A 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. 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. 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. (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. (Example: wood screws, tapping screws.) ASME B18.2.1
CapscrewsCap screws with a hexagonal head are designed to be driven by a wrench (spanner). An ASME B18.2.1 compliant hex cap screw has somewhat tighter tolerances than a hex bolt for the head height and the shank length. The nature of the tolerance difference allows an ASME B18.2.1 hex cap screw to always fit where a hex bolt is installed but a hex bolt could be slightly too large to be used where a hex cap screw is designed in. The bearing surface of a hex cap screw is flat and washer faced.
Carriage BoltsA carriage bolt or coach bolt has a domed or countersunk head, and the shank is topped by a short square section under the head. The square section grips into the part being fixed (typically wood), preventing the bolt from turning when the nut is tightened. A rib neck carriage bolt has several longitudinal ribs instead of the square section, to grip into a metal part being fixed. Carriage bolts are designed to be used in applications that require a low profile head.
Elevator BoltsElevator bolts are similar to a carriage bolt, except the head is thin and flat. There are many variations. Some do not have a square base, but rather triangular sections of the flat head are folded down to form "fangs" that cut into wood and hold it secure.
Eye BoltsEye Bolts are bolts with a looped head that is forged without welds. They can be used in various assemblies or secured in a variety of materials such as wood depending on the specific type of eye bolt used. They are commonly used for tie-downs, lifting and attaching cables.
Flange BoltsFlange Bolts, also called Hex Flange Screws an sometimes Frame Bolts, incorporate a large bearing face (flange) into a one piece head design. The large bearing area under the head distributes clamp load over a larger area to resist fatigue loads; minimizes indentation of the head into the material being bolted together to prevent loosening; increases the heads resistance to rotation to prevent loosening in high vibration applications; and eliminates the need for a flat washer.
Hangar BoltsHangar bolts are a headless fastener that has machine screw threads on one end and self-tapping threads on the other designed to be driven into wood or another soft substrate. Often used for mounting legs on tables.
Hex BoltsHex Bolts are often confused with hex cap screws thanks to their similar hexagonal head. An ASME B18.2.1 compliant hex bolt is built to different tolerances than a hex cap screw and does not have the washer faced bearing surface like a cap screw. Tap Bolts are hex bolts that are fully threaded.
Lag BoltsTypically square or hex headed, lag bolts have a gimlet point and self-tapping threads and are used to secure assemblies to wood.
Sex BoltsA Sex Bolt, also known as a barrel nut, barrel bolt, Chicago screw or post and screw, is a type of fastener (nut) which has a barrel-shaped flange and protruding boss that is internally threaded. The boss sits within the components being fastened, and the flange provides the bearing surface. The sex bolt and accompanying machine screw sit flush on either side of the surfaces being fastened. It is normally chosen because of its low profile compared to other nuts. The sex bolt often has a built-in feature, such as a slot, to aid in tightening the fastener. Some sex bolts, more commonly known as architectural bolts, have knurled barrels to allow one-sided assembly. Binding posts are similar to architectural bolts in that they are designed to be assembled from one side, but they have teeth on the flanged surface to keep them fixed. They were originally designed for the printing industry to bind large catalogs together.
Shoulder BoltsA shoulder bolt, or shoulder screw or stripper bolt differs from machine screws and other bolts in that the shank is ground to a precise diameter, known as the shoulder, and the threaded portion is smaller in diameter than the shoulder. Shoulder bolt specifications call out the shoulder diameter, shoulder length, and threaded diameter; the threaded length is fixed, based on the threaded diameter, and usually quite short. It is usually used for revolving joints in mechanisms and linkages; when used as a guide for the stripper plate in a die set its called a stripper bolt.