How materials impact hose-connector performance

Much has been written about the polymers used in industrial and hydraulic hose, from tube stocks to body compounds and cover materials. But an often-overlooked aspect of the materials used in a hose assembly concerns the coupling metallurgy and plating. Both play major roles in ensuring a hose handles the rated pressure and doesn’t leak, prematurely fail, or compromise the safety of operators and equipment.

 Hose assemblies
Attaching fittings to the ends of a hose creates an assembly. Standard fittings have a stem and ferrule. One end of the stem attaches to the hose and the other is threaded or flanged, facilitating connections to a port, adapter, pipe, or another hose.

The ferrule, a metal sleeve, is compressed by crimping or swaging to securely grip the hose. Couplings made of softer materials such as brass or plastic often attach to the hose with bands or clamps.

Fittings vary widely in construction and materials from one manufacturer to another. Major hose manufacturers maintain R&D centers where engineers, metallurgists, and chemists constantly evaluate hose-fitting materials. Catalogs from these vendors specify which couplings are designed to work with which hoses. Selecting predesigned hose assemblies from major manufacturers virtually guarantees trouble-free service.

However, occasions arise when engineers and field personnel must build hose assemblies using components that differ from predesigned systems due to availability or delivery issues or simply to minimize downtime on expensive equipment. In such cases, it is essential to know the impact coupling and plating materials will have on the hose and application.

Selection criteria
Selecting appropriate coupling materials is guided by formal design standards or, in their absence, sound engineering judgment. Different hoses require different types of couplings and materials, and a multitude of thread configurations, end styles, and adapters are available. Major factors to consider in coupling selection include:

Hose/coupling compatibility. The hose/coupling interface is subject to the greatest stress. It is where pinhole leaks and blowoffs can occur, and they are extremely dangerous in high-pressure applications. Thus, the hose and coupling must be compatible and have the same pressure ratings. For example, never attach a low-pressure brass coupling to a 5,000-psi, spiral-wire hydraulic hose.

Application. The application dictates the coupling’s operating conditions. Factors include temperature, pressure, impulse frequency, amplitude and wave form, vibration, potential risks in the event of a connection failure, installation reliability, and so on. For example, a corrosive environment or handling flammable liquids or abrasive slurries would have a major bearing on the choice of material.

Also consider temperature extremes, both hot and cold; electrical conductivity and resistivity; gas permeability; and aesthetics — the appearance, texture, and color.

Other issues that help identify the right coupling style include: attachment options (ferrule, band, or clamp); hose construction (spiral wire, wire braid, or textile reinforced); required size and thread type; and coupling compatibility with the conveyed material.