Some of these issues are discussed below:ĪSME standards specify a variety of "Machine Screws" in diameters ranging up to 0.75 in (19.05 mm). The issue of what is a screw and what is a bolt is not completely resolved with Machinery's Handbook distinction, however, because of confounding terms, the ambiguous nature of some parts of the distinction, and usage variations. This distinction is consistent with ASME B18.2.1 and some dictionary definitions for screw and bolt. (Example: round head bolts, track bolts, plow bolts.) An externally threaded fastener that has thread form which prohibits assembly with a nut having a straight thread of multiple pitch length is a screw. An externally threaded fastener which is prevented from being turned during assembly and which can be tightened or released only by torquing a nut is a bolt. A screw is an externally threaded fastener capable of being inserted into holes in assembled parts, of mating with a preformed internal thread or forming its own thread, and of being tightened or released by torquing the head. Machinery's Handbook describes the distinction as follows:Ī bolt is an externally threaded fastener designed for insertion through holes in assembled parts, and is normally intended to be tightened or released by torquing a nut. Part of the confusion over this is likely due to regional or dialectical differences. So, as a general rule, when buying a packet of "screws" you would not expect nuts to be included, but bolts are often sold with matching nuts. A simple distinction that is often true, although not always, is that a bolt passes through a substrate and takes a nut on the other side, whereas a screw takes no nut because it threads directly into the substrate (a screw screws into something, a bolt bolts several things together). There is no universally accepted distinction between a screw and a bolt. More generally, screw may mean any helical device, such as a clamp, a micrometer, a ship's propeller or an Archimedes' screw water pump. The right side pedal of a bicycle has a left-hand thread. For example, when the screw will be subject to counterclockwise torque (which would work to undo a right-hand thread), a left-hand-threaded screw would be an appropriate choice. Screws with left-hand threads are used in exceptional cases. If the screw is right-handed (most screws are) and you turn the screw in the direction of your fingers the screw will move in the direction of your thumb.
The majority of screws are tightened by clockwise rotation, which is termed a right-hand thread a common mnemonic device for remembering this when working with screws or bolts is "righty-tighty, lefty-loosey." Another rule is this: curl the fingers of your right hand around the screw with your thumb pointing is the direction you want the screw to go. The distance between each thread is called the "pitch". The cylindrical portion of the screw from the underside of the head to the tip is known as the shank it may be fully threaded or partially threaded. There are exceptions for instance, carriage bolts have a domed head that is not designed to be driven set screws often have a head smaller than the outer diameter of the screw J-bolts have a J-shaped head which is not designed to be driven, but rather is usually sunk into concrete allowing it to be used as an anchor bolt.
The head is usually larger than the body of the screw, which keeps the screw from being driven deeper than the length of the screw and to provide a bearing surface. Common tools for driving screws include screwdrivers and wrenches. The most common uses of screws are to hold objects together and to position objects.Ī screw will usually have a head on one end that contains a specially formed shape that allows it to be turned, or driven, with a tool. Other screw threads are designed to cut a helical groove in a softer material as the screw is inserted. Some screw threads are designed to mate with a complementary thread, known as a female thread (internal thread), often in the form of a nut or an object that has the internal thread formed into it. 3.5 List of abbreviations for types of screwsĪ screw is an inclined plane wrapped around a nail.3.4.1 Superbolt, or multi-jackbolt tensioner.3.1 Fasteners with a tapered shank (self-tapping screws).2.7 Controlled vocabulary versus natural language.2 Differentiation between bolt and screw.
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