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Most engineering efforts focus
on ways to design and manufacture
products that satisfy
application requirements as inexpensively
as possible. But you can make a
satisfactory design better by making it
more reliable and easier to maintain —
and without spending a lot of money.
How? Simply by following simple tips
and recommendations offered by manufacturers.
These fundamentals concern
bending, alignment, motion, and
similar basic guidelines.
Application basics Unlike metal tubing, hose is flexible,
so it is used primarily to allow relative
motion between components at
either end of the hose assembly and to
simplify routing and installation. It is
much easier to route a hose assembly
over, under, around, or through a series
of obstacles than it is to
bend and install a rigid tubing
assembly. Furthermore,
replacing a hydraulic line by
fabricating a rigid tube assembly
often is more costly
and time consuming than
making a hose assembly.
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Ex. #1- Make hose assemblies long enough and routed in a manner that prevents exceeding minimum bend radius recommendations.
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| Most manufacturers offer
hose that can be bent to a
tighter radius than that published
in industry standards.
Still, bending hose to a
smaller radius than recommended
should be avoided to
avoid shortening service life.
Therefore, route hose in a
manners that provides ample
bend radius, Example #1.
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Ex. #2- To prevent excessive strain at hose-to-coupling interfaces, make hoses long enough to allow for contraction and expansion.
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| Because hose is flexible,
you must allow for contraction
and expansion when cutting
the hose to length. Manufacturers
state that, depending on
its type, hose can elongate up
to 2% when pressurized, but,
more importantly, can contract as much
as 4%. This length differential can strain
hose reinforcement wires and eventually
lead to failure, especially at the hose-to-coupling
interface. Therefore, cut hoses
slightly longer than needed to compensate
for contraction, Example #2.
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Ex. #3- Left-hand drawing shows how hose twists because it is bent in one plane while oscillating motion bends in a second plane. Rerouting the hose eliminates multiplane bending.
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| Bend hose in one plane only to avoid
twisting its wire reinforcement, which
would reduce the hose’s pressure capability.
Manufacturers state that twisting
a high-pressure hose only 5o can reduce
service life by 70%, and 7o of twist can
reduce service life up to 90%. Unfortunately,
hose routing usually occurs late
in the design process, so it may be difficult
to find an ideal path. Multi-plane
bending often can be avoided by reorienting
the hose, Example #3. If this is not
possible, install a hose clamp between
bends, Example #4, and provide enough
length on both sides of the clamp to relieve
strain on the hose’s reinforcement
wires. This length depends on the hose
ID, degree of bending, and helix angle
of the particular hose’s reinforcing
wire, so manufacturers prefer to evaluate
each application individually.
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Ex. #4- When multi-plane bending cannot be avoided, install a hose clamp between bends and provide enough hose length on both sides of the clamp to relax torsion and compensate for hose contraction.
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| Another alternative is to use a single
section of hose for each bend and install a
hose-to-hose coupling and hose clamp
between bends. This technique is less
preferred because it not only is more
costly and time consuming to perform,
but increases the number of potential leak
points in the hose assembly. Also, to help
ensure that technicians replace and secure
hose assemblies properly, include
detailed instructions on hose length, use
of hose clamps, and special considerations
in service manuals.
Providing protection Hose manufacturers now offer a variety
of products with abrasion-
resistant covers. No
wonder: manufacturers state
that about 80% of hose failures
are attributable to external
physical damage,
with abrasion cited as the
major culprit. Abrasion is
generated primarily by
hoses repeatedly rubbing
against equipment surfaces
or each other.
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Ex. #5- Hose clamps can prevent abrasion by holding hose away from surfaces it would otherwise rub against.
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To help prevent abrasion,
use clamps to secure hose in
place and keep it from rubbing
against adjacent surfaces,
Example #5. The clamp
should have a snug fit
around the hose to prevent
movement, but not be tight
enough to damage the hose
by squeezing too tightly. Be
sure the hose is slack on
both sides of the clamp to
compensate for contraction
and expansion.
Additional protection can be provided
by sleeves. Metal sleeves resemble
springs that protect the hose from being
crushed. Fabric sleeves help keep
abrasive particles away from hoses, and
both types can serve the added function
of nestling multiple hoses into a compact
bundle.
Some types of sleeves must be installed
from one unconnected end of the
hose and slid along its length. Others
have a longitudinal slit to enable installing
the sleeve without having to disconnect
either end of the hose assembly.
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Ex. #6- Design at left provides ample hose length when cylinder is pivoted, but bends hose in too small a radius when cylinder is vertical. Increasing hose length and providing greater clearance produces much greater bend radii.
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Accommodating movement In addition to causing twisting and
abrasive wear, motion can also quickly
spell doom for hoses that do not properly
accommodate equipment dynamics. For
example, hoses connected to a cylinder
that undergoes pivoting motion, Example #6,
must be of proper length and routed to
avoid becoming kinked or bent beyond
their minimum bend radius.
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| An item that can make a good design
better is a swivel joint, sometimes called a
live swivel. Unlike standard swivel fittings,
which connect hydraulic lines at
any fixed angular position, swivel joints
accommodate relative motion between
the hose and the component to which it is
connected. As Example #7 shows, swivel
joints permit pivoting motion that reduces
the bending transmitted to the hose
assembly and can reduce the length of
hose required.
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Ex. #7- Swivel joints can extend hose life by reducing the amount of bending caused by relative motion between machine elements. They also aid maintenance by simplifying hose installation and replacement.
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| When multiple lengths of hose lie
close to each other, and substantial linear
motion will occur, hose carriers keep
hoses neatly nestled to prevent tangling,
twisting, and rubbing against each other.
Depending on which particular type is
specified, carriers can also isolate the
hoses inside from potentially hostile conditions
outside — impact from falling objects,
abrasive particles, chemicals, or intermittent
high temperatures.
Other important considerations Most hydraulic hose is wire reinforced,
which makes it an electrical
conductor. For equipment that may
be used near power lines or where
hose will be in close proximity to
flammable solutions that could be ignited
by static electricity discharged
from the hose, manufacturers offer
non-conductive hose.
In other applications, static electricity
sometimes may be discharged
through the hose wall to surrounding
surfaces. This is caused by conducting
electrostatic charges from the fluid
through the hose’s metal reinforcement
and cover to adjacent surfaces. Consequences
can include localized burning
that weakens the hose or even produces
pin-size holes in the hose wall. In this
case, hose with a conductive tube may
be called for to conduct electrostatic
charges to hose end fittings rather than
through the hose.
Just as twisting can dramatically
shorten hose life, so can excessive heat.
Heat from external sources, such as exhaust
components on mobile equipment,
can quickly soften or embrittle
the hose wall from the outside in.
Therefore, it is important to keep hose
away from external sources of heat. If
this is not possible, manufacturers offer
insulated protective sleeves to partially
block heat transmitted to the hose.
However, heat from an internal
source — the hydraulic fluid itself —
also can reduce the service life of the
hose. Pumping hydraulic fluid at a temperature
of only about 18o F over the
maximum recommended temperature
for a hose can cut its expected life in
half. What makes this problem even
more serious is that machine operators
often are unaware that fluid temperatures
may exceed manufacturers recommendations
— especially if the high
temperatures occur only intermittently.
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Finally, strive for neat appearance
when routing hoses, Example #8. This
not only prevents tangling, twisting,
and rubbing together (which can
cause abrasive wear), but aids maintenance
by making it easy to remove
and re-install hose assemblies and
trace circuit routing.
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Ex. #8- Lack of planning produces cluttered hose routing, far left, that complicates maintenance and can even reduce hose life. Well-thought-out routing and choice of end fitting configurations, near left, makes assemblies that are more reliable and easier to maintain and troubleshoot.
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| Use adapters sparingly because
they add to the number of components
in an assembly. This increases
assembly time, cost, and the number
of potential leak points. However,
when properly applied, adapters can
simplify hose assemblies that use angled
fittings (such as 90o elbows) at
each end. Hose-end fittings on these
assemblies must be carefully oriented
to prevent twisting the hose during
installation. So using an angled hose
coupling at one end of the hose and a
straight coupling connected to an angled
adapter fitting on the other eliminates
the need to carefully align hose
ends during assembly.
Clean hose prevents early contamination troubles
When cutting hose to length, either
a serrated or abrasive
blade is used. Serrated blades cut
one- and two-wire braid and textile-reinforced
hose cleanly and efficiently,
but usually are not recommended
for use on spiral-reinforced
hose because the blades would wear
quickly or become damaged. Abrasive
wheels cut all types of hose efficiently,
but produce abrasive debris
that usually finds its way into
the hose. If not flushed from the finished
hose assembly, this debris
holds potential for serious wear and
damage to sensitive components of
the hydraulic system. Skiving and
crimping can also produce debris
that must be removed before putting
the finished hose assembly into service.
Skiving involves cutting a
length of the outer jacket from the
hose to prepare it for accepting a
hose-end coupling. Crimping
squeezes the coupling onto the hose’s
inner and outer surfaces, so some residual
plating material could come off and
find its way into the hose.
Compared to skiving and crimping
operations, storage can introduce
more and a wider variety of contaminants.
During storage, dirt, water,
metal particles, rust, abrasive particles,
and any number of other types of
contaminants may migrate into a hose
sitting on a shelf. It should be obvious,
then, that all hose assemblies should
be cleaned before being put into service.
At the very least, finished hose
assemblies should be cleaned with a
strong blast of compressed air. Naturally,
this air should be clean and dry.
The most effective cleaning
technique is high-velocity, bidirectional flushing. This floods the hose
assembly with cleaning fluid until the fluid comes out of the hose as
clean as when it went in. However, the cost of this equipment and the time
required to flush an assembly makes it impractical for many potential
applications.
A relatively new cleaning technique
offers an effective and practical
alternative to these procedures
by using sponge-like projectiles that
are shot through hose and tubing assemblies
by a blast of compressed
air. Equipment for this procedure is
much more affordable than that for
high-velocity flushing. Perhaps
more importantly, though, it cleans
assemblies much more effectively
than compressed air alone and in a
fraction of the time it takes for high-velocity
flushing.
Once the hose assembly
has been cleaned, be sure to install protective caps or plugs at both ends
to prevent contamination from entering the assembly. These should not be
removed until the hose assembly is being installed on the equipment.
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