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screw thread[′skrü ‚thred]
alternating helical grooves and ridges of regular profile formed on the surface of a part. The outline of the grooves and ridges on a plane passing through the axis of the part is called the thread profile. Various types of threads are distinguished according to profile, including V, trapezoidal, buttress, round, and square. Threads are classified as straight and taper (external and internal), depending on the shape of the surface on which they are formed. A distinction is also made
between right-hand and left-hand threads, according to the direction in which the thread winds. Single and multiple (double, triple) threads are distinguished on the basis of the number of threads. On the basis of use, it is possible to distinguish general-purpose threads, used to secure various parts, and special threads, such as threads for microscope lenses, the tubes of geological surveying instruments, or light-bulb bases and sockets.
V threads, which are classified as fasteners, are found most commonly in metric form, although they are also found in inch form in older parts. They are used primarily in plug-and-socket connections of machine parts because they ensure strength and a tight, lasting coupling during prolonged use.
Kinematic threads include trapezoidal threads, which convert rotary motion into progressive motion with minimum friction in screw-nut gears (lead screws in machine tools, screws of tables for measuring instruments). They also include buttress threads, which convert rotary motion into rectilinear motion (for example, in presses and jacks) and are also used for large unidirectional loads (for example, in the couplings of press shafts with crosspieces). Pipe threads—usually taper, occasionally straight—are used in pipelines and fittings of various types to ensure the seal of the coupling.
The principal parameters of both external and internal threads are external diameter d (or D for nuts), internal diameter d1 (or D1), mean diameter d2 (D2), pitch P, profile angle a, angles of inclination of the sides of the profile β and γ (which are equal to half the profile angle α/2 for threads with symmetrical profiles), angle of elevation of the thread, height of the initial profile H, height of the profile H2, working height of the profile H1, and screwing length of the thread or height of the nut.
The metric thread has a profile that is regulated by the All-Union State Standard (GOST) and standardized internationally. Its principal parameters are shown in Figure 1. The metric thread has three series of thread diameters, each with coarse and fine pitches. When a metric thread is selected, the first series is preferred to the second and the second to the third. Metric threads are designated, for example, M12 × 1.5, with the numbers indicating the diameter and pitch of the thread in mm. For threads with a coarse pitch, the pitch is not indicated, for example, M12.
The trapezoidal thread (Figure 2) has a profile that is standardized for all the member countries of the Council for Mutual Economic Asistance (COMECON) and is the same for single and multiple threads. Grades of fit are established on the basis of overall tolerance. Trapezoidal threads are marked as follows: Trap. 40 × 6; 40 is the diameter and 6 is the pitch in mm.
The buttress thread has the profile shown in Figure 3. The grade of this thread is determined by the overall tolerance of the mean diameter. There are first and second grades of fit specified for screw threads and one grade for nuts. The tolerance zone of d2 is arrayed for a sliding fit. Clearance for the mean diameter is obtained by increasing D2. For the external diameter, clearance is created by decreasing the d of the screw, and for the internal diameter, it is obtained by establishing different nominal values of d1 and D1. Buttress threads are designated as follows: B 80 × 10; 80 is the nominal diameter and 10 is the pitch in mm.
The taper thread has the profile shown in Figure 4. Thread diameters are measured in the base plane. Tolerance for the mean diameter of taper threads is not established as it is for straight threads. Deviation in this diameter is restricted indirectly by the limiting deviations of the base distance. In addition, a tolerance is established for the height of the thread profile. The standard designation of taper threads is TP¾”, where ¾” is the diameter of the pipe or tube in inches.
Grades of fit and degrees of precision have been standardized for all screw threads.
REFERENCESKorotkov, V. P., B. G. Kustarev, and A. V. Khnykina. Vzaimozameniaemost’ rez’bovykh sopriazhenii: Spravochnik. Moscow, 1968.
Detail mashin—Raschet i konstruirovanie: Spravochnik, 3rd ed., vol. 1. Edited by N. S. Acherkan. Moscow, 1968.
Iakushev, A. I. Vzaimozameniaemost’, standardizatsiia i tekhnicheskie izmereniia, 3rd ed. Moscow, 1974.
A. I. IAKUSHEV