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.

Accumulators are pressure vessels that contain gases and liquids at high pressures — often at several thousand psi — which demands caution when they are made, handled, and used. To ensure the safety of operators and equipment, various regulations govern their design, manufacture, and installation.

Unfortunately, there is no recognized global certification standard that applies universally, regardless of where and how an accumulator is used. Rather, accumulators and gas bottles are subject to safety laws, regulations, and ordinances valid in the state or country where they are installed. Further complicating matters, other regulations apply to specific industries such as mining, shipbuilding, and aerospace.

Thus, certifications for accumulators vary — often quite significantly — depending on the application and location. Sorting through the requirements that may be encountered is a cumbersome task. So here’s a brief look at the two fundamental design codes and several common certifications engineers are likely to encounter.

Base design codes
Although many countries have their own regulations and quality standards for hydraulic accumulators, most refer to one of two base design codes. The oldest and most referenced design code for pressure vessels is from the American Society of Mechanical Engineers. Originally a standard for manufacturing boilers for steam locomotives, the ASME Boiler and Pressure Vessel Code Section VIII, Division 1 has evolved into requirements for pressure vessels and accumulators in the U. S. This section requires:
• Certification on vessels with internal diameters of 6 in. or greater.
• Certified vessels carry the “U” symbol on them as evidence they were designed and manufactured according to the Code. The “U” symbol is an international designation of design and manufacturing quality.
• Accumulators must be made from materials that meet ASME specifications for traceability.
• A 4:1 ratio of burst pressure to rated pressure. This design factor is with respect to the minimum tensile strength of the material.
• Each vessel must be marked with its design pressure at the Minimum Design Metal Temperature (MDMT) for that vessel.
• Vessels are manufactured under an approved quality system, like ISO 9001.
• An approved third party observes all hydrostatic testing.

The 4:1 design factor is mandatory for all accumulators with ASME Certification, except those that comply with a specific rule within the Code called Appendix 22. Appendix 22 permits accumulators manufactured with forged shells and openings of a specified maximum size to be certified with a 3:1 ratio of burst to rated pressure.

The second base design code is the Pressure Equipment Directive (PED), in force since May 2002 in the European Union. The Directive (designated 97/23/EC) applies to the design, manufacture, testing, and conformity assessment of pressure equipment and related assemblies that operate above 0.5 bar. The directive requires:
• Operating fluids must be in Group 2 (nonhazardous).
• Certified vessels must receive a CE mark if: volume is greater than 1 liter and pressure capacity (the product of service pressure and volume, PS × V) exceeds 50 bar-liter; or service pressure PS exceeds 1,000 bar. In general, all accumulators larger than 1 liter must be CE marked.
• Certified vessels must be made from materials that meet PED specifications for traceability.
• A 2.8:1 ratio of burst to rated pressure. This design factor is with respect to the minimum yield strength of the material.
• Certified products must pass a low-temperature Charpy test (temperature to be determined by application or customer). In Canada, per local inspection authorities, mining applications are specified to have an MDMT of –40°F. Other applications will be determined by the lowest temperature the accumulators will see.
• Vessels are manufactured under an approved quality system, like ISO 9001.
• All hydrostatic testing to be witnessed by an approved supervisory body. Manufacturers can become self‑certified.
• Once installed, national laws govern equipment and accumulator inspection as well as operational safety.

From aerospace equipment to earthmoving machines, hard-chrome plating on piston rods has been used for decades to protect hydraulic cylinders against corrosion and wear. Stricter environmental regulations and the demand for longer service life, however, have led cylinder manufacturers to develop alternative piston-rod coatings in recent years. But how do these new surface treatments affect seal performance, and is there an ideal coating for all applications?

Seal performance
Friction between a cylinder rod and its seals, and the resulting wear, has a crucial influence on the efficiency and service life of hydraulic cylinders. As long as the hydraulic pump generates sufficient power, seal friction is often overlooked with respect to the hydraulic system’s efficiency. In such cases the main concern is that the cylinder seals and wipers prevent leaks and keep dirt out of the system — even under harsh conditions such as when excavating a riverbed.

But the frictional properties of piston seals and piston-rod seals are becoming increasingly important to the operators of modern fluid-power systems. High static and dynamic friction accelerate wear, decrease the efficiency of the entire hydraulic system, and lead to undesirable stick-slip and high breakaway forces after prolonged rest — the so called “Monday-morning effect.” In addition, unsuitable surfaces can cause seals to squeak or creak and make equipment noisy.

Hard-chrome plating
There are two knocks against hard-chrome-plated piston rods. One involves service life. The surface quality or roughness of hard-chrome-plated piston rods changes over time. The surface is typically smoothed which, counterintuitively, increases friction on the piston-rod seals or causes microcracks and abrades the chrome tips on the rod surface. Long score marks also form across the entire running surface. These changes hurt the entire tribological system and can lead to equipment failure.

The other shortcoming of hard-chrome plating relates to the environment. Hexavalent chromium is toxic and heavily regulated by the U. S. Environmental Protection Agency (EPA). It is a human carcinogen and the EPA considers it a hazardous air pollutant under the Clean Air Act, a hazardous substance under the Clean Water Act, and a hazardous waste under the Resource Conservation and Recovery Act. By-products of the plating process cannot be discarded into wastewater

Designing a vacuum-handling system involves many factors. On one hand, engineers must contend with the size, weight, and type of material to be handled, and how fast and precisely it must be moved. On the other, there’s the size and type of vacuum generator, system operating pressure, as well as components such as valves, hoses, and connectors.

But it all comes together at the suction cup or pad. This seemingly simple device must firmly and safely support the load, resist gravitational and acceleration forces, minimize air consumption, and not mar or damage the workpiece. And, of course, it must resist fatigue and abrasion, withstand dirt, contaminants, and temperature extremes, and provide long, economical life with little or no maintenance.

Selection criteria
Obviously, a lot goes into selecting a suction pad. Leading manufacturers offer extensive data on various types of pads and the advantages of each, along with load capacity, temperature limits, chemical compatibility, and so on. Engineers normally select suction pads based on the following criteria:

Operating conditions. Cycle rates, expected life, aggressive surroundings, temperature, and other environmental factors are all considerations when selecting suction pads.

Material. Suction pads come in a wide range of materials to meet specific application requirements. Common materials include nitrile, silicone, and natural rubbers, fluoroelastomers, and polyurethanes. Some materials, for example, are particularly suited for smooth, rough, or oily surfaces, or easily damaged workpieces. There are also special antistatic suction pads for handling electronic components, and pads which won’t mark plastic parts. Environmental conditions can affect the material choice when the pads must resist ozone, chemicals, extreme temperatures, or operate silicone-free.
Surface. The workpiece surface may make certain types of suction pads more suitable than others. The wide range of available products includes flat and bellows suction pads in many sizes and shapes with various types of sealing lips and sealing edges.

Physical factors
Engineers should determine certain physical parameters as part of the selection process.

Coefficient of friction. It is usually not wise to assign a ballpark value for the coefficient of friction, µ, between the suction pad and workpiece. This means designers must determine µ beforehand through testing. However, as a general guide, approximate values for various workpiece surfaces include:
• Oily surfaces: µ = 0.1.
• Moist or wet surfaces: µ = 0.2 to 0.4.
• Glass, stone, plastic (dry): µ = 0.5.
• Wood and metal: µ = 0.5.
• Rough surfaces: µ = 0.6.
• Sandpaper (dry): µ = 1.1.

Holding forces. Calculated holding forces can never exceed the theoretical maximums. In practice, many factors, such as the size and shape of the suction pad and the surface finish and rigidity (deformation) of the workpiece play an important role. For this reason, we recommend that engineers include a safety factor, S, of at least 2. The German accident-prevention regulations demand a minimum safety factor of 1.5. In operations which swivel or turn over the workpiece, use a safety factor of 2.5 or higher to cope with the resulting forces.

Suction pad diameter. The absolute holding force depends on the suction-pad diameter and the workpiece surface finish. Determine the required diameter with the following equations.

With the force applied horizontally:

With the force applied vertically:

Here, d = suction diameter, cm. (For suction pads with a double lip, d = the internal diameter; for bellows suction pads, it’s the minimum internal diameter of the sealing lip.) Also, m = mass of the workpiece, kg; P_U = vacuum, bar; n = number of suction pads; S = safety factor; and μ = coefficient of friction.