COMPARISON OF THREAD CLASS FITS: United and American National Thread Series
The Unified Thread Series represents an effort by the standards bodies of Canada, the United Kingdom, and the United States to establish an interchangeable series of screw threads. These standards are based on and have substantially the same thread form as the American National thread series. It is important to note that the Unified threads are interchangeable with American National threads of the same diameter and pitch.
The essential differences between the two systems are as follows:
a. THREAD DESIGNATION.
Under the American National system, the same thread designation applies to screws and nut, i. e. ½-13 NC-2 is applicable to both the external and internal thread. Under the Unified system this symbol would be written ½-13 UNC 2A if it were an external and ½ -13 UNC 2B if it were an internal thread.
b. DIFFERENCE in TOLERANCE
Pitch diameter tolerance for the American National system are equal on screw and nut for a given class, whereas, the pitch diameter tolerance for the Unified internal thread is 30% greater than the external thread. There is also a variation of major, pitch, and minor diameter tolerance between the two systems.
c. APPLICATION of ALLOWANCES
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. An allowance is defined as an intentional difference in correlated dimensions of mating parts. It is the minimum clearance or maximum interference between mating parts.
CLASS 1 AMERICAN THREADED SERIES LOOSE
CLASS 1A & 1B UNITED THREAD SERIES FIT:
These loose- fit classes are intended to cover manufacture of threaded parts where quick and easy assembly is necessary, and a considerable amount of shake or play is not objectionable. These classes also provide allowance to permit easy assembly even when threads are slightly nick or dirty.
CLASS 2 AMERICAN THREAD SERIES, FREE FIT:
The free-fit class is intended to apply to end users where the threaded members are to assemble nearly or entirely with the fingers where a moderate amount of shake or play between the assembled threaded components is not objectionable, and where no allowance for nick or dirty threads is required.
CLASS 2A & 2B UNITED THREAD SERIES, FREE FIT:
This class is similar to Class 2 except that a moderate allowance is provided which minimizes galling and seizure encountered in assembly and use; it also accommodates plating, or finishes, under certain conditions. The maximum dimensions of threads which are plated or have coatings of similar thickness are increased by the amount of the allowance. Where allowance is not sufficient for thickness of plating or coatings, a special allowance is necessary.
CLASS 3, AMERICAN THREAD SERIES, MEDIUM
CLASS 3A & 3B UNITED THREAD SERIES FIT:
These medium-fit classes are intended to apply to manufactures, that produce a tighter thread fit between the screw and the tapped hole with minimum amount shake or play between the threaded assembly. These two classes are completely interchangeable. If the threads in these classes are to be plated, special allowance is necessary to assure proper limits after plating.