GENERAL PURPOSE ACME THREAD DIMENSIONS


Acme thread form is a trapezoidal screw thread profile used for lead screws (power screws). There are different types of Acme threads such as General purpose, Stub Acme screw and Centralizing. General purpose and Centralizing Acme threads are defined in the ASME/ANSI B1.5-1997 standard and Stub Acme screw thread is defined in the ASME/ANSI B1.8-1988 standard. The differences between these types are summarized in the “Definitions” section.

The calculator given below can be used for the dimensional calculations of general purpose single and multiple start Acme threaded rods and nuts. Calculation can be done for tolerance classes of 2G, 3G and 4G. Tolerance class 2G is the preferred class but if less play and backlash is needed, 3G or 4G can be selected. Angle of thread of general purpose Acme thread is 29°. The thread height is half of the thread pitch as mentioned above.

Schematic representation of the thread form of general purpose Acme thread is given in the “Definitions” section.

Note: For more information on ANSI standard General Purpose Acme screw threads including thread designation, basic dimensions, formulas for determining diameters, multi start Acme threads and limiting dimensions for different classes , please refer to pages 1921 - 1928 of Machinery's Handbook, 29th.

Tolerances Of Acme Screw Thread
Allowances, Tolerances, and Clearances for General Purpose Single-start Acme Threads (All Classes)

Calculator:

INPUT PARAMETERS
Nominal diameter (inches) - threads per inch
Thread Tolerance Class 1
Number of Start 2

Note: 1 Preferred class is 2G.
Note: 2 Single or multiple start Acme threads.

 


RESULTS
Parameter Symbol Value Unit
Designation -- ---  
Nominal Diameter D --- in
Threads per inch n --- ---
Thread Tolerance -- ---
LIMITS OF SIZES FOR ACME THREADED ROD
Maximum Major Diameter dmax --- in
Minimum Major Diameter dmin ---
Maximum Pitch Diameter d2max ---
Minimum Pitch Diameter d2min ---
Maximum Minor Diameter d1max ---
Minimum Minor Diameter d1min ---
LIMITS OF SIZES FOR ACME THREADED NUT
Maximum Major Diameter Dmax --- in
Minimum Major Diameter Dmin ---
Maximum Pitch Diameter D2max ---
Minimum Pitch Diamter D2min ---
Maximum Minor Diameter D1max ---
Minimum Minor Diameter D1min ---
BASIC DIMENSIONS
Basic Major Diameter  D  --- in
Basic Pitch Diameter D2 ---
Basic Minor Diameter D1 ---
Pitch P ---
Basic Thread Thickness t ---
Basic Thread Height h ---
Basic Flat at Crest
(internal thread)
Fcn ---
Basic Flat at Crest
(external thread)
Fcs ---
Basic Flat at Root
(internal thread)
Frn ---
Basic Flat at Root
(external thread)
Frs ---
Basic Lead Angle λ --- --
STRESS AREAS
Minimum Stress Area At  --- in^2
Minimum Shear Area per
Unit Length
As --- in

Definitions:


Centralizing Acme Threads: The tolerances of major diameter of external and internal threads are tighter with respect to general purpose Acme threads. In case of a lateral force, external and internal threads contact each other at the major diameter of the thread. This helps to keep alignment of the thread axis and prevents wedging.

Chamfers: General Purpose external threads (threaded rods) may have the crest corner chamfer as shown in the first figure at the top of the page.

Design Profile: The shapes of the external and internal threads in which the limits of dimensions are applied. The thread form of General Purpose Acme Thread is given in the following figure.

General Purpose Acme Thread Form
General Purpose Acme Thread Form

Gages for General Purpose Acme Threads: Gages for general purpose acme threads are defined in ASME B1.5 standard. Gage tolerances, gages for Acme threaded rods and nuts including GO/NOT GO thread ring and plug gages and necessary information related with gauging are given in this standard.

Lead: The distance that nut advances in one revolution of the Acme threaded rod.

Length of Engagement: The tolerances specified in the ASME B1.5-1997 standard are applicable to lengths of engagement not exceeding twice the basic major diameter.

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.

Multi Start Acme Thread: Provides fast relative traversing motion with lower mechanical advantage. For multiple start Acme threaded rods: major, minor, and pitch diameter sizes for each class are identical to those for single-start threads. In case of multiple start Acme threaded nuts: maximum major, minor, and pitch diameter sizes are increased by a percentage of external pitch diameter allowance (es) ; 50 % of es for two-start threads, 75% of es for three-start threads and 100% for four-start threads. [Ref-2]

Stub Acme Thread: The thread height is less than general purpose and centralizing acme threads. The thread height is 0.3 pitches for Stub Acme thread. In general purpose form this is 0.5 pitches. Stub Acme thread form is provided for unusual applications where a coarse-pitch thread of shallow depth is required due to mechanical or metallurgical reasons.

Thread class: A classification system to classify the threads for interchangeability and manufacturability.

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

List of Equations:

Parameter/Condition Symbol Equation
Pitch P Equation for Acme Thread Pitch
Basic Pitch Diameter D2 Equation for Acme Thread Basic Pitich Diameter
Basic Minor Diameter  D1 Equation for Acme Thread Basic Minor Diameter
Thickness at Pitch Line t Equation for Acme Thread Thickness at Pitch Line
Basic Height of Thread h Equation for Acme Thread Basic Height of Thread
Basic Flat at Crest
(internal thread)
Fcn Equation for Acme Thread Basic Flat at Crest
Basic Flat at Crest
(external thread)
Fcs Equation for Acme Thread Basic Flat at Crest Screw
Basic Flat at Root
(internal thread)
Frn Equation for Acme Thread Basic Flat at Root Nut
Basic Flat at Root
(external thread)
Frs Equation for Acme Thread Basic Flat at Root Screw
Minimum Stress Area At Equation for Acme Thread Minimum Stress Area
Minimum Shear Area
per Unit Length
As Equation for Acme Thread Minimum Shear Area

List of Parameters
Symbol Definition
PDAET Pitch diameter allowance on external thread
MDAIT Major diameter allowance on internal thread
MDAET Minor diameter allowance on external thread

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.
  • ASME B1.5-1997, Acme Screw Threads