Threaded Fasteners BOLTS NUTS AND SCREWS Threaded Fasteners

Threaded Fasteners BOLTS , NUTS AND SCREWS

Threaded Fasteners n n n n Fastener is any device used to connect or join two or more parts/components. Threaded fasteners ~ bolts, screws- machine screws (cap screws), lag screws, setscrews, sheet-metal screws, self tapping screws and wood screws, nuts and studs. Bolts is threaded fastener designed to pass through holes in the mating members and to be secured by tightening a nut from the end opposite the head of the bolt Screw is threaded fastener designed to be inserted through a hole in one member to be joined and into threaded hole in the mating member. Most fasteners are made of steel because of its high strength, high stiffness, good ductility and good machinability and formability. Grade used for bolts and screws are determined by the strength of steels, tensile strength and yield strength plus proof strength which is between 0. 90 to 0. 95 times the yield strength. 3 system of grading in used n n n SAE ~ uses grade number from 1 to 8 ( higher number indicating greater strength) e. g. . J 429 Grade 1 ASTM ~ 5 standard relating to bolt strength e. g. . A 307 Grade A Metric ~ uses numerical code system ranging from 4. 6 to 12. 9 ( higher number indicating greater strength; the number before the decimal point are approximately 0. 01 times the tensile strength in MPa and the last digit with the decimal point is approximate ratio of the tensile strength to the tensile strength ) e. g. . Grade 5. 8

Thread Standard and definitions d ~ Major diameter is the largest diameter of a screw thread dr ~ Minor diameter is the smallest diameter of a screw thread dm ~ Mean diameter is the smallest diameter of a screw thread p ~ pitch is the distance between adjacent thread forms measure parallel to the thread axis. l ~ lead is the distance the nut moves parallel to the screw axis when the nut is given one turn. For single thread, the lead is the same as the pitch. For multiple-thread having two or more thread cut beside each other, the lead is the multiple of the pitch e. g. . Triplethread, lead is equal to 3 times the pitch.

Stress area of thread and Clamping Load preload or pre-tension) When design considerations are in terms of strength and size, the designer must know basic major diameter, thread pitch, and area under tensile loads. The area under tensile load for American Standard Thread (UNC or UNF) and Metric Thread are as follow: American Standard Thread At = (0. 7854) [ d – (0. 9743)p]2 ; p = 1/n where n = number of thread/in E. g. . Thread designation ~ 10 -24 UNC or 10 -32 UNF ; d= Metric Thread At = (0. 7854) [ d – (0. 9382)p]2 E. g. . Thread designation ~ M 3 x 0. 5 or M 10 x 1. 5 ; d=3 mm, p=0. 5 mm or d=10 mm, p=1. 5 mm Clamping Load T = k dn P where k is constant dependent on lubrication present =0. 15 to 0. 20 dn is nominal outside diameter P is clamping load

Constant dependent on the lubrication present For average commercial conditions, use k 1 = 0. 15 if any lubrication at all is present. Even cutting fluids or other residual deposits on the threads will produce conditions consistent with k 1 = 0. 15. If the threads are well cleaned and dried, k 1 = 0. 20 is better. Of course, these values are approximate, and variations among seemingly identical assemblies should be expected. Testing and statistical analysis of the results are recommended.

Bolted connection in shear ~ Eccentrically loaded