The nut of a 3/4-inch screw bolt is usually 3/4 inch thick, as it is considered that when the threads are in good contact, and collectively equal to the diameter of the bolt, that the mutual hold of the threads exceeds the strength either of the bolt or nut; and therefore that the bolt is more likely to break in two, or the nut to burst open, rather than allow the bolt to draw out of the hole, from the thread stripping off.
When screws fit into holes tapped directly into the castings or other parts of mechanism, it is usual to allow still more threads to be in contact, even to the extent of two or more times the diameter of the screw, so as to leave the preponderance of strength greatly in favour of the hold; that the screw, which is the part more easily renewed, may be nearly certain to break in two, rather than damage the casting by tearing out the thread from the tapped hole.
Should the internal and external screws be made in the same material, that is both of wood, brass or iron, the nut or internal screw is somewhat the stronger of the two. For example, in the screw fig. 615, the base of the thread is a continuous angular ridge, which occupies the whole of the cylindrical surface represented by the dotted line. Therefore the force required to strip off the thread from the bolt, is nearly that required to punch a cylindrical hole of the same diameter and length as the bottom of the thread; for in either case the whole of the cylindrical surface has to be stripped or thrust off laterally, in a manner resembling the slow quiet action of the punching or shearing engine.
But the base of the thread in the nut, is equal to the cylindrical surface measured at the top of the bolt, and consequently, the materials being the same, and the length the same, considering the strength of the nut for 615 to be 75, the strength of the bolt would be only 55, or they would he respectively as the diameters of the top and bottom of the thread; although when the bolt protrudes through the nut, the thread of the holt derives a slight additional strength, from the threads situated beyond the nut, and which serve as an abutment.
It is however probable that the angular thread will not strip off at the base of the threads, either in the screw or nut, but will break through a line somewhere between the top and bottom: but these results will occur alike in all, and will not therefore materially alter the relation of strength above assumed.
Comparing 614 t, 615, and 016, upon the supposition that the bolts and nuts exactly fit or correspond, the strengths of the three nuts are alike, or as 75, and those of the bolts are as 05, 55, and 35, and therefore the advantage of hold lies with the bolt of finest thread; as the finer the thread, the more nearly do the bolt and nut approach to equality of diameter and strength.
Supposing however for the purpose of explanation, that instead of the screws and nuts being carefully fitted, the screws arc each one-tenth of an inch smaller than the diameters of the reductive taps employed in cutting the three nuts; 614 would draw entirely out without holding at all; the penetration and hold of 615 would be reduced to half its proper quantity; and that of 016 to three-fourths; and the last two screws would strip at a line more or less elevated above the base of the thread; and therefore the more easily than if the diameters exactly agreed.
The supposed error, although monstrous and excessive, shows that the finer the thread, the greater also should be the accuracy of contact of such screws; and it also shows the impolicy of employing fine threads in those situations where they will be subjected to frequent screwing and unscrewing, and also to much strain. As although when they fit equally well, fine threads are somewhat more powerful than coarse, in hold as well as in mechanical power; the fine are also more subject to wear, and the] receive from such wear, a greater and more rapid depreciation of strength, than threads of the ordinary degrees of coarseness.
In a screw of the same diameter and pitch, the ultimate strength is diminished in a twofold manner by the increase of the depth of the thread; first it diminishes the transverse area of the bolt, which is therefore more disposed to break in two; and secondly, it diminishes the individual strength of each thread, which becomes a more lofty triangle erected on the same base, and is therefore more exposed to fracture or to be stripped off.
But the durability of machinery is in nearly every case increased by the enlargement of the bearing surfaces, and therefore as the thread of increased depth presents more surface bearing, the deep screw has consequently greater durability against the friction or wear, arising from the act of screwing and unscrewing. The durability of the screw becomes, in truth a fourth condition, to be borne in mind collectively with those before-named.
It frequently happens that the diameters of screwed works are so considerable, that they can neither break nor burst after the manner of bolts and nuts; and if such large works yield to the pressures applied, the threads must be the part sacrificed. If the materials are crystalline, the thread crumbles away, but in those which are malleable and ductile, the thread, instead of stripping off as a wire, sometimes bends until the resisting side presents a perpendicular face, then overhangs, and ultimately curls over: this disposition is also shown in the abrasive wear of the screw before it yields.
Comparing the square with the angular thread in regard to friction, the square has less friction, because the angular edges of the screw and nut, mutually thrust themselves into the opposed angular grooves in the manner of the wedge. The square thread has also the advantage of presenting a more direct thrust than the angular, because in each case the resistance is at right angles to the side of the thread, and therefore in the square thread the resistance is very nearly in the line of its axis, whereas in the angular it is much more oblique.