Screw
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.

The screw as simple machine
The screwpump was invented by Sennacherib (r. 705–681 BC) and improved by Archimedes (c. 287–212 BC).[1] The device is a large screw in a pipe used in antiquity and to the present day to pump liquids.A screw may be used to move one object, with a threaded hole right through it, relative to another object to which one end of the screw is fastened allowing it to rotate but not to move axially. A vise tool is an example of this use. In effect, a wood screw also acts this way.
Because it moves forward in a screwing motion, the propeller of a ship is called a screw. Likewise, the propeller of an aircraft is called an airscrew.

Threaded fastener
The remainder of this article is mainly concerned with the screw as threaded fastener.
A screw used as a threaded fastener consists of a shaft, usually mainly cylindrical and in many cases tapering to a point at one end and with a helical ridge or thread formed on it, and a head at one 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).Threaded fasteners were made by a cutting action such as dies provide, but recent advances in tooling allow them to be made by rolling an unthreaded rod (the blank) between two specially machined dies which squeeze the blank into the shape of the required fastener, including the thread. This method has the advantages of work hardening the thread and saving material. A rolled thread can be distinguished from a thread formed by a die as the outside diameter of the thread is greater than the diameter of the unthreaded portion of the shaft. Bicycle spokes, which are just bolts with long thin unthreaded portions, always use rolled threads for strength.

Differentiation between bolt and screw

A bolt passes through a hole of larger diameter than its thread, and is held in place by a nut or similar device; it is not designed to be turned. What is often referred to as a bolt is in fact a cap screw, which is designed to be turned (or screwed). Cap screws may, or may not be used with nuts. The distinction is subtle, but significant in the design of the fastener. If threaded all the way to the back of the head a cap screw becomes a machine screw.

Anchor bolts :: Bolt head T Arranged by Brooks LTD

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