Centralizing ACME Screw Threads

Centralizing ACME screw threads are defined by ANSI/ASME B1.5.

The thread form for Centralizing Acme threads is the same as General Purpose Acme Threads with the exception that a Centralizing Acme Threads have a limited clearance at the major diameter of both the internal and external threads, and are used extensively for feed screws. The restricted clearance enables a bearing at the major diameter of the thread to preserve alignment of the thread axis preventing wedging on the flanks and part sagging.

Five classes are used with the Centralizing Acme threads: 2C, 3C, 4C, 5C, and 6C. For any combination of the five classes of threads Covered within the standard (ANSI B1.5) some end play or backlash will result. Although 5C and 6C are intended to allow for restricted backlash they are not recommended for new designs.

A series of diameters and pitches is recommended in the Standard as preferred. These Diameters and Pitches have been chosen to present needs with the fewest number of items to reduce inventory of both tools and gages. The tolerances specified in this standard are applicable to lengths of engagement not exceeding twice the nominal major diameter as well as to insure interchangeability and maintain a high grade of product. The tolerances on the diameters of internal threads are plus, being applied from the minimum sizes to above the minimum sizes. The tolerances on the diameters of external threads are minus, being applied from the maximum sizes to below the maximum sizes. The pitch diameter tolerance for internal or external threads for any given class is the same.

Below are examples of the proper nomenclature for single and multi-start Centralizing Acme Threads:

Single Start: 1.250"-5 ACME-2C or 1-1/4"-5 ACME-2C
1.250-inch major (nominal) diameter, 5 threads per inch, single thread, Class 2C fit, right hand

Multi-Start: 2.750"-0.4P-0.8L-ACME-3C or 2-3/4"-0.4P-0.8L-ACME-3C
2.750-inch major diameter, pitch 0.4 inch, lead 0.8 inch, double thread, Class 3C, right hand.

Single Start: 1.250"-5 ACME-2C LH or 1-1/4"-5 ACME-2C LH
1.250-inch major (nominal) diameter, 5 threads per inch, single thread, Class 2C fit, Left hand

Multi-Start: 2.750"-0.4P-0.8L-ACME-3C LH or 2-3/4"-0.4P-0.8L-ACME-3C LH
2.750-inch major diameter, pitch 0.4 inch, lead 0.8 inch, double thread, Class 3C, Left hand.
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For more information click the links below.
Centralizing Thread Profile
Centralizing Thread Data
3-Gage System VS. 4-Gage System

Originally Posted: 7/14/2009
Last Revised: 10/21/09

This page is still under construction. If you have a pressing need for this information contact Wayne at:gageguy@gagecrib.com