# ASME/ANSI BUTTRESS INCH SCREW THREADS DIMENSIONS

ANSI/ASME buttress Inch screw thread is a form of thread with load angle of 7 degrees and a clearance angle of 45 deg from the normal to the axis. The basic height of thread engagement for buttress thread is 0.6p. The buttress thread form has certain advantages for the applications where high stress occurs only in one direction along the thread axis. The buttress form of thread is particularly applicable when tubular members screwed together. Breech assemblies of large guns, airplane propeller hubs and columns for hydraulic presses are some of the examples of buttress threaded rods and nuts.

Following calculator has been developed in accordance to ANSI B1.9-1973 “Buttress Inch Screw Threads” standard. The standard provides information about preferred diameter-pitch combinations, formulas for calculation pitch diameter tolerances, tolerances for major and minor diameters, a system of allowances between external and internal threads, recommended methods of measuring and gauging, dimensional acceptability of buttress threaded rods and nuts.

Note: For more information on buttress threads including diameter-pitch combinations, thread designation, basic dimensions, formulas, thread tolerances and allowances for easy assembly , please refer to pages 1945 - 1952 of Machinery's Handbook, 29th.

 Standard 7° / 45° Buttress Thread with 0.6p Basic Height of Thread Engagement and Round Root

##### Calculator:

 INPUT PARAMETERS Nominal diameter (inches) - threads per inch 1/2-12 1/2-16 1/2-20 5/8-12 5/8-16 5/8-20 3/4-12 3/4-16 3/4-20 7/8-10 7/8-12 7/8-16 1-10 1-12 1-16 1 1/4-8 1 1/4-10 1 1/4-12 1 3/8-8 1 3/8-10 1 3/8-12 1 1/2-8 1 1/2-10 1 1/2-12 1 3/4-6 1 3/4-8 1 3/4-10 2-4 2-6 2-8 2-10 2 1/4-6 2 1/4-8 2 1/4-10 2 1/2-6 2 1/2-8 2 1/2-10 2 3/4-5 2 3/4-6 2 3/4-8 3-5 3-6 3-8 3 1/2-5 3 1/2-6 3 1/2-8 4-5 4-6 4-8 4 1/2-4 4 1/2-5 4 1/2-6 5-4 5-5 5-6 5 1/2-4 5 1/2-5 5 1/2-6 6-4 6-5 6-6 Thread Tolerance Class 1 Class 2 Class 3

###### Note: 1 Class 2: Standard grade, Class 3 : Precision Grade

 RESULTS Parameter Symbol Value Unit Nominal Diameter D --- in Threads per inch n --- --- Thread Tolerance -- --- LIMITS OF SIZES FOR BUTTRESS THREADED ROD Maximum Major Diameter Ds_max --- in Minimum Major Diameter Ds_min --- Maximum Pitch Diameter Es_max --- Minimum Pitch Diameter Es_min --- Maximum Minor Diameter Ks_max --- Minimum Minor Diameter Ks_min --- LIMITS OF SIZES FOR BUTTRESS THREADED NUT Maximum Major Diameter Dn_max --- in Minimum Major Diameter Dn_min --- Maximum Pitch Diameter En_max --- Minimum Pitch Diameter En_min --- Maximum Minor Diameter Kn_max --- Minimum Minor Diameter Kn_min --- TOLERANCES & ALLOWANCES Tolerance 1 tol. --- in Allowance G --- BASIC DIMENSIONS Pitch p --- in Height of sharp-V thread H --- Basic height of thread engagement h --- Max. root radius (theoretical) rmax --- Min. root radius (theoretical) rmin --- Max. root truncation smax --- Min. root truncation smin --- Max. flat width for flat root form Smax --- Min. flat width for flat root form Smin --- Crest truncation f --- Crest width F --- Height of thread of internal thread hn --- Height of thread of external thread hs ---

##### Definitions:

Gages for Buttress Screw Threads: Recommended gages and gauging practice for external and internal buttress thread are defined in ASME B1.9 standard. Gage maker's tolerances, gages for buttress threaded rods and nuts including GO/NOT GO plain ring gages, threaded rings, setting plug gages and necessary information related with gauging is given in this standard.

Allowance: An allowance (clearance) should be provided on all external threads to secure easy assembly of parts.

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.

Roots of Threads: There are 2 alternatives for thread roots. These are rounded and flat roots.

Rounded Roots: Equal radii, at the roots of the external and internal basic thread forms tangential to the load flank and the clearance flank. The rounded root form of the external and internal thread shall be a continuous, smoothly blended curve within the zone defined by 0.07141p maximum to 0.0357p minimum radius. The resulting curve shall have no reversals and sudden angular variations, and shall be tangent to flanks of the thread. There is, in practice, almost no chance that the thread forms will be achieved strictly as basically specified, that is, as true radii.

Flat Roots: Equal flat root of the external and internal thread. In instances where absence of root radius is not detrimental to the requirements for strength, and where it is more economical to provide tools which do not produce a radius at root, flat root buttress threads may be specified.

 Standard 7° / 45° Buttress Thread with 0.6p Basic Height of Thread Engagement and Flat Root

Threads designation: When only the designation, BUTT is used, the thread is "pull" type buttress (external thread pulls) with the clearance flank leading and the pressure flank 7° following. When the designation, PUSH-BUTT is used, the thread is a push type buttress (external thread pushes) with the load flank 7° leading the 45° clearance flank following.

##### List of Equations:

 Parameter/Condition Symbol Max. Material (Basic) Min. Material Height of sharp-V thread H H = 0.89064p --- Basic height of thread engagement h h = 0.6p --- Root radius (theoretical) r r= 0.07141p rmin = 0.0357p Root truncation s s = 0.0826p smin = 0.0413p Flat width for flat root form S S = 0.0928p Smin = 0.0464p Height of thread engagement he he_max = h - 0.5G he_min = he_max - (0.5 tol. on major dia. external thread + 0.5 tol. on minor dia. internal thread) Crest truncation f f = 0.14532p --- Crest width F F = 0.16316p --- Major diameter of internal thread Dn Dn = D + 0.12542p Dn_max = En_max+ 0.80803p Major diameter of external thread Ds Ds = D -G Ds_min = D - G - Dtol Pitch diameter of internal thread En En = D - h En_max = D - h + PDtol Pitch diameter of external thread Es Es = D - h - G Es_min = D - h - G - PDtol Minor diameter of external thread Ks Ks = D - 1.32542p - G Ks_min = Es_min - 0.80803p Minor diameter of internal thread Kn Kn = D - 2h Kn_min = D - 2h + Ktol Height of thread of internal thread hn hn = 0.66271p --- Height of thread of external thread hs hs = 0.66271p ---

 List of Parameters Symbol Definition p Pitch G Allowance D Major Diameter Dtol Major diameter tolerance E Pitch diameter PDtol Pitch diameter tolerance K Minor diameter Ktol Minor diameter tolerance

##### Reference:

• Oberg, E. , Jones ,F.D. , Horton H.L. , Ryffel H.H., (2016) . Machinery's Handbook . 30th edition.  Industrial Press Inc.
• Oberg, E. , Jones ,F.D. , Horton H.L. , Ryffel H.H., (2012) . Machinery's Handbook . 29th edition.  Industrial Press Inc.
• ANSI B1.9-1973 Buttress Inch Screw Threads