''A short, slender, sharp-pointed metal pin with a raised helical thread running around it and a slotted head, used to join things together.'' <ref>https://en.oxforddictionaries.com/definition/screw</ref> <br \>The As can be seen in the image below, the cylindrical portion of the screw from the underside of the head to the tip is known as the shank; it . The shank may be fully threaded or partially threaded. The Pitch is the distance between each thread is called the pitch. <ref>https://books.google.co.in/books?isbn=1317938100</ref><br \>The majority of screws have a right handed thread, meaning that they are tightened by clockwise rotation, which is termed a right-hand thread; a common . A very good mnemonic device for remembering this when working with screws or bolts is '''righty-tighty, lefty-loosey'''.<ref>http://www.solidswiki.com/index.php?title=Screws&mobileaction=toggle_view_mobile</ref>

Both screws and bolts are externally threaded fasteners. <br \>A bolt is an externally threaded fastener designed for insertion through holes in assembled parts, and . It is normally intended to be tightened or released by torquing a nut. <br \>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 . It can be tightened or released by torquing the head. A nut is not required. <ref>http://www.threadall.com/bolts </ref> <br \>

== Screw Drives <ref>http://www.fastenersuperstore.com/fastener-guides/screws-drive-styles</ref>==

Phillips drives are among the most common type of screw drive. They were developed as one of the first alternatives to the slotted drivedrives.

The slotted drive is , though the oldest drive , is still commonly used today. They are subject to slippage and provide less torque lesser torques than other drives and are subject to slippage.

Socket drive screws contain These drives have a hex shaped internal socket which is tightened or loosened using a . A hex key or allen wrenchis used to tighten or loosen it.

One This one is interesting. It is one of the few drives which is turned by using the outside of the head rather than the inside. Hex head screws are installed with a socket, usually connected to a ratcheting tool or a power driver.

Square drives These have a square shaped socket which has a slight taper as it goes deeper tapers slightly with depth into the screw head. The Strong torque is provided by the four 90° angles provide strong torque and . They decrease the possibility of slippage.

A U drive screw is also Also known as a hammer drive screw. It , it is set in a properly sized pilot hole and . It is then driven into place with a hammer. The Quite interestingly, the threads of the this screw turn as it is driven, thereby creating the fastening action.

Single slotted and Phillip Phillips screws tend to slip off while using a screwdriver to screw them.These fasteners are easily damaged at the slot. In addition, these are hard to use with a torque wrench. <br \>

== How tight should a screw be? <ref name = "allabout">https://www.elexp.com/Images/AllAboutScrews.pdf</ref> ==

Screws are very quite likely to come loose if any relative motion occurs takes place between the threads. Static friction is substantially very much higher than sliding friction, so once movement in one a direction startsis onset, it becomes much easier for unscrewing motion to happen. <br \>As a follow up, bolts Bolts are less resistant to shear (sideways) loads, so more higher clamping force and friction helps should be used to reduce these loads. <br \>As long as joint separation does not occur, additional Additional tightening preload does not buy any additional fatigue protectionas long as joint separation does not happen.

According to the Machinery's handbook, tightening by feel is only +-35% accurate, and using . Using a torque wrench only improves the accuracy to only about +-25%. These uncertainties are massivehuge, and give good reason thus, we should not to tighten too close to yield, or too loose, and also . They give a good reason to design a joint so that it will still work with a huge span wide range of possible pre-loads. A There's a method called "turn-of-nut" which can supposedly get within +-10%accuracy, but this it relies heavily on a reliable starting point from which to start counting turns . (see Machinery's handbook).

Metric fasteners are marked with two numbers separated by a decimal point, like 10.9. The 10 is 1/100th of tensile strength in MPa, and the .9 represents the ratio of yield to tensile strength. So 10.9 represents a tensile strength of 1000 MPa and yield of 900 MPa.<br \><br \>

== References ==*httpSee https://www.timcoboltdepot.co.ukcom/screwfastener-types*http:information//store.curiousinventor.com/guides/Metal_Working/Screws*https://en.wikipedia.org/wiki/Screw*Unbrako Engineering Guide"*Fundamentals of Machine Component Design," by Robert C. Juvinall, Kurt M. Marchek*The Machinery's Handbook, 27th ed.*"Handbook of Bolts materials-and Bolted Joints," by John Herbert Bickford, Sayed Nassar*"What Every Engineer Should Know About Threaded Fasteners: Materials and Design," by Alexander Blake*http://esmat.esa.int-grades/ecssbolt-qgrade-st-70-46cchart.aspx for more information on these marking schemes.pdf