Modern Machine-Shop Practice Part 223

[49] From "_Steam Boilers_."

"For instance, take three cylindrical boilers each made of 1/2 inch plate, the first one 2 feet 6 inches in diameter; the second twice that, or 5 feet in diameter; and the third twice that again, or 10 feet in diameter; and if the 2 foot 6 inch boiler is fit for a safe working pressure of 180 lbs. per square inch, then the 5 foot boiler will be fit for exactly one-half that amount, or 90 lbs. per square inch; and the ten foot boiler will be fit for half the working pressure of the five foot boiler, hence we have:

-----------------+------------+-------------------------- Diameter of | Thickness |Relative working pressure.

boiler sh.e.l.l. | of plate. | -----------------+------------+-------------------------- 2 feet 6 inches.| 1/2 inch. | 180 lbs. per square inch.

5 " | " " | 90 " " " "

10 " | " " | 45 " " " "

"The reverse applies to the thickness of the plate. For instance, if we take two cylindrical boiler sh.e.l.ls, each 5 feet in diameter, the first one made of plate 1/2 inch thick, and the second twice that, or 1 inch thick, and if the first is equal to a safe working pressure of 90 lbs.

per square inch, then the second is equal to a safe working pressure of twice as much, or 180 lbs. per square inch, providing, of course, that the riveted seams are of equal strength in each case, and that both boilers are allowed the same margin for safety; hence we have:

----------------+------------+-------------------------- Diameter of | Thickness | Safe working pressure.

boiler. | of sh.e.l.l. | ----------------+------------+-------------------------- 5 feet. | 1/2 inch. | 90 lbs. per square inch.

5 " | 1 " | 180 " " " "

[Ill.u.s.tration: Fig. 3237.]

[Ill.u.s.tration: Fig. 3238.]

"These principles (namely, that the strength of a boiler is, all other things or elements being equal, inversely proportional to its diameter, and directly proportional to its thickness) afford us a groundwork upon which we may lay down rules for determining by calculation the strength of the solid part[50] of any boiler sh.e.l.l, and the bases of these calculations are as follows:

[50] In the case of the riveted joints or seams other considerations come in, as will be shown hereafter.

"If the sh.e.l.l plate of a cylindrical boiler is 1/2 inch thick, there is one inch section of metal to be broken before the boiler can be divided into two pieces, that is to say there is 1/2 inch on each side of the sh.e.l.l, as shown in Fig. 3237, and the two together will make 1 inch. If we take a ring an inch broad, as, say, at A in Fig. 3238, we shall obviously have a section of 1 square inch of metal to break before the ring can be broken into two pieces.

"The next consideration is, what is the average strength of a plate of boiler iron? Now suppose we have a strip of boiler iron 2 inches wide and 1/2 inch thick, or, what is the same thing, a bar of boiler iron 1 inch square, and that we lay it horizontally and pull its ends apart until it breaks, how many lbs. will it bear before breaking? Now for our present purpose we may a.s.sume this to be 47,040 lbs., and if this number of lbs. be divided by the diameter of the boiler in inches, it will give the bursting pressure in lbs. for any square inch in the ring, or any other square inch in the cylindrical sh.e.l.l of the boiler.

"The reason for dividing by the diameter of the boiler is as follows:

[Ill.u.s.tration: Fig. 3239.]

"Of course the steam pressure presses equally on all parts of the interior surface of the sh.e.l.l, and may be taken as radiating from the centre of the boiler, as in Fig. 3239, which represents an end view of a strip an inch wide, of one half of a boiler. Now leaving the riveted seam out of the question, and supposing the sh.e.l.l to be truly cylindrical, and the metal to be of equal quality throughout, it will take just as much pressure to burst the sh.e.l.l apart in one direction as it will in another, hence we may suppose that the boiler is to be burst in the direction of arrow _a_, and it is the section of metal at _b_ _b_ that is resisting rupture in that direction.

"Now suppose we divide the surface against which the steam presses into six divisions, by lines radiating from the centre C, and to find the amount of area acting on each division to burst the sh.e.l.l in the direction of arrow _a_, we drop perpendicular lines, as line _e_, from the lines of division to the line _b_ _b_, and the length of the line divided off (by the perpendicular) on the diameter represents the effectiveness of the area of that division to burst the boiler in the direction of arrow _a_; thus for that part of the boiler surface situate in the first division, or from _b_ to line _e_, the area acting to burst the boiler in the direction of _a_ is represented by the length of the line _k_, while the general direction of the pressure on this part of the sh.e.l.l is represented by arrow _m_.

"Similarly, for that part of the sh.e.l.l situate between vertical line _e_ and vertical line _f_, the general direction of the steam pressure is denoted by the arrow _l_, while the proportion of this part that is acting to sever the boiler in the direction of _a_ is represented by the distance _n_, or from the line _e_ to line _f_ measured on the line _b_ _b_.

[Ill.u.s.tration: Fig. 3240.]

"By carrying out this process we shall perceive that, although the pressure acts upon the whole circ.u.mference, yet its effectiveness in bursting the boiler in any one direction is equal to the boiler diameter. Thus in Fig. 3240, the pressure acting in the direction of the arrows _a_ (and to burst the boiler apart at _b_ _b_) is represented by the diametral line _b_ _b_, while the pressure actually exerted upon the whole boiler sh.e.l.l is represented by the circ.u.mference of the boiler.

"To proceed, then, it will now be clear that the ultimate strength of the boiler material, multiplied by twice the thickness of the boiler sh.e.l.l plate in inches or decimal parts of an inch, and this sum divided by the internal diameter of the boiler, in inches, gives the pressure (in lbs. per square inch) at which the boiler sh.e.l.l will burst."

We have here only considered the strength of the solid plate of the sh.e.l.l, and may now consider the strength of the riveted joints, because, as the boiler cannot be any stronger as a whole than its weakest part is, and as the riveted joints are the weakest parts of a cylindrical boiler,[51] therefore the strength of the riveted joint determines the strength of the boiler.

[51] It may be here noted that the riveted joint of a flat plate is stronger than the flat surface of the plate, because at the joint the plate is doubled, or one plate overlaps the other.

[52]"The strains to which a riveted joint is subjected are as follows: That acting to shear the rivet across its diameter is called the _shearing_ strain. But the same strain acts to tear the plate apart; hence, when spoken of with reference to the action on the plate, it is called the _tearing_ strain.

[52] From "_Steam Boilers_."

"The same strain also acts to crush and rupture the plate between the rivet hole and the edge of the plate, and in this connection it is called the _crus.h.i.+ng_ strain.

[Ill.u.s.tration: Fig. 3241.]

"Thus, Fig. 3241 represents a single riveted lap joint, in which the joint at rivets A, B, and C is intact, the metal outside of D has crushed, the rivets E, F have sheared, and the plate has torn at H, leaving a piece J on the rivets K L.

"It is obvious that, since it is the same strain that has caused these different kinds of rupture, the joint has, at each location, simply given way where it was the weakest.

[Ill.u.s.tration: Fig. 3242.]

"If a riveted joint was to give way by tearing only, the indication would be that the proportion of strength was greatest in the rivets, which might occur from the plate being of inferior metal to the rivets, or from the rivets being too closely s.p.a.ced. If the rivets were to shear and the plate remain intact, it would indicate insufficient strength in the rivets, which might occur from faulty material in the rivets, from smallness of rivet diameter, or from the rivets being too widely s.p.a.ced.

"The object then, in designing a riveted joint is to have its resistance to tearing and shearing proportionately equal, whatever form of joint be employed."

The English Board of Trade recommends that the rivet section should always be in excess of the plate section, whereas, in ordinary American practice, for stationary engine boilers, the plate and rivet percentages are made equal.

The forms of riveted joints employed in boiler work are as follows:

[Ill.u.s.tration: Fig. 3243.]

[Ill.u.s.tration: Fig. 3244.]

Fig. 3242 represents a single riveted lap joint. Fig. 3243 represents a double riveted lap joint, chain riveted; and Fig. 3244, a double riveted lap joint, with the rivets arranged zigzag.

[Ill.u.s.tration: Fig. 3245.]

[Ill.u.s.tration: Fig. 3246.]

Fig. 3245 represents a single and Fig. 3246 a double riveted b.u.t.t joint, so called because the ends of the boiler plate abut together. The plates on each side of joint are called b.u.t.t straps.

The advantages of the b.u.t.t joint are, first, that the boiler sh.e.l.l is kept more truly cylindrical, and the joint is not liable to bend as it does in the lap joints, in the attempt of the boiler (when under pressure) to a.s.sume the form of a true circle, and second that the rivets are placed in double shear. That is to say, if in a lap joint the rivet was to shear between the plates, the joint would come apart, whereas, in a b.u.t.t joint, the rivet must shear on each side of the plate, and therefore in two places.

[Ill.u.s.tration: Fig. 3247.]

Fig. 3247 represents a form of joint much used in locomotive practice in the United States. It is a lap joint, with a covering plate on the inside of the joint; rivets E and F are in single and rivets D in double shear.

[53]"When we have to deal with comparatively thin boiler plates, there is no difficulty in obtaining a sufficiently high percentage of strength in the joints, by using the ordinary double riveted joint, but when we have to deal with thick plates, as in the case of large marine boilers, as 1 inch or upwards, a more costly form of joint must be employed, in order to obtain the required percentage of strength at the joint; hence the ordinary double riveted joint is replaced by various other forms as follows:

[53] From "_Steam Boilers_."

[Ill.u.s.tration: Fig. 3248.]

"First, a triple zigzag riveted lap joint, such as shown in Fig. 3248, or a chain riveted joint as in Fig. 3249, in both of which the third row of rivets enables the rivet pitch to be increased, thus increasing the plate percentage, while the third row of rivets also increases the rivet percentage.

"Second, by employing b.u.t.t joints with b.u.t.t straps, either double or treble riveted.