Threads and size designation: Terminology

Machines and equipment are designed and manufactured in every corner of the world. The hydraulic systems alone use countless different types of fittings and adapters with different sealing methods and thread forms. While the sealing method can often be distinguished by appearance, thread forms all seem to look the same, making it difficult and time consuming to identify them when equipment modifications or repairs are needed…

Most connectors have a threaded stud to secure them either to a system component or to another fitting. Press-fitting or welding may also be used in some applications. The variety of available stud threads makes the identifica- tion and selection of fittings complicated. Therefore, some thread related terms need to be defined.

helix the spiral-like pattern of the thread, comparable to a coil formed by winding a wire around a tube.

major diameter – the largest diameter of a screw thread (2, Figure 1)

minor diameter the smallest diameter of a screw thread (3, Figure 1)

pitch – the distance between the crest of one thread to the same spot on the crest of the next thread (1, Figure 1) ; the smaller the pitch, the greater the number of threads per inch. The pitch, or number of threads per inch can best be determined by using a thread pitch gauge (see Figures 2 and 3). Metric thread pitch is denoted as the distance in milli- meters (mm) between two adjacent crests. Inch thread pitch is expressed as the number of threads per inch.

thread angle the included angle between the adjacent flanks of a screw thread (6, Figure 1).

thread, coarse thread with the crests far apart; generally used for threading into cast iron or aluminum. Coarse thread has a larger and less critical shoulder bearing area, screws in and out more quickly, and is less subject to stripping and galling.

thread crest – the top or peak of the thread (4, Figure 1).

thread, fine thread with the crests close together.

thread height the difference between the major diameter and the minor diameter.

thread, left hand studs with left-hand threads should be turned in a counterclockwise direction to tighten.

thread, operations  the male part (stud) must advance as it turns through the female part (port), and when the hex flange bot- toms, further tightening produces a tensile stress and a clamping force in the stud.

thread, parallel cylindrical thread; the major diameter is the same over the entire thread length.

thread, right hand with common right-hand threads, the stud should be turned clockwise to tighten.

threads-centre-point-hydraulic

Figure 1: The parts of a thread

thread-pitch

Figure 2: Using a thread-pitch gauge to de- termine the number of threads per inch of a stud

thread-pitch-two

Figure 3: Checking a nut for number of threads per inch