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.

Buttress Screw Thread Form
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
Thread Tolerance Class 1

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 ---

Note: 1 Tolerance on major diameter of external thread, pitch diameter of external and internal thread, and minor diameter of internal thread.

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.

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

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

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