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Technical Handbook
Hydraulic Cylinders
Service
9279 02-09
Publ. Gr. S.
370
Clean hydraulics
Hydraulic systems are extremely sensitive to impurities - a hy-
draulic system is just as sensitive as a diesel engine’s fuel sys-
tem, with its pumps, filters and lines.
It is therefore extremely important that hydraulic systems are kept
clean and free from impurities. A clean system means that there
are no impurities in the system, whether it be in the hydraulic flu-
id, component parts of the system or hydraulic lines and hoses.
By impurity, we refer to everything and anything that is not intend-
ed to be in the system:
z
solid particles such as dust, grit, threads of cotton waste,
flakes of rubber from hoses or seals, etc.
z
liquids
z
gasses
Possible sources of impurity
z
Existing impurities
– Casting sand, welding cinder, dust, water, etc., from com-
ponents, hydraulic lines and hoses.
z
Generated impurities
– Scaling. When particles that come between two moving
surfaces attach themselves to one of the surfaces and
thereafter act as a cutting surface to the opposite surface
where it cuts or grinds off material. The newly formed par-
ticles then harden and cause even more wear and tear as
they circulate in the system.
– Erosion
– Cutting or “cold welding”
– Cavitation, insufficient flow to pump
– Corrosion that generates particles
– Fatigue/wear and tear that generates particles
z
Accidentally added impurities
– During repair operations
– As a result of topping up with impure hydraulic fluid
z
Penetrative impurities
– Penetration via breather air to the hydraulic tank
– Penetration via the piston rod’s lubricating film
– Penetration through poorly sealed inspection hatches
– Condensation
– Penetration via rod seals
Solid particles
It is possible to see a particle of 40
µ
in size with the naked eye,
but not one of 10
µ.
(1
µ
= 1/1000 mm).
By comparison, a strand of human hair is approx.70
µ
thick. The
smallest particles are less than 25
µ
in size and are not visible to
the naked eye. If the number of small particles is sufficiently large,
they merely discolour the hydraulic fluid - by which time there are
far too many impurities in the system.
It is not, therefore, possible to judge, with the naked eye, whether
or not hydraulic fluid is sufficiently clean! A particle count using
special machinery or magnification equipment must be made to
establish the degree of impurity.
Solid impurities in hydraulic systems cause many different types
of problem. Large quantities of impurity dramatically reduce the
operative lifespan of the system. Hydraulic components break
down as a result of internal wear. Particles cause directional and
servo valves to seize, which, in turn, leads to the burning out of
the magnetic spools.
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