Unified screw thread  form, series, class, allowance, tolerance, and designation are specified in ASME B1.1 standard. General information about unified inch screw thread.

- Basic profile for UN and UNR thread

The basic profile for Unified inch screw threads

- The design profile of the unified screw thread form for classes 1A, 2A, 1B, and 2B is shown in the following figure. Refer to ASME B1.1:2003 standard for the profile of class 3A and class 3B.

The design profile for Unified inch screw threads

Unified inch screw thread (UN and UNR) design profile for classes 1A, 2A, 1B, and 2B according to ASME B1.1-2003

- ASME B1.1-2003 standard covers for UN and UNR thread forms with sizes from 0.0600 inch to 6 inch.

- The difference between UN and UNR thread forms is the form of root section. For UN thread form , the root of the thread is flat. Optionally a rounded root contour is allowed. For UNR, the root of the thread is only rounded root contour. UNR thread form is applicable to external threads only. UNR internal thread does not exist.

- The design profiles of external thread of UN and UNR screw is flat according to standard but it’s allowed to have some corner radius as shown in the figure. The same situation is valid for root of the internal threads.

- Standard series is given in the standard screw thread series table for information.

- The basic profile of UN, UNR and ISO profiles are same .

- Unified thread form and ISO metric form are not interchangeable

- In case to determine the thread pitch quickly, tools named thread pitch gauges can be used to find the pitch of the internal and external threads. Thread gauges are not used for precise measurements but allows user to determine thread pitch without losing time.

Terms and Definitions:

Design Profile: The shapes of the external and internal threads in which the limits of dimensions are applied.

Gages and Gauging : Specifications and dimensions for the gages used on Unified inch screw thread is defined in the ANSI/ASME B1.2-1983 standard. Types of gages including thread plug gages, snap gages, thread ring gages and plain diameter gages for internal and external threads are covered in same standard. Screw thread gauging systems for acceptability is covered in the ASME B1.3-2007 standard.

Major Diameter: The maximum diameter of a thread which is diameter of the crest of a male thread or root of a female thread.

Minor Diameter: The minimum diameter of a thread which is diameter of the root of a male thread or crest of a female thread.

Unified Screw Thread: A thread form used by the United Kingdom, Canada, and the United States to obtain screw thread interchangeability among these three nations. It has 60° thread angle and dimensions are stated in inch units.

UN form screw threads: The UN thread is intended for general purpose fastening operations.

UNC (Unified National Coarse): Most commonly used type for general engineering applications. This thread form is used in materials with low tensile strength which makes threads more resistance against stripping (internal threads) .UNC give possibility for quick assembly.

UNF (Unified National Fine): External threads of this Fine Series have greater tensile stress area than comparable sizes of the Coarse series. The Fine series is suitable when the resistance to stripping of both external and mating internal threads equals or exceeds the tensile load carrying capacity of the externally threaded member. It is also used where the length of engagement is short, where a smaller lead angle is desired, where the wall thickness demands a fine pitch, or where finer adjustment is needed.

UNEF (Unified National Extra Fine): Used when finer pitches than UNF are needed (Ex: Short engagement length).

UNS (Unified National Special): UNS are special series. Bolt / screw selection shall be done from standard series unless there is a special requirement to use a special series.

Thread class: A classification system to classify the threads for interchangeability and manufacturability. Class 1 threads are loosely fitting thread and used for ease of assembly. Most commonly used class is 2 and this class is designed for general use. Class 3 is used for closer tolerances.

Threads per inch: Number of full thread per an inch length.

Unified Thread Formula:

Parameter Equation
Height of fundamental triangle [H] $$H=\frac { \sqrt { 3 } }{ 2 } P$$
Thread basic pitch diameter [d2 , D2] $${ d }_{ 2 }={ D }_{ 2 }=d-2\times \frac { 3 }{ 8 } H=D-2\times \frac { 3 }{ 8 } H$$
Thread basic minor diameter [d1 , D1 ] $${ d }_{ 1 }={ D }_{ 1 }=d-2\times \frac { 5 }{ 8 } H=D-2\times \frac { 5 }{ 8 } H$$
External thread maximum major diameter [dmax] $${ d }_{ max }=d-es$$
External thread minimum major diameter [dmin] $${ d }_{ min }={ d }_{ max }-{ T }_{ d }$$
External thread maximum pitch diameter [d2max] $${ d }_{ 2max }={ d }_{ 2 }-es$$
External thread minimum pitch diameter [d2min] $${ d }_{ 2min }={ d }_{ 2max }-{ T }_{ d2 }$$
External thread maximum minor diameter [d1max] $${ d }_{ 1max }={ d }_{ 1 }-es$$
External thread UNR reference minor diameter [d3] $${ d }_{ 3 }={ d }_{ 1 }-\frac { H }{ 8 }$$
External thread UNR maximum minor diameter [d3max] $${ d }_{ 3max }={ d }_{ 3 }-es$$
Internal thread minimum major diameter [Dmin] $${ D }_{ min }=D$$
Internal thread maximum pitch diameter [D2max] $${ D }_{ 2max }={ D }_{ 2min }+{ T }_{ D2 }$$
Internal thread minimum pitch diameter [D2min] $${ D }_{ 2min }={ D }_{ 2 }$$
Internal thread maximum minor diameter [D1max] $${ D }_{ 1max }={ D }_{ 1min }+{ T }_{ D1 }$$
Internal thread minimum minor diameter [D1min] $${ D }_{ 1min }={ D }_{ 1 }$$

es: Allowance, Td :Major diameter tolerance of external thread, Td2:Pitch diameter tolerance of external thread, TD1: Minor diameter tolerance of internal thread, TD2 : Pitch diameter tolerance of internal thread



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