1、 Chapter 8 Threaded Joints 8.1 Selection of Joint Type and Fastening Method8.2 Threaded Fasteners8.3 Screw Thread Standards and Terminology 8.4 Strength under Static Load8.5 Load Distribution between Threaded Parts in a Group Joint8.6 Tightening and Locking 8.1 Selection of Joint Type and Fastening

2、MethodVirtually all machines and structures, both large and small, comprise an assemblage of individual parts, separately manufactured, and joined together to produce the complete article. Most joint types permit choices among a variety of different permanent and removable fastening techniques. The

3、basic challenge is to design the joint so that the components may be assembled and secured economically, with maximum joint integrity. Several basic types of joints Selection of the joint type and the fastening methodSelection of the type of joint to be used and the method of fastening depends upon

4、many factors, including the loading direction, magnitude, and spectrum characteristics, The sizes, thickness, geometries, and weights of the parts to be joined, the precision of alignment and dimensional tolerances required, and the cost of assembly. Whether the load is symmetric or eccentric?Whethe

5、r materials to be joined are the same or different?Whether the joint is to be permanent or detachable?Whether the joint must be pressure sealed? Potential fastening methodsPotential fastening methods include interference fits, threaded fasteners, welding, bonding (brazing, soldering, adhesive bondin

6、g), crimping, staking, clamping, or the use of pins, retaining rings, clips, or other specialty fasteners. Which method is selected by the designer depends largely upon the choice of basic joint type, the forces to be transmitted, whether a detachable fastener is required or desired, and the cost of

7、 assembly. For permanent joints, welding is widely used, but rivets are also frequently chosen. Various bonding techniques are used in suitable applications. Pins, retaining rings, and set screws are chosen for special applications, such as retaining shaft-mounted components.Interference fits are wi

8、dely used for mounting rolling element bearings, gears, flywheels, and other similar components. They may be used to join components of the same or different materials, for simple or complex joint configurations, in factories, or in the field.They may be readily and safely installed with standard ha

9、nd tools or power tools, and if maintenance or repair is required they may be removed or replaced just as easily. Because threaded fasteners are so widely standardized, interchangeability and low unit cost are virtually guaranteed, irrespective of manufacturer. Advantages of threaded fastening8.2 Th

10、readed FastenersThe main standard threaded fasteners include bolts, studs, screws and nuts. They are used in combination with washers and nut locks (retainers) of various design.The basic threaded fastening system consists of an external (male) threaded element such as a bolt, machine screw, or stud

11、, assembled to a mating internal (female) threaded element, such as a nut, threaded insert, or tapped hole. A bolt is a bar with a thread for a nut at one end and a head at the other.A stud is a bar threaded at both ends; the mating end of the stud is screwed into a tapped hole in one of the parts t

12、o be connected while the nut is screwed onto the other end.Studs are used when the design of a joint provides no space for the bolt head or when a blind hole has to be drilled. A screw differs from a bolt in that its threaded portion is screwed into one of the fastened parts (without a nut).A nut ha

13、s a threaded hole to engage the threaded end of a bolt or stud and is the locking part of the system: bolt (stud)fastened parts-nut.A washer is placed under a nut or the head of a bolt (screw) to transmit and distribute evenly the force exerted on the connected parts or to lock them. Many different

14、threaded fastener styles are commercially available off the shelf.Various standard head styles and thread configurations and materials and grades (standardized strength levels) are available (Sec.8.3). Many different types of nuts, locknuts, washers, and lock washers may be obtained (see above figur

15、e).The pitch, p, is the axial distance between corresponding points on adjacent threads. The major diameter, d, is the largest diameter of the (male) thread and the minor diameter, d1, is the smallest (root) diameter of the thread. The lead, l, is the axial displacement of the mating nut for one nut

16、 rotation. For a single thread the lead is equal to the pitch. For double threads or triple threads, the lead is equal to twice the pitch or three times the pitch, respectively. Multiple threads are rarely used in threaded fastener applications because they tend to loosen more easily due to the stee

17、per lead angle.8.3 Screw Thread Standards and Terminology The size of a screw thread refers to the nominal major diameter, or, for a male thread, the nominal diameter of the stock on which the helical thread is cut. Selected dimensional information is shown in Table 8.1 for Metric standard threads.S

18、crew thread profile and terminology Manufacturing of Thread fasteners Threads are manufactured either by cutting or forming. Smaller sizes may be produced by utilizing cutting tools called taps for internal threads or dies for external threads. Larger sizes are usually lathe-turned. Thread forming i

19、s accomplished by rolling a blank between hardened dies that cause the metal to flow radially into the desired thread shape. This cold-rolling process produces favorable residual stresses that enhance fatigue strength, and results in a smoother, harder, more wear-resistant surface. High-production-r

20、ate automatic screw machines utilize the thread-forming process.Heads are typically formed by a cold-forming process called upsetting in which the fastener blank is forced to flow plastically into a head die of the desired shape. Thread series and classesCoarse-series threads are advantageous where

21、rapid assembly or disassembly is required, or to reduce the likelihood of cross-threading. Fine-series threads are used where higher bolt strength is important, since smaller thread depth and larger root diameter provides higher basic tensile strength. Fine threads have less tendency to loosen under

22、 vibration than coarse threads. An extra-fine series may be used in special cases where more precise adjustments must be made or for thin-wall tubing applications. Thread classes distinguish standard specified ranges of dimensional tolerance and allowance. Classes 1A, 2A, and 3A apply to external th

23、reads only, and classes 1B, 2B, and 3B apply to internal threads only. Tolerances decrease (higher precision) as class number increases. Classes 2A and 2B are the most commonly used. Materials of threaded fastenersBecause of the many advantages of selecting a steel material, carbon steels and steel

24、alloys are by far the most widely used materials for threaded fastener applications. Common steels used for threaded fasteners include 10, 20 , 35 (high-strength fasteners), 45 (special high-strength requirements) or 40Cr.In addition to the standard steel materials, fasteners may be made of alloys o

25、f aluminum, brass, copper, nickel, stainless steel, or beryllium, from plastics, and, for high-temperature applications Specifications of threadsScrew threads are specified by designating in sequence the nominal size, pitch, series, class, and hand. MS 101.5, defines a 10-mm-diameter thread with thr

26、ead pitch of 1.5 mm, rounded root (MS profile), external thread (since rounded root profiles are used only on external threads). For a left-hand thread the designation LH would be appended.A system of head marking that identifies material class or grade and minimum strength level for each individual

27、 fastener has been standardized, as shown in Figure 8.5 for metric bolts and cap screws. Table 8.2 provides corresponding material and property information.Number to left of decimal point designates approximately minimum tensile strength, Su , in hundreds of mega-pascals; approximate yield strength,

28、 Syp , in each case, is obtained by multiplying Su times decimal fraction to the right of (and including) the decimal point. In this table the proof of strength is defined as the minimum tensile stress that must be sustained by the fastener without significant deformation or failure, and generally c

29、orresponds to about 85 percent of the yield strength. 8.4 Strength under Static LoadIn designing threaded joints it is important not only to assess correctly the size of parts in order to ensure proper strength in accordance with the accepted design, but also to choose such structural features as wo

30、uld preclude the appearance of additional loads during operation which are not provided for by the design.Depending on the nature of loading and the method of assembly threaded joints are either prestressed or used unstressed.In most cases threaded joints are prestressed, i.e., their threaded parts

31、and, consequently, the connected members are tightened with a certain torque before external load is applied to prevent the separation of the joint and loss of tightness. Under static load threaded parts seldom fail. Under heavy overloads the bolt shank may be shorn off in the solid or threaded sect

32、ion and the thread stripped, bent of crushed. Some typical cases of bolt failure due to fatigueStatistical analysis of thread failure shows that about 90% of all failures are due to fatigue.Most bolt failures occur in the first or second thread of engagement (counting from the nut thrust face, Fig.a

33、); rupture in the imperfect thread portion (Fig.b) and near the bolt head (Fig.c) is much rarer.Breakdowns can be avoidedBy proper calculation which takes into account the specific features of the behavior of threaded parts under respective load conditions, by improving their design, by taking prope

34、r processing and operational measures and strictly observing assembly rules.Assembly prestressing causes more of less considerable tensions to arise in the threaded parts, which makes the method of designing such joints radically different from that used for joints which are not prestressed.1) Joint

35、s designed without initial stress The fastening of a hook to the traverse of a block tackle is an example of this kind of joints, in which no subsequent tightening of the joint is needed. In this case threaded bolt carries an axial load F. The condition for strength precluding the tear of the danger

36、ous section is:for unhardened bolts, screws and studs;for hardened and cyanided bolts. is the yield strength of the bolt material.2) Joints designed for axially loaded threaded partsSuch load conditions arise if, after the bolt has taken the force F, it is tightened (always in prestressed joints). I

37、n this case, the bolt shank will carry in addition, between the head and the nut, the torque on the thread T, whose value is determined by the ratio we know from the theory of mechanics.An example provided by a turnbuckle.Design of axially loaded threaded partsFor bolts made from ordinary carbon ste

38、el, =(0.20.25) for d = (616) mm and =(0.250.4) for d = (1630) mm.The resultant stress The minor diameter of thread 3) Joints designed with initial stressThis design is typical of the majority of group joints used in mechanical engineering to fasten covers, flanges, base plates, etc. In some cases th

39、ese joints should also be tight, in other cases the joint must not be allowed to yawn if this impairs the conjoint operation of the parts in the unit. Connecting-rod and foundation bolts, etc., must comply with these conditions.The joint between the cover and cylinder of an internal combustion engin

40、e.General analysis of such caseBy prestressing the threaded joints sufficiently, the joint yawn or leakage after the external load is applied can be prevented This means that after the external load F which tends to reduce the effect of the prestressing force F has been applied, the joint parts shou

41、ld be pinched together with the force F termed residual pressing load.The drop of the tightening force (F- F) is determined by the value of the external load F and the elastic properties of all parts in the joint (both threaded and held together).They are the external load imposed on the cover and b

42、olts and the tensile axial force carried by each bolt. The axial tensile force in the bolt is a function both of the initial preload force , due to tightening, and the subsequently applied operating force, F, which tends to separate the clamped members. To ensure normal operating conditions the join

43、t should be compressed with the force F provided by the initial prestressing force F.Such a preloaded bolted joint constitutes a statically indeterminate elastic system as shown above.The elastic deformation of the bolt due to the force F and the elastic deformation of the joint, respectively, are:A

44、fter an external load has been imposed on the joint (Fig. c) the bolt will carry an additional force which will stretch it further and relieve the jointthe force holding the joint tightly compressed will drop to F. Thus, the force acting on the bolt equals the sum of the external load and the residu

45、al load in the joint after the external load has been applied:The drop in the tightening force: According to the force-deformation relation of the joint parts shown above, Determination of residual load and the relative stiffnessThe residual stressing force is estimated on the basis of designing pra

46、ctice as a factor of the external load F according to the formula where is the experimental coefficient between 0.2-1.8 depending on operating conditions. To ensure proper sealing of pipe joints we take =1.5-1.8.The values of the coefficients of relative stiffness can be found in Table 8.3. The desi

47、gn procedure of the joints with initial stressDetermine the number of bolts and external load F carried by bolt (generally that resisting the heaviest load);Determine the required magnitude of residual stress:Find the design axial load:Calculate the cross-section of the bolt from the formulaAccordin

48、g to the design of the unit, decide the relative stiffness by Table 8.3;Determine the required prestressing force: When the bolt fits the hole with some play, the joint should be tightened before the external lateral load is applied in order to prevent inadmissible relative displacement of the eleme

49、nts in the direction of the acting forces. 4) Joints designed for shear-loaded threaded partsThe value of is determined by the required friction force Ff acting between the contact surfaces according to the condition: where f is the coefficient of friction and i -the number of joints. So For the des

50、igns shown (in Fig.8.12a) (i =2) and taking f = 0.2 for dry steel and cast-iron surfaces we obtain If the number of joins =1, the required stress =5. The result obtained shows that such designs cannot be considered rational because of the large diameter of bolts required ( ).It is expedient to provi

51、de for special devices in the joints which relieve the bolt from shearing forces and ensure relative immobility of the fastened parts.In the designs shown in Fig.8.12b, the shearing load is taken by a bush (sleeve) inserted into the hole with a small interference; the bolt is entirely free from shea

52、ring load. Fig.8.12c, d shows other possibilities (key and pin).Special devices resisting shearing forceWhen the bolt is inserted into a reamed hole with a small interference (Fig. 8.13) the shearing load is taken by the bolt shank. Such a joint is not necessarily prestressed.In this case the design

53、 equation will have the form given above; . The dimensions of the bolt shank should be checked for compression. . In these formulae: -for steel; -for cast iron.In the joints with a group of blots, the load is not necessarily uniformly spread between the bolts.In a joint fastened by several bolts the

54、 law of external load distribution depends on the design of the joint and the nature of load it carries.In group joints the bolts most frequently have equal diameters with a view to cut down the range of items required. In this case the diameter of all bolts in a joint is taken as equal to the diame

55、ter of the bolt resisting the heaviest load.8.5 Load Distribution between Threaded Parts in a Group JointSome of the most typical designs of group joints 1) A set of bolts carrying loads whose resultant is perpendicular to the contact surface and passes through its center of gravity.This occurs in s

56、ecuring round and rectangular pressure vessel covers. When bolts of equal length are arranged symmetrically it can be assumed that all bolts carry the same load.With prestressing the design load on the bolt is found from formulae (8-6) and (8-10) and the bolt diameter from the formula (8-13).2) A se

57、t of bolts resisting a shearing force acting in the plane of the joint along its axis of symmetry.We assume that the external load is distributed evenly while the design load depends on whether the bolts are fitted into the holes with an interference or clearance.In the first case In the second case

58、 . In these formulae z is the number of bolts, is the reliability coefficient considering the instable nature of friction transmission.3) A set bolts carrying design torque acting in the plane of the joint.Example: designing of a joint between two discs of a flange coupling:When the bolts are fitted

59、 with an interference the diameter of the bolt shank is found from the work in shear; the rated force acting on the bolt is When the bolts are inserted into the holes with a clearance the tightening force necessary to transmit torque from one disc of the coupling to the other through friction alone

60、is: 1) Tightening TorqueTo obtain the maximum benefits of preloading, it is essential that the designer-specified initial preload actually be induced in the bolt by the tightening process. The most accurate method of inducing the desired axial preload involves direct measurement of the axial force-i