Hydraulic lines is leaking. Slow dripping on your shop floor is not due to too much hydraulic pressure but a poorly installed line. A tech simply used force instead of technique, or they guessed the fitting size.
The SAE J514 Standard for JIC fittings take the guessing out of it and makes assembling a hydraulic hose a very precise engineering task. Each part serve a specific purpose. That’s where the thirty-seven-degree flare come into play for the system. As shown in the chart above, it forms a seal with the geometry. Pipes use compound or tape to close gaps in their threads, but here cone seat of metal-to-metal seal depends on near-perfect mating surfaces. The flaring of the tube end has to match exactly the shape of body of the fitting.
How to Stop Hydraulic Leaks
The result is binary. Either the angles line up and you have a solid seal…or they don’t and there will be a leak. The design can handle three thousand PSI without any degrading gasket-type seals.
Understanding their dash numbers system are important for selecting hardware. These is not random numbers. They correspond directly to outer diameter of tubing you are using. For example, a Dash 2 will accept an eighth-inch of tubing. A Dash 32 accepts two-inch tubing. Because larger tubes also has thinner walls with less structure, they can’t handle as much pressure. In general, stainless steel hold less pressure than steel of the same size.
Match the outer diameter of your tube to your dash size first. That determines everything else. And there it goes….out of control. Tools are everything. If you have a good flare tool, manual or hydraulic press, then the result is consistently good. If you don’t use the correct die set, your flare won’t be even and therefore won’t seal propery.
How tight? Not tight enough to feel like the nut is going to break loose but just snug. Torquing too hard distort the soft metal in the flare and causes little cracks in the metal which become leaks over time with vibration. Flats From Finger Tight is the name of the industry standard for tightness. That sounds vague but really isn’t. It means you take the nut and thread it on by hand until it’s seated snug. Next you pick up your wrench and give it one flat (a specific amount depending off the size). Generally, smaller fittings -2 through -4 is only a single flat. A little larger fittings is a single and a half flats. And then they go up to two flats for larger ones.
The nice thing about this approach is you get the same clamping force every time and don’t have to depend on torque wrenches which can vary from shop to shop. For comparison, here’s the difference between JIC and some other standards. Aerospace tends to go with the same-looking but different-pitch AN fitting. ORFS are sometimes used where vibration is high. They have a flat face and use an O ring as a seal rather than a metal flare. Hydraulic power transmission doesn’t work well with NPT which uses taper threads that depend upon tape and compression. People commonly mix standards, they look alike but don’t work more better.
You want a system that will hold pressure when under stress. Whether it’s a custom race car or industrial machinery, it’s all about the principles. Follow the rules, respect the flare angle. Size the dash to match the tube. Tighten with precision not power. Do this and the leaks goes away. Then the system does what it was designed to do. That’s why pro hydraulic work stays clean and reliable for years.
