Rubber hydraulic hose and ... - Flipbook - Seite 311
Analyzing failures
Cause: This could be simply old
age. The crazed appearance is
the effect of weathering and
ozone over a period of time.
Try to determine the age of
the hose. Some manufacturers
print or emboss the cure date
on the outside of the hose. As
an example, Aeroquip hose
would show “4Q01” which
would mean that the hose was
manufactured during the fourth
quarter (October, November or
December) of 2001.
15. Symptom: Hose is leaking at
the fitting because of a crack in
the metal tube adjacent to the
braze on a split flange head.
Cause: Because the crack is
adjacent to the braze and not in
the braze this is a stress failure
brought on by a hose that is
trying to shorten under pressure
and has insufficient slack in it to
do so. We have cured dozens of
these problems by lengthening
the hose assembly or changing
the routing to relieve the forces
on the fitting.
16. Symptom: A spiral reinforced
hose has burst
and literally split open with
the wire exploded out and badly
entangled.
Cause: The hose is too short
to accommodate the change
in length occurring while it is
pressured.
17. Symptom: Hose is badly
flattened out in the burst area.
The tube is very hard down
stream of the burst but appears
normal up stream of the burst.
Cause: The hose has been kinked
either by bending it too sharply
or by squashing it in some way
so that a major restriction was
created. As the velocity of the
fluid increases through the
restriction the pressure decreases
to the vaporization point of the
fluid being conveyed. This is
commonly called cavitation, and
causes heat and rapid oxidation
to take place which hardens the
tube of the hose down stream of
the restriction.
18. Symptom: Hose has not
burst but it is leaking profusely.
A bisection of the hose reveals
that the tube has been gouged
through to the wire braid for a
distance of approximately two
inches.
Cause: This failure would indicate
that erosion of the hose tube has
taken place. A high velocity needle
like fluid stream being emitted
from an orifice and impinging at a
single point on the hose tube will
hydraulically remove a section of
it. Be sure that the hose is not bent
close to a port that is orificed. In
some cases where high velocities
are encountered particles in the
fluid can cause considerable
erosion in bent sections of the
hose assembly.
19. Symptom: The hose fitting
has been pulled out of the hose.
The hose has been considerably
stretched out in length. This
may not be a high pressure
application.
Cause: Insufficient support of
the hose. It is very necessary to
support very long lengths of hose,
especially if they are vertical. The
weight of the hose along with the
weight of the fluid inside the hose
in these cases is being imposed on
the hose fitting. This force can be
transmitted to a wire rope or chain
by clamping the hose to it much
like the utilities support bundles
of wire from pole to pole. Be sure
to leave sufficient slack in the hose
between clamps to make up for
the possible 4% shortening that
could take place when the hose is
pressurized.
20. Symptom: The hose has not
burst but it is leaking profusely.
An examination of the bisected
hose reveals that the tube has
burst inwardly.
Cause: This type of failure is
commonly referred to as hose
tube blow down. It is usually
associated with very low viscosity
fluids such as air, nitrogen, freon
and other gases. What happens
is that under high pressure
conditions the gases will effuse
into the pores of the hose tube
charging them up like miniature
accumulators. If the pressure is
very suddenly reduced to zero
the entrapped gases literally
explode out of the tube often
tearing holes in it. In some hose
constructions a second hose tube
made from a plastic such as nylon,
is inserted into the hose.
A small leak will allow the gaseous
fluid to seep between the two
inner liners and when pressure
is reduced to zero the innermost
liner will collapse because the
entrapped pressure around its
inner diameter.
21. Symptom: PTFE hose
assembly has collapsed internally
in one or more places.
Cause: One of the most common
causes for this is improper
handling of the PTFE assembly.
PTFE is a thermoplastic material
which is not rubber-like. When
bent sharply it simply collapses.
This type of collapse is localized in
on area and is radical. When the
PTFE tube is folded longitudinally
in one or more places this could be
the result of heat (which softens
the hose) along with vacuum
conditions inside of it. Because of
the additional tension of the wire
braid, reinforcement inherent with
this type of hose, there is always a
radial tension on the tube trying
to push it in. Rapid cycling from
a very hot agent in the hose to a
very cold agent in the hose can
produce the same type of failure.
Danfoss Aeroquip offers an internal
support coil that will eliminate this
problem.
22. Symptom: A PTFE hose
assembly has developed a pin
hole leak or several pin hole leaks.
Cause: This situation occurs when
a petroleum based fluid, with low
viscosity, is flowing at high velocity.
This condition can generate high
voltage use to static electricity. The
high voltage is seeking a ground
connection and the only ground
connection available is the braided
stainless steel reinforcement.
This causes an electric arc, which
penetrates through the PTFE tube
as it travels to the reinforcement.
Specially constructed PTFE tubes
are available that have enough
carbon black in them so as to be
conductive. They will “drain off”
the static electricity and preclude
this problem.
EMEA Danfoss rubber hydraulic hose, fitting, tooling and accessories category catalog
311
Danfoss | Safety information
Analyzing failures
