Rubber hydraulic hose and ... - Flipbook - Seite 309
Analyzing failures
Everyone in maintenance
encounters hose failures.
Normally, there is no problem.
The hose is replaced and
the equipment goes back in
operation. Occasionally the
failures come too frequently –
the same equipment with the
same problems keep popping
up. At this point the task is to
determine and correct the cause
of these repeated failures.
Improper application
Beginning with the most
obvious, the most common
cause of hose failures – Improper
application – compare the
hose specifications with the
requirements of the application.
Pay particular attention to the
following areas:
•
The maximum operating
pressure of the hose.
• The recommended
temperature range of
the hose.
• Whether the hose is rated for
vacuum service.
• The fluid compatibility of
the hose.
Check all of these areas
against the requirements of
the application. If they don’t
match up, you need to select
another hose. It’s a good idea at
this point to call on your local
hose distributor for assistance
in selecting the proper hose.
Danfoss’ distributors, for
example, are well equipped to
perform this service for you.
Distributor personnel attend
special training courses in
hydraulics and hose application
conducted by the company. Or,
if your problem is particularly
difficult, the distributor can call
on the services of Danfoss’ field
engineering staff. The company
will send in a hose and hydraulic
specialist to study the problem
and come up with a solution.
Improper assembly and
installation
The second major cause
of premature hose failure
is improper assembly and
installation procedures. This can
involve anything from using the
wrong fitting on a hose, to poor
routing of
the hose.
Danfoss provides excellent
training material that you can
use to combat this problem.
A little time spent in training
your maintenance people could
pay big dividends in reduced
downtime.
Contact Danfoss to register for a
training session today.
External damage
External damage can range from
abrasion and corrosion, to hose
that is crushed by a lift truck.
These are problems that can
normally be solved simply once
the cause is identified. The hose
can be re-routed or clamped, or
a fire sleeve or abrasion guard
can be used.
In the case of corrosion, the
answer may be as simple as
changing to a hose with a more
corrosion resistant cover or
re-routing the hose to avoid the
corrosive element.
Faulty equipment
Too frequent or premature hose
failure can be the symptom of a
malfunction in your equipment.
This is a factor that should
be considered since prompt
corrective action can sometimes
avoid serious and costly
equipment breakdown. Reprints
of an article on “Troubleshooting
hydraulic systems,” which tells
you how to spot problems in a
hydraulic system are available
from Danfoss.
Faulty hose
Occasionally a failure problem
will lie in the hose itself. The
most likely cause of a faulty
rubber hose is old age. Check
the lay line on the hose
to determine the date of
manufacture. (2Q99 means
second quarter 1999.) The
hose may have exceeded its
recommended shelf life. If
you suspect that the problem
lies in the manufacture of the
hose (and don’t jump to this
conclusion until you have
exhausted the other possibilities)
contact your distributor.
Given effective quality control
methods, the odds of a faulty
batch of hose being released
for sale are extremely small.
So make sure that you haven’t
overlooked some other problem
area.
Analyzing failures
A physical examination of the
failed hose can often offer a
clue to the cause of the failure.
Following are 22 symptoms
to look for along with the
conditions that could cause
them:
2. Symptom: The hose is
cracked both externally and
internally but the elastomeric
materials are soft and flexible at
room temperature.
Cause: The probable reason is
intense cold ambient conditions
while the hose was flexed. Most
standard hoses are rated to
–40°C (–40°F). Some hoses are
rated at –49°C (–55°F). Military
specified hoses are generally
rated to –54°C (–65°F). PTFE hose
is rated to –73°C (–100°F). Some
Everflex Polyon thermoplastic
hoses are rated at –54°C (–65°F).
3. Symptom: The hose has burst
and examination of the wire
reinforcement after stripping
back the cover reveals random
broken wires the entire length of
the hose.
1. Symptom: The hose tube is
very hard and has cracked.
Cause: Heat has a tendency to
leach the plasticizers out of the
tube. This is a material that gives
the hose its flexibility or plasticity.
Aerated oil causes oxidation to
occur in the tube. This reaction
of oxygen on a rubber product
will cause it to harden. Any
combination of oxygen and
heat will greatly accelerate the
hardening of the hose tube.
Cavitation occurring inside the
tube would have the same
effect.
Cause: This would indicate
a high frequency pressure
impulse condition. SAE impulse
test requirements for a double
wire braid reinforcement
are 200,000 cycles at 133%
of recommended working
pressure. The SAE impulse
test requirements for a four
spiral wrapped reinforcement
(100R12) are 500,000 cycles
at 133% maximum operating
and at +121°C (+250°F). If the
extrapolated impulses in a
system amount to over a million
in a relatively short time a spiral
reinforced hose would be the
better choice.
EMEA Danfoss rubber hydraulic hose, fitting, tooling and accessories category catalog
309
Danfoss | Safety information
Hose selection
