be positioned manually,
mechanically, or electrically. Often two or more
methods of operating the same valve will be used in
different phases or its action.