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Designing Hydraulic Hoses for Extreme Conditions

May 27, 2016
Hydraulic hoses must withstand extreme temperatures to minimize downtime and maintain productivity.
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Hydraulic hoses can be found in the most remote parts of the world, or right around the corner. They are commonly used under the hoods of automobiles, as well as in aircraft-support and forestry equipment, heavy construction vehicles, and the drills, vehicles, and rigs used in mining and the oil and gas industries.

Hoses for All Fluids

Hoses transmit any number of industrial fluids, including motor oils, heavy-duty coolants, and transmission and brake fluids. Most of these fluids tend to be petroleum-based oils, but engineers also work with many obscure fluids as well, including antifreeze and other glycol-based coolants.

The challenge in dealing with such a wide variety of fluids is finding the right hose for a particular fluid. On top of that, fluid manufacturers and oil companies regularly update their products, says Nathan Groves, a chemical engineer and project manager with Parker Hannifin Corp. Fluids are usually changed after five to 10 years, with a typical update being a change to the additive package, he says. Although the base fluid remains the same, changing the additives often affects the fluid’s compatibility with the hose’s inner elastomer tube. Hose manufacturers are aware of these changes and maintain compatibility databases based on material testing.

The nylon-fiber-braid cover of Parker’s 293-10 Air Brake hose offers abrasion resistance with operating temperatures ranging from -58°F to 302F° (-50°C to 150°C). Hose designers determine the best materials for particular hose covers based on the intended operating environment and fluid it will carry.

“If an OEM changes a fluid’s base materials, such as going from a distillate to a synthetic, that can be more of a concern than changing additives because the chemical makeups of the two types of oils differ and the fluid may now be more aggressive,” Groves says, referring to its effects on a hose’s tube.

Aggressiveness refers to a fluid’s chemical makeup, which is reflected in the solubility of the polymer in the fluid. This is one of the key variables engineers must consider when designing hoses for particular applications, Groves says. Some components within a blended fluid may tend to be absorbed into the hose’s rubber compound or extract oils out of the rubber, changing the hose’s physical properties.

Such changes in physical property can include swelling or contraction. Swelling restricts fluid flow through the hose and softens the rubber, which can lead to fittings leaking at elevated temperatures. Extracting oils makes the hose much stiffer at low temperatures, which causes cracking when the hose is stressed or flexed.

“Some end-users actually immerse hoses in fluids they are trying to transfer, whether by accident or on purpose,” Groves says. “This can degrade the properties of the hose cover.”

The hose cover, usually rubber, primarily acts as a protective barrier against external conditions such as abrasions and ozone. In most cases, the cover layer should never be exposed to fluids because it can cause the hose to either crack or lose its abrasion resistance. This could expose the hose’s reinforcement layers to the environment, weakening the hose.

It’s up to hose designers to determine the best materials for a particular hose based on the fluid it will carry and its intended operating environment.

Parker’s GlobalCore 722TC (ToughCover) works at temperatures ranging from -40°F to 257°F (-40°C to 125°C). Several layers of precisely oriented wire reinforcement create a hydraulic balance, which keeps the hose from moving too much when under pressure.

Beating the Heat

The second most important variable to consider is the temperature of the fluid being transmitted. This is especially true with hoses carrying oil in high-temperature applications, says Greg Reardon, business development manager with Parker. If the oil temperature exceeds the hose’s threshold temperature, usually between -40°F and 257°F, the heat can bake the inner tube, causing it to harden and crack. This happens when running a system for too long without giving the oil a chance to cool, he says.

“This is a common issue with heavy mobile machinery,” Reardon notes. “In a perfect world, all mobile equipment would come with oil regulators and coolers to keep oil temperatures in check. However, this is not always the case due to size or cost restraints. Without cooling, and with the vehicles running for eight to 16 hours at a stretch, the oil heats up and starts baking the hose.”

Some companies, including Parker, design hoses with inner tubes that handle transfer-fluid temperatures of up to 302°F. But Reardon still cautions customers to closely monitor the temperature of any transfer fluid during operation.

Hoses must be designed to meet strict requirements for each application, including bend radius, flexibility, and life. Variables, including environment temperature, application pressure, and fluid being transferred, can all affect the hose’s ability to meet those requirements.

Ambient heat is also of significant concern, he adds. Hoses in the engine compartment of a vehicle, for example, see higher temperatures than those in open areas where there is some natural cooling and the heat has an avenue of escape. Because engine compartments are generally closed off, heat is prevented from escaping. The resulting high temperatures affect hoses from the outside in.

In addition to oil temperature and ambient heat, nearby operations can expose hoses to dangerously high temperatures. Anything from vats of molten metal to stray welding spatter can heat up unprotected hoses and increase the possibility of failure.

For these situations, Parker and other companies have developed a variety of sleeves to cover and further protect hoses. Sleeve materials range from fabrics to fire-resistant polymers to steel braid, and some also protect against injuries in the event a hose bursts.

All Parker hoses and tubing exhibit their best physical properties at room temperature, 68°F (20°C). As elastomeric materials are exposed to higher temperatures, they soften and their physical properties, such as flexibility, may change.

Couplings attached to hoses are held in place in part by the rubber compounds’ pliability. If a hose becomes brittle due to excessive operating temperatures, the coupling can break free. To avoid this, conscientious end-users institute preventive maintenance and planned replacements for at-risk components.

“When selecting and constructing hoses for customers, we complete a thorough analysis of how the hoses will be used and under what conditions,” Groves says. “We ensure all hoses are tested in a controlled, safe environment. Hoses must perform to strict specifications, such as SAE or ISO industry standards, under operating conditions dictated by the product’s planned operating environment before we deliver them to customers.”

Fighting the Cold

Although temperatures above 257°F are a cause for concern, customers also experience issues with temperatures at the other extreme, -40°F and lower. When exposed to extremely cold temperatures, rubber hoses stiffen and cannot flex like they should, causing them to crack. This leads to potential hose failures. Two industries that rely on low-temperature hoses are land-based oil rigs and the logging industry.

Hoses are designed in layers that get stacked on top of each other, starting with an inner tube layer that resists the transmitted fluid, a reinforcement layer for pressure carrying capability, and a cover (perhaps an elastomer) that makes helps it resist abrasions.

Fortunately, hoses can be designed for specific operating temperature ranges and there are versions that operate at temperatures as low as -70°F and with working pressures as high as 6,000 psi. Designing hoses for extreme cold is a matter of getting the right combination of fluid compatibility and the chemistry of the hose layers, according to Derek Garceau, engineering manager for Parker’s Hose Products Division.

“Variables ranging from temperature, working pressure, and fluid compatibility, along with restrictions inherent in the customer’s application, make it challenging to determine the right hose for an application,” he says.

“It’s a balancing act between the compatibility of the inner tube with the fluid being used, and the conditions of the specific application, such as repeated flexing or high operating temperatures,” Garceau says. “Both can affect each other’s performance. For example, having more material in the polymer to resist the fluid often decreases its ability to withstand extreme temperatures.”

Cold temperatures can also affect the fluid being transferred through the hose. For example, cold temperatures may thicken the fluid, decreasing its flow rate. And adding more insulation or heated sleeves to the hose isn’t always viable. In many of today’s vehicles, hose are routed to save space and cost. This does not leave a lot of room to install an insulated hose or additional sleeving.

“We can’t make the hose two inches in diameter if the client needs it to be half-an-inch in diameter,” Garceau says. “We usually recommend end-users complete a warming procedure prior to use in which they let oil circulate through the hose until it’s up to a working temperature.”

Other Challenges

Not all challenging applications involve extreme temperatures. In one case, for example, Boeing wanted to simulate flight conditions in its 787 Dreamliner while it was still on the assembly line to check the hydraulic pressures. The Dreamliner uses higher hydraulic pressures than most commercial aircraft. For the test, Boeing needed a 6,000 psi hose to validate the aircraft’s functions.

At the time, no hose on the market compatible with aviation fluid carried a 6,000-psi rating. In fewer than four months, Parker designed, tested, and manufactured its F42 hose made out of an ethylene propylene diene monomer (EPDM)-based rubber. Rated at 6,000 psi and compatible with aviation fluid, the hose also meets other Boeing requirements for bend radius, flexibility, and life.

Certain materials let hoses be built with thinner layers, allowing for decreased outside diameters and increased bend radii while maintaining the same level of performance. The chemistry for hoses is chosen based on what they are going to be used for, but the goal for all hoses is the same—maximum durability, high flexibility, and long service life. Hoses are expected to perform in both extreme heat and cold, so they are tested in some of the worst conditions.

EPDM is a base polymer used to make rubber hoses. It is a high-density synthetic rubber compatible with fireproof hydraulic fluids, ketones, hot and cold water, and various alkalis. EPDM hoses resist heat, ozone, and weather, and maintain excellent flexibility at high and low temperatures. Parker’s EPDM hose provides reliable, constant working pressures of up to 6,000 psi in temperatures from -40° to 176°F (-40°C to 80°C).

A second commonly used chemical is a nitrile-based polymer, which is a synthetic rubber copolymer of acrylonitrile (ACN) and butadiene. Nitriles provide good compatibility with oils and standard hydraulic fluids.

“When designing nitrile rubber hoses for customers, engineers pay close attention to the ACN content in the mixture,” Groves notes. “The higher the ACN content, the better the polymer resists certain nonpolar solvents such as mineral and vegetable oils, benzene/petrol, ordinary diluted acids, and alkalis.”

However, there is a tradeoff between ACN content and low temperature flexibility. The higher the ACN content, the more resistant the hose is to fluids, but the worse it will be for low temperature flexing, which can lead to cracking and failure when the hose flexes when it is below its stated operating temperature.

The way chemicals in the recipe bond together gives the hose its different resistances, he says. Hoses are built like sandwiches with several layers of material stacked on top of each other, but all those materials must bond with each other.

“How easily the materials bond is important,” Garceau says. “Otherwise, hose failure could lead to delamination while in use. The inner tube could separate and rip itself out.”

A hose’s pressure rating is determined by its reinforcements, he says. Hoses are constructed in layers, starting with an inner tube, then alternating layers of rubber and reinforcements, such as steel wire. Each reinforcement layer is placed to work with the layer above or below it, so when the hose is pressurized, it creates a hydraulic balance. This balance restricts the hose from moving too much under pressure. If there is no balance, the hose twists or “bounces” under pressure and could start to come apart due to the forces from the pressurized fluid, he explains.

While hoses under the hood of vehicles are usually fairly stationary and don’t move around very much, hoses used on a piece of equipment that has a flexible joint, like a backhoe, must flex reliably in all conditions.

Parker also engineers its hoses with a number of SAE-tested and approved additives to increase resistance to ozone and ultraviolet (UV) light. Without such additives, constant exposure to direct sunlight (like on a piece of construction equipment) degrades the rubber, causing it to break apart and fall off in chunks, which is made worse by any flexing motion.

“Additives impart their characteristics and properties to the rubber compound,” he says. “Other additives include processing aids such as waxes and oils, and abrasion-resistant and fire-resistant polymers added to the hose cover.

Kyri McDonough, Marketing Services Manager

Parker Hannifin Corp., Hose Products Division, Wickliffe, OH

Two Tips for Installing Hoses

Here are some best practices for installing and routing hoses that should prolong their life and prevent failure:
  • Many hoses, including those from Parker, have continuous labels, or “lay lines” printed on the outer cover. Paying close attention to these lay lines during installation prevents the hose from being twisted.
  • To prevent abrasion, a primary cause of premature hose failure, clamp hoses in place as applicable. But don’t clamp them so tightly they can’t move when pressurized. They should also be clamped individually to keep them from rubbing against each other, which leads to abrasion failures.

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