Predicting the life of hydraulic hose

March 3, 2005
A variety of service factors affect how long a hose assembly will last.

Ron Mramor
Technical Service Engineer
Parker Hannifin Corp.
Cleveland, Ohio

Factors such as operating conditions, ambient temperature, pressure, and the type of hydraulic fluid all play a role in how long a hose assembly survives in service.

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 212°F. 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 –67°F and high-temperature versions for applications exceeding 300°F.

Application indicates the system function and duty cycle. In general, mobile applications face harsher conditions than industrial ones. Nature’s 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.

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, and other potential perils.

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 manufacturer’s 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.

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 maintenance schedules. 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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