Checks for Contamination
Whenever it is suspected that a hydraulic system has
become excessively contaminated or the system has
been operated at temperatures in excess of the specified
maximum, a check of the system should be made. The
filters inmost hydraulic systems are designed to remove
most foreign particles that are visible to the naked eye;
however, hydraulic liquid which appears clean to the
naked eye may be contaminated to the point that it is
unfit for use.
Thus, visual inspection of the hydraulic liquid does
not determine the total amount of contamination in the
system. Large particles of impurities in the hydraulic
system are indications that one or more components in
the system are being subjected to excessive wear.
Isolating the defective component requires a systematic
process of elimination. Liquid returned to the reservoir
may contain impurities from any part of the system. In
order to determine which component is defective, liquid
samples should be taken from the reservoir and various
other locations in the system.
FLUID SAMPLING. Liquid samples should be
taken according to the instructions provided in
applicable technical publications for the particular
system and the contamination test kit. Some hydraulic
systems are provided with permanently installed bleed
valves for taking liquid samples; while on other systems,
lines must be disconnected to provide a place to take a
sample. In either case, while the liquid is being taken, a
small amount of pressure should be applied to the
system. This ensures that the liquid will flow out of the
sampling point and thus prevent dirt and other foreign
matter from entering the hydraulic system. Hypodermic
syringes are provided with some contamination test kits
for the purpose of taking samples.
CONTAMINATION TESTING. Various proce-
dures are recommended to determine the contaminant
level in hydraulic liquids. The filter patch test provides
a reasonable idea of the condition of the fluid. This test
consist basically of filtration of a sample of hydraulic
system liquid through a special filter paper. This filter
paper darkens in degree in relation to the amount of
contamination present in the sample and is compared to
a series of standardized filter disks which, by degree of
darkening, indicates the various contamination levels.
The equipment provided with one type of contamination
test kit is illustrated in figure 10-37.
When you are using the liquid contamination test
kit, the liquid
filter disk (fig.
samples should be poured through the
10-37), and the test filter patches should
be compared with the test patches supplied with the test
kit. A microscope is provided with the more expensive
test kits for the purpose of making this comparison.
Figure 10-38 shows test patches similar to those
supplied with the testing kit.
To check liquid for decomposition, pour new
hydraulic liquid into a sample bottle of the same size and
color as the bottle containing the liquid to be checked.
Visually, compare the color of the two liquids. Liquid
which is decomposed will be darker in color.
At the same time the contamination check is made,
it may be necessary to make a chemical analysis of the
liquid. This analysis consists of a viscosity check, a
moisture check, and a flash point check; however, since
special equipment is required for these checks, the liquid
samples must be sent to a laboratory where a technician
will perform the test.
Flushing the System
Whenever a contamination check indicates
impurities in the system or indicates decomposition of
Figure 10-38.Hydraulic fluid contamination test patches.