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1、Chapter 8 Threaded Joints,8.1 Selection of Joint Type and Fastening Method 8.2 Threaded Fasteners 8.3 Screw Thread Standards and Terminology 8.4 Strength under Static Load 8.5 Load Distribution between Threaded Parts in a Group Joint 8.6 Tightening and Locking,8.1 Selection of Joint Type and Fasteni
2、ng Method,Virtually 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.
3、The 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 method,Selection of the type of joint to be used and the method of fastening depends
4、upon 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?
5、Whether 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 methods,Potential fastening methods include interference fits, threaded fasteners, welding, bonding (brazing, soldering, adhesi
6、ve bonding), 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
7、 cost of 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 fi
8、ts are widely 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 s
9、tandard hand 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 faste
10、ning,8.2 Threaded Fasteners,The 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 sc
11、rew, or stud, 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
12、 the parts to 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 n
13、ut). A nut has 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.,M
14、any different 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 (
15、see above figure).,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
16、 nut for one nut 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
17、 due to the steeper 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.,
