Types of screws and bolts

Types of screws and bolts

Types of screws and bolts

Threaded fasteners either have a tapered shaft or a non-tapered shaft. Fasteners with tapered shafts are designed to either be driven into a substrate directly or into a pilot hole in a substrate. Mating threads are formed in the substrate as these fasteners are driven in. Fasteners with a non-tapered shaft are designed to mate with a nut or to be driven into a tapped hole.

 Fasteners with a tapered shaft (self-threading screws)

• Screw 

• There is not a universally accepted definition of the word, screw. It generally refers to a smaller threaded fastener with a tapered shaft. See the section Differentiation between bolt and screw above for a more detailed discussion.

• Wood screw 

• Generally has an unthreaded portion of the shaft below the head. It is designed to attach two pieces of wood together. Lag screw (lag bolt) 

• Similar to a wood screw except that it is generally much larger running to lengths up to 15 inches (381 mm) with diameters from ¼" to ½" (6.4–12.25 mm) in commonly available (hardware store) sizes (not counting larger mining and civil engineering lags and lag bolts) and it generally has a hexagonal head drive head. Lag bolts are designed for securely fastening heavy timbers (post and beams, timber railway trestles and bridges) to one another, or to fasten wood to masonry or concrete.

• Lag bolts are usually used with an expanding insert called a lag in masonry or concrete walls, the lag manufactured with a hard metal jacket that bites into the sides of the drilled hole, and the inner metal in the lag being a softer alloy of lead, or zinc amalgamated with soft iron. The coarse thread of a lag bolt and lag mesh and deform slightly making a secure near water tight anti-corroding mechanically strong fastening.

• Coach bolts 

• rather like lag bolts, but normally have a square (4 sided) head, rather than a hexagon. Formally used in (horse-drawn) coaches, and have a large thread with good holding force, and an un-threaded part, rather like a giant wood screw.

• Sheet metal screw (self-tapping screw, thread cutting screws) 

• Has sharp threads that cut into a material such as sheet metal, plastic or wood. They are sometimes notched at the tip to aid in chip removal during thread cutting. The shaft is usually threaded up to the head. Sheet metal screws make excellent fasteners for attaching metal hardware to wood because the fully threaded shaft provides good retention in wood.

• Self-drilling screw (Teks screw) 

• Similar to a sheet metal screw, but it has a drill-shaped point to cut through the substrate to eliminate the need for drilling a pilot hole. Designed for use in soft steel or other metals.

• Drywall screw 

• Specialized screw with a bugle head that is designed to attach drywall to wood or metal studs, however it is a versatile construction fastener with many uses. The diameter of drywall screw threads is larger than the shaft diameter.

• Particle board screw (chipboard screw) 

• Similar to a drywall screw except that it has a thinner shaft and provides better resistance to pull-out in particle board, while offset against a lower shear strength.

• Deck screw 

• Similar to drywall screw except that it is has improved corrosion resistance and is generally supplied in a larger gauge.

• Double ended screw (dowel screw) 

• Similar to a wood screw but with two pointed ends and no head, used for making hidden joints between two pieces of wood.

• Screw eye (eye screw) 

• Screw with a looped head. Larger ones are sometimes call lag eye screws. Designed to be used as attachment point, particularly for something that is hung from it.

 Fasteners with a non-tapered shaft

• Bolt 

• There is no universally accepted definition of the word bolt. It generally refers to a larger threaded fastener with a non-tapered shaft. See the section Differentiation between bolt and screw above for a more detailed discussion.

• Cap screw 

• In places the term is used interchangeably with bolt. In the past the term cap screw was restricted to threaded fasteners with a shaft that is threaded all the way to the head, but this is now a non-standard usage.

• Hex cap screw 

• Cap screw with a hexagonal head, designed to be driven by a wrench (spanner). An ASME B18.2.1 compliant cap screw has somewhat tighter tolerances than a hex bolt for the head height and the shaft 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.

• Hex bolt 

• At times the term is used interchangeably with hex cap screw. An ASME B18.2.1 compliant hex bolt is built to different tolerances than a hex cap screw.

• Socket cap screw 

• Also known as a socket head cap screw, socket screw or Allen bolt, this is a type of cap screw with a hexagonal recessed drive. The most common types in use have a cylindrical head whose diameter is nominally 1.5 times (1960 series design) that of the screw shank (major) diameter. Counterbored holes in parts allow the screw head to be flush with the surface or recessed. Other head designs include button head and flat head, the latter designed to be seated into countersunk holes. A hex key (sometimes referred to as an Allen wrench or Allen key) or hex driver is required to tighten or loosen a socket screw. Socket screws are commonly used in assemblies that do not provide sufficient clearance for a conventional wrench or socket.

• Machine screw 

• Generally a smaller fastener (less than ¼ inch in diameter) threaded the entire length of its shaft that usually has a recessed drive type (slotted, Phillips, etc.). Machine screws are also made with socket heads (see above), in which case they may be referred to as socket head machine screws.

• Self-tapping machine screw 

• Similar to a machine screw except the lower part of the shaft is designed to cut threads as the screw is driven into an untapped hole. The advantage of this screw type over a self-tapping screw is that, if the screw is reinstalled, new threads are not cut as the screw is driven.

• Set screw (grub screw) 

• Generally a headless screw but can be any screw used to fix a rotating part to a shaft. The set screw is driven through a threaded hole in the rotating part until it is tight against the shaft. The most often used type is the socket set screw, which is tightened or loosened with a hex key or hex driver.

• Tap bolt 

• A bolt that is threaded all the way to the head. An ASME B18.2.1 compliant tap bolt has the same tolerances as an ASME B18.2.1 compliant hex cap screw.

• Stud 

• similar to a bolt but without the head. Studs are threaded on both ends. In some cases the entire length of the stud is threaded, while in other cases there will be an un-threaded section in the middle. (See also: screw anchor, wedge anchor.)

• Eye bolt 

• A bolt with a looped head.

• Toggle bolt 

• A bolt with a special nut known as a wing. It is designed to be used where there is no access to side of the material where the nut is located. Usually the wing is spring loaded and expands after being inserted into the hole.

• Carriage bolt (coach bolt) 

• Has a domed or countersunk head, and the shaft 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.

• Stove bolt 

• A type of machine screw that has a round or flat head and is threaded to the head. They are usually made of low grade steel, have a slot or Philips drive, and are used to join sheet metal parts using a hex or square nut.

• Shoulder screw 

• Screw used for revolving joints in mechanisms and linkages. A shoulder screw consists of the shaft, which is ground to a precise diameter, and a threaded end, which is smaller in diameter than the shaft. Unlike other threaded fasteners, the size of a shoulder screw is defined by the shaft diameter, not the thread diameter. Shoulder screws are also called stripper bolts, as they are often used as guides for the stripper plate(s) in a die set.

• Thumb screw 

• A threaded fastener designed to be twisted into a tapped hole by hand without the use of tools.

• Tension control bolt (TC bolt) 

• Heavy duty bolt used in steel frame construction. The head is usually domed and is not designed to be driven. The end of the shaft has a spline on it which is engaged by a special power wrench which prevents the bolt from turning while the nut is tightened. When the appropriate torque is reached the spline shears off.

• Plow bolt 

• A bolt similar to a carriage bolt, except the head is flat or concave. There are many variations, with some not using a square base, but rather a key, a locking slot, or other means. The recess in the mating part must be designed to accept the particular plow bolt.

 Other threaded fasteners

• Thread rolling screws 

• These have a lobed (usually triangular) cross section. They form threads by pushing outward during installation. They may have tapping threads or machine threads.

• Superbolt, or multi-jackbolt tensioner 

• Alternative type of fastener that retrofits or replaces existing nuts, bolts, or studs. Tension in the bolt is developed by torquing individual jackbolts, which are threaded through the body of the nut and push against a hardened washer. Because of this, the amount of torque required to achieve a given preload is reduced. Installation and removal of any size tensioner is achieved with hand tools, which can be advantageous when dealing with large diameter bolting applications.

• Hanger screw 

• 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.