MRO Today - Get the right hose

Get the right hose
When you select the correct hose for your application,
you build reliability and long life into your repair

by Sherrie Swadburg

The devil is in the detail when it comes to selecting hydraulic hose assemblies. Taking shortcuts in selecting hose can affect the performance and life of equipment; it also creates a potentially hazardous situation for machine operators.

Any time you must specify a hose assembly, follow the “STAMPED” acronym:
Size: inside diameter (I.D.) and outside diameter (O.D.).
Temperature of the fluid conveyed.
Application in which the hose assembly is used.
Material to be conveyed.
Pressure that’s adequate for the application.
Ends that are compatible with the selected hose.
Delivery of assembly components.

S = Size
Most American-made hydraulic hoses and many imported hoses are built to conform to Society of Automotive Engineers (SAE) specifications. These specs detail size, tolerances, construction and minimum performance characteristics of each major hose type. The SAE J517 standard provides general, dimensional and performance specifications for the 100R hose series, which are the most common hoses used in hydraulic systems on mobile and stationary equipment.

In order to select the proper hose size for an application, you must identify the correct hose length and its inside and outside hose diameters.

To determine hose size, read the lay line printed on the side of the original hose, which includes (from left to right):

• The dash size numeral, which is measured in sixteenths of an inch. In all but 100R5 and 100R14, the dash size is the hose I.D.

• The SAE 100R specification number.

• Letters indicating the cover type (A = thick cover, AT = thin cover).

If the original hose lay line is painted over or worn off, you need to cut the hose and measure the inside diameter, preferably with a plug gauge. Make sure you first measure total hose length before you cut it apart.

Hose outside diameter is important when hose routing clamps are used, or when hoses are routed through bulkheads. Check individual hose specification tables for outer diameters in suppliers’ catalogs.

When replacing a hose assembly, always cut the hose the same length as the one being removed. Too long a hose can lead to severing or pinching in moving equipment components. If the hose is too short, pressure may cause it to contract and stretch, which can reduce service life. Hose length can change from plus-2 to minus-4 percent under pressure.

T = Temperature
Fluid and ambient temperature can greatly affect hose and coupling selection. All hoses are rated to withstand a maximum fluid working temperature range of 200 degrees F to 400 F (93 degrees C to 204 C). Step above that temperature range and you can dramatically shorten the hose life. For example, using a hydraulic hose at 18 F above its maximum rated temperature may cut hose life in half. If you operate at higher temperatures and also use the wrong viscosity hydraulic oil, you’ll further reduce hose life.

If you must operate in unusual temperature ranges, consider nitrile and winterized rubber compounds which can handle temperatures down to minus-40 F or Teflon-style hose that can handle temperatures up to plus-400 F.

You will find hose service life drops dramatically when exposed to high external and internal temperatures. Use insulating sleeves to help protect hose from hot equipment and other high- temperature sources. Often, an additional barrier may be needed to shield fluid from a possible source of ignition.

A = Application
You must know your application thoroughly to select the right hose. Consider these factors:

Bend radius: Do not exceed the manufacturer’s recommended minimum bend radius.

Routing: High-pressure lines should be routed parallel to machine contours whenever possible. This can help save money by reducing line lengths and minimizing the number of hard-angle, flow-restricting bends.

Electrical conductivity: Some equipment requires the use of non-conductive hose if there’s a chance of contacting a power source.

Government and industry standards: Do any agency specifications or requirements apply to this application? Industry agencies include the SAE, the Deutche Industrial Norm (DIN), which are German standards accepted throughout much of Europe and the International Standard Organization (ISO).

M = Material
All hose assembly components — the hose tube, cover, couplings and O-rings — must be compatible with the fluid used. The characteristics shown in Table 2 on page 18 are for the normal or usual range of specific hose stocks. Also, consult the hose manufacturer’s chemical resistance tables to determine hose compatibility.

Be wary of some of the newer environmental fluids available today. While they may be kinder from an ecological standpoint, they may not be totally compatible chemically with the hose, causing premature deterioration. New elastomeric tubing compounds are being tested for compatibility with the environmentally friendly hydraulic fluids and additives. They are showing great promise.

If your application uses the hose to convey special oils or chemicals, permeation or effusion may occur. Here, the fluid or gas seeps through the skin of the hose. If permeation or effusion is a risk, consider the potential safety hazards such as explosions, fires and toxicity. Pin-perforated hose covers can help prevent fluid build-up.

Tube and cover stocks may help prevent hose degradation problems. The new rubber/ synthetic thermoplastic compounds found in abrasion-resistant hydraulic hose can offer greater resistance to abrasion than neoprene and nitrile.

These hoses also have a very slick surface that resists dirt and oil residues, resulting in a cleaner appearance.

Abrasion-resistant hose covers have been tested and found to last up to 300 times longer than standard rubber covers. This feature increases service life, lowers maintenance and eliminates the need for costly hose protectors such as guards, sleeves and bundling.

P = Pressure
Knowing system pressure is as important as knowing system spike pressure. Hose working pressure must always be greater than or equal to the maximum system pressure, including pressure spikes.

In situations where equipment has been modified, hydraulic pressure spikes are common and subject the hose and coupling to pressures not engineered into the unit. Allow a generous margin of safety in these situations. Typically, for dynamic hydraulic applications, the minimum burst pressure rating is four times that of the maximum working pressure rating.

Pressure drop greatly affects fluid performance. Friction, fluid viscosity and temperature, couplings and adapters and flow rate all affect pressure drop.

It’s not easy to figure pressure drop, but your hose and coupling distributor can be a great resource. Be prepared to describe the type of application, fluid type and viscosity, fluid and ambient temperature, fluid flow rate, hose size and length, routing requirements, government and industry standards being met and the number and type of fittings.

E = Ends
As the United States becomes even more global in its manufacturing processes, you’ll see different standards on different equipment. While U.S. equipment conforms to SAE guidelines, off-shore equipment may follow another set of standards. This greatly affects hose assemblies.

The primary difference between a conventional SAE coupling and a foreign coupling is the thread configuration and the seat angle.

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 will provide a clue as to what type of thread end is used. 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. You need to know three measurements to select the correct replacement coupling:
• Seat: Inverted (BSPP & DIN), regular (JIS & Komatsu) or flat (flange, 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).

The coupling or hose interface must match the hose you select. Follow the hose manufacturer's coupling recommendations only to assure you get proper mating for leak-free sealing.

Couplings can be affected by machine motion, vibration or temperature. Use split flange or O-ring couplings in high-vibration applications; O-ring couplings can withstand extreme temperature fluctuations.

Avoid interchanging couplings and hoses from different manufacturers. SAE allows a whole range of materials and different manufacturers use different materials that result in a variety of tube styles.

D = Delivery
In downtime situations, product availability may become the top criteria in selection. Let your local distributor know what your needs are to ensure that the required products are available for you when you need them.

Sherrie Swadburg is a hydraulic products application engineer at the Gates Rubber Company.

This article appeared in the June/July 1998 issue of MRO Today magazine. Copyright, 1998.