Grade Markings and Mechanical Properties of Steel Fasteners

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Steel and stainless steel fasteners, such as screws, bolts and studs are, like many manufacturing tools, governed by an official grading system developed by the International Organization for Standardization (ISO). In screws, bolts and studs, these standards refer to thread grade markings, or the standard measurements for thread sizes and dimensions. But there are a number of de facto standards among these fasteners which govern a wider variety of aspects, such as head size and shape, shaft length, and drive type.
The Differences Between Bolts, Screws and Studs
One aspect of steel fasteners, for which there is no set of standards is a proper differentiation definition for bolts, screws and studs. A screw is a metal cylinder around which is wrapped a helical inclined plane. The cylinder is tapered at one end to a point, while the other is capped by a head with a groove for driving the screw into place. A bolt generally does not feature the tapered, pointed end, so it must be driven into a pre-tapped or drilled hole. A stud consists only of the cylindrical shaft portion of the screw, featuring neither the tapered point nor the drive head. Studs can be threaded all along the length of the shaft, or only at one or both ends with a smooth section in the middle. Because they have no tapered point, they need to be inserted into pre-tapped holes.
Head Size and Shape
Bolt and screw head shapes are different and function based on different application needs, as well as which company is using them. Certain companies and patented technologies use specific fasteners that are exclusive to a given product. The common head shapes are:
• Pan heads are circular and flat on top with beveled edges on the sides.
• Button heads have a rounded top with beveled edges on the sides.
• Round heads resemble half a sphere.
• Truss heads are flattened discs.
• Flat (Countersunk) heads resemble an upside down cone, with the tapered edges meeting at the cylindrical top of the shaft. This shape helps the head surface remain level with the surface of the material into which it's been screwed.
• Oval heads are similar to countersunk heads in that they taper inward, but the head surface is more convex.
Other head types include hex head, which is hexagonally shaped, socket head, for socket bolts, and cheese head.
Shaft Length and Thread Standards
Fastener thread standards are commonly judged by ISO standards, as laid out in ISO 68-1, ISO 261, ISO 262, and ISO 965-1. These standards assign measurements to stand combinations of height and pitch of thread depth for screws, bolts and studs. It is the international standard generally adhered to, but the United States uses its own system, known as the Unified Thread Standard. Basically, this standard exists as two numbers separated by a dash—the first number representing the diameter and the second number representing threads per inch. They are differentiated by fine and course varieties.
Mechanical Properties
The main mechanical property concerns for fasteners are the proof load, the yield strength and the tensile strength. The proof load, measured in pounds per square inch, is the minimum acceptable load a fastener will withstand. The yield strength is the load at which the fastener will become permanently deformed. The tensile, or ultimate, strength, is the load at which a fastener will break. These properties will be different depending on the type of material used, the shaft length and other reasons. It is best to find what loads specific fasteners can endure.

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