In many cases, the choice between
hose or tubing has been clear: in environments
subjected to high temperatures
or damage from sharp or
heavy objects, a metal tubing assembly
was specified. If the fluid line needed
to be flexible, a hose assembly became
the choice. The development of better
tubes and covers for hydraulic hose —
combined with more-convenient meth-ods
of fabricating tube assemblies —
makes the decision between using a
hose assembly or tubing assembly in an
application tougher than ever.
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On one hand, tubing has several distinct
advantages. Tubing:
- can handle higher pressures (exceeding
6000 psi)
- dissipates heat more readily
- can cost less (due to high-volume
purchasing)
- has a tighter bend radius
- weighs less (which is important in
long boom arms)
- can be used in higher ambient temperatures,
and
- it potentially has a longer life than hose.
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However, having to braze a fitting
onto a tube is labor-intensive and
costly, and bending a tube assembly to
fit an application, and trying to install it
into an existing machine can be a real
nightmare. Hose assemblies, on the
other hand, are flexible, can easily be
routed around obstacles, and can be
fabricated more easily using any of
several popular crimping techniques
and machines. In the replacement aftermarket,
hose is considerably easier to
obtain and more cost-effective than
bent tubing, which can require custom
tube benders, special fittings, and considerable
skill to install. Given the
availability and routing advantages of
hose, it should not be surprising, then,
that maintenance workers usually replace
a hard-to-reach failed bent-tubing
assembly with a hose assembly.
Hose is also less susceptible to vibration
than tubing and is preferred in
applications involving port to port
movement or odd routing configurations.
These advantages often out-weigh
those of tubing. In fact, hose assemblies
frequently are used for
convenience where a tube assembly
would be the better choice — a long,
straight rigid run, for example. In this
case, the hose assembly costs more,
but may be easier to fabricate. The tubing
assembly may cost less and offer
better overall performance, but the
convenience of making a hose assembly
using standard tools wins out.
A hose assembly can be more economical
than tubing for a straight, rigid
run if the tube connects to hose at both
ends. In this instance, the total cost
(parts, labor, etc.) of making a hose assembly,
connecting it to a tube assembly,
and ending in another hose assembly
can far exceed the cost of a single
hose assembly covering the entire
length of the branch.
Allowing for flexibility In critical routing situations, hose
replacement is much easier than tube
replacement because the hose is flexible
enough to bridge port to port connections.
With bent tubing assem-blies,
the connecting section must be
exactly the same size as the original
section to avoid placing stress on the
fittings, which are often brazed or
welded to the tube. If stress on the
joint is high enough, the joint becomes
especially susceptible to vibration
damage and cracking.
| Increasingly, designers of hydraulic
equipment are integrating
bent tube assemblies and hose assemblies
into hybrid bent tube/hose assemblies.
These assemblies provide
the weight and bend advantages of
bent tube with the vibration-damping
characteristics of hose.
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Combination hose/tube assemblies prevent problems caused by vibration, kinking, and abrasive failure.
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| These bent-tube/hose assemblies are
particularly effective on mobile applications
where vibration is often severe.
A feasible alternative to port-to-port
bent tube assemblies is to connect a
port to a location with tubing, then attach
a hose.
A weighty problem An illustration of how bent tube/hose
assemblies can work together can be seen
on the mobile agricultural applications
that include 60-ft wide boom attachments.
In the past, manufacturers preferred
to use bent-tube assemblies on
these attachments to minimize weight
and prolong life of the tool bar. At the
same time, equipment manufacturers
recognize that these attachments can be
a source of vibration — a major factor
leading to system failure. Their solution
to this problem has been to use primarily
hose assemblies where the tool
bar will stand up to the weight and a
bent tube/hose combination where light
weight is important.
Today’s hydraulic hoses have been
designed to be much lighter and provide
smaller bend radii than earlier
products. With the introduction of
these new hoses, the weight advantage
of bent tubing has been eliminated, and
the small bend radius advantage has
been cut in half (see Tables 1 and 2).
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Avoid overheating The thinner walls in today’s hoses
also reduce the insulation properties
that can contribute to heat buildup
under certain situations. This insulation
effect is an advantage in cold
weather applications where designers
prefer the insulation properties of
hose over tubing in order to retain
heat in hydraulic oil.
On the other hand, tubing is considered
a better heat dissipater than hose
and is preferred for applications that
are marginal in terms of sensitivity to
heat. This preference extends to the replacement
aftermarket, where conventional
wisdom has held that substituting
hose for bent tubing may result in
an overheated system.
In reality, engineers have been successful
in designing cool-running hydraulic
systems, so overheating is not
a major problem. In certain applicaions,
using hose can actually result
in less heat buildup because of improved
laminar flow through the
more gradual bends created between
hose connections.
Even in situations where overheating
is a problem, replacement of tube assemblies
with hose will have a negligible
effect, especially when newer
model hoses are used.
High-pressure applications One area in which bent tube maintains
an advantage over hose is in high-pressure
applications, such as injection
molders that operate at working pressures
up to 10,000 psi. Hose manufacturers
are making major strides toward
developing new products that will
equal or surpass these ultra-high pressure
requirements.
When considering hose for replacement
of a bent-tube assembly, it
is important to check the manufacturer’s
specifications for both pressure
and temperature ratings, and determine
the correct inside and outside
hose diameters using a precision-engineered
caliper. Hose OD is especially
important when hose routing
clamps are used, or when hoses are
routed through bulkheads. Check individual
hose specification tables for
ODs in suppliers’ catalogs.
The ID must be capable of handling
the required fluid flow without generating
backpressure. It is not uncommon
for pumps to deliver more than 200
gpm to hydraulic cylinders and motors
in various types of equipment, which is
why it is important to know the type of
hose and the working pressure in a system
when making replacements. Flow
and pressure intensification should also
be considered. For example, double-acting
telescopic cylinders can generate
return flows and pressures during
retraction that exceed source flows and
pressures by far.
In situations where hydraulic equipment
has been modified to perform
special operations, it is not uncommon
to see pressure spikes that manufacturers
of the equipment and the hose and
fittings did not anticipate. As a general
rule, when choosing hose as a replacement
for tubing, it’s best to allow a generous
margin of safety.
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