must be enough space to hold all the oil when the
cylinders retract with some space to spare for
expansion of hot oil.
An air vent allows the air to be drawn in and
pushed out of the reservoir by the ever-changing fluid
level. An air filter is attached to the air vent to prevent
drawing atmospheric dust into the system by the ever-
changing fluid level. A firmly secured filling strainer
of fine mesh wire is always placed below the filler cap.
The sight gauge is provided so the normal fluid
level can always be seen, as it is essential that the fluid
in the reservoir be at the correct level. The baffle plate
segregates the outlet fluid from the inlet fluid.
Although not a total segregation, it does allow time to
dissipate the air bubbles, lessen the fluid turbulence
(contaminants settle out of nonturbulent fluid), and
cool the return fluid somewhat before it is picked up by
Reservoirs used on CESE may vary considerably
from that shown in figure 3-6; however, manufacturers
retain many of the noted features as possible
depending on design limits and use.
Strainers and Filters
Strainers are constructed of fine mesh wire screens
or of screening elements, consisting of specially
processed wire of varying thickness wrapped around
metal frames. They do NOT provide as fine a
screening action as filters, but they offer less resistance
to flow and are used in pump suction lines where
pressure drop must be kept to a minimum. If one
strainer is not large enough to handle the supply of the
pump. two or more strainers can be used in parallel.
The most common device installed in hydraulic
systems to prevent foreign particles and contamination
from remaining in the system are called filters. They
may be located in the reservoir, in the return line, in the
pressure line, or any other location in the system where
the designer of the system decides they are needed to
safeguard the system against impurities.
Filters are classified as full flow and partial flow.
In the full-flow filter, all fluid that enters the unit
passes through the filtering element, while in the
partial-flow filter. only a portion of the fluid passes
through the element.
The purpose of a hydraulic pump is to supply a
flow of fluid to a hydraulic system. The pump does not
create system pressure, since only a resistance to the
flow can create pressure. As the pump provides flow, it
transmits a force to the fluid. As the fluid flow
encounters resistance, this force is changed into
pressure. Resistance to flow is the result of a resistance
or obstruction in the path of flow. This restriction is
normally the work accomplished by the hydraulic
system, but can also be restrictions of lines, fittings,
and valves within the system. Thus the load imposed
on the system or action of a pressure-regulating device
controls the pressure.
Pumps are rated according to their volumetric
output and displacement. Volumetric output is the
amount of fluid a pump can deliver to its outlet port in a
certain period of time at a given speed. Volumetric
output is usually expressed in gallons per minute
(gpm). Since changes in pump speed affect volumetric
output, some pumps are rated by their displacement.
Pump displacement is the amount of fluid the pump
can deliver per cycle. Since most pumps use a rotary
drive, displacement is usually expressed in terms of
cubic inches per revolution.
Many different methods are used to classify
pumps. Terms, such as nonpositive displacement,
positive displacement, fixed displacement, variable
displacement, fixed delivery, variable delivery,
constant volume, and others are used to describe
pumps. The first two of these terms describe the
fundamental division of pumps because all pumps are
either nonpositive displacement or positive
displacement. Basically pumps that discharge liquid in
a continuous flow are referred to as nonpositive
displacement, and those that discharge volumes
separated by a period of no discharge are referred to as
Pumps may also be classified according to the
specific design used to create the flow of fluid.
Practically all-hydraulic pumps fall within three
designs classificationscentrifugal, rotary, and
reciprocating. Since the use of centrifugal pumps is
limited, we will only discuss rotary and reciprocating.
ROTARY PUMPS.All rotary pumps have
rotating parts that trap the fluid at the inlet (suction) port
and force it through the discharge port into the system.
Gears (figs. 3-7, 3-8, and 3-9), screws (fig. 3-10), lobes
(fig. 3-11), and vanes (fig. 3-12) are commonly used to
move the fluid. Rotary pumps are positive
displacement of the fixed displacement type.
Rotary pumps are designed with very small
clearances between rotating parts and stationary parts