Page:Miscellaneous Papers on Mechanical Subjects.djvu/37

Rh modified; but practically no one character can be determined irrespective of the others. The pitch of the square thread is generally twice that of the angular for the same diameter, to retain similar proportions of power and strength. Coarse threads should be deep as compared with fine, to provide against the wear from friction. A coarse angular thread will also require additional depth to preserve the due proportion of power, and to prevent the longitudinal strain from being thrown too much sideways on the nut. Hence, each character acts as a limit to the variation of the others, and in some instances (that is, in the case of certain diameters), it will be found that the leading consideration in fixing one character is the resulting effect on another. Thus, in some of the smaller sizes, the pitch is determined principally by reference to the depth—a coarser thread being objectionable, because the extra depth would too much weaken the centre part of the bolt—while the necessary shallowness of a finer thread would render it too liable to wear with friction.

The proportional strength of the thread and centre part of the screw is regulated mainly by the depth of the nut, which is generally of the same measure as the diameter of the bolt. Assuming that dimension as fixed, the proportion of strength between the two parts will vary with