Thread Fit And Designations.

and do not wedge, friction between the matching threads is reduced to a

minimum under heavy pressure.

d.

Thread Fit and Designations.

(1) Thread Fit.

(a) The Unified and American (National) Standard thread forms designate

classifications for fit to insure that mated threaded parts fit to the

tolerances specified.

(b) The Unified screw thread form specifies several classes of threads

which are classes 1A, 2A, 3A for screws or external threaded parts, and 1B,

2B, and 3B for nuts or internal threaded parts. Classes 1A and 1B are for a

loose fit where quick assembly and rapid production are important and shake

or play is not objectionable. Classes 2A and 2B provide a small amount of

play to prevent galling and seizure during assembly and use, and sufficient

clearance for some plating. Classes 2A and 2B are recommended for standard

practice in making commercial screws, bolts, and nuts.

Classes 3A and 3B

have no allowance, and 75 percent of the tolerance of classes 2A and 2B. A

screw and nut in this class may vary from a fit having no play to one with a

small amount of play.

Only high grade products are held to class 3

specifications.

(c) Four distinct classes of screw thread fits between mating threads

(as between bolt and nut) have been designated for the American (National)

Standard screw thread form.

Fit is defined as "the relation between two

mating parts with reference to ease of assembly."

These four fits are

produced by the application and by the tolerances which are listed in the

standards. The four fits are described as follows:

1 Class 1 Fit. This fit is recommended only for screw thread work

where clearance between the mating parts is essential for rapid assembly and

where shake or play is not objectional.

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