Page:Miscellaneous Papers on Mechanical Subjects.djvu/40

32 between the pitch and the diameter varies throughout the entire scale. Thus the pitch of the ¼ inch is one-fifth of the diameter—that of the ½-inch, one-sixth—of the 1-inch, one-eighth—of the 4-inch, one-twelfth—of the 6-inch, one-fifteenth. It is obvious that more power is required as the diameter increases. But this consideration alone will not account for the actual deviation, which is much less than it would be if the scale were calculated with reference to the power required. The amount of power necessary must be determined in relation to the muscular force of the human arm, aided by the leverage of the screw key. Now, in the case of smaller screws, there is a considerable excess of force. Again, in the larger, there will be found a deficiency of power, for with all the leverage which can generally be applied, it requires the force of several men to fix a bolt of six inches diameter. Hence it is evident that at the two extremes of the scale the amount of power required is not the leading consideration in fixing the pitch of the thread. In the smaller sizes the necessary depth of a coarser thread—as before observed—would too much weaken the centre part of the screw. It may also be mentioned that coarse threads would render small screws apt to work loose for want of sufficient hold to prevent the effect of jarring.