Screw
A screw is a combination of simple machines—it is, in essence, an inclined plane wrapped around a central shaft, but the inclined plane (thread) also comes to a sharp edge around the outside, which acts a wedge as it pushes into the fastened material, and the shaft and helix also form a wedge in the form of the point. 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 object that has the internal thread formed into it. Other screw threads are designed to cut a helical groove in a softer material as the screw is inserted. The most common uses of screws are to hold objects together and to position objects.
A screw will usually have a head on one end that allows it to be turned with a tool. Common tools for driving screws include screwdrivers and wrenches. 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. There are exceptions. 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. Headless set screws are also called grub screws. J-bolts have a J-shaped head that is not designed to be driven but rather is usually sunk into concrete allowing it to be used as an anchor bolt. 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. The distance between each thread is called the "pitch".
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".
Screw Head Types
Flat Countersunk HeadThe flat countersunk head has a flat top surface and a conical bearing surface with a head angle for one style of approximately 82-deg and for another style of approximately 100 deg.
Oval Countersunk Head The oval countersunk head has a rounded top surface and a conical bearing surface with a head angle of approximately 82-deg.
Undercut Flat and Oval Countersunk Heads For short lengths, 82-deg flat and oval countersunk head screws have heads undercut to 70% of normal side height to afford greater length of thread on the screws.
Flat and Oval Countersunk Trim Heads Flat and oval countersunk trim heads are similar to the 82-deg flat and oval countersunk heads except that the size of head for a given size screw is one or two sizes smaller than the regular flat and oval countersunk head size, and oval countersunk trim heads have a controlled radius where the curved top surface meets the conical bearing surface. Trim heads are furnished only in cross recessed head types.
Pan Head The slotted pan head has a flat top surface rounded into cylindrical sides and a flat bearing surface. The recessed pan head has a rounded top surface blending into cylindrical sides and a flat bearing surface.
Fillister Head The fillister head has a rounded top surface, cylindrical sides, and a flat bearing surface.
Truss Head The truss head has a low rounded top surface with a flat bearing surface, the diameter of which for a given screw size is larger than the diameter of the corresponding pan head.
Binding Head The binding head has a rounded top surface and slightly tapered sides. The bearing surface is flat and, where so specified by purchaser, slotted heads shall have an annular undercut adjacent to the shank. Binding heads are only applicable to machine screws.
Hex Head The hex head has a flat or indented top surface, six flat sides, and a flat bearing surface.
Hex Washer Head The hex washer head has an indented top surface and six flat sides formed integrally with a flat washer that projects beyond the sides and provides a flat bearing surface.
Round Head The round head has a semielliptical top surface and a flat bearing surface. In recognition of superior slot-driving characteristics of pan head screws over round head screws, and the overlap in the dimensions of cross recessed pan heads and round heads, it is recommended that pan head screws be used in new designs and, wherever possible, substituted in existing designs.
Round Washer Head The round washer head has a semielliptical top surface formed integrally with a flat washer that projects beyond the crown of the head and provides a flat bearing surface.
Flat Countersunk HeadThe flat countersunk head has a flat top surface and a conical bearing surface with a head angle for one style of approximately 82-deg and for another style of approximately 100 deg.
Oval Countersunk Head The oval countersunk head has a rounded top surface and a conical bearing surface with a head angle of approximately 82-deg.
Undercut Flat and Oval Countersunk Heads For short lengths, 82-deg flat and oval countersunk head screws have heads undercut to 70% of normal side height to afford greater length of thread on the screws.
Flat and Oval Countersunk Trim Heads Flat and oval countersunk trim heads are similar to the 82-deg flat and oval countersunk heads except that the size of head for a given size screw is one or two sizes smaller than the regular flat and oval countersunk head size, and oval countersunk trim heads have a controlled radius where the curved top surface meets the conical bearing surface. Trim heads are furnished only in cross recessed head types.
Pan Head The slotted pan head has a flat top surface rounded into cylindrical sides and a flat bearing surface. The recessed pan head has a rounded top surface blending into cylindrical sides and a flat bearing surface.
Fillister Head The fillister head has a rounded top surface, cylindrical sides, and a flat bearing surface.
Truss Head The truss head has a low rounded top surface with a flat bearing surface, the diameter of which for a given screw size is larger than the diameter of the corresponding pan head.
Binding Head The binding head has a rounded top surface and slightly tapered sides. The bearing surface is flat and, where so specified by purchaser, slotted heads shall have an annular undercut adjacent to the shank. Binding heads are only applicable to machine screws.
Hex Head The hex head has a flat or indented top surface, six flat sides, and a flat bearing surface.
Hex Washer Head The hex washer head has an indented top surface and six flat sides formed integrally with a flat washer that projects beyond the sides and provides a flat bearing surface.
Round Head The round head has a semielliptical top surface and a flat bearing surface. In recognition of superior slot-driving characteristics of pan head screws over round head screws, and the overlap in the dimensions of cross recessed pan heads and round heads, it is recommended that pan head screws be used in new designs and, wherever possible, substituted in existing designs.
Round Washer Head The round washer head has a semielliptical top surface formed integrally with a flat washer that projects beyond the crown of the head and provides a flat bearing surface.