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Technical
Service Bulletin 96-2R1
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Diagnosing Hydraulic Filters with Collapsed Center Tubes
Today's
hydraulic systems commonly incorporate some sort of filtration in the
system design. The most common filtration used in hydraulic systems involves
cartridge or spin-on filter elements. When diagnosing hydraulic system
malfunctions or performance problems, or when conducting routine maintenance,
a review of the system's filter(s) should be conducted. If, during the
inspection of the filter the element's center tube is found to have collapsed,
further system inspection and servicing must be conducted.
Hydraulic
systems that make use of a cartridge or spin-on filter should incorporate
a filter by-pass valve that is located somewhere between the inlet and
out let flow pattern of the filter. The purpose of the by-pass valve is
to relieve excessive differential pressure. Differential pressure is the
difference in fluid pressure between a filter's inlet (dirty) side, and
the filter's outlet (clean) side. The most common cause of high differential
pressure is the filter becoming filled with contamination, a condition
commonly known as "plugged". As the filter's media becomes more
and more contaminated, it becomes more restrictive to the flow of fluid,
and thus causes an increase in differential pressure. By-pass valve settings
differ from system to system. Most hydraulic systems have the by-pass
valve located somewhere in the vicinity of the filter's mounting base.
If
a filter is found to have a collapsed center tube, the diagnosis can be
one of a by-pass valve malfunction, whether permanent or temporary. If
the valve does not relieve the differential pressure, this pressure will
increase to the point where center tube damage occurs. High differential
pressure may also be caused by an intermittently or permanently malfunctioning
fluid pressure regulating valve. If both or either of the valves malfunctions,
the increased differential pressure may result in damage to the filter.
Malfunction of the by-pass valve and fluid pressure regulating valve may be caused by any one or a combination of the following:
- Sticky surfaces caused by cold hydraulic fluid
- Hydraulic fluid high in chemical contamination (water, acids, etc.)
- Improper servicing
(cleaning) of the by-pass valve assembly during filter change
At times the filters
used on a hydraulic system may be spin-on type filters that incorporate
a by-pass valve assembly within the filter. These valves are designed
to allow for the appropriate amount of flow through the filter, should
the differential pressure presented to it be of sufficient quantity to
activate the valve.
Hydraulic filters
can be subjected to periods of very high flow. If this flow is excessive,
the by-pass valve may not be able to handle all of t he flow that is suddenly
presented to it. This would cause very high differential pressure. This
too could cause a collapsed center tube. Additionally, this could also
occur on a system equipped with a cartridge style filter. If the flow
of hydraulic fluid suddenly overwhelms the by-pass valve assembly, excessive
differential pressure will occur, as will possible filter damage.
High flow rates may be caused by the following:
- Malfunctioning valves or valve bodies
- Sudden shift in direction of the fluid within the system
- A rapid return of fluid to the tank (i.e., a load being dropped quickly)
- Momentary malfunction
of the system's pressure regulating valve
Any time a filter
is discovered to have a collapsed center tube, the system's components
should be checked carefully, and serviced as necessary. In addition, the
center tube collapse probably resulted in excessive contamination being
presented to the entire system. We recommend that the system be serviced
and cleaned thoroughly prior to the equipment being returned to use.
For additional information, contact:
Filter Manufacturers
Council
P.O. Box 13966
Research Triangle Park, NC 27709-3966
Phone: 919/406-8817 Fax: 919/406-1306
www.filtercouncil.org
Administered by Motor & Equipment
Manufacturers Association
Revised September 1996
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