Different types of screw threads have evolved for fastening, and hydraulic systems. Of special concern are plastic-to-metal, taper/parallel threaded joints in hydraulic circuits. A discussion and recommendations are provided to create an awareness of different types of threads and how they are used.
Over time many different types of screw threads have been developed. Applications include fastening components, and hydraulic and pneumatic circuits. In the nineteenth century, manufacturers needing fasteners would devise their own systems. This resulted in compatibility problems. The English mechanical engineer and inventor, Sir Joseph Whitworth devised a uniform threading system in 1841 to deal with these difficulties. The Whitworth thread form is based on a 55 degree thread angle with rounded roots and crests.
In America, William Sellers set the standard for nuts, bolts, and screws which became the National Pipe Tapered Thread (NPT) in 1864. His 60 degree thread angle, in common use by early American clockmakers, enabled the American Industrial Revolution. These thread forms later became the American National Standard.
The Whitworth thread form was selected as a connecting thread for pipes, which was made self sealing by cutting at least one of the threads on a taper. This became known as the British Standard Pipe thread (BSP Taper or BSP Parallel thread). The Whitworth thread is now used internationally as a standard thread for jointing low carbon steel pipes.
The best known and most widely used connection where the pipe thread provides both the mechanical joint and the hydraulic seal is the American National Pipe Tapered Thread, or NPT. NPT has a tapered male and female thread which seals with Teflon tape or jointing compound…