scope:

INTRODUCTION

The Unified screw thread standards shown in this section constitute the basic thread standards used in the United States for the screw threads used on threaded fasteners. Unified screw threads are a completed and integrated system of threads for fastening purposes in mechanisms and structures. Their outstanding characteristic is general inter-changeability of threads achieved through the standardization of thread form, diameter-pitch combinations, and limits of size.

The standards have as their original basis the work done about a century ago by William Sellers in the United States and Sir Joseph Whitworth in Great Britain. Throughout the intervening years there have been many further developments and revisions, culminating in the system of United Threads approved and adopted for use by all inch-using countries.

Unification of screw thread standards receive its impetus from the need for interchangeability among the billions of fasteners used in the complex equipment of modern warfare which equipment was, and continues to be, made in different countries. Equally important, however, are international trade in mechanisms of all kinds and the servicing of transportation equipment which moves from country to country. These have made unification not only highly advantageous but practically essential.

United screw threads had their origin in an Accord signed at Washington, D.C., on November 18, 1948, by representatives of Standardizing Bodies of Canada, the United Kingdom, and the United States. The Unified standard threads generally supersede the American standard threads. Threads are classed as Unified if they have the basic Unified thread form and have limits of size and tolerances based on the Unified formulations. Such threads are identified by the letter combination "UN" in the thread symbol. In relation to previous American practice, Unified threads have substantially the same thread form and are mechanically interchangeable with American National threads of the same diameter and pitch.

The principal differences between the two systems relate to the application of allowances, the variation of tolerances with size, difference in amount of pitch diameter tolerance on external and internal threads, and differences in thread designations. Under the Unified system, an allowance is provided on both the classes 1A and 2A external threads, whereas under the American National system only the class 1 external thread has an allowance. Under the Unified system, the pitch diameter tolerance of an internal thread is 30 percent greater than that of the external thread, but such tolerance are equal under the American National system. Since the tolerances differ, the letter "A" is used in the thread symbol to denote an external thread and the letter "B" is used to denote an internal thread. Unified tolerances and allowances for both standard and special diameter-pitch combinations are derived from the same formula, but American National tolerances for special threads have a different basis from that for some standard threads.