Fluid lines and fittings

Fuel, oil, hydraulic fluid, air and oxygen move through rigid tubing and flexible hose. Rigid tubing is aluminum alloy for low pressure (5052-O is easy to bend) and corrosion-resistant steel or titanium for high pressure. Its size is the outside diameter in sixteenths of an inch: a -6 tube is 6/16 = 3/8 inch.

Tubing is bent with a bender to the radius the manual gives: smooth, with no kinks, wrinkles or flattening past the limit. Lines are clamped at the spacing given, kept from rubbing and labelled with tape bands that carry the fluid's name and a symbol as well as a colour, so they can be read by anyone.

Fittings

• Flared fittings (AN): the tube end is flared to 37° and squeezed between a nut and a cone. Automotive flares are 45°, and the two must never be mixed.
• Single flares are common; a double flare is stronger and used on soft aluminum tubing in smaller sizes.
• Flareless fittings (MS) use a sleeve that bites into the tube when the nut is first tightened (presetting).
• Overtightening a flare thins or cracks it; tighten to the torque given, and don't use a sealant on the flare itself.

Flexible hose goes where parts move or vibrate, like an engine. Its size is roughly the inside diameter in sixteenths. Each hose has a lay line printed along it: if it spirals after installation, the hose is twisted, which weakens it. Hoses get a little slack (about 5 to 8% extra length), because a hose gets shorter under pressure. PTFE (Teflon) hoses take a set in service and shouldn't be straightened.

O-rings seal fittings and actuators. Use the exact part number for the fluid in the system, because seals are made for one type. Never reuse one. A backup ring goes on the side away from the pressure, so the O-ring can't squeeze into the gap.