FASTENERS. Aylin YENİLMEZ GÜRKÖK

Transcription

1 FASTENERS Aylin YENİLMEZ GÜRKÖK

2 FASTENERS A fastener is a hardware device that mechanically joins or affixes two or more objects together. Welding, Soldering, Nuts & Bolts, Washers, Screws, Clips, Clamps, Studs, Rivets, Studs, Splines/Keys/Roll Pins, Glue/Silicone/Adhesives, Cluthes, etc.

3 FASTENERS The fundamental operation in manufacture is the creation of shape - this includes assembly, where a number of components are fastened or joined together either permanently by welding for example or detachably (nonpermanent) by screws, nuts and bolts and so on. Since there is such a variety of shapes in engineering to be assembled, it is hardly surprising that there is more variety in demountable fasteners than in any other machine element. Fasteners based upon screw threads are the most common, so it is important that their performance is understood, and the limitations of the fastened assemblies appreciated.

4 FASTENERS We will deal with threaded fasteners that can be classified as "assembly elements". These elements may be used for aligning one part to another or may be used to transmit motion or force. The use of threaded fasteners is so common that they receive the most attention. A threaded fastener is a discrete piece of hardware that has internal or external screw threads. They are usually used for the assembly of multiple parts and facilitate disassembly. The most common types are the screw and nut.

5 Threaded Fasteners Did you know that? the Boeing 747 uses about 2.5 million fasteners 70,000 titanium costing $150, ,000 other fasteners costing about $250,000 30,000 squeeze rivets, 50cents each installed In certain applications (such as an engine head), you should tighten the bolt as much as possible, if it does not fail by twisting during tightening, there is a very good possibility that the bolt will never fail

6 Why are Fasteners Used? Advantages Removable Easy to install Wide variety of standard parts Disadvantages loosening failure cost

7 Fasteners External Threads (Male Threads) Bolts, Studs, Screws Internal Threads (Female Threads) Nuts, Threaded holes

8 Fasteners a Threaded Fastener similar to a nut and bolt which joins a number of components together again by transforming rotary motion into linear motion, though in this case the translation is small.

9 Screw Thread Threaded components are the primary means of assembly for interchangeable connections.

10 Screw thread Continuous helical ribs on a cylindrical shank. Screw threads are used principally for fastening, adjusting, and transmitting power To perform these specific functions, various thread forms have been developed. Pitch Pitch Notice the pitch can be measured from crestto-crest OR Root-to-Root

11 Screw thread A thread on the outside of a cylinder or cone is an external (male) thread; a thread on the inside of a member is an internal (female) thread

12 Terms Screw thread can be defined as a ridge of uniform section in the form of a helix on the external or internal surface of a cylinder or hole. 1. External thread (screw): A thread on the external surface of a cylinder. 2. Internal thread (nut): A thread on the inernal surface of a cylinder. 3. Right-hand thread: A thread that when viewed axially winds in a clockwise and receding direction. Threads are always right-hand unless otherwise specified.

13 Terms 4. Left-hand thread: A thread that when viewed axially winds in a counterclockwise and receding direction. All left-hand threads are designated LH. 5. Major diameter (nominal diameter), D: The largest diameter of a screw thread 6. Minor diameter (tap drill diameter): The smallest diameter of a screw thread. 7. Pitch diameter: The diameter of an imaginary cylinder, the surface of which cuts the thread forms where the width of the thread and groove are equal.

14 Terms The major diameter can be measured with a micrometer, caliper or steel ruler. Major diameters are generally the first numbers found in thread designations. Therefore, it is good practice to measure the major diameter over the least used section of the screw.

15 Terms The minor diameter can be determined by direct measurement on an optical comparator or by measuring the depth of the thread with a depth micrometer and subtracting twice the measured depth of thread from the major diameter.

16 Thread Profile Parameters Terminology of screw threads. Threads shown for clarity; the crests and roots are actually flattened or rounded during the forming operation Lead=L=n p

17 Thread Forms The Acme form (29 degree included angle) is only 1 degree different from the ISO Metric Trapezoidal form (30 degree included angle). Many thread forms such as Unified, Metric ISO and ACME are subject to published standards while others, including Ballscrew and Worm threads, are not defined in detail by any standards organizations.

18 Thread Forms Imperial Threaded Fasteners UNC Unified National Coarse. UNF Unified National Fine.

19 Thread Forms

20 Thread Forms Power screw thread forms. [Note: All threads shown are external (i.e., on the screw, not on the nut); d m is the mean diameter of the thread contact.

21 Thread Forms The Whitworth thread was the world's first standard, devised and specified by Joseph Whitworth in Until then, every industry had used their own screw threads. The new standard specified a 55 thread angle. 55

22 Obtaining Helical Curve External Thread Internal Thread

23 Single And Multiple Threads Most screws have single threads. Assume single thread unless otherwise noted. Multiple threads are used where parts must screw together or apart quickly. For example, the compass adjustment bar and toothpaste tube caps have multiple threads. Notice in the picture above that: Single Thread - The lead is equal to the pitch. Double Thread - The lead is twice the pitch Triple Thread - The lead is three times the pitch.

24 Single And Multiple Threads (a) Single (STANDARD), (b) double, and screws. (c) triple threaded

25 Metric Thread Specification M20 x 1.5-4g6g, 45 LG M20: The major (nominal) diameter of the thread. Appendix 13 in textbook lists metric screw thread series. 1.5: is the pitch of the thread. 4g6g: is the tolerance class. It is a combination of tolerance grade and tolerance position. It indicates the allowance and tolerance for the pitch and major diameters of external threads, and pitch and minor diameters of internal threads. Tolerance grade (e.g. 4 close tolerance, 6 medium 8 coarce tolerance); Tolerance position (e.g. g,h for external threads; G,H for internal threads ; h,h stands for zero allowance) 45 LG: Length of thread is the choice of designer. M20 x 1.5, M16 x 2 are some examples. Such notations are sometimes called basic designation. Threads are always assumed to be right handed threads unless marked with the initials LH (Left Hand) at the end of the thread note.

26 Metric Thread Specification Metric Threaded Fasteners ISO Metric International Organization for Standardization Most new vehicles have metric fasteners M6, M7, M8, M10, M12, M14 M8 X 25 X 1 Metric 8mm 25mm long Pitch (threads are 1mm apa

27 Design of Screws and Screw Symbols Screw Type Dimension Symbol Example Metric Nominal Diameter M M 24 Metric Thin Nominal Diameter x Pitch M M 24 x 2 Whitworth Nominal Diameter [inch] W W 84 x ¼ Stub Nominal Diameter x Pitch Tr Tr 20 x 4 Buttress Nominal Diameter x Pitch Te Ts 50 x 8 Square Nominal Diameter x Pitch Kr Kr 20 x 2 Knuckle Nominal Diameter x Pitch Yv Yv 20 x 2 Pipe Screw Nominal Diameter [inch] R R 1/8 Left Screw Nominal Diameter [Left] sol M 24, sol Left Thin Screw Nominal Diameter x Pitch sol M 24 x 2, sol [Left] Multiple Threads Nominal Diameter [Number of threads] Number of threads M 10, 2 threads

28 Thread Symbols

29 Thread Symbols

30 Thread Represenation A true representation of a thread is not needed on a working drawing. Symbols are used instead. Three types of symbols are in use: detailed, schematic and simplified. We will prefer simplified symbolic representation during our studies.

31 Thread Represenation Threads are drawn with thin lines as shown in this illustration. When drawn from end-on, a threaded section is indicated by a broken circle drawn using a thin line.

32 Screw Thread Male thread Threads are drawn with thin lines as shown in this illustration. When drawn from end-on, a threaded section is indicated by a broken circle drawn using a thin line. (0.25) Thin continues

33 Screw Thread Frequently a threaded section will need to be shown inside a part. The two illustrations to the left demonstrate two methods of drawing a threaded section.

34 Screw Thread Note the conventions. The hidden detail is drawn as a thin dashed line. The sectional view uses both thick and thin line with the hatching carrying on to the very edges of the object.

35 Screw Thread (0.25) Thin continues Female thread Through Usually drilled and tapped. Blind Usually drilled and tapped. (0.25) Thin continues

36 Screw Thread Male & F thread e.g. a fastener in a tapped hole. Note here that the tapped hole is sectioned, the fastener is not.

37 Fasteners Bolts, screws & studs: Threaded fasteners. Bolts have a shank partially threaded, whereas screws are threaded along the entire length.

38 Dr. Aylin Gurkok - Florida International University - College of Engineering - Department of Mechanical and Materials Engineering - Miami Fasteners This diagram gives approximate dimensioning methods for drawing hexagon headed metric bolts, nuts and plane washers.

39 Speed Nut Push-on (mostly screws) Nuts Hex Nut Most common Castle Nut locks with a cotter pin Wing Nut No tool required Locking Nut Nylon Patch (center, top) Lug Nut Taper face to help center the wheel

40 Screws Machine Screws no nut required ( usually less then ¼ Passes through one piece and threads into other. Sheet Metal Screws (self tapping) makes its own hole/threads Set Screw (allen wrench/no head) Used for aligning pullies on the shaft.

41 Washers Flat Washer Distributes pressure over a wider surface area. Lock Washer - Prevents the fastener from loosening. Used at high vibration places. Bites into the material and the fastener.