It will be noted in the formulae for bolt strengths that different values for S are assumed. This is necessary on account of the uncertain initial stresses which are produced in setting up the nuts. For cases of mere fastening, the safe tension is high, as just before the joint opens the tension is about equal to the load and yet the fastening is secure. On the other hand, bolts or studs fastening joints subjected to internal fluid pressure must be stressed initially to a greater amount than the working pressure which is to come on the bolt. As this initial stress is a matter of judgment on the part of the workman, the designer, in order to be on the safe side, should specify not less than 5/8-inch or |-inch bolts for ordinary work, so that the bolts may not be broken off by a careless workman accidentally putting a greater force than necessary on the wrench handle. In making a steam-tight joint, the spacing of the bolts will generally determine their number; hence we often find an excess of bolt strength in joints of this character.
Through bolts are preferred to studs, and studs to tap bolts or cap screws. If possible, the design should be such that through bolts may be used. They are cheapest, are always in standard stock, and well resist rough usage in connecting and disconnecting. The threads in cast iron are weak and have a tendency to crumble; and if a through bolt cannot be used in such a case, a stud, which can be placed in position once for all, should be employed - not a tap bolt, which injures the thread in the casting every time it is removed.
The plain portion of a stud should be screwed up tight against the shoulder, and the tapped hole should be deep enough to prevent bottoming. To avoid breaking off the stud at the shoulder, a neck, or groove, may be made at the lower end of the thread entering the nut.
To withstand shearing forces the bolts must be fitted so that no lost motion may occur, otherwise pure shearing will not be secured.
Nuts are generally made hexagonal, but for rough work are often made square. The hexagonal nut allows the wrench to turn through a smaller angle in tightening up, and is preferred to the square nut. Experiments and calculations show that the height of the nut with standard threads may be about ½ the diameter of the bolt and still have the shearing strength of the thread equal to the tensile strength of the bolt at the root of the thread. Practically, however, it is difficult to apply such a thin wrench as this proportion would call for on ordinary bolts. More commonly the height of the nut is made equal to the diameter of the bolt so that the length of thread will guide the nut on the bolt, give a low bearing pressure on the threads, and enable a suitable wrench to be easily applied. The standard proportions for bolts and nuts may be found in any handbook. Not all manufacturers conform to the United States standard; nor do manufacturers in all cases conform to one another in practice.
If the bolt is subject to vibration, the nuts have a tendency to loosen. A common method of preventing this is to use double nuts, or lock nuts, as they are called (see Fig. 55 A). The under nut is screwed tightly against the surface, and held by a wrench while the second nut is screwed down tightly against the first. The effect is to cause the threads of the upper nut to bear against the under sides of the threads of the bolt. The load on the bolt is sustained therefore by the upper nut, which should be the thicker of the two ; but for convenience in applying wrenches the position of the nuts is often reversed.
The form of thread adapted to transmitting power is the square thread, which, although giving less bursting pressure on the nut, is not as strong as the V thread for a given length, since the total section of thread at the bottom is only ½ as great. If the pressure is to be transmitted in but one direction, the two types may be combined advantageously to form the buttress thread of the proportions shown in Fig. 59. Often, as in the carriage of a lathe, to allow the split nut to be opened and closed over the lead screw, the sides of the thread are placed at a small angle, say 15°, to each other, as illustrated in Fig. 60.
The practical commercial forms in which we find screwed fastenings are included in five classes, as follows:
1. Through bolts (Fig. 61), usually rough stock, with square upset heads, and square or hexagonal nuts.
2. Tap bolts (Fig. 62), also called cap screws. These usually have hexagonal heads, and are found both in the rough form, and finished from the rolled hexagonal bar in the screw machine.
3. Studs (Fig. 63), rough or finished stock, threaded in the screw machine.
4. Set screws (Fig. 64), usually with square heads, and case-hardened points. Many varieties of set screws are made, the principal distinguishing feature of each being in the shape of the point. Thus, in addition to the plain hoveled point, we find the " cupped," rounded, conical, and "teat" points.
5. Machine screws (Fig. 64d), usually round, " button," or countersunk head. Common proportions are indicated relative to diameter of body of screw.
1. Calculate the diameter of a bolt to suatain a load of 6,000 lbs.
3. With a wrench 16 times the diameter of the bolt, and an efficiency of 10 per cent, what axial load can a man exert on a standard ¾-inch bolt, if he polls 40 lbs. at the end of the wrench handle ?
4. A single, square-threaded screw of diameter 2 inches, lead ¼ inch, depth of thread 1/8 inch, length of nut 3 inches, is to be allowed a bearing pressure of 300 lbs. per square inch. What axial load can be carried ?
5. Calculate the shearing stress at the root of the thread in problem 4.