UNIT 9b: SCREW FASTENERS Introduction A mechanical screw is a cylinder or cone that has a helical ridge called a thread. A helix has one or more turns, so a screw can have several turns. If the helix is on the outside surface of a cylinder or a cone, it is an external thread. If the helix is on the inside surface of a hollow cylinder or cone, it is an internal thread. Threaded components or screws are of two types: screw fasteners and power screws. Screw fasteners are used to hold two or more components together in a detachable joint. Power screws are designed to transmit power and they provide a large mechanical advantage in devices. The ends of external threads are normally chamfered at 45 o for easier starting and engagement. Fastening devices are used in almost all manufactured products to hold two or more components together in an assembly. Fasteners may be divided into two groups: threaded and unthreaded. Threaded fasteners use screw threads to develop the gripping force that hold components together. Unthreaded fasteners use other means like friction to develop the grip that keeps components together in the assembly. Examples of threaded fasteners are bolts and nuts. Examples of unthreaded fasteners are rivets, pins, keys, etc. Screw threads have been standardized nationally and internationally. Functions Screws are used to: 1) fasten two or more parts together in an assembly 2) transmit power in some equipments 3) control devices for precision measurements. Screw Features The features of a thread are the elements of its profile. A thread profile describes the shape of the thread. The International Standards Organization initiated standardization of screw threads in 1946.This led to the establishment of the Metric Screw Thread that allows interchangeability of screw threads at the international level and should be the first choice in design selection. In 1948, the Unified Screw Thread Standard was established by the United States, Britain and Canada. It allowed these countries to interchange screw parts. Fig. 1 shows the elements of an external and internal thread from. a) External thread b) Internal thread Fig. 1: Thread nomenclature Elements Terms of a Thread Profile Axis : the center line that passes through the screw thread cylinder in the longitudinal direction. Chamfer : the angular relief at the last thread. It allows easier assembly. Crest : the peak or top surface of a thread. Root : the bottom of a screw thread. Depth : the distance between the crest and the root of a thread in a direction normal to the axis. Major diameter : the largest diameter on a screw thread. Minor diameter : the smallest diameter on a screw thread. Pitch diameter : the diameter of an imaginary cylinder between the major and minor diameters. Pitch: the distance between corresponding points along the axis of a thread. TPI : threads per inch (English threads) 1 Lead: the axial distance a point on a screw thread moves when it is rotated through 360 o. Thread angle : the angle between two adjacent surfaces of a thread. Helix angle : the angle of a screw thread helix from the normal to the axis. Common Types of Thread Profiles Screw threads have been standardized nationally and internationally. The international standard is the Metric thread. The national standard is the Unified National thread in the U.S.A. Metric Thread This is the international standard thread profile. The profile and proportions of external Metric screw are shown in Fig. 2. The flat root profile has flat crest and root while the rounded root profile has flat crest and rounded root. The internal profile for Metric screws is the flat root type. Flat root profile screws are identified by the letter “M” while rounded root profile screws are identified by letters “MJ”. The MJ profile is preferred in high fatigue stress environment and is popular in the aerospace industry. Fig. 2: Profile and proportions of external Metric screws Unified National Thread This is the standard agreed on by United States, United Kingdom and Canada on November 18, 1949. The standard is identified by the letters “UN”. The profile and proportions of UN or English screws are shown in Fig. 3. The external thread has a flat crest and a round root. A variation has a rounded crest. Fig. 3: Profile and proportions of English screws Knuckle Thread: Thread has curved flanks, others have straight flanks. Fig. 4 shows the profile and proportions of a knuckle thread. It is used on britle materials such as glass, ceramics, plastics, or thin materials like sheet metals. These materials do not readilly accept densely spaced sharp countors. Knuckle threads are molded or rolled on componentes. Bottle tops glass jars, and base of light bulbs are typical components with knuckle threads. Fig. 4: Knuckle thread 2 Thread Styles: Right- and Left-Hand Thread The direction of inclination of the helix angle of a thread determines whether it is a right- or left-hand thread. A right- hand thread has a helix angle inclined backwards while a left-hand thread has a helix angle inclined forward as shown in Fig. 5. When a right-hand thread is turned clockwise, it tightens and loosens if turned counterclockwise. A left-hand thread tightens when is turned counterclockwise and loosens if turned clockwise. Threads are assumed to be right- handed except specified otherwise. a) Right-hand thread b) Left-hand thread Fig.5: Thread styles Multiple Start Threads Some screws may have more than one thread start. These screws are called multiple start threads and common types are the double and triple start threads. The number thread starts determine the lead of a screw. Fig. 6 shows common start threads. a) Single start a) Double start a) Triple start Fig. 6 Common number of thread starts Thread Series Thread series are based on the type of pitch. The Metric thread has two series: coarse and fine pitch series. For the same major diameter size, the fine pitch series has more threads or smaller pitches. English thread has four series: coarse (UNC), fine (UNF), extra fine (UNEF), and constant pitch. In the constant pitch series, different major diameter sizes have the same pitch or number of threads per inch (TPI). The pitch for English threads is the reciprocal of TPI. Thread Classes A thread class of fit determines the manufacturing precision. Table 1 shows the classes of threads in the Metric system, while Table 2 shows the classes of threads in the English system. Fit Classes Fit Name Applications Internal Threads External Threads Free 7H 8g For quick and easy assembly Medium 6H 6g For general engineering applications Close 5H 4g For precision applications Table 1: Metric thread classes Class Name Applications Class 1 For quick assembly and when play is acceptable Class 2 For general-purpose applications and for threads of mass production Class 3 For precision tools, high stress and vibration applications Table 2: English thread classes 3 Thread Specification A thread specification provides necessary information about the thread for manufacture or purchase. Threads may be specified in basic or detailed form. Fig. 7a shows a basic specification of a Metric thread while Fig. 7b shows a detail specification. Fig. 8a and 8b show the basic and detail specification of threads respectively in the English units. Table 3 gives the interpretations of the thread elements shown in Fig. 7 while Table 4 gives the interpretation of the thread specifications interpretations of the thread elements shown in Fig. 8. The threads per inch (TPI) element of English thread, is the reciprocal of the thread pitch. thread, is the reciprocal of the thread pitch. ) Basic specification ) Detail specification (c) (d) Fig. 7: Metric thread specifications Table 3: Interpreting Metric thread specification ITEM Description ITEM Description 1 Metric thread identifier 5 Major diameter tolerance specification 2 Major diameter (mm) 6 Minor diameter tolerance specification 3 Separator 4 Pitch (mm) ) Basic specification ) Full specification (c) (d) Fig. 8: English thread specifications Table 4: Interpreting English thread specification ITEM Description ITEM Description 1 Major diameter or Number reference 7 Left hand thread (RH = Right hand thread) 2 Threads per inch (TPI) 8 Number of starts 3 Unified National 9 Separator 4 Coarse (Series identifier) 10 Length value 5 Class 11 Length identifier 6 External thread (B = Internal thread) 4 Thread Representations Threads may be represented in three (3) ways: pictorial, schematic and simplified. The pictorial representation is also called detail and may be an isometric drawing or solid model. The schematic representation is an approximation of the pictorial. The simplified representation is quick and easy to drawn and it is the most common in industry. Fig. 9 shows representations for external thread while Fig. 10, Fig. 11, and Fig. 12 show representations for internal thread. a) Pictorial b) Schematic c) Simplified Fig. 9: External thread representations a) Pictorial a) Through hole c) Blind hole Fig. 10: Pictorial internal thread Fig. 11: Schematic internal thread representations a) Through hole b) Through hole c) Blind hole Fig. 12: Simplified internal thread representations Types and Applications of Threaded Fasteners There are three types of threaded fasteners: studs, bolts, and screws. Studs are needed when large forces are present in joints. Bolts are used for medium to large forces are set up in joints. Screws are used in light services and are of three types: cap screw, machine screws, and set screws. Fig. 13 shows applications of a stud, bolt, and cap screw. Fig. 14 shows applications of machine and set screws. Metric bolt heads, cap screw heads and nuts have hexagonal styles; but English screws have more varieties of head styles. Stud: Stud is a headless fastener with cylindrical shank that is threaded on both ends, usually separated by an unthreaded portion.