The magnetic chip detectors for debris monitoring are installed in the
scavenge pump inlets or in the scavenge oil lines upstream of the pumps
where easy access is ensured.
The filter system is a very important element for the reliability of a recirculatory
lubrication system. Because the oil has to pass through small
holes and passages, even very small particles contaminating the oil could
block the oil flow resulting in a lubrication failure. The normal contaminant
is abrasive material and is released by the bearings and gears during
their normal operation. It is flushed away from the bearings and gears by
the oil and carried with the scavenge oilflow away from the sump. In the
filters of the system the contaminants are removed nearly completely from
the oil. Thus the oil can be supplied again to the bearings and gears. If a
bearing or gear failure develops, larger than normal particles will be found
in the filters and on the magnetic chip detectors.
Fresh oil filled into the lubrication system will gradually dissolve contaminants
and holds microscopic particles in suspension. Bearings, seals and
gears wear, erode and corrode introducing traces of these components into
the oil flow. Thus the condition of the oil, which has been circulated in the
lubrication system for some time, very exactly reflects the condition of the
system. If the system operates normally, the oil contains the amount of
particles, which is typical of the system. The size of the particles is typical
of abrasive contamination. During the development of a bearing or gear
damage the size and the amount of the particles become larger. With the
detection of these particles in the oil a bearing or gear damage can be
recognized in its early stage. These particles are also collected by the oil
filter, but the filter inspection intervals are too long to detect a damage
earTlyo. facilitate the check of the oil system for particles in shorter intervals,
magnetic chip detectors are installed in the scavenge oil flow of each sump
or as a master chip detector in the common scavenge line downstream of
the scavenge pumps. Because the gears and bearings are made of steel, the
magnets of the chip detectors are able to collect the particles (or chips) of
these parts. These chip detectors collect particles from 0.02 to 1 mm in
size. The whole arrangement of magnetic chip detectors is often called debris
monitoring system. In its simplest design the magnetic chip detector is
a tiny bar magnet which protrudes into the scavenge oil flow. Figure 3.9
shows such a chip detector. The check of the chip detectors for collected
particles in fixed time intervals is part of the maintenance checks. To facilitate
the check of these detectors they can be removed from their housing
without a tool. If particles are found on the chip detector, they can be
analyzed in a laboratory to exactly determine the component releasing
these particles into the oil.
The more sophisticated variant of the magnetic chip detector is the electrically
monitored chip detector as shown in Above Figure. Such a chip detector
comprises a set of two magnets. The FADEC ( Full Authority Digital Engine Control ) computer
monitors the resistance between these two magnets. A check and removal of the electrical
monitored chip detector is not necessary until the FADEC computer
sends the corresponding maintenance message.
For the particle monitoring of modern engines Oil Debris Monitors
(ODM) are used. These sensors are based on an inductive measurement
technique which enables the system to detect, count and classify wear
metal particles by size and type (ferromagnetic or non-ferromagnetic).
This allows the system to determine the trend for the amount of particles in
the oil. The ODMs are connected to the FADEC computer or another
computer assigned to this function.
Another tool for the monitoring of the oil-wetted parts is the Spectrographic
Oil Analysis Program (SOAP). Through this analysis the concentration
of particles of the size from 0.001 to 0.02 mm and its specific elements
can be identified. The metal type and concentration may indicate to
the analyst and engineers which part of an engine is failing if a direct assignment
to an engine part is possible. SOAP is used if this program is part
of the engine maintenance schedule. It may be used temporarily if uncertainties
exist about the reliability of engine bearings or gears. Some turbine engine manufacturers mandate SOAP in certain turbine engine maintenance
schedules.
For the monitoring of particle concentration in the oil a periodic analysis
of oil samples, taken from the engine, is made in a laboratory. When the
amount of particles for an element or an element combination increases,
this indicates an increase in wear. If the trend continues, the development
of a damage is imminent. In this phase the particle size also increases and
the presence of the particles can be verified by the magnetic chip detectors.
The verification with the help of a chip detector is important if an assignment
of the analyzed elements to an engine part is not possible.
If SOAP is used, the airline engineering has more time for the observation
of a failure development. This allows a longer planning period for the
engine removal necessary in such case.
