To quote Air Force Systems Command/Space Division (AFSC/SD), SD Regulation 540-9:

“1. Background. In numerous aerospace programs, standard AN, MS, and MC tubular flare fittings are used as separable plumbing joints. These have been found to be a potential source of single-point failures. Metal-to-metal seal integrity depends upon maintaining locally high clamping stress. Required force is generated and transmitted across the mated parts by application of torque to the threaded elements. Tests have shown that some threaded fittings torqued to a proper sealing value tend to undergo strain deformation and leaks may develop with time, temperature cycling, or vibration…”


A typical separable fluid fitting is dependent on coupling nut preload to provide the clamping stress required to achieve and maintain a seal. The amount of preload is directly proportional to the number of degrees of arc the nut is tightened following mating surface contact. This angle must be selected to provide sufficient stress to seal and prevent loosening but not to approach the yield strength of any of the mating parts. Torque is not necessarily indicative of preload stress due to the effects of mating surface friction.

A typical size of -06 fluid fitting with .5625-18 UNJF-3 threads requires 30º of preload angle. At this level of preload, based on actual measurements, the coupling nut stretches .002″. If all the mating parts were properly designed and machined and no other factors were involved this joint would be adequately stressed and the fluid fitting would never loosen or leak.

However other factors are involved. When the coupling nut is tightened it has a tendency to impart a twisting motion to the hose or tube on which it is installed. The degree of twisting is greater at higher preload angles and with higher levels of friction between the pressure faces of the coupling nut and mating part. Worn coupling nuts or coupling nuts with insufficiently lubricated pressure faces impart more twist to the tube. Consequently, the hose or tube is constantly exerting a torsional force against the coupling nut, in the CCW loosening direction. Secondly, if the mating parts are subjected to varying levels of temperature, thermal expansion can cause a reduction in clamping stress applied to the pressure surfaces.

For example, take our size -06 fluid fitting manufactured from Inconel 625. The linear coefficient of thermal expansion of this material within a typical operating temperature range is 7.5 X 10 –6 inch per inch per º F. This coupling nut is .725″ long. Therefore, if this coupling nut is heated by 368ºF, it will lengthen .002″. In a fluid fitting the male threaded coupling, the tubes, and the ferrule, (if used) are all in contact with the fluid. The coupling nut is never in contact with the fluid. In many applications (i.e. aircraft engine nacelle area) the fluid fitting is subjected to a considerable amount of external heat. The fluid is cooling the coupling and tubes. The nut is cooled by contact across the mating surfaces only, which is a relatively small area and frequently coated with dry film lubricant. The coupling nut will be hotter that the mating parts. If this temperature delta reaches 368º F. the nut will expand by .002″ more that the mating parts. This is equal to the .002″ stretch imparted by the tightening angle preload, and the clamping stress will no longer exist. Also note that radial expansion will occur, which further amplifies the problem. While this temperature delta is usually not reached and the lessening of clamping stress is usually insufficient to cause substantial leaking in itself, it definitely contributes to coupling nut loosening which will cause substantial if not catastrophic leakage.

Vibration can also cause loosening even if a joint is tightened or maintained at an adequate level of preload. The continual flexing of all contact surfaces of the fitting as a result of vibration causes microyeilding and strain deformation. Eventually this will become sufficient to reduce the clamping forces to a level below that required to keep the coupling nut tight. Testing indicated that greater vibratory acceleration levels due to amplitude or frequency increases would cause loosening in lessor numbers of vibratory cycles.

Other factors which can also accelerate coupling nut loosening include residual linear and axial stress due to tube misalignment, dynamic torsional loading caused by adjacent bends in the tube vibrating in a non-coaxial vector, pressure pulsation, and micro-yielding of the material at mating surfaces of the fluid fitting. In combination with the factors described above even properly designed and adequately preloaded fluid fittings can and do loosen and leak.


First, to address the tube twist-up problem multiple actions should be considered. The use of oil on the threads and pressure surfaces of the coupling nut/ferrule or compression sleeve interface is important. When the joint is tightened this area is subjected to the highest level of stress as it has comparatively the least surface area. If possible, at least one of the surfaces should have oil re-applied frequently, ideally every time the joint is separated. Even though the pressure surfaces of the coupling nut may inaccessible once installed on the tube, oil should still be used on the threads and if possible to the back of the nut where the tube exits.

Dry film lubricants are often used on coupling nuts. When the nuts are subjected to high temperatures silver plating can be effective. In both cases it is important that the dry film lubricant or silver plating is applied to the pressure flank of the thread as well as the pressure face inside the nut. The use of oil is still recommended even with dry film lubricant or silver plating.

Another technique to minimize the effects of twist-up is to loosen the adjacent tube clamps prior to tightening the fluid fitting and then re-tighten them after the fluid fitting has been tightened. This effectively lengthens the torsional beam and lowers the torsional stress against the coupling nut.

The next contributing factor to loosening is thermal expansion. The obvious approach here is to avoid the use of a coupling nut material with a higher coefficient of thermal expansion that the coupling and ferrule. Ideally all fluid fittings components should be manufactured from the same material or materials with similar coefficients of thermal expansion.

Vibration is the most apparent contributing factor in loosening of all types of threaded fasteners, fluid fitting included. During our vibration testing we have found typical fluid fittings and tube sections can have an extremely high “Q” factor. At one point we measured acceleration at the fluid fitting of over 500 G with an input acceleration of less that 3 G. We also found test specimens can be induced to vibrate or vibrate at increased amplitude when subjected to sonic input at the specimen’s resonant frequency. We also found the tube clamps to be of prime importance in respect to the specimen’s Q factor. A tube clamp design incorporating a damping material or configuration would greatly help to reduce the vibratory acceleration and thus loosening of the fluid fittings. Additionally, if practical, the tube clamps adjacent to the fluid fitting should be spaced such that the resonant frequency of the active span including the fluid fitting is well outside of the natural frequency of the system to which the fluid fittings are connected.

Finally, it is important to follow good maintenance practice while servicing the fluid fittings and lines. Misalignment should be avoided. If possible, tightening should be done by angle rather than torque because the measured torque values are highly dependent on the coefficient of friction of the mating surfaces. Once a typical fluid fitting has been used several times, especially if there is no lubrication on the pressure surfaces, the coefficient of friction will rise. Repeated tightening of the joint to a specific torque level will result in increasingly less angular preload of the coupling nut. This effect is magnified as the torque level is increased. Over-tightening a fluid fitting causes galling of the pressure surfaces and therefore subsequent re-tightening, even to the increased torque level, will result in less angular preload at the joint. This will contribute to joint loosening. Lubricating the coupling nuts every cycle greatly extends their life and improves the correlation between tightening torque and angular preload.

In conclusion, many conditions can affect the integrity of a threaded fluid fitting. Proper component design and maintenance practices can help to reduce the effects of some of these conditions. However, some fluid fittings will always be subjected to adverse environmental conditions, and human factors will always be present.

For many years the practice has been to apply lockwire to critical fluid fittings. This is known as a secondary locking feature (correct tightening is considered the primary). The concept of preventing coupling nut back-off by the means of a secondary locking feature is a good one. However, lockwire is widely recognized as a leading cause of Foreign Object Damage (FOD) and it is not conducive to easy maintainability. In some applications lockwire will fail due to fatigue cracking. Other secondary locking features for fluid fittings include thread locking compounds, deformed threads, and various clips and tab washer type devices. These products all offer limited or zero reusability, and/or require the user to perform an additional action, and/or are a loose part and potential FOD source. The industry-leading Moeller Click-LocÔ is a secondary locking feature with very high reusability, no loose parts, no additional action required, and an astounding perfect record of reliability – Zero Failures in over 13 years and hundreds of millions of component service hours.

This report was compiled by Moeller Mfg. Co., Inc., and is based on testing performed at Entela Laboratories, Inc., (an independent testing facility) as well as information from other sources including the U.S. Air Force, the Society of Automotive Engineers, and various commercial concerns.

Copyright 2006
Moeller Mfg. Co., Inc.
30100 Beck Rd.
Wixom, MI 48393
Tel: 248-960-3999
Fax: 248-960-1593

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