Different kinds of screws
2008-08-28

A screw is a shaft with a helical groove or thread formed on its surface and provision at one end to turn the screw. Its main uses are as a threaded fastener used to hold objects together, and as a simple machine used to translate torque into linear force. It can also be defined as an inclined plane wrapped around a shaft.

A screw used as a threaded fastener consists of a cylindrical shaft, which in many cases tapers to a point at one end, and with a helical ridge or thread formed on it, and a head at the other end which can be rotated by some means. The thread is essentially an inclined plane wrapped around the shaft. The thread mates with a complementary helix in the material. The material may be manufactured with the mating helix using a tap, or the screw may create it when first driven in (a self-tapping screw). The head is specially shaped to allow a screwdriver or wrench (British English: spanner) to rotate the screw, driving it in or releasing it. The head is of larger diameter than the body of the screw and has no thread so that the screw can not be driven deeper than the length of the shaft, and to provide compression. Screws can normally be removed and reinserted without reducing their effectiveness. They have greater holding power than nails and permit disassembly and reuse.

The vast majority of screws are tightened by clockwise rotation; we speak of a right-hand thread. Screws with left-hand threads are used in exceptional cases, when the screw is subject to anticlockwise forces that might undo a right-hand thread. Left-hand screws are used on rotating items such as the left-hand grinding wheel on a bench grinder or the left hand pedal on a bicycle (both looking towards the equipment) or hub nuts on the left side of some automobiles.

Wood screws are usually made of steel, although brass, nickel, bronze, and copper screws should be used if there is potential for corrosion. Use this type of screw to fasten pieces of metal together.

Sheet metal screws form threads in the metal as they are installed. There are several different types of sheet metal screws. Pointed panhead screws are coarse-threaded; they are available in gauges from 4 to 14 and lengths from 1/4 inch to 2 inches. Pointed panheads are used in light sheet metal. Blunt panhead screws are used for heavier sheet metal; they are available in gauges from 4 to 14 and lengths from 1/4 inch to 2 inches. Both types of panhead screws are available with either plain or Phillips-head slots.

 Partial-tapping roundhead screws have finer threads; they can be used in soft or hard metals. They are available in diameters from 3⁄16 inch to 11/4 inches. Self-tapping roundhead screws are used for heavy-duty work with thick sheet metal and are available in diameters from 1/4 inch to 2 inches and in lengths from 1/8 to 3/4 inch. Both types of roundhead screws are available with either plain or Phillips-head slots.

Machine screws are blunt-ended screws used to fasten metal parts together. They are commonly made of steel or brass. Like other fasteners, they are also made with coatings -- brass, copper, nickel, zinc, cadmium, and galvanized -- that help deter rust. Machine screws are manufactured with each of the four basic types of heads -- flathead, ovalhead, roundhead, and fillister-head -- and with both plain and Phillips-head slots. They are typically available in gauges 2 to 12 and diameters from 1/4 inch to 1/2 inch and in lengths from 1/4 inch to 3 inches. For light work, lead, plastic, or fiber plugs (called anchors) can be used to hold screws. But for larger jobs and more holding power, lead expansion anchors and lag screws are used.

Lag screws are heavy-duty fasteners. They are driven with a wrench and are used primarily for fastening to masonry or wood framing. The anchors are inserted into holes drilled in the masonry, and the lag screws are driven firmly into the anchors.