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UNIT 5 DESIGN OF SCREWS, FASTENERSAND POWER SCREWSDesign of Screws,Fasteners and ometry of Thread5.3Mechanics of Screw and Nut Pair5.4Power Screw Mechanics5.5Application of Power Screw5.6Standard Threads5.7Design of Screw and Nut5.8Threaded Fastener5.9Failure of Bolts and Screws5.10 Permissible Stresses in Bolts5.11 Summary5.12 Key Words5.13 Answers to SAQs5.1 INTRODUCTIONScrews are used for power transmission or transmission of force. A screw is a cylinderon whose surface helical projection is created in form of thread. The thread will havespecified width and depth, which bear some ratio with the diameter of the cylinder. Thescrew rotates in a nut, which has corresponding helical groove on the internal surface.Thus a nut and a screw make a connected pair in which one remains stationary whileother rotates and translates axially. The helical surface of the screw thread makes surfacecontact with the helical groove surface of the nut. If an axial force acts on, say screwmoving inside stationary nut, the point of application of the force will move as the screwadvances in axial direction. This will result in work being done and hence power beingtransmitted. Both types – one in which screw rotates and advances in a stationary nut orone in which screw rotates between fixed support and nut is free to move axially – areused in practice. In the latter case the force acting on nut will move as nut translates.However, the friction between the surfaces of contact will require some power to beovercome. Hence the power delivered by the screw-nut pair will be less than the powersupplied.The contact surfaces of screw thread and nut groove are made perpendicular to theoutside and inside cylindrical surfaces. They are sometimes given a small inclination.Such provision keeps coefficient of friction to a reasonable low level. The coefficient offriction may be further reduced by lubrication. However, by creating considerablyinclined surfaces in nut and screw the effective coefficient of friction is increased. Suchscrew thread joint will make advancing of threaded part difficult. This combination willbe used as fastening device.ObjectivesAfter studying this unit, you should be able to describe geometry of screw and nut, define mechanics of screw and nut,105

Machine Design determine forces on screw and nut threads, calculate dimensions of screw and nut for transmission of force, and find the force on screw fastener and load transmitted to parts jointed byfasteners.5.2 GEOMETRY OF THREADLook at Figures 5.1(a) and (b) and you will get a fair idea how would a screw and a nutappear. The screw will pass into nut by rotating either of them. For understanding how ahelical thread can be formed on a cylinder you can take a plane sheet of paper and drawan inclined line on it. Then roll the paper to form a cylinder by bringing two oppositeedges of the paper together. The line which you drew will appear like a helix on thesurface of the cylinder. A line drawn as AC, inclined at angle with horizontal line AA1,will be wrapped on the cylinder to AA , looking like a helix. AA1, becomes thecircumference of the cylinder and A coincides with C. In Figure 5.2, you can see twoparallel lines AC and A C drawn inclined at to horizontal and then paper wrapped toform a cylinder and thus two threads are formed on cylinder. p is the vertical distancebetween A and C or between C and C . If the paper is wrapped such that the lines drawnare on the inside surface, you can get the idea of internal thread.dP/2d1DopDp/2(a) A Screw(b) A NutFigure 5.1 : Screw and Nut with Helical Surfaces Cut on Outside and InsideSurfaces of Cylinders, respectivelyC’C’C”A’CPA”dAdA1Figure 5.2 : Formation of a Helix on the Cylindrical Surface106The distance A1C which is equal to AA and A C is called the pitch of the screw. Pitch isapparently the distance between two corresponding points on two consecutive threads.The angle between the base of the triangle and hypotenuse becomes the angle of helix.Obviously,