A screw thread is a helical structure wrapped around a cylindrical or cone. In case of screws this surface is external, and in case of nuts it is internal. There is a particular case when the sprial thread is disposed on a flat surface. Geometrically speaking, the thread can be generated by a square, triangular, trapezoidal or semi-circle figure, which has a spiral motion around the axis of the surface of rotation.

	p = pitch of thread α (alpha) = angle of thread
External thread carried on a cylindrical surface

  The helical part described by a point on the surface line making a complete rotation is called spiral. The pitch (p) of the helical (thread) is the distance between two spirals which are measured on the same surface line. The thread angle α (alpha), is the angle between the flanks and is the same with the inclination angle of the helical. Dmin is the diameter of the bottom of the thread and Dmaj is the external diameter of the thread which is measured at the peak tangential. At a 360 ° rotation, a spiral is gone through, and the screw or nut is moving forward or backward along the axis with a distance equal to the length of pitch p.

• after the thread profile: triangular, square, trapezoidal, saw, rectangular, round
• after the revolution surface on which is achieved: cylindrical or taper
• after the number of beginnings: one start point(simple thread), several start points (multiple thread)
• after the measuring unit: metric thread (mm), imperial thread (inch)

  The metric thread is the most common type of thread around the world, used for joining various components. This was one of the first international standards agreed when the International Organization for Standardization (ISO) was set up in 1947.

P: Pitch H: Thread height h: Thread depth Dmaj: Major diameter (the largest diameter of the screw) Dmin: Minor diameter (the smallest diameter of the screw, Dmin= Dmaj – 2xh) Dp: Effective pitch diameter (the thread diameter where the thickness is equal to the space between the spirals, Dp = Dmaj – h)

• thread angle: 60°;
• H = 0.866025 x P;

• UNC (coarse pitch) de la 1/4" la 4" dia.
• UNF (fine pitch) de la 1/4" la 11/2" dia.
• UNEF (extra fine pitch) de la 1/4" la 111/16" dia.
• UNS (unified national special)
• UN (constant pitch) pentru 4,6,8,12,16,20,28, 32 TPI

  The UNIFIED thread has the same 60° profile as the ISO metric screw thread but the characteristic dimensions of each UTS thread (Unified Thread Standard) were chosen in inch fraction rather than a round millimeter value. The UTS thread is most commonly used in USA and Canada.

• thread angle: 60°;
• H = 0.866025 x P;

  The BA thread was formulated in 1884 and standardised in 1903 (England) with the use in electrical fittings and accessories. It is slowly being replaced by the ISO metric thread and now is mainly used for fine threaded connections especially in instruments.

BA thread properties: thread angle: 47.5° H = 1.13634 x p h = 0.60000 x p r = 0.18083 x p s = 0.26817 x p

• BSC (BSCy) - British Standard Cycle
• CEI - Cycle Engineers Institute - replaced by BSC

  The BSC thread is a fine thread well suited to cycle and motorcycle applications because it allows high torque settings and resists the tendancy to loosen under vibration. Also, the larger core diameter gives greater shear strength. The most popular sizes are with a pitch of 26 TPI although other pitches are also possible 24, 32 and 40 TPI.

BSC thread properties: thread angle: 60° h = 0.5327 x p r = P/6

• W - Whitworth
• BSW – British Standard Whitworth
• BSF - British Standard Fine
• BSC - British Standard Cycle - used for motorcycles because of high torque and vibration resitance

  The Whitworth thread is used especially in USA and England, being devised by Sir Joseph Whitworth in 1841. It is marked with the letter 'W' followed by a number expressed as a fraction: W 1 1/2". This means that the diameter is expressed in inches and the pitch in number of threads per inch (TPI) - 1 inch = 25.4 mm. Because of the more inclined flank angle, it loosens much more difficult under vibrations.

Whitworth thread properties: thread angle: 55° H = 0.960491 x p h = 0.640327 x p r = 0.137329 x p

• BSPP (G) - British Standard Pipe Parallel Thread
• BSPF - British Standard Pipe Fastening Thread
• BSPT – British Standard Pipe Taper Thread

  The Gas thread (G) is a straight thread used for interconnecting pipes, also known in the trade as BSPP, NPS or G. It is technically a Whitworth thread which is actually a British Standard Pipe Parallel (BSPP) thread known as "G". Per the ISO standard, the proper designation is "G" in front of the fraction.

Whitworth thread properties: thread angle: 55° H = 0.960491 x p h = 0.640327 x p r = 0.137329 x p

• NPT – National Pipe Thread Taper
• FPT – female (internal) National Pipe Taper threads
• MPT – male (external) National Pipe Taper threads
• PTF – SAE short tape pipe thread
• NPTF – Dryseal American National Standard Taper Pipe Thread (ANSI B1.20.3)
• NPSM – American National straight pipe thread for mechanical joints
• NPSI – American National straight intermediate pipe thread
• NPSF – American Standard National Pipe Straight Fuel (Dryseal)
• NPSL – American Standard Straight Locknut Pipe Thread

  The NPT thread is a U.S. standard for tapered threads used on threaded pipes and fittings. The ANSI/ASME B.1.20.1 standard includes thread angles of 60° and sizes from 1/16" to 24" nominal pipe diameter.

H = 0.866025p = height of 60° sharp V thread h = 0.800000p = height of thread on product p = 1/n = pitch (measured parallel to axis) n = number of threads per inch a = 30° = thread flank angle β = 1° 47' = thread taper angle for 1/16 taper fc = depth of truncation at crest fr = depth of truncation at root Fc = width of flat at crest Fr = width of flat at root

• tapered thread 1° 47'
• truncation of roots and crests are flat
• thread angle: 60°
• pitch is measured in threads per inch (TPI)

  The ACME thread is a parallel thread most commonly used for transversal motion on industrial machines. The trapezoidal thread form provide high strength and ease of manufacture. The ACME thread is similar to the trapezoidal thread, the only difference being the reference system (ACME - imperial and Trapezoidal - metric 30°).

ACME thread properties: thread angle: 29° H = 1.93335 x p h = p/2 Fcn = 0.3707 x p

  The STUB ACME thread came into being early in the 1900's and it's use has been generally confined to those unusual applications where a coarse-pitch thread of shallow depth is required.

STUB ACME thread properties: thread angle: 29° H = 1.93335 x p h = 0.3 x p Fcn = 0.4224 x p

  The BUTTRESS thread has a saw tooth shaped profile and is basically used on transversal motion of industrial machines, hydraulic sealings (oil fields) or gun barrels so that it can withstand recoiled impact.

BUTTRESS thread profile: thread angle: 7°/45° H = 0.89064 x p h = 0.6627 x p Fcn = 0.1632 x p

  The round thread (Rd) is insensitive to dirt and damage due to the round profile and is used on clutches, train braking systems and gate valves.

Round thread properties: thread ange: 30° h = 0.5 x p

  The PG thread - is a thread as per German standards used mainly for electrical connections instead of the B.S. Conduit thread. The depth of the thread is smaller than NPT or Metric but larger flank angle.

PG thread properties: thread angle: 80° h = 0.595875 x p

• Pg7: Dmaj = 12.5mm; TPI = 20
• Pg9: Dmaj = 15.2mm; TPI = 18
• Pg11: Dmaj = 18.6mm; TPI = 18
• Pg13.5: Dmaj = 20.4mm; TPI = 18
• Pg16: Dmaj = 22.5mm; TPI = 18
• Pg21: Dmaj = 28.3mm; TPI = 16
• Pg29: Dmaj = 37mm; TPI = 16
• Pg36: Dmaj = 47mm; TPI = 16
• Pg42: Dmaj = 54mm; TPI = 16
• Pg48: Dmaj = 59.3mm; TPI = 16