CH 8 - CH 8: Screws, Fasteners, and the Design of...

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Shigley’s Mechanical Engineering Design, 8 th Ed. Class Notes by: Dr. Ala Hijazi CH 8 Page 1 of 14 CH 8: Screws, Fasteners, and the Design of Non-Permanent Joints This chapter introduces non-permanent joining elements such as bolts, nuts, setscrews rivets, pins, keys, etc. It also introduced power screws which changes angular motion to linear motion, where it is similar in principle to screws and bolts. Thread Standards and Definitions The terminology of screw threads is illustrated in the figure. Pitch ( ݌ ): the distance between adjacent threads measured parallel to thread axis. Major diameter ( ܦ ): the largest diameter of the screw thread. Minor diameter ( ܦ ): also called “ root diameter ”, is the smallest diameter of the screw thread. Mean diameter ( ܦ ): also called “ pitch diameter ”, the average diameter of the screw thread ( considering the theoretical full height of the threads ). Lead ( ݈ ): the distance a nut moves parallel to the screw axis when it rotates one full turn. For a single thread screw the lead is same as the pitch. For multiple thread screws ( two or more threads run beside each other ) the lead equals the pitch multiplied by the number of threads All threads are usually right-handed unless otherwise is indicated. Tensile tests showed that a thread road has a tensile strength equal to that of an unthreaded rod having diameter equal to the average of the pitch diameter and minor diameter of the threaded rod. Threads are standardized and there are two standards: Metric ( ISO ) and American ( Unified ). In both standards the thread angle is ͸Ͳι .
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Shigley’s Mechanical Engineering Design, 8 th Ed. Class Notes by: Dr. Ala Hijazi CH 8 Page 2 of 14 Metric ( ISO ) : There are two standard profiles M and MJ where both have a similar geometry but the MJ has a rounded fillet at the root and a larger minor diameter and therefore it has a better fatigue strength . Metric bolts are specified by the major diameter and the pitch ( both in mm ). h Example : ܯͳͲ ൈ ͳǤͷ ( 10 mm diameter and 1.5 mm pitch). x Table 8-1 gives the standard sizes of Metric bolts along with the effective tensile stress area and the root diameter area ( which is used when the bolt is subjected to shear loading ). h Note that there is Coarse-pitch and Fine-pitch ( more threads ) where the fine- pitch has better tensile strength. American ( Unified ) : There are two standard profiles UN and UNR where the UNR has a filleted root and thus better fatigue strength . Unified threads are specified by the major diameter ( in inch ) and the number of threads per inch ( ܰ ). h Example : ͲǤʹͷ–ʹͲ ܷܰܥ x Table 8 -2 gives the standard sizes along with the tensile stress areas and root diameter areas ( used for shear loading ) for Unified bolts ( Coarse and Fine series ) h Note that for diameters smaller than 1/4 inch , the size is designated by size numbers rather than diameter. For screws used to transmit power (
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This note was uploaded on 10/25/2010 for the course MECHINCAL 2010 taught by Professor علاءحجازي during the Spring '10 term at Hashemite University.

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CH 8 - CH 8: Screws, Fasteners, and the Design of...

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