You're Over-Tightening Hydraulic Fittings

Published on: 11:25AM Apr 11, 2010
 I've seen a steady parade of broken or leaky hydraulic hoses through the parts department in the past weeks. Spring must finally have arrived. Here are a few tips on correctly reconnecting those hoses once you get them repaired:

-Use the correct fittings. Plain ol' National Pipe Tapered (NPT) pipe fittings are designed for low-pressure fluids of 300 psi or less. They are NOT designed for, nor safe, when installed in high-pressure hydraulic situations. National Pipe Tapered Fuel (NPTF) hydraulic connectors are physically similar to NPT fittings, but are plated with zinc dichromate, which gives them a silver or gold tint. NPTF fittings ARE safe for use in high-pressure applications. If you don't know for certain if a fitting is NPT or NPTF, DON'T USE IT IN A HIGH-PRESSURE HYDRAULIC APPLICATION.

-When installing an NPTF fitting in a high-pressure situation, don't tighten the fitting then back it off to align it with a hose, line or other fitting. Because of the unique design of NPTF threads, those fittings should be tightened into alignment, and never loosened.

-Straight-Thread O-Ring Fittings are the ones you thread into place, then tighten a nut on the fitting against an 0-ring built into the fitting. FYI, to 'test' an old, used straight-thread o-ring fitting, shake the fitting. If the washer slides back and forth on the fitting and clicks, it's worn out. To correctly install a straight-thread o-ring fitting, twist the retaining nut away from the washer and o-ring then install the threaded portion of the fitting until it is hand-tight. You can can "back-off" this sort of fitting from hand-tight up to one revolution to align it with other plumbing. Once aligned, tighten the nut until it is "firm" against the washer and o-ring, with the nut becoming snug with a feel of metal-on-metal in less than one revolution.  Over-tightening causes as many leaks as under-tightening.

-Flared hydraulic fittings come in 37- and 45-degree designs and may share similar thread pitches. This means a 45-degree male fitting may thread into a 37-degree female, but they won't seal correctly. Beyond making certain the cone-angles of fittings match, don't over-tighten flared fittings. Over-tightened flared fittings leak as badly as under-tightened fittings because the immense torque of over-tightened threads cracks or distorts the precision-machined flared portions of the fittings.

I can always tell when I take apart a machine whether the hydraulic fittings are "factory" of if they're "farmer." "Factory fittings" break loose with a good smack of my hand to the end of a wrench. "Farmer Fittings" usually require a cheater pipe and possibly profanity to break them loose. I'm not sayin' that's good or bad, I'm just sayin'...
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