Because hose assemblies are relatively maintenance free, heavy-duty equipment personnel can reduce, if not eliminate, premature hydraulic hose failure by giving maximum consideration to the product's selection and installation.

Following are some major considerations offered by The Gates Rubber Company.

Pressure Ratings

Each and every hose in a hydraulic system must be able to handle the highest pressure produced by the system. Hose designs range form non-hydraulic applications such as fuel, air, oil, and coolant lines up to 250 psi, to extremely high-pressure hose, rated at 10,000 psi, and built to carry the highest pressures found in hydraulic systems. Pressure surges or peaks exceeding the hose rated working pressure are destructive and must be considered when selecting a hose.

Temperature Range

Hydraulic hoses must tolerate both the external ambient and internal fluid temperatures. These temperatures range from -65oF to +300oF in the most severe cases. Exposing hose to the maximum temperature and maximum rated working pressure, concurrently, will cause degradation of material compounds, and reduced service life. Moreover, the fluid manufacturer's recommended maximum operating temperature for any given fluid must not be exceeded, regardless of hose temperature range, warns Gates.

Fluid Compatibility

All components of the hose assembly -- tube, cover, and couplings -- must be compatible with the hydraulic fluid being used. Many hoses are not compatible with all of the common fluids that include petroleum-based, phosphate esters, water-based and diesters. Remember, too, that variables, such as elevated temperature, and fluid contamination and concentration, will affect compatibility. If in doubt, consult the hose manufacturer. Minimum Bend Radius

Subjecting a hose to a bend radius smaller than the minimum recommendation places excessive stress on the reinforcement, opens larger gaps between strands of reinforcement and severely reduces the ability of the hose to withstand pressure. Also, hose bends immediately behind the couplings result in undue stress at the couplings. These are common causes of hose failure. Gates suggests that if the bend must be sharper than the minimum radius, adapters or angle fittings should be used. The hose manufacturer's specifications should be checked to determine the straight hose section, between the couplings, needed for a particular I.D. As a rule of thumb, a 1/4-in. I.D. hose requires a 5-inch minimum straight section, a 3/4-in. I.D. hose needs seven inches, and a 2-in. I.D. hose must have at least 11 inches of hose length between the couplings.

Hose Size

Hose size (inside diameter) must be capable of handling the required flow volume. Too small an I.D. for a given volume of flow results in velocity that is too high, excessive fluid turbulence, pressure drop, heat generation, and tube damage. A hose I.D. larger than the recommended size will not affect performance of the machine. It is an impractical replacement, however, because the hose will have reduced flexibility, and it will add unnecessary weight and cost to the piece of equipment.

Hose Routing

Replacement hoses must be restrained, protected, or guided (using clamps), says Gates, to minimize risk of damage due to excessive flexing and whipping. Flat metal armor and wire spring guards, or a sheath made of abrasion, temperature, or chemical resistant material will help protect a hose from cuts, abrasion from other moving parts, severe blows, hot components, and corrosives.

Hose Lengths

When determining correct hose length, consideration must be given for length changes under pressure (about 5 percent), machine vibration and motion, and hose assembly routing. When a hose is routed from one moving part to another, enough length must be allowed so that fittings are not subjected to pull-off forces when the parts reach their limits of travel.

Even if a hose is not subjected to pull-off, stretching the hose restricts fluid flow. To allow for length changes during pressurization, a hose should not be clamped at the bend. This prevents the curve from absorbing the change. Also, high and low pressure lines should not be clamped together.

Hose and Coupling Compatibility

Hoses, couplings, assembly equipment, and crimping tolerances vary from one manufacturer to another, and are not interchangeable. Gates says that if components are mixed, coupling retention could be adversely affected. Mixing components not only causes unnecessary downtime, but also, personal injury can result. Also, remember that the thread or flange end of a coupling must be matched properly to a mating component to achieve a leakproof connection.

The best hydraulic hose ever made is comparatively useless if it is the wrong hose for the job or if it is installed improperly.

A workable hydraulic system, asserts Gates, requires a properly mated hose and coupling assembly installed in such a way as to retain its flexibility while also preventing abuse. The only other requirements for the equipment mechanic or operator are to inspect hoses regularly for leaks and visible damage, and follow a good maintenance-replacement program to head-off problems before they arise.