MAINTENANCE
46
Contamination Levels
Clean fluid implies the absence of impurities such
as solid particles, water and air. Contamination control
includes proper design, installation, and maintenance
of hydraulic components.
The reservoir is designed to limit entry of contami-
nation during servicing and operation. A sealed reser-
voir with a low pressure relief valve reduces the intro-
duction of contaminants while maintaining inlet and
case drain pressures below the maximum recom-
mended.
The purpose of the filter in a system is to clean the
oil at initial start up and to maintain acceptable levels
of particle contaminants as they are ingested or gener-
ated during operation of the system. Filtration is a mat-
ter of controlling the particle sizes and their respective
quantities to appropriate levels so as to avoid degrada-
tion of the system components and thereby assure the
desired life.
Fluid contaminant profile requirements for Etnyre
hydraulic units have been determined empirically based
on measuring fluid quality in successful application,
and in laboratory tests designed to evaluate hydraulic
unit contaminant sensitivity. General contaminant lev-
els have been developed based on this experience and
are shown in Figure 25.
Acceptable contamination levels at machine start up
for the system loop should be equal to or better than
Curve D. The machine may be exercised to 500 psi but
should not be worked (pressure over 500 psi) until the
oil cleanliness meets or exceeds Curve C.
The machine should clean up during a relatively short
period of normal operation to meet the oil cleanliness
level of Curve A or Curve B. A system that meets Curve
A will provide the user with a longer trouble-free oper-
ating life than one that meets Curve B. The Curve A
cleanliness level should be met in systems exceeding
the continuous pressure limit for 1000 hours/years ma-
chine use.
Some control valves may require better contamina-
tion limits than specified in Figure 25. Controls with
small area screens or low force level values may be
susceptible to malfunction from contamination.
The contamination sensitivity of components gen-
erally increases with higher pressure, temperature or
speed. A better fluid contaminant profile may be re-
quired for systems which operate near the extremes of
their ratings. Short life or infrequent operation require-
ments may increase allowable contaminants.
The selection of a filter depends on a number of fac-
tors including the contaminant ingression rate, the gen-
eration of contaminants in the system, the required fluid
cleanliness, and the desired maintenance interval. Fil-
ters are selected to meet the above requirements using
rating parameters of efficiency and capacity.
Filter efficiency may be measured with a Beta ratio.
For simple closed circuit transmissions with controlled
reservoir ingression, a filter with a Beta ratio of 1.5 to
2 has been found to be satisfactory. For some open cir-
Curve C
Limit at machine ship and new
fluid added to transmission
Curve D
Limit at machine start up
Curve A
Desired limit for longer life (3500
PSI continuous system pressure)
Curve B
Continuous operating limit (2000
PSI continuous pressure) system
Figure 25. Fluid Cleanliness Chart
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