ME3180 ME 3180 - Mechanical Engineering Design Screws and Fasteners Lecture Notes #1 The George W. Woodruff School of Mechanical Engineering ME3180 Screws and Fasteners Success or failure of design can hinge on proper selection and use of its fasteners Design and manufacture of fasteners are big businesses and are significant part of our economy Thousands to millions of fasteners are used in single complex assembly such as automobile, ship, train or aircraft Boeing 747 uses about 2.5 million fasteners, some of which cost several dollars each The George W. Woodruff School of Mechanical Engineering ME3180 Screws and Fasteners Cont’d Variety of fasteners are available commercially, such as shown below FIGURE 14 – 1 (Norton) A Sample of the Variety of Commercially Available Fasteners The George W. Woodruff School of Mechanical Engineering Screws and Fasteners Cont’d ME3180 Discussions in this class will be limited to design, analysis and selection of conventional fasteners such as bolts, screws, nuts, etc., used for machine design applications in which significant loads and stresses are encountered Screws/bolts are used both to hold things together as fasteners and to move loads as power screws or lead screws • Will investigate both of these applications FIGURE 14-4 (Norton) An Acme-Thread Power-Screw Jack FIGURE 14-5 (Norton) Servomotor-Driven Lead Screw The George W. Woodruff School of Mechanical For UseEngineering as a Positioning Device ME3180 Screws and Fasteners Cont’d Screws/bolts as fasteners can be arranged to take tensile loads, shear loads, or both Will study application of preloads to bolts/screw and fasteners Preloads can have significant benefit to their load-carrying abilities The George W. Woodruff School of Mechanical Engineering Design ME3180 of Screws, Fasteners and Connections - Definitions Definitions (See Fig. 8-1 or Fig. 14-2) • • • • • • d = major diameter, largest diameter of screw thread dr or d1 = minor diameter, smallest diameter of screw thread At = tensile stressed area. p = Pitch = 1/N, N = # of threads per inch l = Lead is distance nut travels parallel to screw axis per revolution Single threaded l = p, Double threaded l = 2p, Triple threaded l = 3p (NORTON) The George W. Woodruff School of Mechanical Engineering Design of Screws, Fasteners and Connections – Types of Thread Standards ME3180 Types of Thread Standards (See Fig. 8-1 or Fig. 14-2) 1. Unified National Standard (UNS) thread series (inch class) Two major unified thread series are: • UN series • UNR series Difference between them: • Root radius must be used in UNR series. Because of reduced thread stress concentration factors, UNR series thread has improved fatigue strengths. • Unified threads are specified by stating nominal major diameter, number of threads per inch, and thread series. Example: 5''/8 - 18 UNRF or 0.625''-18 UNRF, d-N UNRF 0.1250 - 40 UNC, 0.1250 - 44 UNF Unified thread is used in US and Britain, and has 60 thread angle. Thread Crests may The be flat or rounded. George W. Woodruff School of Mechanical Engineering ME3180 Design of Screws, Fasteners and Connections – Types of Thread Standards Cont’d 2. Metric type of threads (See Fig. 8-2 or Fig 14-2) There are two types: • M profile • MJ profile Differences between them • M profile is replacement of inch class and is basic ISO (International Standard Organization) 68 profile with 60 symmetric threads • ISO 68 is European standard not interchangeable with UNS • MJ profile has rounded fillet at root of external threads and larger minor diameter for external and internal threads. Useful where high fatigue strength is required. Metric threads are specified by writing the major diameter and pitch in millimeters M12 x 1.75 The George W. Woodruff School of Mechanical Engineering Design of Screws, Fasteners and Connections – Types of Thread Standards Cont’d ME3180 Square thread, Buttress and ACME threads (See Fig 14-3) • Used on screws when power is to be transmitted. FIGURE 14-3 (NORTON) – Square, Acme, and Buttress Threads Multiple Threads (Fig 15-1) Multiple threads help nut to latch onto bolt quicker than single threads would. It will take single thread one complete turn, double thread one half turn (2The thread George one W. Woodruff of Mechanical start), triple third turnSchool (3-start) to advanceEngineering the nut a distance p. ME3180 Screws and Fasteners Primary focus here is on fasteners. Since completion of manufacturing intricate parts often requires assembly of components, engineers are then confronted with task of fastening various members together. Number of fastener types can be used for this task including: • thread fasteners • riveted joints • welded joints • adhesive joints Interesting information about fastener materials indicate that bolt, rivet, or welded material should be strong and tough whereas members being fastened should ideally be soft and ductile. The George W. Woodruff School of Mechanical Engineering ME3180 Screws and Fasteners Cont’d Common element among screw fasteners is their thread. In general, thread is helix that causes screw to advance into the work-piece or nut when rotated. Threads may be external (screw/bolt) or internal (nut or threaded hole). After World War II, threads were standardized in Great Britain, Canada and United States to what is now called Unified National Standard (UNS) series as shown in Figure 14.2. thread angle 2α dm (NORTON) 45o chamfer angle The George W. Woodruff School of Mechanical Engineering ME3180 Screws and Fasteners Cont’d European standard is also defined by International Standard Organization (ISO) and has same thread cross-sectional shape, but uses metric dimensions, so is not interchangeable with UNS threads. Both UNS and ISO threads are in general used in United States. Both use 60 degrees included angle and define thread size By nominal thread (major) diameter d of external thread. Pitch p shown in Figure 15.1 is distance from one point on one thread to same point on an adjacent thread, and its unit is meters or inches. Parameter that can be used instead of pitch is threads per inch, n. Important relationship between pitch and threads per inch is P = 1/n dc = crest diameter. The George W. Woodruff School of Mechanical Engineering Figure 15.1 Parameters used in defining terminology of thread profile. ME3180 Screws and Fasteners Cont’d The George W. Woodruff School of Mechanical Engineering ME 3180 ME3180 Screws and Fasteners Cont’d The George W. Woodruff School of Mechanical Engineering ME3180 Screws and Fasteners Cont’d The George W. Woodruff School of Mechanical Engineering Screws and Fasteners Cont’d ME3180 Crests and roots are defined as flats to reduce stress concentration from that of sharp corner. Pitch diameter dp and root diameter dr are defined in terms of thread pitch p with slightly different ratios used for UNS and ISO threads. Lead l of thread is distance that screw would advance relative to the nut in one revolution. Figure 15.2 shows differences between single-, double-, and triple- threaded screws. If it is single thread, as shown in Figure 15.2a, l = p. Screws can also be made with multiple threads, also called multiplestart threads. Double thread (2-start) has two parallel grooves wrapped around diameter, like pair of helical “railroad tracks,” as shown in Figure 15.2b. In this case l = 2p. Triple thread (3-start) will have l = 3p, etc. Figure 15.2 (a) Single-, (b) double-, and (c) triple-threaded screws. The George W. Woodruff School of Mechanical Engineering ME3180 Screws and Fasteners Cont’d Advantages of multiple threads are smaller thread height and increased lead for fast advance of the nut. Some automotive power-steering screws use 5-start threads. However, most screws are made with only single thread (1-start). Fishing reels use 2-start threads. Different number of thread profiles can be used for wide variety of applications. Figure 15.3 shows two types. ACME thread profile is used for power screws and machine tool threads. ACME profile has thread angle of 29 degrees, whereas UNS profile has thread angle of 60 degrees. UNS, is used extensively today. Metric profile (M) is popular and is quite similar to the UNS profile. The George W. Woodruff School of Mechanical Engineering ME3180 Screws and Fasteners Cont’d Figure 15.4 shows more details of M and UNS thread profiles that are shown in Figure 15.3. from this figure ht = 0.5p/tan 30 = 0.8660p Once pitch p and largest possible thread height ht are known, various dimensions of the UNS and M thread profiles can be obtained. The George W. Woodruff School of Mechanical Engineering ME3180 Tape Handling Mechanism Tape handling mechanism is used to move 8mm data tapes between tape storage bins and tape reader. Mechanism is from a computer data backup device The George W. Woodruff School of Mechanical Engineering ME3180 Tensile Stress Area If threaded rod is subjected to pure tensile loading, one might expect its strength to be limited by area of its minor (root) diameter, dr. However, testing of threaded rods in tension shows that their strength is better defined by the average of minor and pitch diameters. Tensile-stress area At is defined as: d d At 4 ( p r 2 )2 Where for UNS threads: dp = d – 0.649519/N dr = d – 1.299038/N and for ISO threads: dp = d – 0.649519P dr = d – 1.226869P d = Outside diameter (major diameter) N = Number of threads per inch P = Pitch in mm Stress in a threaded rod due to pure axial tensile load F is then: The George W. Woodruff School of Mechanical Engineering F t At ME3180 Standard Thread Dimension Table 14-1 shows the principal directions of UNS threads and Table 14-2 shows the same for ISO threads. UNS threads smaller than 0.25-in diameter are designated by a gage number. A useful algorithm for determining the major diameter of numbered threads is to multiply the gage number by 13 and add 60. The result is its approximate major diameter in thousandth of an inch. The minor diameter = major diameter – pitch. The George W. Woodruff School of Mechanical Engineering ME3180 The George W. Woodruff School of Mechanical Engineering ME3180 The George W. Woodruff School of Mechanical Engineering ME3180 The George W. Woodruff School of Mechanical Engineering ME3180 UNS and ISO Standard Thread Series UNS has three standard series of thread pitch families: coarse-pitch (UNC), fine-pitch (UNF), and extra-fine-pitch (UNEF). ISO has two standard thread series: Coarse and fine series of threads Coarse Thread Series: Most common Recommended for ordinary applications, especially where • Repeated insertions and removals of screws are required • Where screw is threaded into soft material Coarse threads are less likely to cross or strip soft material on insertion. Fine Threads Series: More resistant to loosening from vibration than coarse threads because of their smaller helix angle and so are used in automobiles, aircraft, and other applications that are subject to vibration Extra Fine Thread Series Used where wall thickness is limited and their short threads are an advantage The George W. Woodruff School of Mechanical Engineering ME3180 UNS and ISO Thread Standards Cont’d Tolerance ranges for internal and external threads used to control their fits UNS Series: There are three fit classes, labeled Classes 1, 2, and 3 Class 1 has the broadest tolerances and is used for “hardware quality” (i.e., inexpensive) fasteners intended for casual use around home, etc. Class 2 defines closer tolerances for better quality fit between mating threads and is suitable for general machine design applications. Class 3 is highest precision and can be specified where closer fits are needed. Cost increases with higher class of fit. Letter designator indicates an external (A) or internal (B) thread. The George W. Woodruff School of Mechanical Engineering ME3180 UNS and ISO Thread Standards Cont’d Example of UNS Thread Specifications ¼ -20 UNC – 2A -> 0.250 in major diameter by 20 threads per inch, coarse series, Class 2 fit, external thread Example of ISO (Metric) Thread: M8 X 1.25 -> 8-mm diameter by 1.25-mm pitch thread in ISO coarse series All standard threads are right-hand (RH) by default unless specified as left-hand by adding LH to specification. Right-hand thread will advance the nut (or screw) away from you when either is turned clockwise. The George W. Woodruff School of Mechanical Engineering ME3180 Power Screws Power screw is device used to change angular (rotational) motion into linear motion, and it is used in lathes, and screws for vices, presses, and jacks. • Usually to transmit power • Also called lead screws The George W. Woodruff School of Mechanical Engineering ME3180 Power Screws Cont’d More specifically, power screws are used: 1. To obtain greater mechanical advantage in order to lift weight, as in screw type of jack for cars 2. To exert large forces, as in home compactor or press 3. To obtain precise positioning of axial movement, as in micrometer screw or lead screw of lathe In each of these applications, torque is applied to ends of screws through set of gears, thus creating load on the device. Fasteners we have discussed thus far may not be strong enough for all power screw applications. Power screws use the following thread profiles: square, ACME and buttress. FIGURE 14-3 (NORTON) Square, Acme, andEngineering Buttress Threads The George W. Woodruff–School of Mechanical Power Screws Cont’d ME3180 Square Thread • Provides greatest strength and efficiency (than other two) and eliminates any radial component of force between screw and nut. • More difficult to cut because of its perpendicular face • Modified square thread (not shown) is made with a 10 degrees included angle to improve its manufacturability. ACME Thread • Has 29 degrees included angle, making it easier to manufacture and also allowing use of split nut that can be squeezed radially against screw to take wear • Common choice for power screws that must take loads in both directions. Tensile stress area is: d d At 4 ( p r 2 )2 The George W. Woodruff School of Mechanical Engineering ME3180 Power Screws Cont’d Table 14.3 and Figure 15.5 should be used jointly in evaluating ACME power screws. The pitch diameter of ACME power screw thread is: dp = dc – 0.5p – 0.01 This equation is valid only when inches are used for dc and p. Buttress Thread • If axial load on screw is unidirectional, buttress thread can be used to The George W. Woodruff School of Mechanical Engineering obtain greater strength at root than either of other two. ME3180 The George W. Woodruff School of Mechanical Engineering ME3180 Power Screw Application Figure 14-4 shows one possible arrangement of power screw used as jack to lift load. Nut is turned by applied torque T and screw translates up to lift load P or down to lower it. There needs to be some friction at load surface to prevent screw from turning with load. In either case, there will be significant friction between screw and nut as well as between nut and base, requiring that thrust bearing be provided as shown If plain (i.e., non-rolling) thrust bearing is used, it is possible for bearing interface to generate larger friction torque than threads. FIGURE 14-4 An Acme-Thread The Power-Screw Jack Ball-thrust bearings are often used in this application to reduce these losses. George W. Woodruff School of Mechanical Engineering ME3180 Other Applications of Power Screws In linear actuators, which operate on same principle as shown in Figure 144, either motorize nut rotation to translate screw or motorize screw rotation to translate nut, shown in Figure 14-5. FIGURE 14-5 Servomotor-Driven Lead Screw for Use as a Positioning Device These devices are used in machine tools to move table and workpiece under cutting tool, in assembly machines to position parts, and in aircraft to move control surfaces as well as in many other applications. If rotation input is provided by servomotor or stepping motor in combination The George W. Woodruff School of Mechanical Engineering with precision lead screw, very accurate positioning can be obtained. ME3180 Power Screw Force and Torque Analysis This section is copied from pages 897 through pages 904 of Machine Design - An Integrated Approach, by Robert L. Norton, published by Prentice Hall, 1996. Note: Material covered in pages 400-403 in Shigley The George W. Woodruff School of Mechanical Engineering ME3180 The George W. Woodruff School of Mechanical Engineering ME3180 The George W. Woodruff School of Mechanical Engineering ME3180 The George W. Woodruff School of Mechanical Engineering ME3180 (for a square thread) * The George W. Woodruff School of Mechanical Engineering ME3180 * The George W. Woodruff School of Mechanical Engineering (8-1) ME3180 (8-6) * * The George W. Woodruff School of Mechanical Engineering ME3180 The George W. Woodruff School of Mechanical Engineering ME3180 14.5o * * The George W. Woodruff School of Mechanical Engineering ME3180 General Expression The George W. Woodruff School of Mechanical Engineering ME3180 * Note: total torque including collar torque The George W. Woodruff School of Mechanical Engineering * ME3180 Has a larger torque than 14.5b ACME Square * * The George W. Woodruff School of Mechanical Engineering ME3180 The George W. Woodruff School of Mechanical Engineering ME3180 The George W. Woodruff School of Mechanical Engineering ME3180 The George W. Woodruff School of Mechanical Engineering ME3180 The George W. Woodruff School of Mechanical Engineering