Figure 5-12.—Hose assemblyINSTALLATIONFlexible hose mustidentification tags (aircraft).not be twisted duringinstallation, since this reduces the life of the hoseconsiderably and may cause the fittings to loosenas well. You can determine whether or not a hoseis twisted by looking at the layline that runs alongthe length of the hose. If the layline does not spiralaround the hose, the hose is not twisted. If thelayline does spiral around the hose, the hose istwisted (fig. 5-13, view B) and must be untwisted.Flexible hose should be protected from chafingby using a chafe-resistant covering wherevernecessary.The minimum bend radius for flexible hosevaries according to the size and construction ofthe hose and the pressure under which thesystem operates. Current applicable technicalpublications contain tables and graphs showingminimum bend radii for the different types ofinstallations. Bends that are too sharp will reducethe bursting pressure of flexible hose considerablybelow its rated value.Flexible hose should be installed so that it willbe subjected to a minimum of flexing duringoperation. Support clamps are not necessary withshort installations; but for hose of considerablelength (48 inches for example), clamps should beplaced not more than 24 inches apart. Closer5-11Figure 5-13.—Correct and incorrect installation of flexiblehose.supports are desirable and in some cases may berequired.A flexible hose must never be stretched tightlybetween two fittings. About 5 to 8 percent of thetotal length must be allowed as slack to providefreedom of movement under pressure. Whenunder pressure, flexible hose contracts in lengthand expands in diameter. Examples of correct andincorrect installations of flexible hose areillustrated in figure 5-13.PFTE hose should be handled carefully duringremoval and installation. Some PFTE hose is pre-formed during fabrication. This type of hose tendsto form itself to the installed position in the sys-tem. To ensure its satisfactory function and reducethe likelihood of failure, anyone who works withPFTE hose should observe the following rules:1.2.3.4.Do not exceed recommended bend limits.Do not exceed twisting limits.Do not straighten a bent hose that hastaken a permanent set.Do not hang, lift, or support objects fromPFTE hose.
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