Shock-resistant, flexible hose assemblies are required to absorb the movements of mounted equipment under both normal operating conditions and extreme conditions. They are also used for their noise-attenuating properties and to connect moving parts of certain equipment. The two basic hose types are synthetic rubber and polytetrafluoroethylene (PTFE), such as Du Ponts Teflon fluorocarbon resin.

pressure ranges: low, medium, and high. The outer cover is designed to withstand external abuse and contains identification markings. Synthetic rubber hoses with rubber covers are identified with the military specification number, the size by dash number, the quarter and year of cure or manufacture, and the manufacturers code identification number or federal supply code number printed along their layline (fig. 5-10, view A). The layline is a legible marking parallel to the longitudinal axis of a hose used in determining the straightness or lay of the hose. Synthetic rubber hoses with wire braid cover are identified by bands (fig. 5-10, view B) wrapped around the hose ends and at intervals along the length of the hose.

The size of a flexible hose is identified by the dash (-) number, which is the ID of the hose expressed in 16ths of an inch. For example, the ID of a -64 hose is 4 inches. For a few hose styles this is the nominal and not the true ID.

Synthetic rubber hoses will deteriorate from aging. A cure date is used to ensure that they do not deteriorate beyond material and performance specifications. The cure date is the quarter and year the hose was manufactured. For example,

1Q89 or 1/89 means the hose was made during the first quarter (1 Jan to 31 Mar) of 1989. The cure date limits the length of time a rubber hose can be stored, in bulk or as an assembly, prior to being placed into service. The storage or shelf life for rubber hose is 4 years. For the hose manufactured in 1Q89, the storage or shelf life will end on the 31st of March 1993. At this point, the hose is no longer considered usable and should be discarded or downgraded. The Aviation Hose and Tube Manual, NAVAIR 01-1A-20, and the Technical Directive for Piping Devices and Flexible Hose Assemblies, NAVSEA S6430-AE-TED- 010. volume 1. provide detailed instructions on discarding and downgrading of rubber hoses exceeding their shelf life.

PFTE hose is a flexible hose designed to meet the requirements of higher operating pressures and temperatures in present fluid power systems. This type of hose is made from a chemical resin, which is processed and extruded into a tube shaped to a desired size. It is reinforced with one or more layers of braided stainless-steel wire or with an even number of spiral wrap layers with an outer wire braid layer.

PTFE hose is unaffected by all fluids presently used in fluid power systems. It is inert to acids, both concentrated and diluted. Certain PFTE hose may be used in systems where operating temperatures range from 100F to +500F. PTFE is nonflammable; however, where the possibility of open flame exists, a special asbestos fire sleeve should be used.

PFTE hose will not absorb moisture. This, together with its chemical inertness and anti-adhesive characteristics, makes it ideal for missile fluid power systems where noncontamination and cleanliness are essential.

In lieu of layline marking, PTFE hoses are identified by metal or pliable plastic bands at their ends and at intervals along their length. Figure 5-10, view C, shows a hose label for a PTFE hose. Usually the only condition that will shorten the life of PTFE hose is excessive temperature. For this reason there is no manufacture date listed on the identification tag.

As mentioned earlier, flexible hose is available in three pressure ranges: low, medium, and high. When replacing hoses, it is important to ensure that the replacement hose is a duplicate of the one removed in length, OD, material, type and contour, and associated markings. In selecting hose, several precautions must be observed. The selected hose must

3. be designed to give adequate performance and service for infrequent transient pressure peaks up to 150 percent of the working pressure of the hose, and

4. have a safety factor with a burst pressure at a minimum of 4 times the rated working pressure.

There are temperature restrictions applied to the use of hoses. Rubber hose must not be used where the operating temperature exceeds 200F. PTFE hoses in high-pressure air systems must not be used where the temperature exceeds 350F. PTFE hoses in water and steam drain applications must not be used where the operating temperature exceeds 380F.

The fabrication of flexible hose assemblies is covered in applicable training manuals, technical publications, and NAVAIR 01-1A-20. After a hose assembly has been completely fabricated it must be cleaned, visually inspected for foreign materials, and proof tested.

A hose assembly is proof tested by the application of a nondestructive pressure for a minimum of 1 minute but not longer than 5 minutes to ensure that it will withstand normal working pressures. The test pressure, known as normal proof pressure, is twice the rated working pressure. While the test pressure is being applied, the hose must not burst, leak, or show signs of fitting separation. NAVAIR 01-1A-20 and NAVSEA S6430-AE-TED-010, volume 1, provide detailed instructions on cleaning of hoses, cleaning and test media, proof pressure and proof testing. After proof testing is completed, the hose must be flushed and dried and the ends capped or plugged to keep dirt and other contaminants out of the hose.