Machine Design

"Tight" is not necessarily leaktight

Keeping separable fluid fittings secure can be a challenge. Here are a few helpful tips.

Kurt Bynum
Vice President,
New Product Development
Moeller Mfg. Co. Inc.
Wixom, Mich.

Jet engines use secondary locking mechanisms to secure fluid fittings.

Most separable fluid fittings are of a flare or flareless design. These, as well as some face-sealing types, rely on metal-to-metal contact between the sealing surfaces of a male connector and a tube or hose end. Typically, a coupling nut applies compressive clamping stress or preload that maintains the metal-to-metal seal. This preload also keeps the coupling nut from backing off. The amount of preload varies directly with the number of degrees of arc that the nut is tightened beyond finger tight, after the sealing surfaces touch, or about 30° in most cases.

Though it is common practice to tighten coupling nuts to a spec-ified torque value or range, torque is not necessarily indicative of preload stress. One reason: Friction at the mating surfaces can lower preload stress for a given tightening torque. A torque wrench simply sums torque from preload and friction and cannot distinguish between the two. Potential sources of friction include the threads and the pressure face where the bottom internal face (or thrust wire for swivel nuts) of the coupling nut touches the back of the ferrule.

Repeated tightening of a coupling nut to a specified torque lessens angular preload with each on-off cycle. Angular pre-load will eventually drop to a level where it cannot maintain a metal-to-metal seal and keep the nut from backing off. Overtightening boosts preload, but also accelerates wear and galling of threads and pressure surfaces, aggravating the problem in subsequent cycles. Extreme over-tightening can permanently distort the nut and sealing surfaces, ruining the fitting.

The tightening of a coupling nut imparts twisting force to the hose or tube on which it is installed. Consequently, the hose or tube exerts a constant CCW (loosening) torque against the coupling nut. This "twist-up" comes from friction between the pressure faces, as described above. Higher preload angles and friction levels, as well as worn coupling nuts or galling at the pressure faces, all contribute to twist-up.

For best results, tighten coupling nuts by angle rather than torque, whenever possible. This ensures the correct amount of preload, regardless of friction levels. Minimize twist-up by loosening the adjacent tube clamps before tightening the fluid fitting. Then, retighten the clamps after the fitting has been tightened. This effectively lengthens the torsional beam and lowers torsional stress against the coupling nut. Avoid misalignment, which can lead to loosening of fittings and introduce residual linear and axial stress.

Be sure to use lubricating oil on threads and on the nut back where the tube exits. Oil helps lower friction and improves the correlation between measured torque and actual pre-load, extends component life, and minimizes twist-up. Oil should be reapplied each time a joint is separated. Use system fluid as a lubricant when lubricating oil is incompatible with the system fluid itself, because this is still preferable to dry assembly.

The use of dry-film lubricants further improves lubricity. Silver-plating works for fittings run at temperatures exceeding 600°F (316°C). Apply dry-film lubricant or silver-plating to the thread pressure flank as well as the pressure face inside the nut. For reference, the pressure flank is the one you don't see when peering into the threaded end of the nut. The pressure face is the bottom inside face adjacent to the hole where the tube exits. Be sure to use liquid lubricant in addition to dry films or silver plating.

Vibration can loosen a fluid fitting even when it is properly preloaded. Vibration continually flexes the contact surfaces, causing local microyielding and strain deformation. Eventually, this action lowers clamping stress to a level below that which is needed to keep the coupling nut tight. Vibration tests show that higher acceleration levels — coincident with increases in vibration amplitude or frequency — tend to loosen fittings in fewer vibratory cycles, as expected.

Resonance can greatly amplify the effects of even moderate vibration input. Rigidly mounted tube sections are especially vulnerable to resonance. Vibration tests of one particular tube assembly measured over a 500 g acceleration at an input acceleration of less than 3 g. Resonant response is not limited to tube assemblies. Hoses become increasingly rigid with pressure and are more prone to resonate than those operated at low pressure.

In addition to direct vibrational input, certain test specimens can be made to vibrate when subjected to acoustic and hydraulic inputs. For example, a hydraulic pump running at 6,000 rpm (100 Hz) or a hydraulically actuated vane fluttering at 100 Hz can trigger resonance in a connected hose or tube section that has a comparable natural frequency, even when the components are located relatively far apart.

In practice, designs should minimize vibrations that can hurt integrity of fluid-fitting connections. Tube clamps, for example, should incorporate damping material. Try to space tube clamps adjacent to a fluid fitting such that the resonant frequency of the active span, including the fluid fitting, is well outside the natural frequency of the system to which the fittings connect.

Thermal expansion is yet another source of preload loss. In some applications, such as an aircraft-engine nacelle, fluid fittings see a considerable amount of external radiant heat.

Consider a Size -06 fluid fitting made of Inconel 625. At a 30° preload angle, the 0.725-in.-long coupling nut stretches 0.002 in. Inconel 625 has a linear coefficient of thermal expansion of about 7.5 10 6 in./in.-F°. The fluid — fuel in this case — helps cool parts that touch it. But the nut itself does not touch the fluid and is instead exposed to radiant heat from the engine casing. Should the temperature difference between the cooled and noncooled parts reach 368°F, the nut will expand 0.002 in. more than the mating parts. This expansion equals the stretch from preload, so clamping stress goes to zero and the fitting can leak. Heating the nut also expands it radially, further adding to the problem.

Such high temperature differentials are unlikely in practice, though even moderate levels of radiant heating may loosen coupling nuts and cause leaks. Note that Inconel 625 has a fairly low thermal-expansion coefficient compared with other common coupling-nut materials, including 300-Series stainless steels, brass, and aluminum.

Avoid using coupling-nut materials with a higher coefficient of thermal expansion than that of the male fitting and ferrule. Ideally, fluid-fitting components in a system should be of the same material, or materials with similar coefficients of thermal expansion. When large temperature differences exist between the fluid and external environment, consider thermal-management strategies such as heat shields and supple-mental cooling to stop excess heating of coupling nuts.

When failure is not an option

Click-Loc fittings on fuel manifolds of a Pratt & Whitney PW127 turboprop engine.

Mission-critical applications such as jet-engine-fuel manifolds typically incorporate a secondary locking mechanism to secure fluid fittings. Lockwire was once the method of choice and still finds use in many cases. But lockwire is widely recognized as a leading cause of foreign object damage (FOD). It's also difficult to maintain and only minimizes the chance of loosening, but doesn't prevent it. This is because (ideally) lockwire is not installed in tension. Lockwire can fatigue crack and fail in some cases, especially when it is under tension. For these reasons, many newer military weapons systems specify "no lockwire."

Other secondary locking techniques for fluid fittings include safety cable (similar to lockwire but made from cable with crimpedon ends), thread-locking compounds, deformed threads, clips, and tab washers. Most of these products can't be reused, and clips and tab washers are yet another potential FOD source.

Fuel manifolds for PW127 turboprop engines from Pratt & Whitney Canada Corp., use Moeller Click-Loc technology on the fluid couplings. Fingerlike springs made of Inconel 718 engage a multitooth cam of the same material on the mating fitting, securely locking the two halves together.

Click-Loc secondary locking technology can be integrated into fittings or added to existing ones. Click-Loc-equipped fittings can be reused 100 times or more before replacement and have a perfect record of reliability (no failures) in hundreds of millions of component service hours.

Photo: Jean Claude Belanger

Moeller Mfg. Co. Inc.,

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