leak source not over ¾ of an inch in diameter.
A seep is a leak that wets an area from ¾ inches to 1½ inches in diameter. A heavy seep is a fuel leak that wets an area around the leak source from 1½ inches to 3 inches in diameter. In none of these three leak classifications does the fuel run, flow, drip, or resemble any of these conditions at the end of the 30-minute time period.
The last classification, a running leak, is the most severe and the most dangerous. It may drip from the aircraft surface, it may run down vertical surfaces, or it may even run down your finger when you touch the wet area. The aircraft is unsafe for flight and must be grounded for repair.
When possible, the fuel from the leaking tank should be removed after you mark the leak location.
If it is impossible to defuel the tank immediately, the aircraft should be isolated in an approved area.
Place appropriate warning signs around the aircraft until qualified personnel can defuel the leaking tank.
Grounding of the aircraft for slow seeps, seeps, and heavy seeps, is determined by the applicable aircraft handbook. This determination may depend on the location of the fuel leak. For example, can the leakage progress to a potential fire source?
The number of fuel leaks in a given area is also a contributing factor. There is no rule of thumb for determining if the aircraft is to be grounded.
Running leaks ground the aircraft regardless of location.
You may only have to make appropriate entries on the aircraft forms and periodically observe the progress of the fuel leak if it is determined that the aircraft is airworthy and no repair is required.
When repair is required, you must find the cause of the fuel leak and make an effective repair.
Leak Repairs Repair of leaks in integral fuel tanks must be accomplished in accordance with the aircraft manufacturer's specifications. No attempt will be made in this handbook to discuss integral tank repairs further.
Fire Safety The first and most difficult step in the achievement of fire safety is to correct the misconceptions about the "safety" of turbine fuels. At the time these fuels were first introduced many people said, "fire problems in aircraft are over, turbine fuel is completely safei This is obviously nonsense but it has been persistent nonsense.
Flight line personnel have agreed that aviation gasoline will burn, and therefore they have exercised reasonable care and caution in handling it.
However, it has been difficult to convince them that under some circumstances turbine fuels are just as dangerous from the fire standpoint.
The characteristics of turbine fuel do vary from those of gasoline. Kerosene, for example, has a slow flame propagation and burning rate, which makes it less hazardous in the event of spill or a ground accident. However, it does ignite readily when vaporized or when misted, as when sprayed through a small leak in a service hose.
One disadvantage of the low volatility fuels is that they will not evaporate readily and completely if spilled on the ramp, so special treatment of the spill area is required. Small spills of kerosene should be removed with a commercial absorbent cleaning agent. On large spils it is better to apply an approved emulsifier and then flush away the resulting mixture with large volumes of water.
This will prevent or appreciably lessen any oily residue.
Just as with gasoline, an electrostatic charge may be built up in pumping turbine fuel through a service hose. In fact, the amount of the charge is higher in kerosene because of the higher specific gravity and wider boiling range, Also, the amount of the charge increases with high linear rate of fuel flow, such as is required for servicing turbine powered aircraft.
In consequence, all of the fire safety precautions observed in the handling of gasoline must be followed with equal care in the handling of turbine fuels. These precautions are well known and have been detailed by the National Fire Protection Association in its bulletin No. 407. It is recommended that this bulletin be made required reading for all personnel handling turbine fuel.
CHAPTER 5
FLUID LINES AND FITTINGS
GENERAL The term "aireraft plumbing" refers not only to the hose, tubing, fittings, and connectors used in the aircraft, but also to the processes of forming Occasionally it may be necessary to repair or replace damaged aircraft plumbing lines. Very often the repair can be made simply by replacing the tubing. However, if replacements are not available, the needed parts may have to be fabricated. Replacement tubing should be of the same size and material as the original line. All tubing is pressure tested prior to initial installation, and is designed to withstand several times the normal operating pressure to which it will be subjected.
If a tube bursts or cracks, it is generally the result of excessive vibration, improper installation, or damage caused by collision with an object.
All tubing failures should be carefully studied and the cause of the failure determined.
PLUMBING LINES Aircraft plumbing lines usually are made of metal tubing and fittings or of flexible hose. Metal tubing is widely used in aircraft for fuel, oil, cooloxygen, instrument, and hydraulic lines.
Flexible hose is generally used with moving parts or where the hose is subject to considerable vibra- Generally, aluminum alloy or corrosion-resistant steel tubing have replaced copper tubing: The high fatigue factor of copper tubing is the chief reason for its replacement. It becomes hard and brittle from vibration and finally breaks, however it may be restored to its soft annealed state by heating it red hot and quenching it in cold water.
Cooling in air will result in a degree of softness but not equal to that obtained with the cold water quench: This annealing process must be accomplished if copper tubing is removed for any reason.
Inspection of copper tubing for cracks, hardness, brittleness and general condition should be accomplished at regular intervals to preclude failure. The workability, resistance to corrosion, and lightweight of aluminum alloy are major factors in its adoption for aircraft plumbing.
In some special high-pressure (3,000 p.s.i.)
hydraulic installations, corrosion-resistant steel tubing, either annealed or ¼-hard, is used. Corrosionresistant steel tubing does not have to be annealed for flaring or forming; in fact, the flared section is somewhat strengthened by the cold working and strain hardening during the flaring process.
higher tensile strength permits the use of tubing with thinner walls; consequently the final installation weight is not much greater than that of the thicker-wall aluminum alloy tubing.
IDENTIFICATION OF MATERIALS Before making repairs to any aircraft plumbing, it is important to make accurate identification of plumbing materials. Aluminum alloy or steel tubing can be identified readily by sight where it is used as the basic plumbing material. However, it is difficult to determine whether a material is carbon steel or stainless steel, or whether it is 1100, 3003, 5052-0, or 2024-T aluminum alloy.
It may be necessary to test samples of the material for hardness by filing or scratching with a scriber. The magnet test is the simplest method for distinguishing between the annealed austenitic and the ferritic stainless steels. The austenitic types are nonmagnetic unless heavily cold worked, whereas the straight chromium carbon and low alloy steels are strongly magnetic. Figure 5-1 gives the methods for identifying five common Magnet Nitric acid Material test test Carbon steel- - Strongly Slow chemical magnetic. action, brown.
18-8_. Nonmagnetic. No action.
Pure nickel. Strongly Slow action, magnetic. pale green.
Monel-- Slightly Rapid action, magnetic. greenish blue.
Nickel steel__. Nonmagnetic. Rapid action, greenish bluę.
FIGURE 5-1. Identification of metallic materials.
metallic materials by using the magnet and concentrated nitric acid tests.
By comparing code markings of the replacement tubing with the original markings on the tubing being replaced, it is possible to identify definitely the material used in the original installation.
The alloy designation is stamped on the surface of large aluminum alloy tubing. On small aluminum alloy tubing, the designation may be stamped on the surface, but more often it is shown by a color code. Bands of the color code, not more than 4 inches in width, are painted at the two ends and approximately midway between the ends of some tubing. When the band consists of two colors, one-half the width is used for each color.
Painted color codes used to identify aluminum alloy tubing are:
Aluminum aloy number 1100 3003 Green 2014 Gray 2024 Red 5052 Purple 6053 Black 6061 7075 Aluminum alloy tubing, 1100 (/½-hard) or 3003 (½-hard), is used for general purpose lines of low or negligible fluid pressures, such as instrument lines and ventilating conduits. The 2024-T and 5052-0 aluminum alloy materials are used in general purpose systems of low and medium pressures, such as hydraulic and pneumatic 1,000 to 1,500
p.s.i. systems and fuel and oil lines. Occasionally, these materials are used in high-pressure (3,000
p.s.i.) systems.
Tubing made from 2024-T and 5052-0 materials will withstand a fairly high pressure before bursting. These materials are easily flared and are soft enough to be formed with handtools. They must be handled with care to prevent scratches, dents, Corrosion-resistant steel tubing, either annealed or ¼-hard, is used extensively in high-pressure hydraulic systems for the operation of landing gear, flaps, brakes, and the like. External brake lines should always be made of corrosion-resistant steel to minimize damage from rocks thrown by the tires during takeoff and landing, and from careless ground handling. Although identification markings for steel tubing differ, each usually includes the manufacturer's name or trademark, the SAE number, and the physical condition of the metal.
Metal tubing is sized by outside diameter, which is measured fractionally in sixteenths of an inch.
Thus Number 6 tubing is 16 (or ⅜ inch) and Number 8 tubing is 16 (or ½ inch), etc.
In addition to other classification or means of identification, tubing is manufactured in various wall thicknesses. Thus, it is important when installing tubing to know not only the material and outside diameter, but also the thickness of the wall.
FLEXIBLE HOSE Flexible hose is used in aircraft plumbing to connect moving parts with stationary parts in locations subject to vibration or where a great amount of flexibility is needed. It can also serve as a connector in metal tubing systems.
Synthetics Synthetic materials most commonly used in the manufacture of flexible hose are: Buna-N, Neoprene, Butyl and Teflon (trademark of DuPont Corp.). Buna-N is a synthetic rubber compound which has excellent resistance to petroleum products. Do not confuse with Buna-S. Do not use for phosphate ester base hydraulic fluid (Skyrol ®).
Neoprene is a synthetic rubber compound which has an acetylene base. Its resistance to petroleum products is not as good as Buna-N but has better Do not use for phosphate ester base hydraulic fluid (Skydrol ®). Butyl is a synthetic rubber compound made from petroleum raw materials. It is an excellent material to use with phosphate ester based hydraulic fluid (Skydrol ® ).
Do not use with petroleum products. Teflon is the DuPont trade name for tetrafluoroethylene resin.
It has a broad operating temperature range (-65° F. to +450° F.). It is compatible with nearly every substance or agent used. It offers little resistance to flow; sticky viscous materials will not adhere to it. It has less volumetric expansion than rubber and the shelf and service life is practically limitless.
Rubber Hose Flexible rubber hose consists of a seamless synthetic rubber inner tube covered with layers of cotton braid and wire braid, and an outer layer of rubber-impregnated cotton braid. This type of hose is suitable for use in fuel, oil, coolant, and hydraulic systems. The types of hose are normally classified by the amount of pressure they are designed to withstand under normal operating conditions.
1. Low pressure, any pressure below 250 p.s.i.
Fabric braid reinforcement.
Two cotton braids - impregnated with synthetic compound MIL H-8794 Size 6-2/68 Mfg Symbol
- Yellow Single wire braid - Synthetic inner tube A. Flame- and aromatic-resistant hose White numerals, letters and stripe - RED 7 MIL-H-6000-Size- 3-4/68 to Sanbole as (Views showing opposite sides of hose) B. Nonself-sealing, Aromatic and Heat-resistant hose White T Mfg Symbol:
Red numerals and letters C. Flame-, Aromatic-, and Oil-resistant hose Yellow numerals, letters and stripe Red numerals, letters and stripe IL H-5593- D. Nonself-sealing, Aromatic-resistant hose.
E. Self-sealing, Aromatic-resistant hose FIGURE 5-2. Hose identification markings.
2. Medium pressure, pressures up to 3,000
p.s.i. ap One wire braid reinforcement.
Smaller sizes carry pressure up to 3,000
p.s.i.
Larger sizes carry pressure up to 1,500
p.s.i.
3. High pressure (all sizes up to 3,000 p.s.i.
operating pressures).
Identification markings consisting of lines, letters, and numbers are printed on the hose. (See figure
5-2.) These code markings show such information as hose size, manufacturer, date of manufacture, and pressure and temperature limits. Code markings assist in replacing a hose with one of the same specification or a recommended substitute.
Hose suitable for use with phosphate ester base hydraulic fluid will be marked "Skydrol " use". AULIC In some instances several types of hose may be suitable for the same use. Therefore, in order to AULIC make the correct hose selection, always refer to the maintenance or parts manual for the particular airplane.
Teflon Hose ROC Teflon hose is a flexible hose designed to meet LUBRICATION CATA the requirements of higher operating temperatures and pressures in present aircraft systems. It can ILUBRICATION ROC generally be used in the same manner as rubber hose. Teflon hose is processed and extruded into LUBRICATION tube shape to a desired size. It is covered with stainless steel wire, which is braided over the tube for strength and protection.
Teflon hose is unaffected by any known fuel, petroleum, or synthetic base oils, alcohol, coolants, or solvents commonly used in aircraft. Although D D it is highly resistant to vibration and fatigue, the principle advantage of this hose is its operating Size Designation The size of flexible hose is determined by its inside diameter. Sizes are in one-sixteenth-inch increments and are identical to corresponding sizes of rigid tubing, with which it can be used.
Identification of Fluid Lines Fluid lines in aircraft are often identified by markers made up of color codes, words, and geometric symbols. These markers identify each line's function, content, and primary hazard, as well as the direction of fluid flow. Figure 5-3 illustrates the various color codes and symbols used to designate the type of system and its contents.
In most instances, fluid lines are marked with 1-inch tape or decals, as shown in figure 5-4 (A).
WARNING SYMBOL FIGURE 5-3. Identification of aircraft fluid lines.
On lines 4 inches in diameter (or larger), lines in oily environment, hot lines, and on some cold lines, steel tags may be used in place of tape or decals, as shown in figure 5-4 (B). Paint is used on lines in engine compartments, where there is the possibility of tapes, decals, or tags being drawn into the engine induction system.
FUEL FLAM VEL Red FUEL Red FLAM (B) FIGURE 5-4. Fluid line identification using:
(A) tape and decals and (B) metal tags.
In addition to the above-mentioned markings, certain lines may be further identified as to specific function within a system; for example, DRAIN, VENT, PRESSURE, or RETURN.
Lines conveying fuel may be marked FLAM;
lines containing toxic materials are marked TOXIC in place of FLAM. Lines containing physically dangerous materials, such as oxygen, nitrogen, or freon, are marked PHDAN.
The aircraft and engine manufacturers are responsible for the original installation of identification markers, but the aviation mechanic is responsible for their replacement when it becomes necessary.
Generally, tapes and decals are placed on both ends of a line and at least once in each compartment through which the line runs. In addition, identification markers are placed immediately adjacent to each valve, regulator, filter, or other accessory within a line. Where paint or tags are used, location requirements are the same as for tapes and decals.
PLUMBING CONNECTORS Plumbing connectors, or fittings, attach one piece of tubing to another or to system units.
There are four types: (1) Flared fitting, (2) flareless fitting, (3) bead and clamp, and (4) swaged.
The amount of pressure that the system carries is usually the deciding factor in selecting a connector. The beaded type of joint, which requires a bead and a section of hose and hose clamps, is AN819 AN818 nut sleeve Tubing FIGURE 5-5. Flared-tube fitting.
AC
AN FEATURE AC 37° ANGLE 35°
- BODY LENGTH YES RECESS NO BODY LENGTH
COARSER THREADS FINER
BLUE GRAY
OR COLOR OR
BLACK YELLOW BODY
LONGER LENGTH SHORTER
BLUE OR BLACK GRAY OR YELLOW FIGURE 5-6. AN and AC fitting differences.
used only in low- or medium-pressure systems, such as vacuum and coolant systems. The flared, flareless, and swaged types may be used as connectors in all systems, regardless of the pressure.
5-6.) The AC fitting does not have this shoulder.
Other differences between the AC and AN fittings include the sleeve design, the AC sleeve being noticeably longer than the AN sleeve of the same size. Although certain flared-tube fittings Flared-Tube Fittings are interchangeable, the pitch of the threads is A flared-tube fitting consists of a sleeve and a nut, as shown in figure 5-5. The nut fits over the sleeve and, when tightened, draws the sleeve and different in most cases. Figure 5-7 shows the AN and AC811 fittings that can be safely interchanged. Combinations of end connections, nuts, tubing flare tightly against a male fitting to form a seal. Tubing used with this type of fitting must Type End Type Typc be flared before installation. Sizes OD Tube Connection (Male (Female Sleeve Nut Type Flare Tub‹ The male fitting has a cone-shaped surface with Thread) Thread) the same angle as the inside of the flare. The sleeve supports the tube so that vibration does All Sizes All Sizes ANI 8112 ANI 8112 ANI 8112 AN 8112 not concentrate at the edge of the flare, and All Sizes AN AN AN 811 All Sizes AN AN 811 811 distributes the shearing action over a wider area All Sizes AN AN 811 for added strength. Tube flaring and the instal- All Sizes 811 811 811 All Sizes 811 811 AN lation of flared-tube fittings are discussed in detail All Sizes 811 811 AN 811 later in this chapter. AN 811 AN 811 The AC (Air Corps) flared-tube fittings have 78,16> 4. 516, 1¾, 2 16, 14, 2 AN AN 811 811 811 AN AN been replaced by the AN (Army/Navy) Standard 78, 16. 4, 16, 14, 2 AN 811 811 and MS (Military Standard) fittings. However, 1 This is the normal assembly of AN fittings.
since AC fittings are still in use in some of the 2 This is the normal assembly of AC811 fittings.
older aircraft, it is important to be able to identify them. The AN fitting has a shoulder between the FIGURE 5-7a. Interchangeability of AN and AC811 end of the threads and the flare cone. (See figure fittings.
TYPICAL CONNECTION CT 45 MT OT FIGURE 5-7 b. AC811 solderless fittings.
- AN744 to AN932 —- Material:
Aluminum alloy. (code D) Steel.. (code, absence of letter) Brass- (code B) Aluminum bronze. (code Z-for AN819 sleeve) Size:
The dash number following the AN number indicates the size of the tubing (or hose) for which the fitting is made, in 16ths of an inch. This size measures the O. D. of tubing and the I. D. of hose.
Fittings having pipe threads are coded by a dash number, indicating the pipe size in 8ths of an inch.
The material code letter, as noted above, follows the dash number.
BOLT AN775 ELBOW AN777 BOLT AN774 ELBOW AN776 ELBOW AN778 TEE AN779 PLUG AN806 TEE AN804 PLUG AN814 ADAPTOR AN807 UNION AN815 CAP AN820 NUT AM817 SLEEVE NUT AN818 AN819 NIPPLE AN816 ELBOW AN821 ELBOW AN822 ELBOW AN823 TEE AN824 FIGURE 5-8. AN plumbing fittings.
TEE AN826 TEE CROSS AN827 AN825 UNION AN832 TEE ELBOW AN833 AN834 ELBOW AN837 ELBOW AN839 ELBOW AN838 AN840 HOSE NIPPLE HOSE NIPPLE AN841 * HOSE ELBOW AN842 HOSE ELBOW AN843
HOSE ELBOW AN844 ELBOW AN847 · ELBOW AN848 * ELBOW AN845 *AN867 ELBOW AN846 *AN871 BUSHING CONNECTOR AN849 AN893 BUSHING AN894 * Inactive for new design.
FIGURE 5-8. AN plumbing fittings. Continued 106
AN COUPLING NIPPLE AN911 BUSHING AN912 PLUG AN913 ELBOW AN914 AN910 ELBOW AN915 ELBOW AN916 TEE AN917 REDUCER AN919 NUT AN924 CROSS AN918 TEE AN926 ELBOW AN927 Y AN ELBOW AN928 CROSS AN925 PLUG AN932 CAP AN92 FIGURE 5-8. AN plumbing fittings.-Continued sleeves, and tube flares are allowed to make up a complete fitting assembly. The use of dissimilar metals should be avoided since their contact will cause corrosion.
straight threads, but they have different pitch for the various types.
Flared-tube fittings are made of aluminum alloy, steel, or copper base alloys. For identifi- When combining AC and AN end connections, nuts, sleeves, or tube flares, if the nut will not move more than two threads by hand, stop and investigate for possible trouble.
The AN standard fitting is the most commonly used flared-tubing assembly for attaching the tubing to the various fittings required in aircraft plumbing systems. The AN standard fittings include the AN818 nut and AN819 sleeve. (See figure 5-8.) The AN819 sleeve is used with the AN818 coupling nut. All these fittings have cation purposes, all AN steel fittings are colored black, and all AN aluminum alloy fittings are colored blue. The AN 819 aluminum bronze sleeves are cadmium plated and are not colored The size of these fittings is given in dash numbers, which equal the nominal tube outside diameter (O.D.) in sixteenths of an inch.
Threaded flared-tube fittings have two types of ends, referred to as male and female. The male end of a fitting is externally threaded, whereas the female end of a fitting is internally threaded.
Cone angle Sleeve pilot Sleeve Tube shoulder. Bevel Tube Sleeve cutting edge Body • Nut FIGURE 5-9. Flareless-tube fitting.
The MS (Military Standard) flareless-tube fittings are finding wide application in aircraft plumbing systems. Using this type fitting eliminates all tube flaring, yet provides a safe, strong, dependable tube connection. The fitting consists of three parts: a body, a sleeve, and a nut. The body has a counterbored shoulder, against which the end of the tube rests. (See figure 5-9.) The angle of the counterbore causes the cutting edge of the sleeve to cut into the outside of the tube when the two are joined. Installation of flarelesstube fittings is discussed later in this chapter.
Quick-disconnect Couplings Quick-disconnect couplings of the self-sealing type are used at various points in many fluid systems. The couplings are installed at locations where frequent uncoupling of the lines is required for inspection and maintenance.
Quick-disconnect couplings provide a means of quickly disconnecting a line without loss of fluid or entrance of air into the system. Each coupling assembly consists of two halves, held together by a union nut. Each half contains a valve that is held open when the coupling is connected, allowing fluid to flow through the coupling in either direction. When the coupling is disconnected, a spring in each half closes the valve, preventing the loss of fluid and entrance of air.
The union nut has a quick-lead thread which permits connecting or disconnecting the coupling by turning the nut. The amount the nut must be turned varies with different style couplings. One style requires a quarter turn of the union nut to lock or unlock the coupling while another style requires a full turn.
Some couplings require wrench tightening;
others are connected and disconnected by hand The design of some couplings is such that they must be safetied with safety wire. Others do not require lock wiring, the positive locking being assured by the teeth on the locking spring, which engage ratchet teeth on the union nut when the coupling is fully engaged. The lock spring automatically disengages when the union nut is unscrewed. Because of individual differences, all quick disconnects should be installed according to instructions in the aircraft maintenance manual.
Flexible Connectors Flexible connectors may be equipped with either swaged fittings or detachable fittings, or they may be used with beads and hose clamps. Those equipped with swaged fittings are ordered by correct length from the manufacturer and ordinarily cannot be assembled by the mechanic.
They are swaged and tested at the factory and are equipped with standard fittings.
The fittings on detachable connectors can be detached and reused if they are not damaged;
otherwise new fittings must be used.
The bead and hose clamp connector is often used for connecting oil, coolant, and low-pressure fuel system tubing. The bead, a slightly raised ridge around the tubing or the fitting, gives a good gripping edge that aids in holding the clamp and hose in place. The bead may appear near the end of the metal tubing or on one end of a fitting.
TUBE FORMING PROCESSES Damaged tubing and fluid lines should be replaced with new parts whenever possible. Sometimes replacement is impractical and repair is necessary. Scratches, abrasions, and minor corrosion on the outside of fluid lines may be considered negligible and can be smoothed out with a burnishing tool or aluminum wool. Limitations on the amount of damage that can be repaired in this manner are discussed later in this chapter under "Repair of Metal Tube Lines." If a fluid line assembly is to be replaced, the fittings can often be salvaged; then the repair will involve only tube forming and replacement.
Tube forming consists of four processes: (1) Cutting, (2) bending, (3) flaring, and (4) beading.
If the tubing is small and of soft material, the assembly can be formed by hand bending during installation. If the tubing is ¼-inch diameter, or larger, hand bending without the aid of tools is impractical.
Tube Cutting When cutting tubing, it is important to produce a square end, free of burrs. Tubing may be cut with a tube cutter or a hacksaw. The cutter can be used with any soft metal tubing, such as copper, aluminum, or aluminum alloy. Correct use of the tube cutter is shown in figure 5-10.
Cutting a tube open side of sues Tube Clean cut → FIGURE 5-10. Tube cutting.
A new piece of tubing should be cut approximately 10 percent longer than the tube to be replaced to provide for minor variations in bending.
Place the tubing in the cutting tool, with the cutting wheel at the point where the cut is to be made.
Rotate the cutter around the tubing, applying a light pressure to the cutting wheel by intermittently twisting the thumbscrew. Too much pressure on the cutting wheel at one time could deform the tubing or cause excessive burring. After cutting the tubing, carefully remove any burrs from inside and outside the tube. Use a knife or the burring edge attached to the tube cutter.
When performing the deburring operation use extreme care that the wall thickness of the end of the tubing is not reduced or fractured. Very slight damage of this type can lead to fractured flares or defective flares which will not seal properly.
fine tooth file can be used to file the end square and smooth.
If a tube cutter is not available, or if tubing of hard material is to be cut, use a fine-tooth hacksaw, preferably one having 32 teeth per inch. The use of a saw will decrease the amount of work hardening of the tubing during the cutting operation.
After sawing, file the end of the tube square and smooth, removing all burrs.
An easy way to hold small-diameter tubing, when cutting it, is to place the tube in a combination flaring tool and clamp the tool in a vise. Make the cut about one-half inch from the flaring tool. This procedure keeps sawing vibrations to a minimum and prevents damage to the tubing if it is accidentally hit with the hacksaw frame or file handle while cutting. Be sure all filings and cuttings are removed from the tube.
Tube Bending The objective in tube bending is to obtain a smooth bend without flattening the tube. Tubing under one-fourth inch in diameter usually can be bent without the use of a bending tool. For larger sizes, a hand tube bender similar to that shown in figure 5-11 is usually used.
To bend tubing with the hand tube bender, insert the tubing by raising the slide bar handle as far as it will go. Adjust the handle so that the full length of the groove in the slide bar is in contact with the tubing. The zero mark on the radius block and the mark on the slide bar must align. Make the bend by rotating the handle until the desired angle of bend is obtained, as indicated on the radius block.
Bend the tubing carefully to avoid excessive flattening, kinking, or wrinkling. A small amount of flattening in bends is acceptable, but the small diameter of the flattened portion must not be less than 75 percent of the original outside diameter. Tubing with flattened, wrinkled, or irregular bends should not be installed. Wrinkled bends usually result from trying to bend thin-wall tubing without using a tube bender. Examples of correct and incorrect tubing bends are shown in figure 5-12.
Tube bending machines for all types of tubing are generally used in repair stations and large maintenance shops. With such equipment, proper bends can be made on large diameter tubing and on tubing made from hard material. The production tube bender is an example of this type of machine.
The ordinary production tube bender will accommodate tubing ranging from /2-inch to 12- inch outside diameter. Benders for larger sizes are available, and the principle of their operation is similar to that of the hand tube bender. The radius blocks are so constructed that the radius of bend will vary with the tubing diameter. The radius of bend is usually stamped on the block.
When hand or production tube benders are not available or are not suitable for a particular bending operation, a filler of metallic composition or of dry sand may be used to facilitate bending.
When using this method, cut the tube slightly longer than is required. The extra length is for inserting a plug (which may be wooden) in each end.
After plugging one end, fill and pack the tube with fine, dry sand and plug tightly. Both plugs must be tight so they will not be forced out when the bend is made. The tube can also be closed by flattening the ends or by soldering metal disks in them. After the ends are closed, bend the tubing 109