The Nuts and Bolts of the Screw

In previous articles we’ve discussed types and grades of stainless steel and special alloys such as A286, Inconel, Monel, MP35N, Invar 36, and others. However, it’s only when these materials get made into useful products that their value becomes evident.

The lynch pin of products is that of the fastener, for without fasteners everything would of course fall apart. The most basic of all fasteners is likely the simple but invaluable screw. From the humble beginnings of the screw, has arisen an impressive variety of fasteners meeting the needs of today’s petroleum, medical, aerospace, energy, and computer industries.

A short history of the screw: It is often said that necessity is the mother of invention. Well a Greek named Archimedes needed a better way to pump water. He produced what was a giant screw inside a hollow tube. When turned, “the screw” would draw water up.

The basic shape proved to be very functional and by the first century people were soon employing hand-made wooden screws in the workings of presses for wine and olives. Unfortunately, like snowflakes, no two screws were alike. This limited the screw’s usefulness for the time being. However, the Industrial Revolution saw mass and standard production. Additionally, the transition of screws from wood to metal completed the process of making fasteners a mainstay in the pursuit of manufacturing advances. By the late 18th century the world saw fasteners with strength and consistency of production.

Now, what makes a screw a screw (and not a bolt)? First, it has a spiral threading around a shank. Next, the head of the screw is typically wider than the shank. Screws are typically tapered and self-threading whereas a bolt is tightened or released by torquing a nut and usually isn’t pointed or even tapered at all.

Huge Variety: Some of the basic categories of screws and fasteners include machine screws, cap screws, tapping screws, nuts, bolts, flat washers, locking washers, and pins. Within those categories, the combination of options and material type (e.g. stainless steel), make the total number of unique products almost endless.

As an example, the basic cap screw is available with a number of heads, including, button head, hex head, socket head, flat socket head and Torx drive. There are a variety of widths and diameters. The product can be made to meet certain specifications (ASTM, ANSI, AISI, AN, MS, NAS, NASM). Finally, it can consist of various metals, stainless steel, or other special alloy materials such as 18-8, A286, Monel, or Alloy 20 to list just a few (see the Ocean State Stainless stock item web page for just a small sampling of what is typically stocked and available). Top off with lines of commercial grades such as 302, 303, 304 and 316 stainless and there become too many variations to list.

Innovation: We’ll see innovation surrounding this simple yet vital fastener continue to be pushed as the precision requirements of high-tech industries, such as aerospace and medicine, demand more strength and versatility.

Today’s high-end fasteners are often made from special materials, such as A286 (a favorite of the aerospace industry) Inconel 600, 625, 718 and various other exotic metals. In tomorrow world, we are likely to see refinements and possibly even a whole new generation of alloys.