General Purpose ACME Screw Threads

There are 3 different forms of ACME Screw Thread: General Purpose ACME, Centralizing ACME, and Stub ACME. On this page we are talking about the General Puropse ACME Screw Thread as defined in: ANSI B1.5.

The ACME screw thread form has a 29° included angle. ACME screw threads are perceived as a bilateral transverse motion screw thread. The ACME screw thread form offers high strength with ease of machining and assembly. Acme screw theads are largely used for feed screws and adjusting screws. The ACME screw thread may be used in assemblies with the internal thread rigidly fixed and movement of the external thread in a direction perpendicular to its axis limited by its bearing or bearings.

The class-of-fit defines the relational fit between the various attributes of the internal and external screw thread. The class-of-fit for ACME screw threads allows clearance on all diameters for varing degrees of free movement. The General Purpose ACME screw thread has three suggested classes-of-fit: 2G, 3G, and 4G which are defined in the ANSI B1.5 standard. Class 2G is the specification's preferred choice. If less backlash or end play is desired, Classes 3G and 4G are provided. There is also an obsolete class-of-fit (5G) which is not recommended for new designs. As a general reccommendation; it is suggested that external and internal threads of the same class be used together for general purpose assemblies.

The General Purpose threads consist of 29° included angle and a series of diameters and related pitches, as recommended in the ANSI B1.5 Standard. These diameters and pitches have been chosen to meet current requirements with the least number of items in order to condense amount of inventory of both tools and gages. The tolerances specified in this standard are relevant to lengths of engagement not greater than twice the nominal major diameter.

The tolerances contained by the standard are designed to ensure interchangeability and maintain a high grade of product. The tolerances on diameters of the internal thread are plus, being applied from minimum sizes to above the minimum sizes. The tolerances on diameters of the external thread are minus, being applied from the maximum sizes to below the maximum sizes. The pitch diameter (or thread thickness) tolerances for an external or internal thread of a known class are the same. The thread thickness tolerance is 0.259 times the pitch diameter tolerance.

Below are examples of the proper nomenclature for single and multi-start General purpose ACME Threads:

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

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

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

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

For more information click the links below.
General Purpose ACME Screw Thread Profile
General Purpose Screw Thread Data
Centralizing ACME
Stub ACME

Originally Posted: 12/9/2008
Last Revised:

If you discover error or wish to comment on this page contact Wayne at:gageguy@gagecrib.com