== 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