Thread ProtrusionCharlie ChongHow much protrusion is acceptable for fastener assembly?On the question raised on amount of thread protrusion above the nut to be considered asacceptable;Specific requirements were not mentioned in the standards (API / ASME) as far as I know.By common sense, the all the threads of the nut shall be engaged by the bolt or stud.Thus a minimum thread protrusion length is given to ensure that all the threads are engaged!(we do not need to convince this point by showing code requirements because this is requiredto ensure a full strength fastener assemblies as given in the design itself)At site, when installed and tightened, the nut shall protrude a distance of at least one threadbeyond the top of nut as sign of full thread engagement. Two threads engagement is usuallyadopted for double sure. Excessive protrusion should be avoided in view of clearance,accessibility and safety is important and this could damage to machinery, hinder operation orharm to personnel.Full engagement of threads could not be positively ascertained.
Full engagement of threads could not be positively ascertained on the left fastener assembly.On the right fastener assembly full thread engagement ensuring full fastener assemblycapacity can be positive ascertained, thus acceptable.Some Codes and Standards Requirements. API RP 686-2009 “Recommended Practice for Machinery Installation and Installation Design”- 11.10 Anchor bolts shall project a minimum of 2 threads above the fully engaged nut(s). API 570-1998 Piping Inspection Code "Inspection, Repair, Alteration, and Rerating of In-service Piping Systems" 5.11 INSPECTION OF FLANGED JOINTS…Fasteners should extend completely through their nuts. Any fastener failing to do so is considered acceptably engaged if the lack of complete engagement is not more than one thread.What if the threads were not fully engaged?API 570-1998 Piping Inspection Code "Inspection, Repair, Alteration, and Rerating ofIn-service Piping Systems"5.11 INSPECTION OF FLANGED JOINTSThe markings on a representative sample of newly installed fasteners and gaskets should be examinedto determine whether they meet the material specification. The markings are identified in the applicableASME and ASTM standards. Questionable fasteners should be verified or renewed.Fasteners should extend completely through their nuts. Any fastener failing to do so is consideredacceptably engaged if the lack of complete engagement is not more than one thread.(See Notes on Nuts and Bolts may shed some light on the relaxation)
Notes on Nuts and BoltsA screw thread is a helical groove on a shaft. When used for delivering power, it is called adrive screw. Drive screws arent really all that efficient, as they loose a significant amount ofpower to friction. However, this friction can be put to use in the case of threaded fasteners.You might say that a drive screw is an inclined plane wrapped around a post, while a fasteneris a wedge wrapped around a post.Bolt Terms A 1/2-13UNC-2A-3 bolt, with a 2" thread and a 1" shank.As nuts and bolts are not perfectly rigid, but stretch slightly under load, the distribution ofstress on the threads is not uniform. In fact, on a theoretically infinitely long bolt, the firstthread takes a third of the load, the first three threads take three-quarters of the load, and thefirst six threads take essentially the whole load. Beyond the first six threads, the remainingthreads are under essentially no load at all. Therefore, a nut or bolt with six threads acts verymuch like an infinitely long nut or bolt (and its a lot cheaper). Thread % %Sum 1 34% 34% 2 23% 55% 3 16% 71% 4 11% 82% Stress on bolt threads. Note how the majority of 5 9% 91% the stress is on the first thread to the left. 6 7% 98% Image from Spiralock.
There is little point in having more than six threads in anything. Nuts with National Coarsethreads typically have 5 threads in them, whereas nuts with National Fine threads have about8 threads. Nuts are usually stronger than the bolts they are on, which is to say that the boltwill usually break before the nut strips.It is often said that two threads must be exposed above a nut. The reason for this is that thefirst two threads of a bolt are often poorly formed, and may not engage the nut properly. Iftheyre not doing their share, the other threads in the nut will be overloaded, and the nut maystrip.Thread TermsMetric and American threads both conform to the same profile, a series of equilateraltriangles with the crests chopped off and the roots rounded. External Standard Thread ProfileThe depth of the threads is 54.127% of the distance between threads, and the radius of therounded root is 14.434% of the distance between threads. Another way of looking at it wouldbe to say that 1/8 of the height of each equilateral triangle is chopped off the top, and 1/4 of theheight off the bottom, leaving only 5/8 of the height available. (The height of an equilateraltriangle is equal to the width times half of the square root of three; 5/8 of this is 0.54127.)The root diameter of the thread is the nominal diameter minus 108.3% of the pitch of thethread. This means that fine threads have larger root diameters than coarse threads, and thuslarger tap drill sizes. For threading using a tap or die, most threads are not cut to full depth,but to 75% or so. The resulting threads are not quite as strong, but full depth threading is veryhard on the tap or die. Threading on a lathe presents no difficulty cutting to full depth.Thread Specifications
Thread specifications are written thus: 1/2-13UNC-2which means: bolt diameter threads per inch thread type fit classThere are four Fit Classes, ranging from falling-off-loose to scientific-instrument-tight. Class 1 = Loose Class 2 = Free Class 3 = Medium Class 4 = CloseThe class is followed by an A for external (screw) threads and a B for internal (nut) threads.Most are class 2. 3 is for precision assembly, and 4 is used for things like lathe lead screwsand measuring instruments.In November 1948, NATO issued a new standard for threads, the Unified National system.American bolts had flat-bottomed groves between threads, which interfered with Britishround-topped threads. Likewise, British bolts wouldnt fit American nuts. The Unified systemuses a round-bottom grove to fit the British threads, and a flat-topped thread to fit theAmerican threads, so it not only fit itself, but both existing systems. American/United National Threads Hex SAE SAE SAE TPI TPI Root Dia. Size Diameter Head Washer Washer Washer Coarse Fine Coarse Size ID OD Thickness #0 0.0600 - 80 0.0447 #1 0.0730 64 72 0.0560 3 1 1 #2 0.0860 56 64 0.0668 /32" /4" /32" #3 0.0990 48 56 0.0771 1 5 1 #4 0.1120 40 48 0.0813 /8" /16" /32" #5 0.1250 40 44 0.0971 5 3 3 #6 0.1380 32 40 0.1073 /32" /8" /64" 3 7 3 #8 0.1640 32 36 0.1299 /16" /16" /64" 7 1 1 #10 0.1900 24 32 0.1570 /32" /2" /16"
1 9 1 #12 0.2160 24 28 0.1722 /4" /16" /16" 1 3 9 5 1 /4" 0.2500 20 28 0.1850 /8" /32" /8" /16" 5 1 11 11 1 /16" 0.3125 18 24 0.2400 /2" /32" /16" /16" 3 9 13 13 1 /8" 0.3750 16 24 0.2940 /16" /32" /16" /16" 7 5 15 15 1 /16" 0.4375 14 20 0.3440 /8" /32" /16" /16" 1 3 17 /2" 0.5000 13 20 0.4000 /4" /32" 1-1/16" 3 /32" 9 7 19 /16" 0.5625 12 18 0.4540 /8" /32" 1-3/16" 3 /32" 5 15 21 /8" 0.6250 11 18 0.5070 /16" /32" 1-5/16" 3 /32" 3 /4" 0.7500 10 16 0.6200 1-1/8" 13 /16" 1-1/2" 1 /8" 7 /8" 0.8750 9 14 0.7310 1-5/16" 15 /16" 1-3/4" 1 /8" 1" 1.0000 8 12 0.8370 1-1/2" 1-1/16" 1-3/4" 1 /8"A much more complete table is available here.http://www.usi.edu/science/engrtech/COURSWRK/met371/UN_thrds.htm Metric ThreadsMetric threads use the same thread profile as SAE threads. The biggest difference is that thethread pitch (distance between consecutive threads) is given instead of threads per unitdistance. Hex ISO ISO ISO Coarse Fine Root Dia. Head Washer Washer Washer Diameter Pitch Pitch Coarse Size ID OD Thickness mm mm mm mm mm mm mm 1 0.25 0.7294 1.1 0.25 0.8294 1.2 0.25 0.9294 1.4 0.30 1.075 1.6 0.35 1.221 3.2 1.8 0.35 1.421 2 0.40 1.567 4 2.2 0.45 1.713 2.5 0.45 2.013 5 3 0.50 2.459 5.5 3.4 7.0 0.6 3.5 0.60 2.850 4 0.70 0.50 3.242 7 4.5 9.0 0.9
4.5 0.75 0.50 3.688 5 0.80 0.50 4.134 8 5.5 10 11 5.5 0.50 6 1.00 0.50 4.917 10 6.7 12.5 1.8 7 1.00 0.75 5.917 8 1.25 0.75 6.647 13 8.7 17 1.8 9 1.25 0.75 7.647 10 1.50 0.75 8.376 16 10.9 21 2.2 11 1.50 0.75 9.376 12 1.75 0.75 10.11 18 13.4 24 2.7 14 2.00 1.00 11.83 21 16 2.00 1.00 13.83 24 17.4 30 3.3 18 2.50 1.00 15.29 20 2.50 1.00 17.29 30 21.5 37.9 3.3Bolt StrengthThe Society of Automotive Engineering has issued standard J429, which sets forth standardsfor both strength. The SAE grade of a bolt is marked on its head in the form of short radiallines, the number of lines being two less than the SAE grade (i.e.. 3 lines for grade 5). SAE Grade Size Range Strength (psi) 1 1 /4" to 1-1/2" 60,000 1 3 2 /4" to /4" 74,000 7 1 2 /8" to 1- /2" 60,000 1 5 /4" to 1" 120,000 1 1 5 1- /8" to 1- /2" 105,000 1 1 7 /4" to 1- /2" 133,000 1 8 /4" to 1-1/2" 150,000ASTM standards are sometimes used as well; A325 bolts are the equivalent of SAE 5, andA490 bolts are the equivalent of SAE 8.PreloadA very misunderstood part of bolting stuff together is preload, which is the tension placed onthe bolt by the nut (as opposed to the load). A sufficiently high preload will protect the boltfrom fatigue as the load changes, as the varying load will change the clamping force on thebolted components, rather than the tension on the bolt. (This is not strictly true, but for a
tinkerer like me, its adequate.) As a rule of thumb, the preload should exceed the maximumload by 15% or so.In order for this to work, however, the joint must be stiffer than the bolt. For this reason, theshank of high-tech bolts are often necked down to the same diameter of the root of the thread.As long as it isnt thinner than the root of the thread, it isnt any weaker than the thread, andtherefore doesnt effect overall bolt strength, but it is significantly less stiff than the originalshank.There are two ways to measure preload on a bolt; a torque wrench, and by measuring theangle the nut has turned. Of the two, the latter is more accurate, as friction plays a significant- and more importantly, indeterminate - role when using a torque wrench. Torque = K × preload × diameterK, the so-called Nut Factor, usually varies between 0.3 and 0.1, and is very sensitive to anumber of factors, ranging from temperature to thread condition, even to how fast the bolt istightened.Measuring the angle the nut has turned is simply measuring how much the bolt is stretching,equal to the pitch (distance between threads) times the number of turns. Using this requiresthat the components being bolted dont compress much (or compress a known amount), andthat the "spring rate" of the bolt be known. Turns = preload ÷ (spring rate × pitch)For example, if the "spring rate" of a 1/2-13 bolt is 50,000 pounds per inch (note that I madethat up, and that most bolts will yield long before stretching an inch), and you need 500pounds of preload, youll need to stretch the bolt 500 ÷ 50000 = 0.01 inch. At 13 threads perinch (0.0769 inches per thread), this would equate to 0.13 turns, or about 45° past snug.If more than one bolt is used in a joint, and those bolts are closer together than about fourdiameters, the preload on one bolt will effect the preload on the other bolts by compressingthe joint. This effect is called "crosstalk", and then all bets are off. Joints that are significantlyless stiff than the bolts, such as joints involving gaskets, suffer much worse from crosstalk.The best way to control crosstalk is to use a carefully thought out tightening sequence(usually a spiral starting at the center, or for circular patterns, alternating bolts), and to tightenthe bolts in small steps. Even so, its a crap shoot.
S9086-CJ-STM-010/CH-075R2 REVISION 2 NAVAL SHIPS’ TECHNICAL MANUAL CHAPTER 075 FASTENERS THIS CHAPTER SUPERSEDES NSTM CHAPTER 075 DATED 15 SEPTEMBER 1997 DISTRIBUTION STATEMENT A: APPROVED FOR PUBLIC RELEASE; DISTRIBUTION IS UNLIMITED. PUBLISHED BY DIRECTION OF COMMANDER, NAVAL SEA SYSTEMS COMMAND 1 DEC 1997 TITLE-1@@FIpgtype@@TITLE@@!FIpgtype@@
S9086-CJ-STM-010/CH-075R2 CHAPTER 75 FASTENERS SECTION 1. INTRODUCTION TO THREADED FASTENERS075-1.1 SCOPE075-1.1.1 GENERAL. This chapter covers the use and installation of threaded fasteners to repair and maintainshipboard equipment. The selection of proper threaded and locking methods shall be found in applicable partslists, drawings or technical manuals. The design and selection of threaded fasteners for speciﬁc applications isbeyond the scope of this chapter; however, references to design and selection procedures are provided. Guide-lines for selecting and tightening fasteners are also provided. The guidance in this manual may be used whenspeciﬁc information is unavailable or when emergency repairs are required.075-18.104.22.168 Many nonthreaded mechanical fasteners such as rivets, bands, lock rings, and clamps are notincluded in this document.075-22.214.171.124 Unless otherwise stated, all threaded fasteners discussed in this document have right-handedthreads; that is, they tighten (advance) when turned to the right (clockwise).075-126.96.36.199 The information in this manual covers the following:a. Identifying fasteners by their markings.b. How to distinguish between different types of similar fasteners.c. Properly using thread inserts.d. Identifying threaded fasteners by their markings, and by their thread class, ﬁt, series, and designation.e. Proper thread lubricants.f. Proper tightening requirements and recommended torque limits for fastener tightening. This manual also includes guidelines for calculating fastener torque requirements when they are unavailable.g. Thread fastener locking using self-locking fasteners and applying chemical thread-locking compounds.h. Selecting a suitable fastener system when the equipment speciﬁcations or the original fasteners or both are unavailable.i. Inspecting fasteners to see if they are suitable for reinstalling and inspecting installed fasteners.j. Repairing damaged fasteners.k. Removing stubborn or damaged fasteners.075-1.1.2 REFERENCE DOCUMENTS. To assist in developing Controlled Work Packages and to have adirect reference to the chemical and physical properties of fasteners as well as their speciﬁc manufacturingrequirements, the following documents will be of assistance: 75-1
S9086-CJ-STM-010/CH-075R2a. NAVSEA 0948-045-7010, Material Control Standard (Non-Nuclear) Volumes 1 and 2.b. NAVSEA 0948-LP-103-6010, Catalog of Level I/Subsafe Components.c. NAVSEA S9085-AM-GYD-010, Submarine Fastening Criteria (Non-nuclear). This contains additional infor- mation on design and maintenance techniques associated with submarine fasteners in pressure vessels and piping. Although not speciﬁed for surface ships, it contains helpful technical data.d. FED-STD-H28, Screw Thread Standards for Federal Services. (NOTE : Many of the FED-STD-H28 slash sheets reference ASME/ANSI (American Society of Mechanical Engineers/American National Standards Insti- tute) documents. Companion industry standards to FED-STD-H28 are included in Table 075-1-1.)e. VHS video tape titled Fastener Selection and Installation (PIN 805737). This VHS format video tape pro- vides information on how to identify and order replacement fasteners along with information on the proce- dures to be followed when installing fasteners.075-1.2 GENERAL075-1.2.1 USING FASTENERS. At ﬁrst glance nuts and bolts may appear to be simple devices. It is true thatthey are not complex mechanisms. If you consider the jobs they are required to do, however, they are verysophisticated devices, especially when you realize that a single 1/4-inch, SAE Grade 8 bolt and nut can hold upa full-size car (if the ﬁxture through which the bolt passes also can support the required load). A 1/4-20 UNC3A, SAE Grade 8 bolt has a tensile stress area of 0.0318 square inch with a proof load capacity of 120,000pounds per square inch. This gives the bolt an axial load capacity of 3,816 pounds, the approximate weight of acar.075-188.8.131.52 The point of all this is that the 1/4-inch bolt will hold up the car only if the proper thread classand material grade are selected and the bolt is installed properly. For example:a. The material strength and thickness of the nut has to be able to develop the full strength of the bolt.b. The nut has to be screwed on the bolt so that all the threads in the nut are fully engaged. If the bolt is screwed into a tapped hole, instead of using a nut, the hole should be tapped deep enough to develop the full strength of the bolt.c. The bolt holes in the ﬁxture that is bolted to the car have to be the proper size and drilled square with the nut and bolt bearing surfaces to prevent putting a bending load on the bolt head.d. If you expect to hold up the car for any length of time, you have to select a fastener with the proper coating to prevent corrosion.e. Grade 8 bolts are sensitive to hydrogen, which causes hydrogen embrittlement. You have to be careful, there- fore, about what corrosion protection coating you use in what environment: in certain environments some coatings will release atomic hydrogen into the fastener.f. A Grade 8 bolt gets a signiﬁcant part of its strength from heat treating. If you try to weld it to something, the bolt may be weakened and fail.g. If you expect to pick up the car very often, you will be unable to loosen the bolt and retighten it each time, and you will have to design the bolt ﬁxture so that the bolt can be properly preloaded to protect it from fatigue failure.h. Additionally, you should always add a safety factor to the calculated design stress to ensure that the fastener will not fail in service.75-2
S9086-CJ-STM-010/CH-075R2 Of course, no one would use a 1/4-inch bolt and nut to pick up a car. This example just shows you the thingsto consider when you select a fastener to do a job. This manual is designed to make you, the user, aware of someof these considerations and the pitfalls or consequences of choosing the wrong fastener. Table 075-1-1 COMPANION INDUSTRY STANDARDS FED-STD-H28 ASME/ANSI Area Covered Document No. Companion StandardNomenclature, Deﬁnitions, and Letter Symbols FED-STD-H28/1 ASME/ANSI B1.7Mfor Screw ThreadsUniﬁed Inch Screw Threads - UN and UNR FED-STD-H28/2 ASME B1.1Thread FormsControlled Radius Root Screw Threads, UNJ FED-STD-H28/4 ASME B1.15SymbolUniﬁed Miniature Screw Threads FED-STD-H28/5 ANSI B1.10Gauges and Gauging for Uniﬁed Screw Threads - FED-STD-H28/6 ASME/ANSI B1.2UN and UNR Thread FormsInspection Methods for Acceptability of UN, FED-STD-H28/20 ASME/ANSI B1.3MUNR, UNJ, M, and MJ Screw ThreadsMetric Screw Threads FED-STD-H28/21 ASME/ANSI B1.13M and ANSI B1.21MMetric Screw-Thread Gauges FED-STD-H28/22 ASME/ANSI B1.16M, B1.21MClass 5 Interference-Fit Screw Threads FED-STD-H28/23 ASME/ANSI B1.12075-184.108.40.206 Threaded fasteners are used throughout a ship to join individual parts of machinery, piping, andequipment mechanically. Because of this extensive use, threaded fasteners come in a wide variety of types, sizes,and materials. This great variety of fasteners, when combined with the special design requirements of shipboardequipment, requires you to be careful when using, maintaining, and replacing them.075-1.2.2 FASTENER MAINTENANCE PRACTICES. Many shipboard machinery and equipment casualtieshave been caused by improperly maintaining or installing threaded fasteners. The three main causes are substi-tuting a lower strength fastener, failing to use corrosion-resistant fasteners in corrosive environments, and improp-erly preloading. Proper strength and proper preload are especially important when high-impact (HI) shockrequirements need to be considered. A fastener may be entirely satisfactory for normal operating loads yet failwhen subjected to HI-shock loads during combat.075-1.2.3 HI SHOCK. HI-shock is the pressure pulse suddenly applied to a ship by a noncontacting underwa-ter explosion. This pressure pulse has a high intensity, and, although less severe than the shock pulse caused bythe direct impact of a projectile, it is sensed by personnel as a high-intensity shock pulse; hence the term ″HI-shock.″075-220.127.116.11 A signiﬁcant difference in the two forms of shock that is important to fastener design is that, underHI-shock, the stresses and strains (stretching of the bolt) can be calculated by conventional means using normalphysical properties. The speed with which the shock load is applied and the resulting speed at which the fasteneris stretched (strain rate) is lower than for direct impact of a projectile. The resulting strain rates are low enoughso that the mechanical properties of a fastener, such as the modulus of elasticity and the yield and tensile strength,do not increase signiﬁcantly. Under direct impact shock loads, however, the strain rate is high enough to causethese properties to increase signiﬁcantly, requiring more elaborate calculations. 75-3
S9086-CJ-STM-010/CH-075R2075-18.104.22.168 All shipboard equipment, systems, and components are assigned one of three HI-shock grades.Grade A shock is assigned to items that must remain fully functional during and after the application of HI-shockloads. Grade B shock is assigned to items that do not have to remain functional but are not allowed to present ahazard to personnel at assigned battle stations or to Grade A shock items. Essentially, Grade B items shall notcome adrift or rupture their pressure boundary. Grade C shock is assigned to the remaining shipboard items,which, either by their basic design or their location, do not present a hazard.075-22.214.171.124 Because of shock qualiﬁcation requirements, substitution of fasteners of different designs or mate-rials should not be made on equipment subject to Shock Grade A or Shock Grade B requirements without engi-neering analysis and approval.075-1.2.4 GLOSSARY OF TERMS. Be careful when using common names or slang terms. The fastener worldhas become more complicated. Terms that were acceptable, such as Cr-Mo, may now be ambiguous. ASTM A193 Grade B7 bolts are in fact bolts made from a steel alloy containing chromium and molybdenum, whereasASTM A193 Grade B16 bolts are bolts made from a steel alloy containing chromium, molybdenum, and vana-dium. When someone uses the term Cr-Mo, it is very difficult to know which of the two steel alloys they arereferring to. Table 075-1-2 is an alphabetical listing of terms used in this chapter. Table 075-1-2 GLOSSARY OF TERMS Term DeﬁnitionAARH Arithmetic average roughness height (Ra), is the arithmetic average of the height of the grooves or serrations in the ﬂange of a bolted joint measured from the nominal or ideal surface to the peaks of the serrations, not peak to valley.allowance Minimum clearance (positive allowance) or maximum interference (negative allow- ance) between mating parts.alloy steel Steel containing signiﬁcant quantities of alloying elements (other than carbon and the commonly accepted amounts of manganese, silicon, sulfur, and phosphorus) added to obtain speciﬁc mechanical or physical properties, such as toughness, strength at elevated temperatures, and corrosion-resistance.anaerobic thread-locking com- A liquid that solidiﬁes in the absence of air; used to secure threaded fasteners againstpound loosening in service.bearing face or surface The surface that is at right angles to the fastener centerline and that bears against the part or parts that it fastens; the area under the nut or head of a bolt.body The unthreaded portion of the shank of an externally threaded fastener.body-bound bolt See ﬁtted bolt.bolt A threaded fastener with an integral, usually hexagonal, head on one end intended to be used with a nut. Sometimes incorrectly called a capscrew.bolt stud A headless fastener threaded with the same form and ﬁt of thread on both ends or continuously threaded throughout its length. Generally used with a nut on each end.bottoming Screwing a capscrew or stud into a tapped hole until it contacts the bottom of the hole.capscrew A threaded fastener with an integral, usually hexagonal, head on one end that is intended for use in a tapped hole. It has stringent controls on its dimensions so that it can be more easily screwed into a tapped hole.clamping force The force, created by tightening a threaded fastener.class of thread An alphanumerical (letter and number combination) designation to indicate the stan- dard grade of tolerance and allowance speciﬁed for a thread.clearance ﬁt A ﬁt between mating, assembled parts that provides a clearance at their maximum material condition. (See ″ﬁt″).75-4