LESSON 12 INTRODUCTION TO THREADED FASTNERS
In threaded joints two or more machine members are joined together with the help of threaded fastening e.g. a nut and bolt. These are non-permanent type joints i.e. members can be disassembled without damaging the component parts for the purpose of maintenance, checking and replacement. Threads are formed by cutting a helical groove on the surface of a cylindrical rod or cylindrical hole. Threaded fasteners are standardized and a wide variety is available for different operating conditions and applications. These are easy to manufacture and a high accuracy can be maintained. Holes are required in the machine parts to be assembled by threaded joints, which lead to stress concentration. Another disadvantage is that, threaded joints tend to loosen when subjected to vibrations.
12.2 Terminology of Screw Threads
Figure 12.1 shows some important terms used in screw threads
Figure 12.1 Terms used in Screw Threads
Major diameter: It is the largest diameter of an external or internal screw thread. The screw is specified by this diameter. It is also known as outside or nominal diameter.
Minor diameter: It is the smallest diameter of an external or internal screw thread. It is also known as core or root diameter.
Pitch diameter: It is the diameter of an imaginary cylinder, on a cylindrical screw thread, the surface of which would pass through the thread at such points as to make equal the width of the thread and the width of the spaces between the threads. It is also called an effective diameter. In a nut and bolt assembly, it is the diameter at which the ridges on the bolt are in complete touch with the ridges of the corresponding nut.
Pitch: It is the distance from a point on one thread to the corresponding point on the next. This is measured in an axial direction between corresponding points in the same axial plane.
Mathematically, Pitch = 1/ No. of threads per unit length of screw
Lead: It is the distance between two corresponding points on the same helix. It may also be defined as the distance which a screw thread advances axially in one rotation of the nut. Lead is equal to the pitch in case of single start threads; it is twice the pitch in double start, thrice the pitch in triple start and so on.
Crest: It is the top surface of the thread.
Root: It is the bottom surface created by the two adjacent flanks of the thread.
Depth of thread: It is the perpendicular distance between the crest and root.
Flank: It is the surface joining the crest and root.
Angle of thread: It is the angle included by the flanks of the thread.
Slope: It is half the pitch of the thread.
12.3 ISO Metric Screw Threads
In screws used for fastening, generally V-threads are used. They provide higher friction thus reducing the chances of loosening. They have higher strength because of higher thickness at the core and also are convenient to manufacture. Profile of an ISO metric screw thread is shown in Figure 12.2. It consists of an equilateral triangle with side equal to pitch and internal angle 60° as thread angle. Crests and roots of the threads are rounded, which reduces stress concentration in the threads and also increases the life of thread cutting tool.
Figure 12.2 Profile of an ISO Metric Screw Thread
Metric threads are divided into coarse and fine series. Thread profiles in both the series is similar. Coarse threads, considered the basic series, have higher static load carrying capacity, are easier to cut, have less effect on strength because of manufacturing errors and wear and have more even stress distribution. On the other hand fine threads have greater strength against fluctuating loads and have greater resistance to unscrewing because of its lower helix angle. Therefore fine series threads are more dependable in terms of self loosening. Coarse threads are used in members, which are free from vibrations and fine threads are used in parts subjected to dynamic loads and hollow thin walled parts as the coarse threads will weaken the members considerably. Fine threads are also used in the parts where the threads are used for the purpose of adjustment.
A screw thread of coarse series is designated by the letter 'M' followed by the value of the nominal diameter in mm. For example ‘M 12’. A screw thread of fine series is specified by the letter ‘M’ followed by the values of the nominal diameter and the pitch in mm and separated by the symbol ‘×’. For example. M 12 × 1.25.
Threads are produced by rolling or machining. Because of cold work, the rolled threads are stronger and have better fatigue properties. Threads can also be produced using casting. Selection of material for threaded fasteners depends upon type of loading, operating environment and temperature etc. Plain Carbon Steel is used for common applications and Alloy Steels are used in high temperature applications and where high strength, better fatigue and corrosion resistance is required. Aluminium, Brass and Bronze are also used in specific applications. Generally a factor of safety of 2 to 3 on the basis of yield strength is considered in case of carbon steels and 1.5 to 3 for alloy steels.
12.5 Types of Screw Fasteners
Bolt (Through Bolt): It is a cylindrical bar with threads for the nut at one end and head at the other end. The cylindrical part of the bolt is known as shank. It is passed through drilled holes in the two parts to be fastened together and clamped them securely to each other as the nut is screwed on to the threaded end. Bolts have hexagonal or square heads.
Figure 12.3 Through Bolt, Tap Bolt and Stud
Tap bolts: Tap bolt is screwed into a tapped hole of one of the parts to be fastened and nut is not used with it.
Studs: A stud is a round bar threaded at both ends. One end is screwed into a tapped hole of the parts to be fastened, while the other end receives a nut on it.
Cap screws: The cap screws are similar to tap bolts except that they are of small size and a variety of shapes of heads are available.
Figure 12.4 Types of Cap Screws
Machine screws: These are similar to cap screws with the head slotted for a screw driver and are generally used with a nut.
Set screws: Set screws are used to prevent relative motion between the two parts. A set screw is screwed through a threaded hole in one part so that its point (i.e. end of the screw) presses against the other part. This resists the relative motion between the two parts by means of friction between the point of the screw and one of the parts.
Figure 12.5 Types of Set Screws
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