Types of Threads | amiga Monday, 4 February 2013


screw, or bolt, is a type of fastener characterized by a helical ridge, known as a male thread (external thread) or just thread, wrapped around a cylinder. Some screw threads are designed to mate with a complementary thread, known as a female thread (internal thread), often in the form of a nut or an object that has the internal thread formed into it. Other screw threads are designed to cut a helical groove in a softer material as the screw is inserted. The most common uses of screws are to hold objects together and to position objects.



nut is a type of fastener with a threaded hole. Nuts are almost always used opposite a mating bolt to fasten a stack of parts together. The two partners are kept together by a combination of their threads’ friction, a slight stretch of the bolt, and compression of the parts. In applications where vibration or rotation may work a nut loose, various locking mechanisms may be employed: Adhesives, safety pins or lockwire, nylon inserts, or slightly oval-shaped threads. The most common shape is hexagonal, for similar reasons as the bolt head – 6 sides give a good granularity of angles for a tool to approach from (good in tight spots), but more (and smaller) corners would be vulnerable to being rounded off. Other specialized shapes exist for certain needs, such as wing nuts for finger adjustment and captive nuts for inaccessible areas.


Thread Form:

There are many different thread forms in use today. The forms most widely used for power transmission screw threads are illustrated in here. An optical comparator is the easiest method of determining thread form. Profile gages, if available and visual methods can also be used. Great care must be taken as many forms are almost identical. The Acme form (29 degree included angle) is only 1 degree different from the ISO Metric Trapezoidal form (30 degree included angle). Many thread forms such as Unified, Metric ISO and Acme are subject to published standards while others, including Ballscrew and Worm threads, are not defined in detail by any standards organizations.


 d – The diameter identifying the thread often wrongly called the outer diameter of the thread.

d1 The diameter of the core of the thread often called the inner diameter of the thread.

L– The length of the thread.

L1 – The length of the screw.

h – The pitch of the thread.

t – The theoretical depth of the thread, the actual depth is rounded.

φ – The thread angel.

screw d


The helix of a thread can twist in two possible directions, which is known as handedness. Most threads are oriented so that the threaded item, when seen from a point of view on the axis through the center of the helix, moves away from the viewer when it is turned in a clockwise direction, and moves towards the viewer when it is turned counterclockwise. This is known as a right-handed (RH) thread, because it follows the right hand grip rule. Threads oriented in the opposite direction are known as left-handed (LH).



How to identify hydraulic threads?

  • Determine if the thread is tapered or parallel. NPT/NPTF and BSPT are tapered threads while UN/UNF and BSPP are parallel. Metric Tapered and Metric Parallel speak for themselves. In some cases, Step 1 can be accomplished by visual inspection alone. Tapered threads get smaller in diameter toward the end of the fitting while parallel threads maintain the same diameter from start to finish. If this is not obvious by looking at the fitting, use the parallel jaws of a caliper to make a comparison. Furthermore, the presence of an O-ring or the removal of a tube nut is usually an indication that the male thread is parallel. Completion of Step 1 will eliminate three of the six possible thread forms.

thread T_P

  • Determine the pitch. This can be deciphered using a pitch gage for comparison or by accurately measuring and calculating the number of threads within a given distance. It is much easier to compare threads against a lighted background with a pitch gage. Because some thread pitches are relatively similar, it is advisable to try a number of gages before deciding which one fits best. The result from Step 2 will narrow down the possible thread forms even more because most have a distinct pitch.


  • Determine the size. Combining the results of Steps 1 and 2 will determine – or help predict, in some cases – the correct procedure for Step 3. There are two methods for determining the thread size – which to use depends on whether the thread is a pipe thread (NPT/NPTF, BSPT, BSPP) or is not a pipe thread (UN/UNF, Metric Parallel, Metric Tapered). Keep in mind that tapered (as determined in Step 1) does not necessarily mean that it is a pipe thread (e.g., Metric Tapered). Likewise, pipe thread can be parallel (e.g., BSPP). For pipe thread, determine the size by comparing it with a nominal size profile, as shown in Figure 1 (a useful tip – pipe sizes up to 2” nominal size can be determined by measuring the actual outside diameter, subtracting ¼” then rounding-off).for non-pipe thread, the actual size can be determined by measuring the outside diameter (major diameter) with a caliper, as shown in Figure 2.




Standardization of screw threads has evolved since the early nineteenth century to facilitate compatibility between different manufacturers and users. The standardization process is still ongoing; in particular there are still (otherwise identical) competing metric and inch-sized thread standards widely used. Standard threads are commonly identified by short letter codes (M, UNC, etc.) which also form the prefix of the standardized designations of individual threads. Additional product standards identify preferred thread sizes for screws and nuts, as well as corresponding bolt head and nut sizes, to facilitate compatibility between spanners (wrenches) and other tools.

ISO standard threads

The ISO metric screw threads are the world-wide most commonly used type of general-purpose screw thread. They were one of the first international standards agreed when the International was set up in 1947. The “M” designation for metric screws indicates the nominal outer diameter of the screw in millimeters (e.g. an M6 screw has a nominal outer diameter of 6 millimeters).

Unified Thread Standard (UTS)

The Unified Thread Standard (UTS) defines a standard thread form and series—along with allowances, tolerances, and designations—for screw threads commonly used in the United States and Canada. It has the same 60° profile as the ISO metric screw thread, but the characteristic dimensions of each UTS thread (outer diameter and pitch) were chosen as an inch fraction rather than a millimeter value. The UTS is currently controlled by ASME/ANSI in the United States.

This standard includes:

  • Unified Coarse (UNC), commonly referred to as “National Coarse” or “NC” in retailing.
  • Unified Fine (UNF), commonly referred to as “National Fine” or “NF” in retailing.
  • Unified Extra Fine (UNEF)
  • Unified Special (UNS)

National Pipe Thread Taper (NPT)

National Pipe Thread Taper (NPT) is a U.S. standard for tapered threads used on threaded pipes and fittings. In contrast to straight threads that are found on a bolt, a taper thread will pull tight and therefore make a fluid-tight seal. In America, William Sellers set the standard for nuts, bolts, and screws which became the National Pipe Tapered Thread (NPT) in 1864.


British standard:

  • British Standard Withwort (BSWWH, W)
  • British Standard Fine (BSF)
  • British Standard Pipe (BSP)
  • British Standard Pipe Tap (BSTP)
  • British Association (BA)

The form of a Whitworth thread is based on a fundamental triangle with an angle of 55° at each peak and valley. The sides are at a flank angle of Θ = 27.5° to the perpendicular to the axis.

The British Association screw thread (BA) standard is sometimes classed with the Whitworth standard fasteners because it is often found in the same machinery as the Whitworth standard. However it is actually a metric based standard that uses a 47.5° thread angle and has its own set of head sizes. BA threads have diameters of 6 mm (0BA) and smaller, and were and still are particularly used in precision machinery.