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SECTION 8
8-2
TROUBLESHOOTING
DIESELING IN HYDRAULIC SYSTEMS
Any hydraulic system should be a sealed environment
free of foreign material including air. Unlike solid con-
tamination, air is compressible and contains oxygen. It
is these two (2) properties of air contamination that pro-
vide the elements needed to support the phenomenon
known as dieseling. Dieseling can only occur when the
elements of fuel, oxygen and heat are all present at the
same time. In the hydraulic system the hydraulic fluid is
the fuel. The presence of air provides not only the oxy-
gen to support combustion, but also the means for gen-
erating sufficient heat to ignite the fuel/air mixture. When
air is rapidly compressed, heat is generated. A rapid
pressure change of only 600 psi may be enough to gen-
erate the heat required to ignite the fluid and mixture.
The oxygen in the heated compressed air and the hy-
draulic fluid ignite resulting in dieseling. The ignitions
that result from dieseling in the hydraulic system are
small in size and many may be needed to eventually
damage a piston seal to the extent that bypass will oc-
cur.
“Where does the air come from?” The answer is that
the air comes out of the fluid itself. Hydraulic fluid can
contain 10% air by volume. As the pressure on the fluid
increases, the amount of air that can be absorbed also
increases.
Now we know that the air is most likely present in the
fluid, but the next question is, “How does the air get out
of the fluid?” The air, while it is in suspension, will pose
no problem to the operation of the hydraulic system.
But once it is seperated into bubbles then all the factors
are present to support combustion. The air in suspen-
sion can be separated when the fluid is subjected to a
negative pressure (vacuum) of as little as 3.5 psi. This
can occur when the fluid is squeezed through a restric-
tion or an orifice. The resulting pressure decreases can
be sufficient to allow trapped air to separate from the
fluid.
A good example may be a front mounted pump dry valve
system. In the dry mode of operation, system hydraulic
fluid is drawn through a small orifice. This provides lu-
brication and cooling for the pump, but also provides a
perfect situation for air separation. In addition, the flow
regulator bypass at the pump output is returned back to
the pump input. This recirculates the fluid/air and al-
lows for the separation od even more air.
In cases where the seals in cylinders appear to be burnt
or melted, consider the possibility of air ingestion. The
following suggestions may help in eliminating this prob-
lem:
Ensure that the pump suction connections are tight.
It is possible for a suction hose connection to allow
air in without leaking any fluid out.
The pump shaft seal cann allow air into the sys-
tem. Replace if the seal is suspect.
Check for air ingestion around the packings on the
ball valve stem. Replace the ball valve if suspect.
Excessive system flow rate (cycle time too fast) can
agitate the hydraulic fluid. Set the cycle time ac-
cording to specifications.
Do not thin hydraulic fluid with diesel fuel (lowers
the flash point). Weather permitting, use a higher
flash point fluid.
1.
2.
3.
4.
5.
After making any repairs on the hydraulic system, bleed
the system at reduced engine speed and pressure to
remove any trapped air. Depending on the size of the
component, e. g. cylinder, hose, it may be necessary to
cycle the unit several times. Of course the larger the air
pocket, the more cycles are needed. To avoid potential
problems, thoroughly bleed all hydraulic systems and
ensure that all inlet connections are tight and not in-
gesting air.
Summary of Contents for LEACH 2R-III
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Page 26: ...SECTION 2 2 8 ...
Page 28: ...OPERATION SECTION 3 LOCATION OF OPERATING CONTROLS 3 2 4 6 5 7 8 1 2 3 8 9 10 11 12 7 ...
Page 46: ...SECTION 3 3 20 ...
Page 54: ...SECTION 4 4 8 ...
Page 60: ...SECTION 6 6 4 PREVENTIVE MAINTENANCE 1 2 3 4 5 6 7 8 ...
Page 62: ...SECTION 6 6 6 ...
Page 76: ...SECTION 7 7 10 ...