Engineering Essentials: Types of Hydraulic Hose

To say that hose is an important part of a hydraulic system is a huge understatement.
The flexibility of hose enables components to be positioned in the most efficient or
convenient places, because the hose has the ability to bend around corners, through tight
spaces, or across long distances.

Yet these days, there seems to be as many different types of hose as there are
telephone long-distance carriers. How does a designer tell one from the other? Isn't
there an easy way to choose or compare hoses?

The SAE standards

SAE answers those questions with its J517 hydraulic hose standard. This <a
href="http://www.chuanhseng.com/lpg-hose/" target="_self">LPG hose standard serves as
the most popular benchmark in the realm of industrial hydraulics today. More specifically,
J517 is a set of guidelines that applies to the current SAE 100R series of hoses.
Currently, 16 such hose styles exist, and they are designated as 100R1 through 100R16 (see
descriptions, pages A105 and 106). Each of the styles must meet a set of dimensional and
performance characteristics as set forth by SAE. However, SAE issues no approval source
lists, certification, or letters of approval-conformance to these standards by
manufacturers is strictly voluntary. In short, the standards only assure a similarity of
products among different manufacturers.

Hydraulic hose construction

Modern hydraulic hose typically consists of at least three parts: an inner tube that
carries the fluid, a reinforcement layer, and a protective outer layer.

The inner tube must have some flexibility and needs to be compatible with the type of
fluid it will carry. Commonly used compounds include synthetic rubber, thermoplastics, and
PTFE, sometimes called Teflon. The reinforcement layer consists of one or more sheaths of
braided wire, spiral-wound wire, or textile yarn. The outer layer is often weather-, oil-,
or abrasion-resistant, depending upon the type of environment the hose is designed for.

Not surprisingly, hydraulic hoses have a finite life. Proper sizing and use of the
correct type of <a href="http://www.chuanhseng.com/lpg-hose/tpr-lpg-hose/"
target="_self">TPR LPG hose will certainly extend the life of a hose assembly, but
there are many different factors that affect a hose's lifespan. SAE identifies some of
the worst offenses as:

flexing the hose to less than the specified minimum bend radius

twisting, pulling, kinking, crushing, or abrading the hose

operating the hydraulic system above maximum or below minimum temperature

exposing the hose to rapid or transient rises (surges) in pressure above the maximum
operating pressure, and

intermixing hose, fittings, or assembly equipment not recommended as compatible by the
manufacturer or not following the manufacturer's instructions for fabricating hose
assemblies.

Selecting the proper hose

Here are seven recommended steps the system designer should follow during the hose and
coupling selection process. To help determine the proper hose for an application, use the
acronym STAMPED - from Size, Temperature, Application, Materials, Pressure, Ends, and
Delivery. Here is what to consider in each area:

Size - In order to select the proper hose size for replacement, it is important to
measure the inside and <a href="http://www.chuanhseng.com/natural-gas-hose/"
target="_self">natural gas hose diameters exactly using a precision-engineered
caliper, as well as the length of the hose. Hose OD is particularly important when hose-
support clamps are used or when hoses are routed through bulkheads. Check individual hose
specification tables for ODs in suppliers' catalogs. When replacing a hose assembly,
always cut the new hose the same length as the one being removed. Moving components of the
equipment may pinch or even sever too long a hose. If the replacement hose is too short,
pressure may cause the hose to contract and be stretched, leading to reduced service life.

Changes in hose length when pressurized range between +2% to 4% while hydraulic
mechanisms are in operation. Allow for possible shortening of the hose during operation by
making the hose lengths slightly longer than the actual distance between the two
connections.

Temperature - All hoses are rated with a maximum working temperature ranging from 200
° to 300° F based on the fluid temperature. Exposure to continuous high temperatures can
lead to hoses losing their flexibility. Failure to use hydraulic oil with the proper
viscosity to hold up under high temperatures can accelerate this problem. Always follow
the hose manufacturer's recommendations.

Exceeding these temperature recommendations can reduce hose life by as much as 80%.
Depending on materials used, acceptable temperatures may range from -65° F (Hytrel and
winterized rubber compounds) to 400° F (PTFE). External temperatures become a factor when
hoses are exposed to a turbo manifold or some other heat source.

When hoses are exposed to high external and internal temperatures concurrently, there
will be a considerable reduction in hose service life. Insulating sleeves can help protect
hose from hot equipment parts and other high temperature sources that are potentially
hazardous. In these situations, an additional barrier is usually required to shield
hydraulic fluid from a potential source of ignition.

Application - Will the selected hose meet bend radius requirements? This refers to the
minimum bend radius (usually in inches) that a hydraulic hose must meet. Exceeding this
bend radius (using a radius smaller than recommended) is likely to injure the hose
reinforcement and reduce hose life.

Route high-pressure hydraulic lines parallel to machine contours whenever possible.
This practice can help save money by reducing line lengths and minimizing the number of
hard-angle, flow-restricting bends. Such routing also can protect lines from external
damage and promote easier servicing.

Materials - It is mandatory to consult a compatibility chart to check that the tube
compound is compatible with the fluid used in the system. Elevated temperature, fluid
contamination, and concentration will affect the chemical compatibility of the tube and
fluid. Most hydraulic hoses are compatible with petroleum-based oils. Note that new
readily biodegradeable or green fluids may present a problem for some <a
href="http://www.chuanhseng.com/natural-gas-hose/csa-natural-gas-hose/" target="_self">csa
natural gas hose.

Pressure capabilities - Hose working pressure must always be chosen so that it is
greater than or equal to the maximum system pressure, including pressure spikes. Pressure
spikes greater than the published working pressure will significantly shorten hose life.

Hose ends - The coupling-to-hose mechanical interface must be compatible with the hose
selected. The proper mating thread end must be chosen so that connection of the mating
components will result in leak-free sealing.

There are two general categories of couplings to connect most types of hose: the
permanent type (used primarily by equipment manufacturers, large-scale rebuilders, and
maintenance shops) and the field-attachable type.

Permanently attached couplings are cold-formed onto the hose with powered machinery.
They are available for most rubber and thermoplastic hoses and offer a wide range of
dependable connections at low cost. Assemblies made in the field with portable machines
are relatively simple; these machines are economical and easy to operate. In most cases,
it is not necessary to skive the cover. These couplings are less complicated to install
than other types.

Field-attachable couplings are classified as screw-together and clamp-type. The
screw-together coupling attaches to the hose by turning the outer coupling shell over the
outside diameter of the hose. The coupling insert is then screwed into the coupling shell.
A clamp-type coupling has a 2-piece outer shell that clamps onto the hose OD with either
two or four bolts and nuts.

This hose should be used in low pressure and vacuum applications, with petroleum- and
water-based hydraulic fluids, within a temperature range from -40° to 100° C.

It is constructed with an inner tube of oil-resistant synthetic rubber, a
reinforcement consisting of a ply, or plies, of woven or braided textile fibers with a
suitable spiral of body wire, and an oil- and weather-resistant synthetic rubber cover.

This hose should be used with petroleum- and water-based hydraulic fluids, within a
temperature range from -40° to 100° C.

It is constructed with an inner tube of oil-resistant synthetic rubber reinforced with
two textile braids separated by a high-tensile-strength steel-wire braid. All of the
braids are impregnated with an oil- and mildew-resistant synthetic rubber compound.

This high-pressure <a href="http://www.chuanhseng.com/natural-gas-hose/csa-natural-
gas-hose-kit/" target="_self">csa natural gas hose kit should be used with synthetic,
petroleum- and water-based hydraulic fluids within a temperature range from -40° to 93°
C.

It consists of a thermoplastic inner tube resistant to hydraulic fluids with suitable
synthetic-fiber reinforcement and a hydraulic fluid- and weather-resistant thermoplastic
cover. Nonconductive 100R8 is identified with an orange cover and appropriate lay line.
Its pressure capacity is similar to that of 100R2.

This hose should be used with petroleum- and water-based hydraulic fluids within a
temperature range from -40° to 100° C.

It consists of an inner tube of oil-resistant synthetic rubber, six spiral plies of
heavy wire wrapped in alternating directions, and an oil- and weather-resistant synthetic
rubber cover. A ply, or braid, of suitable material may be used over the inner tube and/or
over the wire reinforcement to anchor the synthetic rubber to the wire.

In either case, the coupling has limited potential for reuse because the threads
distort during attachment.

To ensure the correct-size coupling is used when replacing an assembly, the number of
threads per inch and thread diameter of the original coupling must be determined. Thread
pitch gages are available for identifying the number of threads per inch. A caliper can
measure both inside and outside dimensions of the threads. ODs are measured on male
couplings, while IDs are measured on female couplings.

In most situations, the only differences between an SAE coupling and an imported
coupling are the thread configuration and the seat angle. International thread ends can be
metric, measured in mm, but also include BSP (British Standard Pipe) threads, which are
measured in inches. Knowing the country of origin provides a clue as to what type of
thread end is used. DIN (Deutsche Industrial Norme) fittings began in Germany and now are
found throughout Europe, while BSP is found on British equipment. Japanese Komatsu
machinery uses Komatsu fittings with metric threads, while other Japanese equipment most
likely uses JIS (Japanese Industrial StandardBSP threads), or, in some cases, BSP with
straight or tapered threads.

Delivery - How available is the product? Is it unique? How soon can it be delivered to
the distributor or end user? It may be preferable to consider several options to maximize
flexibility and avoid the delays that can result from relying on components that are
unavailable or in short supply.