MT-253 MACHINE DESIGN

1. MT-253 MACHINE DESIGN Lecture #7 Syed EhtishamGillani Lecturer Department of Mechanical Engineering Technology University of Technology Nowshera

2. Fasteners • A machine or a structure is made up of large number of parts and they are required to be joined properly for the machine to be fully functional. • These machine parts are joined with each other with the help of fasteners. • The basic purpose of fasteners are: • To hold parts together • To transfer power

3. Fasteners • Fasteners are divided into two major categories: • Temporary Fasteners • Threaded fasteners – screws, bolts and nuts, studs • Keys and Pins • Permanent Fasteners • Riveted joints • Welded joints

4. Threaded fasteners Common types of threaded fasteners are: • Bolts • Screws • Studs Bolt: Bolts are defined as headed fasteners having external threads and a non-tapered end supplied with a nut with internal threads. Screw: Screws are defined as headed, externally-threaded fasteners that do not require a nut for its function. Screws are generally tapered that cut their own internal threads when installed. Stud: They are externally threaded head-less fasteners whose end usually meets a standard nut.

5. Threaded fasteners Classification of Screws on the basis of Head Types • FlatA countersunk head with a flat top.Abbreviated FH • OvalA countersunk head with a rounded top.Abbreviated OH or OV • PanA slightly rounded head with short vertical sides.Abbreviated PN • TrussAn extra wide head with a rounded top. • RoundA domed head.Abbreviated RH

6. Threaded fasteners • HexA hexagonal Head.Abbreviated HH or HX • Hex WasherA hex head with built in washer. • Slotted Hex WasherA hex head with built in washer and a slot. • Socket Head CapA small cylindrical head using a socket drive. • ButtonA low profile rounded head using a socket drive.

7. Threaded fasteners Classification of Screws on the basis of Drive Types • Phillips An X shaped drive.Abbreviated - PH • SlottedA slot in the head.Abbreviated - SL • CombinationA combination of slotted and Phillips drives.Abbreviated - Combo • Socket, Hex or AllenA hexagonal hole for use with an Allen wrench.

8. Threaded fasteners Definitions related to threads THREAD A thread is a ridge of uniform section in the form of a helix on the external or internal surface of a rod. PITCH The Pitch of a thread P is the distance from a point on one thread to the corresponding point on the next thread. LEAD The Lead L is the parallel distance a bolt/screw will travel during one complete revolution. AXIS OF A THREAD This is an imaginary line running longitudinally through the center of the screw.

9. Threaded fasteners Definitions related to threads

10. Threaded fasteners Definitions related to threads • Crest: Crest of the thread is the top most surface joining the two sides. • Root: Root of the thread is the bottom of the groove between the two flanks. • Flank: Flank of thread are straight edges which connect the crest with the root of the thread. • Depth of thread: Depth of a thread is the distance between the crest and root of the thread.

11. Threaded fasteners Definitions related to threads • Major diameter: It is an imaginary largest diameter of the thread which would touch the crests of an internal or external thread. • Minor diameter: It is an imaginary smallest diameter of the thread which would touch the roots of an external thread. • Pitch diameter: It is a theoretical diameter between the major and minor diameter of screw threads.

12. Threaded fasteners Different types of threads • Isometric Thread • Unified National Screw Thread (UNC and UNF) • American National Screw Thread • Square Thread • Acme Thread • Buttress Thread • Worm Thread • Knuckle Thread

13. Threaded fasteners Different types of threads

14. Threaded fasteners Different types of threads

15. Threaded fasteners Threads representation in drawings • Simplified • Detailed • Schematic

16. Threaded fasteners Right and Left hand Threads Note: Threads are assumed to be right hand unless otherwise designated.

17. Threaded fasteners Single, Double and Triple Threads • Single Thread The type of threads in which Lead equals to Pitch of the thread. • Double Thread The type of threads in which Lead equals to two times of the Pitch. • Triple Thread The type of threads in which Lead equals to three times of the Pitch.

18. LEAD = 1P LEAD = 3P LEAD = 2P Threaded fasteners Single, Double and Triple Threads

19. Threaded fasteners Inch and Metric Threads • Threads are commercially available as inch and metric threads. • Both of these threads are represented in different ways. • Let us see how can we understand the commercially available threads and their representation.

20. Threaded fasteners Inch Threads • Pitch for Inch threads: PITCH =1 / No of threads per inch • Thread Series: Threads series are for fasteners with different diameters and the same No of threads per inch • UNC Unified Course Thread Series • UNF Unified Fine Thread Series • UNEF extra fine thread series

21. Threaded fasteners Inch Threads Thread Classes1A, 2A, 3A = External Threads1B, 2B, 3B = Internal Threads Class of fit 1A,1B: Loosest fit 2A,2B: Good grade fit of commercial product 3A,3B: High-grade fit commercial products

22. Threaded fasteners Metric Threads Pitch Pitch for metric threads is the distance between corresponding points on adjacent teeth. Thread Series • Course-thread is intended for general engineering work and commercial applications • Fine-thread series is also for general use. Compared with coarse, a fine-thread screw is stronger in both tensile and torsional strength Tolerance grades The larger the number, the larger the tolerance • Tolerance position e (large allowance) • Tolerance position g (small allowance) • Tolerance position h (no allowance)

23. Threaded fasteners Metric Threads • Nominal size and pitch expressed in millimeters. • Example 1: M16 x 1.5 – 5g Metric designation Nominal diameter Pitch Tolerance position & grade • Example 2: M10 x 1.5 x 25 Ф10mm 1.5 pitch 25 mm long