184 Chapter 7
- Class 1A and 1B: for work of rough commercial quality where loose fit, quick assembly, and
rapid production are important and shake or play is not objectionable. - Class 2A and 2B: the recognized standard for normal production of the great bulk of commer-
cial bolts, nuts, and screws. Classes 2A and 2B provide a small amount of play to prevent galling
and seizure in assembly and use and sufficient clearance for some plating. - Class 3A and 3B: these 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. This fit represents an exceptionally high quality of commercially threaded product and
is recommended only in cases where the high cost of precision tools and continual checking are
warranted. - Class 4: this is a theoretical rather than practical class and is now obsolete.
- Class 5: for a wrench fit, used principally for studs and their mating tapped holes or a force fit re-
quiring the application of a high torque for semipermanent assembly. It is also a selective fit if ini-
tial assembly by hand is required. It is not as yet adaptable to quantity production.
Figure 7.18 Right- and left-handed thread screw types.