From these reasons, the square thread is commonly selected for presses, and for regulating screws, especially those in which rapidity of pitch, combined with strength, is essential; but as regards the ordinary attachments in machinery, the grasp of the angular thread is more powerful, from its pitch being generally about as fine again, and as before explained, angular screws and nuts are somewhat more easily fitted together.
The force exerted in bursting open a nut, depends on the angle formed by the sides of the thread, when the latter is considered as part of a cone, or as a wedge employed in splitting timber. For instance, in the square thread screw, the thread forms a line at right angles to the axis, and which is dotted in figure 619; it is not therefore a cone, hut simply compresses the nut, or attempts to force the metal before it. In the deep thread tig. 620, the wedge is obtuse, and exerts much less bursting effort than the acute cone represented in the shallow thread screw fig. 621; therefore the shallower the angular thread, the more acute the cone, and the greater the strain it throws upon the nut. The transverse measure of nuts, whether they are square or hexagonal, is usually about twice the diameter of the bolt, as represented in the figures, and this in general suffices withstand the bursting effort of the bolt.*
Those nuts, however, which are not used for grasping, but for the regulating screws of slides and general machinery, are made much thicker, so as to occupy as much of the length of the screw as two, three, or more times its diameter; this greatly increases their surface-contact, and durability.
Should it be required to be able to compensate the nut, or to re-adapt it to the lessened size of the screw when both have been worn, the nut is made in two parts and compressed by screws, or it is made clastic, so as to press upon the screw. The nuts for angular threads are divided diametrically, and re-united by two or more screws, as in fig. 022, in fact, like the semicircular bearings of ordinary shafts; as then by filing a little of the metal away from between the two halves of the nut, they may be closed upon the angular ridges of the thread.
The nuts of square threads, by a similar treatment, would on being closed, fit accurately upon the outer or cylindrical surface of the square thread screw, but the lateral contact would not be restored; these nuts are therefore divided transversely, as shown in fig. 623, or they are made as two detached nuts placed in contact. When therefore a small quantity is removed from between them with the file, or that they are separated by one or more thicknesses of paper, the one half of the nut bears on the right hand side of the square worm, the other on the left.
* In the table of the dimensions of nuts, in Temiiloton's Engineer's Pocket Companion, the transferse measures decrease in the larger nuts; the breadth of the nut for a 1/4 inch bolt is stated as 1 inch, that for a 2 1/2 inch bolt as 4 inches.
Either of these methods removes the "end play," or the "loss of time," by which expression is meant that partial revolution, to and fro, which may be given to a worn screw without producing any movement or traverse in the slide upon which the screw acts. It is usual, before cutting the nuts in the lathe or with screw taps, to divide the nuts, and to re-unite them with soft solder, or it is better to hold them together with the permanent screws whilst cutting the thread.
But the screws of slides are very apt to become most worn in the middle of their length, or at the one end, leaving the other parts nearly of their original size: it is then best to replace them by new screws, as the former method of adjusting the nuts cannot be used; although recourse may occasionally be had to some of the various methods of springing, or the elastic contrivances commonly employed in delicate mathematical and astronomical instruments. Although these should be perfectly free from shake or uncertainty of motion, they do not in general require the firm, massive, unyielding structure of engineering works and machinery.
Two kinds of the clastic nuts alone are shown; in fig. 624 the saw-cut extends throughout the length of the nut, but sometimes a portion in the middle is left uncut; the nut is usually a little set-in, or bent inwards with the hammer, so as to press upon the screw. In fig. 625, the two pieces a and b, bear against opposite sides of the thread, and b only is fixed to the slide, as in fig. 623; the correction is now accomplished by interposing loosely around the screw, and between the halves of the nut, a spiral spring sufficiently strong to overcome the friction of the slide upon the fittings; the same contri\anee is variously modified, sometimes two or four spiral springs are placed in cavities parallel with the screw.
The slide resists firmly any pressure from a to b, as the fixed half of the nut lies firmly against the side of the thread presented in that direction, but the pressure from b to a is sustained alone by the spiral spring; when therefore the pressure exceeds the strength of the spring, the slide nevertheless moves endways to the extent of the misfit in the piece b, and which, but for the spring, would allow the slide to shake endways. In absolute effect the contrivance is equivalent to a single nut such as b alone, which although possessing end play, if pulled towards b by a string and weight, would always keep in contact with the one side of the worm, unless the resistance were sufficient to raise the weight. The method is therefore only suited to works requiring delicacy rather than strength, and the spring, if excessively strong, would constantly wear the two halves of the -nut with injudicious friction and haste.
The several threads represented in figs. 620 to 638, may be considered to be departures from the angular thread fig. 626, and the square thread fig. 635, which are by far the most common.