How Material Selection Dictates Hose Assembly Performance
From body compounds to cover materials, materials play a crucial role in the performance of industrial and hydraulic hoses. However, there is one aspect that is often overlooked when it comes to hose materials: the plating and coupling metallurgy. Both play a significant role in making sure a hose assembly can handle its pressure rating and does not prematurely fail, leak, or risk the safety of equipment and nearby operators.
Thus, catalogues from hose manufacturers specify which of their couplings are compatible with which hoses, be they stainless steel or Teflon hoses. Opting for predesigned or complete hose assemblies is a surefire guarantee of trouble-free usage and the best performance. However, there are occasions when operators need to create a hose assembly using parts that differ from complete systems because of delivery or availability issues. In such situations, it is vital to be aware of the impact of plating and coupling materials on the hoses and application.
Selection criteria
Established design standards typically guide the selection of the right coupling materials as well as sound engineering judgment should the former be unavailable. The key considerations in coupling selection are namely:
1. Hose and coupling compatibility
The interface between the hose and coupling experiences the greatest amount of stress, which is why blow-offs and pinhole leaks commonly occur there. This is why compatibility between the two is paramount, as well as having the same pressure ratings.
2. Application
The hose’s intended application determines the coupling’s working conditions, involving factors like pressure, temperature, installation reliability, vibration, and more.
Other considerations that help select the correct coupling style are the attachment options (band, ferrule, or clamp), required size and thread type, hose construction (wire braid, textile reinforced, or spiral wire), and compatibility with the conveyed materials.
3. Operating environment
Environmental considerations often influence the choice of couplings that will be susceptible to the elements. However, many manufacturers today generally use eco-friendly, non-toxic and nonleaded steel platings.
4. Galvanic corrosion
This type of damage takes place when a coupling and the machinery it attaches to are made of different materials. For instance, a typical carbon-steel coupling that connects to a stainless steel tank might cause the former to corrode.
An overview of coupling materials
Coupling metallurgy is a key factor that should not be understated, as fittings have to be strong enough to handle high amounts of psi yet ductile enough to crimp without cracking. But apart from the metal’s quality, it is also important to evaluate the types of coupling materials and the applications where they work best.
- Carbon steel
Carbon steel is the go-to choice for general-purpose couplings in many hose applications. It can also be formulated for highly specific ones, such as for couplings on agricultural sprayers. While a carbon-steel coupling with greater corrosion resistance can hold up against the effects of fertiliser, it will not be as long-lasting as a stainless-steel coupling. Nevertheless, its cost-effectiveness makes up for it.
Carbon content, heat treatment, and alloying elements determine carbon steel’s physical and mechanical properties – ductility, toughness, strength, corrosion resistance, machinability, and more. This means that no one type of carbon steel suits all design purposes.
Formability is also vital for crimping, as excessively hard steel can damage or deform the dies. The steel also needs to be compatible with the plating, as it may flake off during crimping on some brittle materials.
- Stainless steel
Stainless steel couplings are ideal for use in corrosive environments such as those found in maritime applications and are frequently used in applications with low or extremely high pressures.
Out of the over 150 grades of stainless steel, most coupling manufacturers generally use Type 316 in the 300 Series of austenitic stainless steels, as it contains molybdenum, chromium, and nickel. These are also called marine-grade stainless steel due to their high resistance to saltwater corrosion, compressive strength, and good formability.
- Brass
Brass couplings are ideal for low-pressure and certain fuel line applications and any other use case where you absolutely can’t have a spark. Since brass is too soft for effective crimping, couplings made from it have a stem/hose interface that allows for clamping or banding with no metal-to-metal contact.
Brass is a nonferrous alloy of zinc and copper, and any adjustments to the proportions of these two also change the properties of the metal, allowing for soft and hard brasses. Brass is easy to cast thanks to its good flow characteristics and relatively low melting point while offering good ductility, strength, and corrosion resistance.
Plating materials
There are now many plating options available for couplings, with the most common ones being nickel, zinc, zinc-nickel, zinc-chromate, silver-zinc, trivalent chrome, and various other proprietary materials. Deciding on a coupling based on its plating material should depend on cost and performance while ensuring compatibility with the base metal.
The thickness typically ranges from 2-28 µm, and it cannot be too thick nor too thin. Excessive plating causes flaking when connecting the threads and during the crimping process, making it difficult to disconnect and may damage the dies. In contrast, plating that is too thin prevents the threads from holding and seating properly, causing leakage. Moreover, it may get sheared off in some areas during an aggressive crimp, exposing the underlying metal to corrosion.
Conclusion
Hose manufacturers design their assemblies to ensure their couplings match the hose and the intended application, so it is always best to follow their recommendations. However, should you ever have to design hose assemblies using components from different sources, be sure that the coupling’s materials and plating are well-suited to your hose and application.