Demands on today’s hydraulic hoses vary across the board. Some need to handle only modest pressures, other must perform flawlessly despite extreme pressure spikes. Operating temperatures can ranges from Antarctic cold to extreme heat in steel mills and foundries. And modern hoses must deal with a plethora of fluids, from standard petroleum-based offerings to synthetics, biodegradable, and water-based versions.
Regardless of the application, however, all hoses consist of three basic components: tube, reinforcement, and cover.
The inner tube, made from synthetic-rubber compounds and composites, must be chemically compatible with the working fluid. Special compounds may be required to handle fire-resistant fluids. The tube must also resist corrosion and deterioration and the effects of high or low temperatures.
Reinforcement allows the hose to handle fluid pressures and pressure spikes, and prevents premature hose bursts when properly used. It determines the working pressure of the hose. Hoses with low working pressures normally use textile-fiber reinforcement, while those handling higher pressures generally use high-strength steel wire.
Steel-reinforced hoses, in turn, fall into two categories: braid and spiral. Wire-braided hose handles working pressures to 6000 psi, depending on size, with one or two braid layers. Spiral hose, which generally handles high pressures in larger diameters, has wire spiraled around the tube on a bias, with successive layers laid at opposing angles. There are typically four or six layers of steel reinforcement. In braid and spiral hose, rubber layers separate layers of steel wrap to ensure good adhesion throughout the hose wall.
The cover protects the tube and reinforcement from heat, abrasion, and corrosion, as well as environmental deterioration from heat, cold, UV light, and ozone. Covers are made from synthetic rubber, fiber braids, or a fabric wrap, depending on the application.
When specifying hydraulic hose, experts agree it’s important to understand the relationship between reinforcement and pressure ratings across different classes of hose.
Very-high and extremely high-pressure hose is used in off-highway equipment and heavy-duty machinery subject to extreme impulse or pressure surges. These hoses are reinforced with spiraled, high-tensile steel wire wound over the tube in alternating, even-numbered layers to balance pressure and containment forces. The hoses are often called four-wire for high pressure and six-wire for extremely high-pressure hose, though the actual number of spiral-wire layers varies with hose ID. Most spiral hose with an ID of 1 in. or less has four layers.
Spiral reinforcement is particularly suited to high-pressure impulse applications because individual wires in each layer are parallel, and thin rubber-adhesion layers separate adjacent reinforcement layers and keep the wires from cutting one another.
Spiral construction packs the reinforcement tighter around the tube than does braid reinforcement and, therefore, provides more support. Individual ends or strands can be bound tightly together as opposed to the over-under gaps with braiding. However, braided hose is generally more flexible than spiral hose.
High-pressure hose is mainly two-wire braid construction with high-tensile-strength steel reinforcement. It is frequently used on construction equipment and similar applications. Operating pressures range from 6000 psi for a 316-in. ID to 1825 psi for 2-in. ID. Some proprietary hoses, such as Gates M3K and M4K, have the same pressure rating for all sizes.
Medium-pressure hoses handle hydraulic applications requiring operating pressures of 300 to 3000 psi. They may have one-wire steel-braid reinforcement or multiple wire and/or textile-braid construction.
Low-pressure hydraulic hoses withstand operating pressures to 300 psi. Reinforcement is usually textile. They are found on low-pressure hydraulic applications and transmit petroleum-based fluids, diesel fuel, hot lubricating oil, air, glycol antifreeze, and water. Some, such as Global MegaVac (GMV), are also rated for suction applications.
Specialty hydraulic hoses do not fit well into a particular pressure category. For example, specialty hose might be used with environmentally friendly hydraulic fluids, for operating at extremely high or low temperature, or requiring electrical nonconductivity. They may be specified where weight is a concern or long continuous lengths are required. Reinforcement is generally nonmetallic — usually a rubber-impregnated fabric.
To bring a measure of uniformity to hydraulic-hose manufacturing, minimum standards for construction, dimensions, and performance have long been established in North America by SAE.
In other parts of the world, organizations such as the European Norm/Standard (EN), Deutsche Industrie Norm (DIN), and the International Standards Organization (ISO) also set standards, which may differ from those of SAE. Governmental agencies also set standards. Among them are the Mine Safety & Health Administration (MSHA) and the Department of Transportation’s Federal Motor Vehicle Safety Standards.
SAE Standard J517 provides general, dimensional, and performance specs for the 100R hose series, which are the most common hoses used in hydraulic systems. See the accompanying sidebar, “SAE hose constructions,” for more details.
Exceeding SAE specs
Some manufacturers have developed hoses that far exceed the performance and construction requirements of SAE specifications. Benefits to users include higher pressure and temperature capabilities, greater flexibility, and a bend radius as little as one-half that of the SAE standard.
One such product is Gates M-XP hydraulic hose, which combines the flexibility of wire-braid construction with the strength and performance of spiral-wire reinforcement. The result is an economical, two-braid wire hose that handles 4000-psi high-impulse duty in all sizes.
M-XP hose is rated for 1,000,000 impulse cycles (at 100°C), which exceeds the SAE standard of 200,000 impulse cycles and the Gates minimum requirement of 600,000 impulse cycles for typical wire-braid hoses. A high cycle rating equates to longer service life and makes the hose suited for out-of-sight and hard-to-reach applications such as boom arms and scissor lifts on mobile and construction equipment.
The hose also has half the minimum bend radius of equivalent SAE-rated hose. This means it tolerates tighter bends without compromising performance or life. In some installations, this can reduce the required hose length by nearly 50%. Greater flexibility also makes it easier to install in confined spaces.
Another advantage is that rather than needing expensive spiral-wire couplings, M-XP hose can use less-costly, one-piece MegaCrimp couplings that are also rated for ≥1,000,000 impulse test cycles. The assemblies meet MSHA flame-resistance requirements.
Dennis Kemper recently retired from the Gates Corp., Hydraulic Div. after more than 30 years of service in various technical and management positions, most recently as global product application manager. For more information on Gates MX-P hose, visit www.gates.com/mxp.