ASME B18.2

B18.2 Square and Hexagon Bolts and Nuts

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Organization: ASME
Publication Date: 1 January 1960
Status: inactive
Page Count: 35


SECTIONAL COMMITTEE B18 on Dimensional Standardization of Bolts, Nuts, Rivets, Screws, and Similar Fasteners was organized in March, 1922 under the procedure of the American Standards Association with the Society of Automotive Engineers and The American Society of Mechanical Engineers as joint sponsors.

Following the organization of the sectional committee, subcommittees were appointed to facilitate development of standard dimensions for the several types of bolts and nuts within the scope of its activity. Subcommittee No. 2 after appraisal of the requirements of industry, developed a proposed standard series of bolt head and nut dimensions. This proposal was finally approved and designated as an American Tentative Standard in February, 1927.

The June, 1930 meeting of Subcommittee No. 2 convinced the majority of members that certain fundamental changes and additions should be made to meet the requirements of industry. A comprehensive study was made which resulted in the first revision of the standard which was ultimately approved and designated as an American Standard in March, 1933.

Further refinements were made in successive revised drafts of the standard, dated November, 1937, May, 1938, March, 1939, and March, 1940. A revision was approved and designated as an American Standard in January, 1941.

Following reorganization of the sectional committee, February 20, 1947, Subcommittee No. 2 was asked to expand the standard on head proportions into a complete product standard. Subgroups were appointed to develop the product standards for bolts, nuts, cap screws, and automotive hexagon head bolts. After numerous meetings of the subgroups and Subcommittee No. 2 a proposed standard on these products was prepared and submitted to the sectional committee in April, 1950.

While this draft was under consideration, the Sectional Committee received a proposal from the British Standards Institution for unification of dimensions of products using the new 'unified' screw threads. The reactions of the Committee were not favorable to the first proposals, but in its reply of December 22, 1949, .the Committee welcomed the opportunity of discussing the proposals in order to explain and support its reactions. This invitation was accepted and lead to a British-Canadian-American Conference in New York, June 1-2, 1950.

It was agreed in the Conference that the essentials of unification could be accomplished essentially by selection of mutually satisfactory across-the-flats dimensions, since this would permit the use of the same wrenches and since other features would rarely affect interchangeability. The British delegation agreed on the principle of selection of dimensions for unification on the basis of existing American Standards. It was requested, however, that consideration be given to suggestions for improvement of the American Standards in certain instances. The British delegation, therefore, was invited to select dimensions that they felt would be needed in British standards and in standardization of defense products, with the understanding that the Canadian and American delegations would raise questions where they were not in agreement. After due consideration, across-the-flats dimensions were selected for hexagon products.

It then became the responsibility of the respective standardizing bodies to implement the Conference recommendations. In its meeting of October 13, 1950, the Subcommittee agreed to incorporate in American Standards the conference recommendations on 1/4 inch hexagon bolts, 5/8 inch cap screws and automotive bolts, 5/16 and 3/8 inch regular hexagon and square nuts, and 7/16 inch light and regular hexagon and square nuts. At a subsequent meeting of Subcommittee 2 on February 2, 1951. further changes were made in order to combine the light and regular series of nuts and to combine the automotive hexagon head bolt, hexagon head cap screw, and regular close tolerances- bait.. The dimensions of these combined series were submitted to industry for comment in a draft dated March, 1951. In the meantime, British drafts based upon Institution proposed a London Conference to conclude the activity by making certain hexagon bolts and nuts completely interchangeable. These meetings were held April 26-27, 1951, The British, Chadian, and American delegations unanimously agreed to recommend to their respective standards bodies that dimensions of hexagon products that are shown in bold face in this standard be designated as 'unified,' although it was agreed that individual countries might establish smaller tolerances within those limits to govern local production of the products, providing the acceptance limits established for the unified products are not reduced. Other items in the same tables are based upon formulas that have been accepted and published by the British for sizes outside of the range listed in their standard.

In its meeting of June 8, 1951, Subcommittee No. 2 reaffirmed its acceptance of the unified dimensions since they corresponded with those in the March, 1951 draft. However, the Subcommittee reversed previous action on square bolts and nuts which were not involved in unification and they attempted to select better nomenclature for the unified products. A revised draft incorporating these changes was submitted to the Subcommittee in July, 1951, and a further revision was made in a meeting of August 10, 1951 because of continued dissatisfaction with the nomenclature that was previously selected. A final draft using the nomenclature "Finished Hexagon Bolts and Nuts" and containing numerous editorial changes was submitted for letter ballot in September, 1951.

Following approval by the sectional committee. and the sponsors the proposal was presented to the American Standards Association for approval and designation as an American Standard. This was granted on March 24, 1952. In taking this action, the sectional committee recognized there was need for additional refinements, but it felt that they should be included in a subsequent revision because of the urgency for mutual defense purposes of a standard embodying the ABC agreements. Upon issue of the 1952 revision, therefore, work was started immediately to make these minor improvements. Inconsistencies were removed in respect to fillets, improvements were made in the Length tolerances of heavy bolts, and other corrections and clarifications of an editorial nature were incorporated. The most noteworthy editorial change was a decision to combine the B18.2 standards on Hexagon Cap Screws and Square Head Set Screws with the B18.6 standards on Slotted Head Cap Screws for publication in a separate standard. Since the unified "Hexagon Head Cap Screws", and "Finished Hexagon Bolts" have identical requirements in the size range in which they overlap, the information will now be available in two publications to replace the separate tables previously included in the B18.2 standard.

Following necessary approvals, the revision was submitted to the American Standards Association. It was approved and designated as an American Standard February 2, 1955.

The proposed revision of B18.2 dated May 20, 1959 was approved by the Committee, the sponsors and the American Standards Association, and it was designated as an American Standard on April 18, 1960.

Document History

ASME B18.2
January 1, 1960
B18.2 Square and Hexagon Bolts and Nuts
FOREWORD SECTIONAL COMMITTEE B18 on Dimensional Standardization of Bolts, Nuts, Rivets, Screws, and Similar Fasteners was organized in March, 1922 under the procedure of the American Standards...
January 1, 1955
B18.2 Square and Hexagon Bolts and Nuts
A description is not available for this item.
January 1, 1941
A description is not available for this item.