Introduzione al disegno e alla progettazione delle macchine

Le prime righe

Fig. 7. Fig. 7. Fig. 8. Fig. 8. III. SCREWS, BOLTS, AND NUTS. Screw Threads.—The various forms of screw threads used in machine construction are shown in fig. 9. The Whitworth V thread is shown at (a). This is the standard form of triangular thread used in this country. The angle between the sides of the V is 55°, and one-sixth of the total depth is rounded off both at the top and bottom. At (b) is shown the Sellers V thread, which is the standard triangular thread used by engineers in America. In this form of thread the angle between the sides of the V is 60°, and one-eighth of the total depth is cut square off at the top and bottom. The Square thread is shown at (c). This form is principally used for transmitting motion. Fig. 9. Fig. 9. Comparing the triangular and square threads, the former is the stronger of the two; but owing to the normal pressure on the V thread being inclined to the axis of the screw, that pressure must be greater than the pressure which is being transmitted by the screw; and therefore, seeing that the normal pressure on the square thread is parallel, and therefore equal to the pressure transmitted in the direction of the axis of the screw, the friction of the V thread must be greater than the friction of the square thread. In the case of the triangular thread there is also a tendency of the pressure to burst the nut. The Buttress thread shown at (e) is designed to combine the advantages of the V and square threads, but it only has these advantages when the pressure is transmitted in one direction; if the direction of the pressure be reversed, the friction and bursting action on the nut are even greater than with the V thread, because of the greater inclination of the slant side of the buttress thread. The angles of the square thread are