bubbles that normally occur as the fluid flows from the
reservoir, through the system, and back to the reservoir.
The 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.
Because it is essential that the fluid in the reservoir
be kept at the correct level at all times, the sight gauge
is provided to allow the normal fluid level to always be
seen The baffle plate segregates the outlet fluid from
the inlet. This allows the fluid time to dissipate air
bubbles, contaminants to settle, and the return fluid to
cool before it is picked up by the pump.
The proper hydraulic fluid level must be
maintained. In some systems low fluid level causes
overheating because the fluid does not have enough time
to cool in the reservoir before it gas back into the pump.
Also, some systems will not work at all because the fluid
is so low in the reservoir that air gets into the pump.
Before adding hydraulic fluid, know what type to
use and make sure it is clean. Clean around the filler
cap or tube so there is less chance that dirt can get into
STRAINERS AND FILTERS
Hydraulic systems have a strainer and one or more
filters that remove the impurities that would eventually
contaminate the hydraulic fluid. The strainer is
normally located in the reservoir or in the inlet line to
the pump. The filter is normally located so only a small
amount of fluid is lost when the element is changed. The
falter is equipped with a valve that allows the fluid to
bypass the filter element should it become clogged. The
falter element is usually of the paper cartridge, canister,
or edge type and is similar to those used in engine
Regular filter maintenance,
performed by the mechanics, is necessary to prevent
contaminated fluid from being recirculated in the
The hydraulic pump creates the flow of fluid within
the hydraulic system.
The pressure in a hydraulic
system is caused by a restriction placed in the path of
the fluid as it leaves the pump. Because of the resulting
mechanical drive and positive displacement, the pump
merely moves the fluid regardless of the restriction.
When enough pressure is built up, movement of the
restriction occurs or a relief valve placed in the system
opens, allowing the fluid to return to the reservoir or the
suction side of the pump.
When the pump operates, hydraulic fluid is trapped
between the gear teeth and the pump housing and is
carried to the outlet side of the pump. As the teeth mesh,
a seal is freed by the mating surfaces that prevent the
oil from leaking back to the inlet side of the pump. The
sealing action causes the oil to be forced out of the pump
and into the system.
Control valves are valves accessible to the operator
for directing the flow of fluid within the system to
operate the machine or its attachment. By skillful use
of the control valves, the operator can regulate the speed
and operation of the hydraulic cylinders.
NOTE: Hydraulic controls should be operated
smoothly to eliminate the jerking motion that causes
rapid wear and failure of the mechanical parts of the
Hydraulic cylinders are used to transmit motion in
relation to the volume of fluid directed into the cylinder.
The force created by the cylinder is determined by the
pressure of the fluid and the area of the piston contacted
by the fluid. Thus the larger the piston, the more force
Hydraulic cylinders used on heavy equipment are
either single- or double-acting cylinders.
Single-acting cylinders, similar to the one shown in
figure 4-11, view A, are used to exert force in only one
direction. This means the weight or resistance moved
must be located so it causes the cylinder to return to its
original position when pressure is relieved from the
piston. A common use of this type of cylinder is in a
Double-acting cylinders are used on equipment
where force is needed in two directions. Unlike the
single-acting cylinder, the double-acting cylinder
contains seals at both ends of the piston where the piston
rod passes through the end of the cylinder. With the use
of this cylinder, fluid can be directed to either side of the
piston and cause the piston rod to extend or retract under
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