contraction. Bends are preferred to elbows,because bends cause less of a power loss. A fewof the correct and incorrect methods of installingtubing are illustrated in figure 5-1.Bends are described by their radius measure-ments. The ideal bend radius is 2 1/2 to 3 timesthe ID, as shown in figure 5-2. For example, ifthe ID of a line is 2 inches, the radius of the bendshould be between 5 and 6 inches.While friction increases markedly for sharpercurves than this, it also tends to increase up toa certain point for gentler curves. The increasesin friction in a bend with a radius of more than3 pipe diameters result from increased turbulencenear the outside edges of the flow. Particles offluid must travel a longer distance in making thechange in direction. When the radius of the bendis less than 2 1/2 pipe diameters, the increasedpressure loss is due to the abrupt change in thedirection of flow, especially for particles near theinside edge of the flow.During your career in the Navy, you may berequired to fabricate new tubing to replacedamaged or failed lines. Fabrication of tubingconsists of four basic operations: cutting,deburring, bending, and joint preparation.Tube Cutting and DeburringThe objective of cutting tubing is to producea square end that is free from burrs. Tubing maybe cut using a standard tube cutter (fig. 5-3), achipless cutter (fig. 5-4), or a fine-toothedhacksaw if a tube cutter is not available.When you use the standard tube cutter, placethe tube in the cutter with the cutting wheel at thepoint where the cut is to be made. Apply lightpressure on the tube by tightening the adjustingFigure 5-2.—Ideal bend radius.knob. Too much pressure applied to the cuttingwheel at onetime may deform the tubing or causeexcessive burrs. Rotate the cutter toward its openside (fig. 5-3). As you rotate the cutter, adjust thetightening knob after each complete turn tomaintain light pressure on the cutting wheel.When you use the chipless cutter, take thefollowing steps:1. Select the chipless cutter according totubing size.2. Rotate the cutter head to accept the tubingin the cutting position. Check that the cutterratchet is operating freely and that the cutter wheelis clear of the cutter head opening (fig. 5-4).3. Center the tubing on two rollers and thecutting blade.4. Use the hex key provided with theturn the drive screw in until the cuttertouches the tube.Figure 5-1.—Correct and incorrect methods of installing tubing.5-4kit towheel
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