Copyright Goodheart-Willcox Co., Inc. Chapter 8 Fluid Storage and Distribution 189 Hose Hose is a fl exible conductor made from a combination of different materials that are compatible with the sys- tem fl uid and suffi ciently strong to withstand system operating pressures. It is commonly used in systems to make connections where equipment parts tilt or swivel during operation or where severe vibration is encountered. See Figure 8-13. To achieve the combi- nation of fl exibility and strength, a hose has a fl exible inner tube to conduct the fl uid, a middle layer of rein- forcing materials for strength, and an outer protective layer of tough material to withstand external abrasion and abuse. The inner tube that conducts the fl uid must be smooth and suffi ciently fl exible to freely bend, but strong enough to resist collapse, which would cause a reduction in cross-sectional area. This tube is usually made from synthetic-rubber compounds or thermo- plastics. The middle reinforcement section is formed amount of space while minimizing visual clutter. Care needs to be taken when selecting tubing to ensure the material is compatible with system fl uid and the tub- ing is adequately strong to withstand system pres- sures. For example, copper tubing should not be used in a hydraulic system because of the chemical reaction between copper and the oil and its additives. The size of tubing is indicated by measurement of the actual outside diameter. The inside diameter will vary depending on wall thickness. Most tubing is man- ufactured to the specifi cations of various standardizing organizations. The American National Standards Insti- tute (ANSI), Society of Automotive Engineers (SAE), and American Iron and Steel Institute (AISI) all have standards covering various types of tubing. A general- ization that can be made for any one nominal diameter of tubing is that the smaller the inside diameter, the greater the relative thickness of the wall. A thicker wall allows a higher system operating pressure. However, care must be taken to check the specifi cations of each type of tubing. Construction material and construction technique can produce widely varying specifi cations for any given size. Lengths of tubing are bent to form a conductor between two components in a hydraulic system. The formed tubing is then attached to the components on either end by appropriate connectors and fi ttings. Care must be taken in designing these tubing sections. Sys- tem vibration and thermal expansion and contraction must be considered. Both vibration and expansion/ contraction can be absorbed by placing a bend in the tube between the attachment points, Figure 8-12. Tools are available to form these bends. The tools can be used to form most angles, although angles over 90° are diffi cult. Generally, 90° is the maximum bend angle. The tube tends to collapse at the bend, resulting in a reduced cross section that is not round. Tube inserts made from rubber or coiled wire are sometimes used to help prevent tube collapse during bending. Nominal Size (inches) Burst Pressure (psi) Schedule 40 Schedule 80 Schedule 160 Double Extra Heavy 1/4 16,000 22,000 — — 3/8 13,500 19,000 — — 1/2 13,200 17,500 21,000 35,000 3/4 11,000 15,000 21,000 30,000 1 10,000 13,600 19,000 27,000 1 1/4 8,400 11,500 15,000 23,000 1 1/2 7,600 10,500 14,800 21,000 2 6,500 9,100 14,500 19,000 Goodheart-Willcox Publisher Figure 8-11. The maximum pressure at which a pipe can operate depends on the nominal diameter and schedule number classification. Burst pressures of the various pipe schedules vary between manufacturers. Used with permission of CNH America LLC Figure 8-12. Tube can be safely bent to form angles up to 90°. This allows installations that contain fewer fittings than systems using pipe.