There are three characteristic diameters of threads : " major diameter ", " minor diameter ", and " pitch diameter " : Industry standards specify minimum ( min ) and maximum ( max ) limits for each of these, for all recognized thread sizes.
22.
As shown in the figure at right, threads of equal pitch and angle that have matching minor diameters, with differing major and pitch diameters, may appear to fit snugly, but only do so radially; threads that have only major diameters matching ( not shown ) could also be visualized as not allowing radial movement.
23.
This adds the requirement that the sum of the allowances and tolerances on the pitch diameters ( PD ); the drawback is that, for a given amount of end play ( axial clearance, due solely to PD deviations ), closer tolerances and cleaner work environment are necessitated in the application of a centralizing Acme thread.
24.
Provided that there are moderate non-negative clearances between the root and crest of the opposing threads, and everything else is ideal, if the pitch diameters of a screw and nut are exactly matched, there should be no play at all between the two as assembled, even in the presence of positive root-crest clearances.
25.
The pitch diameter ( PD, or " D " 2 ) of a particular thread, internal or external, is the diameter of a cylindrical surface, axially concentric to the thread, which intersects the thread flanks at equidistant points, when viewed in a cross-sectional plane containing the axis of the thread, the distance between these points being exactly one half the pitch distance.
26.
A belt drive is analogous to that of a chain drive, however a belt sheave may be smooth ( devoid of discrete interlocking members as would be found on a chain sprocket, spur gear, or timing belt ) so that the mechanical advantage is approximately given by the ratio of the pitch diameter of the sheaves only, not fixed exactly by the ratio of teeth as with gears and sprockets.
27.
Examples were a French mechanic surnamed Senot, who in 1795 created a screw-cutting lathe capable of industrial-level production, and Standardization of threadforms ( including thread angle, pitches, major diameters, pitch diameters, etc . ) began immediately on the intra-company level, and by the end of the 19th century, it had been carried to the international level ( although pluralities of standards still exist ).
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