The term “class” in fastener industry usage can have different meanings depending on the characteristic it’s meant to describe, and the context of its usage. For example:
Class of Fit – All bolting components are manufactured to a specific class of fit. When making a purchase, buyers should specify a fit class that’s suitable for the fastener’s planned use. Fit classes range from Class 1 (loose) to Class 5 (very tight or “interference” fit for situations where fastener installation is meant to be permanent).
ASME engineering standard B1.1 defines fit classes and the respective allowances and tolerances permitted during the manufacturing process. The most common fit classes used for oil & gas industry bolting are Class 2 and Class 3.
When customers order bolting, fit class is commonly specified following the fastener’s diameter and thread pitch. For example, 3/4-10 2A x 6-1/4 B7 Stud defines a B7 stud whose major diameter is 3/4", thread pitch is 10 TPI, class of fit is 2A, and length is 6‑1/4”. Remember, an “A” designator as part of thread class always indicates a male thread; a “B” designator always indicates a female thread, as in 3/4-10 2B 2H HH Nut.
Class of Coating Thickness – Coating thicknesses typically are measured in microns for electroplating, and mils for spray-applied or dipped coatings. Why the difference is measurement units? Electroplating thicknesses usually are very thin - less than one mil (25.4 microns, or 0.001”), thus when a thickness is specified in microns (e.g., 5 microns or Class 5), there’s a high probability the protective coating being used on the fastener is electroplating. This thickness designation practice is not universal since some customers specify both electroplating and spray-applied or dip-type coating thicknesses in microns. Conversely, others express coating thicknesses in mils (e.g., 0.5 mil, which is the same as Class 12 or 12 micron coating thickness).
Class also can refer to the thickness of protective chromate conversion treatments applied to zinc or cadmium electroplating. The three main categories of chromate treatments are clear, yellow, or olive drab/black, each progressively thicker, and more protective, up to about Class 20 (20 microns).
Class of Material Condition – The mechanical properties of some steels can be enhanced by a process known as work hardening or “strain hardening.” Stainless grades B8 (304) and B8M (316) are particularly well suited for this process since both yield and tensile mechanical properties are increased by strain hardening. The internal crystalline structure of a steel bar undergoing strain hardening is altered such that it begins to increasingly resist further changes making the material harder and stronger in the process. Round bars typically are strain hardened by pulling the bars through a drawing die similar to wire drawing. In the un-strain hardened condition, stainless materials are called Class 1. After strain hardening, they are categorized as Class 2.