Aluminum Tubing strength for Mechanics
The larger the tubing diameter, the less pressure it can withstand.
A 1/4 inch (0.025) aluminum tubing can hold 3,500 psi of pressure. The same aluminum tubing, but in 1/2 inch can only hold 1,800 psi. If we made a business jet pressurized fuselage out of the same tubing, it could only handle 182 psi.
When working with large pressure vessels, such as aircraft fuselage, don't be fooled by the low pressures. Because of their large size, these pressure vessels are under a lot of stress.
Tubing and hose can be thought of as a cylindrical thin-walled pressure vessel.
The strength of thin walled pressure vessels is determined by:
1. The material strength
2. The wall thickness, and
3. The size of the tubing.
The formula is: strength, psi = yield*(wall thickness/radius)
This last item, tubing size, is unusual. One can understand how strength is related to how strong the material is and how thick it is but size (radius)? The relationship between tubing size and strength is inverse; the larger the tube diameter the less strength it has. When you look at pressure ratings for tubing and hose you will notice that for the same hose, maximum recommended operating pressure goes down as the size goes up.
You can use this property to your advantage. For example, you might have a choice of tubing or hose size for a particular application. Everything else being equal, a smaller diameter line holds more pressure than a larger diameter line. Another advantage is that a smaller size weights less.
When you inspect a hose or line, you are inspecting a pressure vessel. As with all pressure vessels, they should be protected from damage that reduces the wall strength. Inspect for nicks, cuts, chafing, and corrosion. Make sure that the line does not vibrate.
What happens when we bend tubing?
Bending reduces strength due to a reduction in wall thickness. The extrados (outside radius) of the bend stretches and become thinner, while the intrados (inside radius) compresses and gets thicker. The tighter the bend, the greater reduction in wall thickness.
Comparing ultimate tensile strength of three common aluminum tubing alloys used on small aircraft: 5052-0, 6061-0, and 3003-0
Most tubing failures on aircraft are caused by fatigue. You are warned that this chart does not compare fatigue strength.
Tubing on aircraft vibrates. How well your tubing endures when subjected to load reversals, impulses, and vibration is called "fatigue strength". Of the three tubing types shown in the chart, 5052-0 has the best fatigue strength.
Originally aircraft used soft copper tubing. There is even some of this still around. Although copper was strong enough, it was replaced with aluminum and stainless tube because of the the high fatigue failures of copper. For lower pressures, 5052-0 became the tubing of choice because it has the best fatigue strength of any of the non heat-treat aluminum alloys.
Copper tubing on older aircraft should be removed and replaced with 5052-0 before it breaks. There is no warning when copper tubing breaks. One cannot "inspect" it and declare it ok.
Some experimental and light-sport aircraft have hydraulic and fuel lines built with 6061-0 or 3003-0 tubing. Low ultimate tensile strength and low fatigue strength provide a narrow safety margin in dynmic (vibration or impulse) applications. Take extra care in clamping and preventing tube vibration. The aircraft industry's long experience with copper tubing failures proved the importance of fatigue strength. 5052-0 has higher ultimate tensile strength and higher fatigue strength at a small price difference. Both 5052-0, 3003 have the same Cold Workability Rating of A (easy to work with). 6061-T6 has a far lower rating of C.
The aluminum hydraulic lines on the Cessna 404 have experienced 5 reported failures due to metal fatigue. For the mechanic, this means that these lines cannot be inspected for fatigue failure. They will not show fatigue stress before failure. A replacement interval is the only method of prevening failure. The old adage that "if it flew in it will fly out" only works until the next failure. See NTSB Safety Board Recommendation A-83-1-2. Metal fluid lines in aircraft subject to vibration have a potential to fail due to metal fatigue. Using the proper alloy tube, combined with good fabrication techniques, and proper clamping, and hard-time replacement interval is the only protection from sudden failure due to metal fatigue.
Tubing is sized by the Outside Diameter
(not to be confused with pipe that is sized by the inside diameter)
The tubing used to produce rigid tubing assemblies is sized by its outside diameter (OD) and wall thickness. Outside diameter sizes are in sixteenth-of-an-inch increments, the number of the tube indicating its size in sixteenths of an inch. For example, the number 6 tubing is 6/16 or 3/8 inch, number 8 tubing is 8/16 or 1/2 inch, and so forth. Wall thickness is specified in thousandths of an inch.
Typical wall thickness for 5052-0 temper fluid tubing is .035 inch.
Aluminum Tubing Identification
5050-0 temper aluminum tubing (.035 wall thickness) is used on a large number of light aircraft for wheel brake and gear retract hydraulic systems, and fuel systems. In the "0" condition, 5052 is soft and workable. The ends are easily flared and 5052 has higher fatigue strength than most aluminum alloys.
Be careful when replacing tubing that you are using the same type of tubing that you are replacing. 5050-0 is for low pressure systems only. In some applications such as light aircraft fuel injector lines, a stronger stainless steel tubing is used. Some hydraulic systems also use tubing other than 5052-0.
The use of aluminum alloy tubing is limited in certain areas of airborne hydraulic systems by MIL-H-5440. Consult the applicable drawing or illustrated parts breakdown to determine the correct tubing for a particular system. Tubing that conforms to Federal Specifications WW-T700/ 1 and WW-T-700/6 will not be used in hydraulic systems.
Aluminum Alloy Tubing. Aluminum alloy tubing that conforms to Federal Specification WW-T-700/1 is used for general-purpose lines and conduits of low or negligible fluid pressure, such as instrument lines and electrical and ventilating conduits.
Aluminum alloy tubing that conforms to Federal Specifications WW-T-700/4 and WW-T-700/6 is the most widely used for general-purpose lines of low and medium pressures.
It is easily flared and soft enough to be formed with hand tools. Handle it with care to prevent scratches, dents, and nicks. Aluminum alloy tubing is used with either of two connection types: a flared joint for mechanical connectors or the beaded end for clamps and flexible hoses. In hydraulic systems, 5052-O aluminum alloy tubing that conforms to Federal Specification WW-T-700/4 is used for reduced pressure (1,500 psi maximum) and return lines. For high-pressure lines (3,000 psi), MIL-T-7081 aluminum alloy tubing (6061) is in considerable use.
Stainless Steel Tubing. Corrosion-resistant steel (CRES) tubing, MIL-T-8504 and MIL-T-6845, is used in high-pressure hydraulic systems (3,000 psi) such as landing gear, wing flaps, and brakes. It is preferred that external brake lines be made of stainless steel components (i.e., tubing, sleeves, nuts) to prevent damage caused by flying gravel and stones and ground-handling accidents. In other systems where stainless steel tubing is used (regardless of the tube size), it is preferred that only stainless steel nuts be used. This will minimize sleeve and nut cracking, which may be induced by overtorque of "B" nuts and vibration. CRES tubing does not have to be annealed for flaring or forming. In fact, the flared section is somewhat strengthened by the cold working and consequent strain hardening. The high tensile strength of stainless steel tubing permits the use of a thinner wall than does aluminum alloy tubing. Therefore, the weight is about the same as thicker-walled aluminum alloy tubing.
Note: WW-T-700 is now AMS-WW-T-700