Hydraulic Hose Handles the Pressure, and the Heat

Growing demands for equipment that’s productive, efficient, and easy on the environment increasingly means hydraulic systems must work at higher pressures, withstand higher temperatures, and be compatible with environmentally “friendly” fluids, all while lasting longer. These requirements certainly hold true for hydraulic hose.

Engineers designing fluid-power circuits often follow the “STAMPED” process for selecting hydraulic hose and couplings. Developed some years ago by the National Assn. of Hose and Accessory Distributors, the acronym relates to the seven major areas of consideration for hose selection: size, temperature, application, material, pressure, ends, and delivery. As manufacturers respond to tougher applications with a myriad of new hoses, it’s a good idea to revisit these selection factors.

Size
Designers must adequately size the inside diameter of a hose to minimize pressure loss and turbulence. Turbulent conditions reduce efficiency and generate heat that can damage the hose. Nomographic charts (such as the one shown nearby) can help engineers size hoses for given hydraulic conditions.

Fluid velocity should not exceed the values shown in the right-hand column brackets. Higher velocities in pressure lines are generally acceptable for short durations. However, fluid velocity in suction lines should always fall within the recommended range to ensure efficient pump operation.

When replacing a used hose assembly, read the layline printing on the side of the original hose to determine size. If the layline is painted over or worn off, cut the old hose and measure the ID. Prior to cutting, measure the overall length and coupling orientation. This will make it easier to build a replacement and match the couplings to mating ports.

Do not use outside diameter to identify the hose ID — save for a few exceptions. (See “Sizing hose and couplings.”) Hoses vary with the wall thickness and OD, even though ID may be the same. OD is a factor when hose requires clamps or goes through bulkheads. Here, it is best to check individual hose specifications.

Hoses that meet or exceed SAE (Society of Automotive Engineers) performance specs but are smaller than standard hose can be good choices. Thanks to better materials, “hybrid” hoses have thinner tubes and covers, smaller-diameter wire reinforcement, and more-aggressive braid-reinforcement angles. The result: hoses with the same ID but smaller OD, making them more flexible and 10 to 15% lighter than earlier-generation products.

Today, leading manufacturers offer spiral-wire and wire-braid hoses with minimum bend radii one-third that of SAE specifications, reducing the overall length of curved assemblies and cutting costs by as much as 60%. In addition, more-compact and flexible hoses:

• Are easier to install and route in tight applications.
• Require fewer bent-tube fittings.
• Reduce inventory.
• Extend life in bending, flexing applications.

Temperature
Designers must consider fluid and ambient temperatures when selecting hose. One benefit of non-SAE hose is that it is more flexible and more-easily routed away from heat sources. Care must be taken when hose lies near hot manifolds and, in extreme cases, heat shields are advisable.

An important new development for design engineers is the Tier 4 emissions standard. In an effort to deliver cleaner exhaust from diesel engines, redesigns have raised engine-compartment temperatures in mobile equipment. The machines also need larger cooling systems and exhaust-treatment hardware, so there is less room to keep sensitive components away from hot spots. Subsequently, hose manufacturers have had to develop heat-resistant products to meet these demands.

One example is hose that uses chlorinated polyethylene (CPE) to replace traditional nitrile tube. CPE has broad fluid compatibilities and can handle constant temperatures to 275° and intermittent temperatures of 300°F. Nitrile limitations are 212 and 230°F, respectively.

In fact, Gates offers a range of hoses specifically for high temperatures. G2XH is an SAE 100R2 hose rated for 300°F and suitable for high-pressure hydraulics. Other products that handle 250 to 300°F include wire-braid and fiber-braid hoses for medium and high-pressure hydraulics, antifreeze, and water, as well as for air-brakes, engines, and return and suction lines.

At the opposite extreme, Gates PolarFlex hydraulic hose is for severe operating conditions as low as –70°F. It has a low-temperature tube, an abrasion and crack-resistant cover, two-wire-braid reinforcement, and uses standard couplings in sizes from 0.25 to 1.25 in.

Water, water/oil emulsions, and water/glycol solutions sometimes require upper temperature limits of only 225 or 200°F, depending on the hose. Low-pressure applications (such as return lines) require even lower maximum temperatures, usually 180°F.

Never exceed the recommended maximum operating temperature for a given fluid. If fluid and hose maximums differ, the lower limit takes precedence.

Actual service life at temperatures approaching the hose’s recommended limit depends on the application and fluid. But detrimental effects increase with greater exposure to high temperatures. Failure to use hydraulic oil with the necessary high-temperature viscosity can accelerate hose degradation. Simultaneously operating at maximum temperature and maximum working pressure can greatly reduce service life. Using a hydraulic hose at 18°F above the maximum ratings will typically cut hose life in half.

Application
Government and industry regulations or special considerations such as nonconductivity can come into play when selecting hose. Six application factors stand out as being most critical. Pressure and temperature are discussed elsewhere. Here’s a look at four others.

Environmental conditions. External mechanical forces wearing against the hose cover, especially when it exposes the reinforcement, can cause hose failure. The best way to protect an assembly from abrasive wear is to avoid hose-to-hose and hose-to-metal contact. A second option is to specify hose covers made of abrasion-resistant hybrid compounds such as ultra-high molecular weight polyethylene. Tests show some new cover materials last up to 300 times longer than standard rubber-covered hoses. Several different types of metal and plastic spring guards and textile/nylon sleeves can also protect hoses from external damage and wear.

Excessive loads. Though it is flexible, avoid excessive vibrations and external loads that kink or compress the hose. Always take into account the manufacturer’s recommended minimum bend radius and avoid applications that twist the hose or cause it to bend immediately behind the coupling. Avoid bending the hose in more than one plane.

Hose-coupling compatibility. Fluid-power engineers strongly discourage using couplings from one manufacturer and hoses from another. Although most American-made and many imported hoses conform to SAE standards, the specs allow for liberal dimensional tolerances and a wide range of construction materials. As a result, hose and couplings from different manufacturers are likely incompatible. Improperly matched and coupled hoses often fail prematurely, causing downtime and possible injury.

In addition, never recrimp or recouple used hose with permanent or field-attachable couplings, and never reuse field-attachable couplings — even with new hose.

Installation. Improperly installing assemblies is a prime cause of hydraulic leaks. One common error is twisting hoses as they are tightened. Internal pressure applied to a twisted hose can cause failure or loosen connections, sometimes referred to as “detorquing.” Using two wrenches (one on the hex nut and one on the stem nut) to tighten swivel fittings helps prevent twisting.

When a coupling leaks, there is a natural tendency to tighten the fitting. However, overtorquing couplings can also cause leaks. Proper torque is especially critical with flared fittings. Excessive torque can strip threads or crack the cone seat — preventing proper sealing.

Materials
Most hydraulic fluids are petroleum based. Others include water-based, water glycols, and synthetic-based fluids such as phosphate esters. In the past, hydraulic-fluid leaks have sometimes contaminated soil and fouled water supplies. As a result, the industry is moving toward more environmentally friendly fluids.

“Green” fluids are typically synthetic (primarily ester based) or vegetable based. Vegetable oils are gaining popularity because they cost less and are more biodegradable than synthetics. They also have excellent lubricity and a high viscosity index. The downside is their limited temperature range and rapid oxidation at elevated temperatures. And although the base fluid may be biodegradable and nontoxic, the additives may not be.

Biodegradable fluids might be great for the environment, but they’re tough on hoses. They permeate ordinary hose tubes, causing sweating, wetness, and blisters on the cover. The result is premature and expensive hose failure.

Most manufacturers use a nitrile tube for environmentally safe hydraulic fluids. Nitrile is tough enough to handle aggressive biodegradable fluids like synthetic esters, polyglycols, and vegetable oils at operating temperatures to 250°F. Plus, nitrile permits significantly less permeation than neoprene tubes when used with petroleum-based oils. (Permeation, or effusion, is seepage through the tube and hose, resulting in fluid loss.)

Because permeation may expose the entire hose assembly to fluid, check compatibility not only with the tube, but with the reinforcement, cover, fittings, and seals. The same holds for assemblies that convey special oils or chemicals.

Exercise additional caution selecting hose for gaseous applications subject to permeation. Some fluids that raise concerns include: liquid and gas fuels, refrigerants, helium, fuel oil, and natural gas. If gas permeates through the tube, consider pin-perforated covers to prevent gas buildup. Don’t neglect the potentially hazardous effects of permeation, such as explosions, fires, and toxicity. Refer to applicable standards for specific precautions involving fuels and refrigerants.

Pressure
It’s essential to know maximum system pressure — including pressure spikes — when selecting hose. Pressures and spikes greater than the hose’s rated working pressure will shorten its life. As a general rule, allow a generous margin of safety.

Burst pressures are reference pressures only intended for destructive testing purposes and safety factors. Typically, minimum burst pressure is four times the maximum working pressure.

It is also important to consider pressure drop through a hydraulic-hose assembly, particularly when equipment requires a specific output pressure to run efficiently. For instance, 150-psi pressure drop through a hose would cut a 4,000-psi input pressure to a 3,850-psi working pressure.

Factors that influence pressure drop include:

Friction. Due to fluid rubbing against the inside of hose, some plastic and Teflon inner tubes can lower internal friction, compared to rubber.

Fluid. Different fluids behave differently under pressure. Thicker fluids move with greater difficulty and exhibit greater pressure drop.

Temperature. Warming “thins” fluids so they move more easily, as with automotive oil.

Length. Longer assemblies have more internal surface friction.

Inner diameter. Flow area affects fluid velocity for a given flow rate, and higher velocities result in greater pressure drops. Using the proper diameter hose generates less pressure drop.

Couplings and adapters. Any change in bore or direction (such as with 45 or 90° elbows) can increase pressure drop.

Flow rate. Pressure drop increases with flow rate for the same size hose.

Application engineers at hose and coupling manufacturers are a good resource for helping determine pressure drop.

End couplings
The amount and type of machinery being imported has grown dramatically. The primary difference between conventional SAE couplings and those made to other standards is the thread configuration and seat angle. And because couplings seal in three ways — thread interface, mating metal-seat angles, and O-rings — this difference dramatically increases the possibility of mismatching threads and seats on various couplings.

It is important to be aware of these differences and to correctly identify different types of couplings. Manufacturers and distributors offer manuals and tables to help accomplish this task.

A coupling’s male and female thread ends must be compatible to ensure an effective seal and prevent leaks or blowoffs. International thread ends can be metric, measured in millimeters, but also include British Standard Pipe (BSP) threads, which are measured in inches. Knowing the country of origin for a piece of equipment provides a clue as to the type of thread end. Deutsche Industrial Norme (DIN) fittings indicate a German or Swedish manufacturer, while BSP is found on British equipment. Japanese Komatsu machinery uses Komatsu fittings with metric threads, while other Japanese equipment most likely uses Japanese Industrial Standard (JIS) BSP threads or, in some cases, BSP straight or tapered threads.

Identify these couplings by:

Seat: Inverted (BSPP and DIN), regular (JIS and Komatsu), or flat (flange and flat-face).

Seat angle: 30° (JIS, BSP, DIN and Komatsu) or 12° (DIN).

Threads: Metric (DIN or Komatsu), BSP (BSPP, BSPT, or JIS), or tapered (BSPT or JIS tapered).

Also keep in mind that rubber O-rings are not interchangeable with all couplings. A common mistake involves using JIS Komatsu flange fittings with the wrong O-ring. In all sizes, O-ring dimensions differ between Komatsu and SAE flanges. When replacing a Komatsu flange with an SAE-style flange, use an SAE O-ring or it will likely leak. Also note that O-rings are not reusable.

Motion and vibration can affect coupling selection. Split-flange couplings and fittings with O-rings generally better withstand vibrations. They are also preferred on applications with extreme temperature fluctuations.

Delivery
Delivery of replacement hose can be critical, especially if a pricey machine is out of service. But quickly getting the right hose depends on details such as the type, size, length, and end fittings. Identifying hose isn’t always easy, given abrasion, weathering, dirt, paint, and other abuse that render markings difficult to read.

Gates, along with some other manufacturers, uses RFID (radio-frequency-identification) tags on hose assemblies to speed and simplify identification. The tags reference unique data about the assembly stored in a database at distributors and the manufacturer. Should an assembly require service, maintenance personnel merely scan the tag using a PDA with an RFID reader to call up specs from the database. Instant access to hose information lets them quickly make a new assembly or order replacements.

© 2011 Penton Media, Inc.