ACME AND OTHER TRANSLATING THREADS Prepared by the Sectional Committee on the Standardization and Unification of Screw Threads (Bl)
|Publication Date:||1 September 1941|
Scope and Purpose.
This American Standard covers the design and dimensions of Acme and similar single1 screw threads intended primarily for translating screws, for which there is a general industrial demand. The designs included have been chosen with the dual purpose of meeting varied needs of the users to the greatest possible extent and at the same time establishing a product capable of economical production.
The diameters and pitches have been carefully selected with a view to meeting the present needs with the fewest number of items in order to reduce to a minimum the inventory of both tools and gages. The tolerances are such as to produce complete interchangeability and maintain a high grade of product.
Four series of translating screw threads are included in this standard-the General Purpose Acme, the 29 Deg Stub, the 60 Deg Stub, and a Modified Square Thread.
These screw thread series are intended to cover such applications as lathe dogs and clamps, track drills, steel bench vises, machine tool vises, drill press vises, lifting jacks, steel hand clamps, valve stems, piano stools, cross feed screws for lather, letter copy presses, and elevating screws on machine tools and other machines.
The subject of Acme and kindred threads embraces a wide field and it is not possible to combine in a single standard all of the variables of all the uses. The following applications are recognized as common usages but each has special features which prevent inclusion in a general purpose standard.
(1) Feed or lead screws where backlash or end shake are objectionable. In such applications the nut is tapped first and then the screw is threaded to fit. The screw and nut so made are kept as a pair.
(2) Long lead screws where sagging cause threads to seize. In such applications the major diameter clearance is reduced so that bearing takes place at major diameter before seizing can occur.
(3) Assemblies where the thread must maintain some degree of alignment as well as transmit motion. Desk chairs, shop stools, fiano stools, and the like are typical examples. In these applications a reduced major diameter clearance is the most effective and economical means of obtaining satisfactory assemblies.
(4) There is a considerable demand in mechanical industries for threaded assemblies which provide faster advance per revolution and which give greater wear surface. The threaded forms covered by this specification are used frequently, incorporating changes in details to meet parttcular requirements. It ts recommended that no coarser thread for a given diameter than chose listed be used but instead that a multsple thread giving the desired lead be adopted. Many applications in the valve industry are typical.
1 Where it is necessary to use multiple threads the form of single thread corresponding to "crests per inch" oi the multiple thread should be used.