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.
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.
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.
Galvanic corrosion. This can occur when a coupling and the equipment it connects to are made of dissimilar metals. For example, attaching a standard carbon-steel coupling to a stainless-steel tank could cause the coupling to corrode.
Environment. Environmental considerations can play a role in selecting couplings which will be exposed to the elements or people. However, major manufacturers typically use environmentally friendly, nonleaded steel and nontoxic platings.
Cost. Engineers should select the most cost-effective coupling that suits the application. But make sure to select a coupling that maximizes safety and performance; and match the coupling end type to that of the port. Never mix thread types.
Considering that fittings must be ductile enough to crimp without cracking and strong enough to contain several thousand psi of pressure and withstand millions of impulse cycles, the importance of coupling metallurgy cannot be understated. But beyond the quality of the metal itself, consider the types of coupling materials and where each works best. Here’s a look at the leading materials for industrial and hydraulic hose couplings.
Carbon steel is the material of choice for general-purpose couplings in most industrial and hydraulic hose applications. It is usually plated with a corrosion-resistant material to prevent rust. Carbon-steel couplings are suitable for low and high-pressures well beyond 5,000 psi.
However, various grades of carbon steel contain different amounts of iron and carbon. And carbon steel is also a catch-all term for alloy steels that contain traces of chromium, nickel, copper, tungsten, or other metals that alter the characteristics of the end product. Carbon content, alloying elements, and heat treatment determine the mechanical and physical properties of carbon steel — strength, toughness, ductility, machinability, corrosion resistance, and so on.
Thus, no single type of carbon steel suits all design purposes. The ability to machine and work the steel is a key factor for manufacturers. For instance, a single production run of just one type of coupling may involve high-speed forming of hundreds of thousands of parts. A metal that is too hard could destroy the tooling in such operations. Formability is also important for crimping. Excessively hard steel can damage the dies. The steel must also be compatible with the plating. The plating can flake off during crimping on some brittle materials.
Carbon steel can be formulated for highly specific applications. As an example, couplings on agricultural sprayers can be compounded for greater resistance to the corrosive effects of concentrated urea-ammonium-nitrate fertilizer. While a carbon-steel coupling formulated for this application may not last as long as a stainless-steel one, it costs considerably less and does an effective job.
Stainless-steel couplings are a good fit for corrosive environments, such as in saltwater, salt spray, and maritime applications. They see use at low to extremely high (8,000+ psi) pressures. But stainless couplings can cost several times more than standard carbon-steel versions, so they should only be used when necessary.
Stainless is an alloy steel compounded with chromium and, sometimes, additional elements such as nickel and molybdenum. It forms a passive chromium-oxide film that prevents rust. There are over 150 grades of stainless steel. However, major coupling manufacturers use Type 316. Type 316, in the 300 Series of austenitic stainless steels, contains chromium, nickel, and molybdenum. It is also known as marine-grade stainless steel due to excellent resistance to saltwater corrosion, and has good formability, compressive strength, and wear resistance.
Brass couplings are suited for low-pressure applications and some (but not all) fuel lines. If you can’t have a spark, you want brass. The metal is too soft for effective crimping, so the stem/hose interface allows for banding or clamping with no metal-to-metal contact.
Brass is a nonferrous alloy of copper and zinc. Varying the proportions of these elements changes the properties, allowing for hard and soft brasses. With a relatively low melting point and good flow characteristics, brass is relatively easy to cast. And it offers good corrosion resistance, ductility, and strength.
The brass-manufacturing process must be subject to rigid quality controls. Cheaply made brass can be porous, with air pockets in the castings. Because it is a soft, fragile metal, brass can also be easily damaged in transport.
Aluminum weighs up to one-third less than steel or brass and offers excellent corrosion resistance. Fittings on large-diameter industrial hose are often made of aluminum to reduce the weight of the hose assembly. Likewise, in certain mobile-equipment applications, lightweight aluminum hydraulic couplings help reduce fuel consumption.
In marine, agricultural, and other applications that need corrosion resistance, aluminum couplings may also offer a cost advantage over stainless steel. However, technicians must be careful when installing hydraulic hose assemblies with aluminum connections because torque requirements differ from steel or stainless-steel couplings.
Aluminum is also ductile and strong; heat treatment can increase strength and hardness. And aluminum can be coated for extra protection against wear and corrosion.
Not long ago, hexavalent chromium was the plating material of choice for couplings. It has excellent corrosion resistance, forms well, and doesn’t flake off the base metal. However, well-publicized environmental problems with processing hexavalent chromium have led most major manufacturers to find substitutes. Today, there are a myriad of plating options, including trivalent chrome, nickel, zinc, zinc-chromate, zinc-nickel, and silver-zinc, as well as a host of proprietary materials.
Some, like nickel, are expensive. Others don’t work particularly well in demanding environments. Which plating material to use is based on cost and performance, but it must be compatible with the base metal. Thickness typically ranges from 2 to 28 µm, and it can’t be too thick or too thin. Excess plating flakes when male and female threads of a coupling mate, making them difficult to separate. Too thin a layer prevents the threads from seating and holding properly, resulting in a leak path for fluids.
Crimping can also damage improperly applied plating. If the material is too thin, an aggressive crimp can remove the plating in spots and expose the underlying metal to corrosion. If the material is too thick, it may flake off during crimping and damage the dies.
The SAE standard for corrosion-resistant plating on carbon-steel couplings calls for 72 hr of protection against red rust. The industry average is 96 hr, but major coupling manufacturers offer far greater protection. For example, Gates TuffCoat plating provides 400 hr minimum of red-rust protection, per ASTM B117 salt-spray testing.
Major manufacturers design hose assemblies to ensure that coupling metals match the hose and the application. Following their recommendations constitutes best practice. When hose assemblies are designed with components from different sources, however, make certain that the coupling material suits the hose and application.
Specifications and industry standards
Subsequently, ASTM International (American Society for Testing and Materials), SAE International (Society of Automotive Engineers), and several metal-trade associations and societies developed a standard for classifying metals called UNS (Unified Numbering System). UNS is a relatively new standard that is gaining acceptance in North America. It classifies metal alloys by material composition.
Other standards are set by American Society of Mechanical Engineers (ASME), U. S. military specifications (MIL-SPEC), European Committee for Standardization (CEN), International Organization for Standardization (ISO), individual countries, and various metal industry and trade societies.
SAE Standard J517 provides general, dimensional, and performance specs for the 100R hose series — the most common hoses in hydraulic systems. SAE J516 provides general and dimensional specifications for the most common hose fittings used in conjunction with hydraulic hoses specified in SAE J517.
SAE J1273 contains information on application factors affecting fittings, hose, and hose assemblies. SAE J343 establishes testing standards, including those for corrosion resistance.
Standards for hydraulic hose assemblies are also set by U. S. governmental agencies, such as the Mine Safety & Health Administration (MSHA) and the Dept. of Transportation’s Federal Motor Vehicle Safety Standards. Outside the U. S., organizations such as EN, ISO, and DIN (Deutsches Institut für Normung) set standards. These non-U. S. specifications may differ from SAE specs.
These hose assemblies often handle corrosive chemicals, LP gas, petroleum products, and steam. Thus, such applications demand specialized, securely attached couplings. For example, couplings used on hoses conveying flammable liquids or gases should be made of a nonsparking material such as brass or aluminum. Coupling and hose compatibility is predetermined by major manufacturers and listed in their catalogs. As a general practice, don’t mix hoses and couplings from different manufacturers.