When specifying hose for a hydraulic system, engineers mostly focus on internal details like pressure, flow velocity and capacity, ID size and fluid compatibility. But what happens outside the hose is important, too. Hose used improperly or in the wrong setting can quickly fail. Here are some practical considerations to ensure long life.
Mechanical influences
Mechanical loads are one factor. Pulling or stretching action that applies high tensile forces to a hose assembly can significantly reduce life. And remember that a hose can lengthen or contract (typically +2 to -4%) when pressurized, so always specify a hose longer than the distance between connected ports. Likewise, excessive flexing or twisting puts undue loads on a hose; using swivel adapters is one way to alleviate stress.
Cover abrasion is a leading cause of premature hose failure, particularly under rugged conditions found in places like construction sites, mines and quarries. Personnel installing hose assemblies must prevent or protect hose from rubbing against and snagging on sharp edges or rough surfaces on adjacent machine components.
Abrading the cover exposes textile fiber or wire reinforcement. Water or chemicals can then attack, corrode and degrade the reinforcement. Continued rubbing can cut through the fiber or wire. In any case, it hastens failure.
Some manufacturers say improper routing leads to the majority of unwanted failures. Hose clamps help keep lines away from pivot points and other sources of abrasion. And if contact is unavoidable, a number of manufacturers make hoses with abrasion-resistant covers, such as Eaton RhinoHide, Gates MegaTuff, and Parker Supertough versions. Protective covers, such as nylon sleeves, can also provide extra defense against abrasion.
Another source of physical concern is excessive bending. Hydraulic hose is engineered to operate at full pressure at or above its specified minimum bend radius. That’s the smallest radius, measured to the inner surface of the curved section, at which the hose can operate without harm. Bending the hose below its minimum recommended bend radius hurts mechanical strength and hose life, and leads to possible failure. So never bend a hose tighter than the recommended minimum radius.
Excessive bending can also kink or flatten a hose, which can seriously damage the inner tube and weaken the reinforcement. If needed, install bend restrictors at the fitting to prevent damage.
Chemical influences
The hose cover and assembly fittings must stand up to any external substance or influences. Environmental factors that negatively affect a hose include chemicals, fuels and solvents, salt water, air pollution, ozone and ultraviolent radiation. For example, hose used in ag and marine equipment, or in washdown settings, are often exposed to fertilizers, salt water or cleaning agents. These can hasten
For example, hose used in ag and marine equipment, or in washdown settings, are often exposed to fertilizers, salt water or cleaning agents. These can hasten breakdown of rubber compounds. Hose can lose resiliency, harden and, when flexed, crack and fail.
These substances can also cause fittings to rust. Corrosion-resistant platings and stainless steel fittings are good options.
The hose cover must be compatible with any fluid it contacts. Covers are generally designed to resist common mildew, cleaning solvents, oils and fuels. Note that manufacturers typically offer chemical resistance tables in its catalogs and web sites, but those resistance ratings generally relate to the hose inner tube and the respective fluid. Contact the manufacturer’s application engineers if there are any uncertainties regarding surface contact.
Temperature extremes
Hydraulic systems routinely withstand a wide range of temperatures without concern. However, operating at temperatures beyond recommended limits will degrade the hose. Extreme temperatures, high and low, can affect a hose’s elastomeric and mechanical properties and cause problems like softening, discoloration, burning or cracking.
At high temperatures, for example, heat can leach plasticizers out of the rubber, which gives hose plasticity and flexibility, and cause the hose to harden and crack. In some instances, manufactures say increasing external temperature by just 18 °F (8 °C) above maximum rated temperature can cut hose life in half. Likewise, radiant heat from furnaces, hot manifolds and molten metal can heat hose to destruction without physical contact. And welding flames or spatter can burn through a hose. Engineers should select a hose that is designed for temperature extremes and ensure it is properly shielded. Reroute hose away from heat sources if at all possible.
At the other extreme, cold temperatures reduce flexibility of rubber, elastomers and other hose constituents. Hose exposed to intense cold ambient conditions below its rated minimum operating temperature can crack when flexed. Most standard hoses are rated to -40 °F (-40 °C), although special rubber and thermoplastic hoses handle -65 °F (-54 °C) or lower, and some PTFE hoses are rated to -100 °F (-73 °C).
Even under ideal conditions, hoses have a finite life. Due to these external influences, experts recommend that hose assemblies in service should be inspected frequently for leaks, kinking, corrosion, abrasion, and any other signs of wear or damage. Immediately replace damaged assemblies.
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