Predicting the Life of Hydraulic Hose

8/6/2019 Predicting the Life of Hydraulic Hose

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Predicting the life of hydraulic hose

Not a week goes by without someone asking, "How long will the hoses last on our system?"

Society of Automotive Engineers (SAE) specifications state the shelf life of bulk rubber hose is

10 years from the date of manufacture and is considered unlimited for thermoplastic hose, if

stored in subdued light and controlled temperatures with no fittings attached.

But in use, so many variables come into play that it is impossible to exactly predict the service

life of a hose assembly. However, understanding important operating parameters that affect a

hose, combined with a bit of experience, can help guide users as to when an assembly should

be replaced. Key factors to consider include size, temperature, application, media, and

pressure.

Size refers to the fact that every hose is designed for specific pressures, flows, temperatures,

and so on, and deviating from these will shorten its life. This may seem obvious, but users

sometimes undersize a hose to increase flow rate and run equipment faster, or simply because

of space constraints. But excessive flow velocity will damage the inner tube, especially at

bends, and lead to premature failure. It can also raise fluid temperature and have a detrimental

effect on pumps, filters, and other system components.

Some users take the opposite approach and specify a larger-than-required hose to keep flow

velocity low and fluid cool. Such a system increases costs and creates problems such as routing

a larger hose in a smaller envelope. Also, the effects of fluids and aging on hose materials are

independent of size, so spending more on an oversized assembly does not necessarily buy

longer life.

Length is another size-related factor. Most engineers recognize that installing too short an

assembly will compromise life. Stretching to install a hose can stress the reinforcement and

even cause fittings to pull out when pressurized. However, others use too much hose when

plumbing a system. This creates long flow distances and unnecessary bends, where flow forces

degrade the inner tube faster than in straight sections. Jamming more hose into an available

space can bend the hose near the fitting, affecting the connection, and offer more points for

the hose to rub and abrade. A good rule of thumb is to trust your eyes. If an assembly has a

nice, smooth flow path and is visually appealing, chances are the length is correct.

Temperature range recommended for typical rubber hose spans about 40 to 212F. Fluid or

ambient temperatures outside these bounds impact service life. Plasticizers leach out of

elastomers faster at high temperatures, though the rate depends on the actual temperature

and duration. Heat-related failure is evident when the cover shows signs of hardening and

cracking, and the hose shape takes on a permanent set.

Temperatures below recommended will also shorten service life. This problem is evident when

the inner tube shows signs of stiffness and cracks. Specials are available for either extreme:

Low-temperature hoses for service to 67F and high-temperature versions for applications

exceeding 300F.

Application indicates the system function and duty cycle. In general, mobile applications face

harsher conditions than industrial ones. Natures effects can prematurely age hose, so

protection against wind, rain, sun, and temperature extremes can help extend life. Dirt and

water in the hydraulic fluid will also adversely affect the life of all system components.


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One of the most common mobile-equipment problems is abrasion. It is caused by hose rubbing

against the machine structure or other hoses, eventually wearing away the cover and

damaging the reinforcement. Avoid such failures by clamping or rerouting the hose. Protective

sleeves also help. These products fit over a hose and come in a range of types and sizes,

including woven nylon, hard-rubber guards, coil springs, and bands of metal that wrap around

the OD. These offer additional protection against abrasion, debris, heat, weld splatter, andother potential perils.


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Stationary indoor applications are not immune to hazards. Pressure impulses cause hose to

expand and contract. For instance, SAE 100R2 specifications permit the hose to change in

length +2 to 4% when pressurized, which can cause considerable movement depending on

the assembly length and installation. This makes abrasion a concern. Vertical installations put

even more stress on the hose construction, compared with properly supported horizontal

routings.

Indoor applications can expose hose to more-severe temperatures than outdoor ones, if near

heat sources such as ovens and casting equipment. Ozone and UV light can also be concerns,

as is abuse. It is not uncommon to see workers standing on hose or even running lift trucks

over air lines. A general attitude is that hose is relatively indestructible and easy to replace.

But spending the time and effort to properly install and protect the hose can substantially

extend service life.

Media incompatible with a hose will shorten service life. Unfortunately, there is no such thing

as standard hydraulic oil. Many different manufacturers offer a wide range of oils and additive

packages, and suppliers rarely list all the contents of a product. Therefore, always request a

Material Safety Data Sheet (MSDS) for the fluid which shows most, if not all, chemical

constituents, as well as a list of incompatible materials. This, in conjunction with hose

manufacturers material compatibility charts, provides a good guideline on the suitability of a

particular hose-fluid pairing.

And review compatibility whenever a change is made, whether switching vendors or simply

trying a new oil. Failing to do so is an ongoing problem that often leads to hose failure. How

fast depends on the degree of incompatibility, ranging a few days to a year or more. Air and

water are two of the worst fluids for a hose. Both prematurely dry out the plasticizers of an

incompatible rubber hose. EPDM is often recommended in such cases, or adding an oil

lubricator to an air system.

Pressure is another key parameter. Systems designers need to be cognizant of both operating

pressure and pressure spikes that may last only fractions of a second. Pressure spikes that

exceed maximum rated working pressure can permanently damage hose and eventually lead

to failure. Good practice allows a safety factor by specifying hose rated a few-hundred psi

higher than maximum system working pressure. For instance, a 2,500-psi hose might be

recommended for a 2,000-psi system. Adding pressure-relief valves can also save a lot of

headaches.

Even knowing all the operating parameters of a system, it is still difficult to predict the

expected service life of a hose. Probably the best tool to prolong life and predict failures is an

adequate maintenance program. This includes keeping the fluid clean and visually inspecting

hose for signs of abrasion, heat damage, and so on.

Base maintenance and replacement intervals on the previous service life of the equipment, in

conjunction with government or industry recommendations. Depending on the application, a

simple visual inspection once a month is often adequate, with a complete overhaul and

replacement of hoses and other components every year or two. On the other hand, if a failure

would be life threatening, changing the assemblies every month would not be out of line.

Keep in mind the associated costs and repercussions of failures, down time, and potential life-

threatening injuries. It is better to replace a $100 assembly twice a year rather than spend

thousands to clean up or make repairs from a spill.


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Also, a component that fails in service is usually more expensive to rebuild than a working one

removed during a planned maintenance procedure. Hose assemblies that fail in service quite

often will affect the rest of the system. As a consequence, parts that would have been

serviceable must also be repaired or replaced.

Accurate records give a good indication of service life and recommended maintenanceschedules. If a hose assembly lasts a year in a given application, the same type replacement

hose will also likely last a year under the same conditions. Thus, the best predictor of hose life

is past experience, so it is essential to keep good records. ?

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Predicting the Life of Hydraulic Hose