"Tight" is not necessarily leaktight

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