A hose fitting is a connector or adapter for the end of a hose, i.e., a piece that screws on to the end of a hose and allows another piece to be screwed into it.
Hose fittings come in many different types and sizes, depending upon the pressures involved, the substances being transferred (often related to their incompatibility), size of hose, temperature of hose contents, and the need for quick disconnection.
The following are examples of common hose fittings:
A “pipe thread” (NPT) connection is designed to provide a leak-proof connection between two plumbing pipes or male fitting ends without the use of tools. It has almost universal application in plumbing and other applications (e.g., engines and chemical plants) where connections must be made or broken rapidly, but should still be completely leak-proof under normal operating conditions.
The National Pipe Thread (NPT) is a screw thread form that has been used in North America for more than 100 years to facilitate the connection of pipes and hose fittings in plumbing and mechanical systems. The threads form an ellipse, with a 60 angle between the axes of the ellipse and its major axis, measuring 1/8″ (0.125″, or 3/4″). Standard pipe thread tape, similar to thread sealing tape used on other threaded connections, is available for use by hand when joining pipe and fittings, providing a leak-proof connection.
The tape is applied to male pipe threads to provide an airtight seal between the exterior of the pipe (male) and the interior (female) of the fitting or adapter. The tape has many advantages over other techniques for sealing threaded connections like “pipe dope”, which requires a separate tool to dispense and is often messy. Pipe thread tape is essentially a standard pipe thread compound, usually containing some form of mineral oil inside the cloth or paper wrapper.
The best type of tape for each application depends on factors such as temperature range, pressure, chemical compatibility with the pipe, etc., but generally speaking it comes in a wide range of quality grades. Pipe thread tape is not designed to stop leaks at a connection under high pressure or reduce friction, so its suitability for a particular application must be evaluated by the user based upon his needs and tolerances.
Tapered threads of different types are used in plumbing in different parts of the world. One type, known as the “G” thread, has a 60 angle between the axes of the ellipse and its major axis measuring 1/4″ (0.25″, or 3/8″). The other type is the straight 60 “Rg” thread, also called “American Standard Straight Pipe Taper Threads”.
The “G” thread has a slightly larger root diameter than the major diameter, which results in a slight “funneling” effect at the minor diameter. The “Rg” thread is just the opposite – it has a larger major diameter and a smaller minor one.
The “G” system was used to seal water pipe joints for many years in the US, before being replaced by the “R” thread. Today many fixtures have both male and female fittings with “G” threads, so when replacing a broken nipple it is important to use the same system on either side of the joint.