Fittings for Hydraulic Fluid Conductors

Hydraulic Pipes Tubes and Hoses in the SI Units

By Joji Parambath

This book presents the necessary information about the configuration, performance specifications, and other details of pipes, tubing, and hoses and their fittings. The book uses the SI system of units.

Pipes Tubes and Hoses in Hydraulic Systems in the English Units

By Joji Parambath

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Fluid conductors interconnect components of a hydraulic system for the safe and leak-free transmission of high-pressure hydraulic fluid throughout the system. Pipes, tubing, and hoses are the three basic types of fluid conductors used in hydraulic systems.

Pipes are rigid conductors with relatively larger wall thickness used to contain and convey hydraulic fluids. It is difficult to shape rigid pipes into the desired configuration. Many fittings, such as elbows, tees, etc. are needed to be used while routing a hydraulic piping system. Tubing is the most widely used type of conductor in hydraulic systems. Tubing is generally a small-diameter thin wall pipe. It can be bent into almost any shape, thus reducing the number of tube fittings while configuring a conductor system. Hoses are the most flexible and versatile type of conductors. They are capable of bending and flexing easily. The end fittings of a hose assembly are used to connect directly to adjoining pipe-work or fittings.

Pipe Fittings

Pipes are rigid conductors with relatively larger wall thickness used to contain and convey hydraulic fluids. It is difficult to shape rigid pipes into the desired configuration. Pipe connections are coupled through welded joints, flanged joints, or threaded joints.

The welded connections are more commonly used in systems involving severe mechanical load, high pressure, vibration, or high temperature. They can also be employed where leaks cannot be tolerated.  The welded connections can be butt welded joints, socket welded joints, or slip-on welded sleeve joints.

However, in hydraulic piping systems with high-quality requirements, it is recommended to use non-welded connection technologies (fittings, flanges, etc.) due to their reliability and inherent cleanliness. The threaded connections are most common.

Thread joints are used for hydraulic service to produce a leak-proof metal-to-metal seal. They are either tapered or straight. Pipe threads used in hydraulic piping can be divided into two types: (1) Standard pipe threads and (2) Dry-seal pipe thread.

The type of jointing technology is selected based on the working pressure, pipe size, pipe material, fitting standards, and other conditions such as possible pressure shocks in the system, nature of the environment, etc. Fittings are available as per various standards including NPTF, JIC etc.

Tube Fittings

Since the wall sections of tubing are relatively thin, threading cannot be used to seal the tubing connections. There are varieties of tube fittings available for hydraulic applications. Tubes can be joined quickly and easily with flaring, brazing or couplings. Flared or flareless-type fittings are used for tubing end-connections.

Flared tube fitting is made up of a nut and a sleeve over the flared tubing, and a body. The most critical step in making a flare tube fitting is forming the flare without galling, over-thinning, or splitting the end of the tube.  The sleeve and nut are pushed smoothly over the tubing end. When the nut is screwed onto the body, it draws the sleeve and the flare against the body, thus forming a seal.

Compression (flareless) tube fitting consists of a body, ferrule(s) and a nut. First, the ferrules and nut be slipped over the tubing. The tubing is inserted into the body, where it butts up against the shoulder. When the nut is screwed onto the body, the ferrule bites into the skin of the tubing to achieve the holding ability of the connection. This tight connection provides a positive seal.

Hose Fittings

Hose fittings can be either permanent or reusable.   Permanent hose fittings are installed on the hose by crimping and cannot be disassembled.  Next, reusable hose fittings are screwed or clamped on the hose end. Fittings are made to metric or SAE/JIC standards.

Quick Couplings (or Disconnects)

They are used for convenience as they can be installed and removed by hand and in situations where there is a need for the repeated connection and disconnection of the lines. A quick coupling has a male side and a female coupler. Quick couplings can be of the poppet type or flat face type. Based on the valving of the coupling, hydraulic couplings generally fall into one of the two groups: double shutoff, and straight through. 

Double Shutoff couplings

Both halves of the coupler, the body and the nipple, contain shutoff valves. These valves open automatically when the body and nipple are connected, and close automatically when the two halves are disconnected—keeping fluid loss to a minimum.

Straight-Thru couplings

They have no valves in either half and are ideal for maximum flow application. Their smooth, open bore offers the lowest pressure drop of any quick disconnect coupling and allows them to be thoroughly cleaned. Since there are no valves in either half, the fluid flow should be shut off before the coupling is disconnected. Straight-through couplings are used where flow must be unrestricted.

Joji Parambath


Fluidsys Training Centre Pvt Ltd

Bangalore, India

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